Indian Institute of Science Bangalore
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Synthesis and Modulation-Doping of VO2 (001) Epitaxial Thin Film Heterostructures
No full textCorrelated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM that undergoes a temperature-driven metal-to-insulator transition (MIT) at ~340 K accompanied by a symmetry-lowering transition in the crystal structure. However, external control of MIT in VO2 – especially without inducing lattice strain in the parent phases - has been a long-standing challenge. All the previous approaches, for example, the control of MIT in VO2 by elemental doping, strain, oxygen vacancy creation, and hydrogenation affect the lattice parameters of VO2. In these cases, simultaneous lattice strain and carrier concentration change make it challenging and, sometimes, impossible to disentangle the role of carrier concentration changes from the role of lattice strain. On the other hand, the electric field-driven control of MIT could, in principle, enable conductivity modulation without causing lattice strain. However, it is limited by either the lack of sufficient field-induced electron density in the VO2 channel when utilizing solid-state gating or by the oxygen deficiency-induced lattice distortion in VO2 when using ionic liquid gating. We propose a VO2-based modulation-doped heterostructure in which carrier densities can potentially be modulated by remotely injecting carriers into VO2, which, in principle, enables us to control the metal-insulator transition temperature (TMIT) of VO2 without introducing lattice strain.
In Chapter 1, we introduce a comprehensive overview of the area of correlated electron materials and provide a broad explanation of metal-insulator transitions (MIT) phenomena in transition metal oxides. Specifically, we discuss Mott-Hubbard transition and Peierls transitions in the context of MIT in VO2. We also discuss crystal field theory (CFT) which explains the energy landscape near the fermi level depending on the electrostatic interaction between metal ions and the surrounding ligands in transition metal oxides. We further introduce vanadium dioxides as a prototypical example of CEMs and describe the electrical and structural phase transition that occurs at the metal-insulator phase transition temperature. Toward the end of this chapter, we provide a brief discussion of the state-of-the-art studies of MIT behavior in VO2. We also discuss the successes and challenges of controlling MIT by external stimulus including elemental doping, oxygen vacancy creation, hydrogenation, lattice strain, and application of electric field. We conclude that even though modulation of TMIT was successfully achieved in these approaches, it is always accompanied by microscopic changes to the crystal lattices.
In Chapter 2, we provide a detailed demonstration of the experimental techniques and specifically the important experimental methodologies which were utilized in this dissertation. First, we introduce the Pulsed Laser Deposition (PLD) system, the preferred thin film deposition technique for the growth of oxide thin films, which was employed for the growth of all the thin films used in these studies. Next, we discuss the fundamental working principles along with utilized measurement parameters of several characterization techniques which are crucial to measuring the physical properties of VO2 thin films. We discuss in-situ Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), High-Resolution X-ray Diffraction (HR-XRD), Reciprocal Space Mapping (RSM), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray Spectroscopy (EDS), Temperature-dependent electrical transport measurement, Temperature-dependent Hall measurement, X-ray Photoelectron Spectroscopy (XPS), and Wedge bonding. Finally, we present details of an in-house built low-temperature electrical transport measuring system (Dipstick), covering details of instrumentation, calibrations, and measurements.
In Chapter 3, we first introduce an overview of thin film growth mechanisms in general. Further, we comprehensively discuss the optimization conditions to accomplish atomically sharp and single-crystalline VO2 thin film growth on TiO2 (001) substrate using the PLD technique. By combining RHEED, AFM, asymmetrical RSM, and HR-XRD, we confirm the epitaxial growth of VO2 films. Further, the films are atomically flat, single crystalline, and highly oriented to the TiO2 (001) substrates. With the aid of temperature-dependent electrical transport and HR-XRD, we show that VO2 films undergo ~3.5 orders of magnitude change in resistance along with a concomitant structural phase transition across the MIT. Temperature-dependent Hall measurements suggest that such changes in resistance are predominantly contributed by the equivalent change in carrier densities whereas carrier mobility changes little, in comparison. Furthermore, temperature-dependent hard X-ray photoelectron spectroscopy (HAXPES) measurements of the V2p core level and valence band reveal the signature of the non-local screening as well as the shift in the density of states (DOS) across MIT. Finally, we demonstrate that the TMIT of VO2 (001) films remain almost identical at ~295 K throughout the thickness range of 9.5 nm to 1.5 nm. This finding is very crucial since in the following chapters we will demonstrate how this TMIT could be modulated by varying the thickness of VO2 using modulation-doped heterostructures.
In Chapter 4, we demonstrate the control of MIT by varying the carrier concentration in VO2 utilizing modulation-doped heterostructures where Nb:TiO2 layer was used as a dopant layer and the single crystalline VO2 (001) film as a channel layer. Here we show, the carrier densities can be modulated in two pathways – either by varying the dopant densities in the dopant layer (i.e. concentration of Nb in Nb:TiO2) for a constant thickness of VO2 or by varying the thickness of the VO2 (001) layer while maintaining a fixed dopant density in the dopant layer. Using a combination of temperature-dependent transport and hall measurements, we present that a systematic reduction in TMIT up to 40 K was achieved. A remarkable feature of this work is the deposition of the dopant layer at room temperature which ensures two important criteria for achieving high-quality modulation-doped heterostructures. Firstly, the low kinetics of the deposited dopant ions at room temperature prevents the dopant ions (Nb5+) from diffusing into VO2 and preventing the formation of NbxV1-xO2, which could lower the TMIT. Secondly, room temperature deposition of dopant layers also minimizes the migration of oxygen ions from VO2 to Nb:TiO2 layer which could result in oxygen vacancy formation in VO2 along with the induced strain and lowered TMIT. Both the transport and XPS data are not consistent with oxygen vacancy formation in VO2. We further show that, for all the heterostructures, in-plane lattice parameters of VO2 remain unchanged. However, interestingly, we found an asymmetrical shape of VO2 (002) peaks in the XRD spectra, which were not observed in pristine VO2 films. With the help of XRD simulations and time-evolved RHEED images during the growth of Nb:TiO2 dopant layers, we attributed the presence of this asymmetric shape to the formation of a few atomic layers of crystalline Nb:TiO2 layer on VO2 film even for room temperature growth. However, for such thin films, this makes it challenging to distinguish and exactly quantify whether these asymmetric natures in the spectra are due to out-of-plane strain in VO2 (if any) or due to the formation of Nb:TiO2 crystalline layers. Since both VO2 and Nb:TiO2 have a similar rutile crystal structure at room temperature, it's energetically more favorable (due to epitaxial stabilization) to grow a few crystalline atomic layers of Nb:TiO2 on VO2 even for deposition at room temperature. To address this, we propose the use of an amorphous spacer layer which we discuss in the next chapter.
In Chapter 5, we designed an upgraded version of the modulation-doped heterostructure that contains LaAlO3 (LAO) as a spacer layer between the VO2 channel and the dopant layer. Furthermore, to avoid the interdiffusion of higher valence metallic dopants such as (Nb5+) from the Nb-doped TiO2 dopant layer to the VO2, we use oxygen-deficient TiO2-x layer as a dopant layer instead of the Nb-doped TiO2 dopant layer. The crystal structure of LAO is different from that of rutile VO2 which enhances the possibility of the LAO layer being amorphous for growth at room temperature. Besides, it is an insulator with a high band gap of 5.6 eV. In addition, because of low oxygen vacancy-diffusivity in LAO, it prevents the oxygen migration from the VO2 to TiO2-x dopant layer. Here, we synthesize and demonstrate that these modulation-doped heterostructures closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, Hall measurements, and structural characterization, we demonstrate that the insulating state can be doped to achieve carrier densities greater than 5x1021 cm-3 without inducing any measurable structural changes. With the aid of Hall measurements, we find that the TMIT is strongly correlated with carrier concentration and decreases continuously (by ~65 K) with increasing carrier concentration. A remarkable feature of this study is the possibility of bulk metallization in modulation-doped VO2. A sharp MIT is observed for heterostructures with VO2 thicknesses as high as 9.5 nm, which are much higher than the Thomas Fermi screening length of ~1 nm. Additionally, by performing temperature-dependent XRD across the MIT we validate that the structural phase transition is still accompanied by the electrical phase transition. Further, we also discuss the role of spacer layer thickness where we found about 2 nm thickness of LAO is optimal for both allowing significant tunneling current from the dopant to the VO2 and preventing oxygen migration across these two layers.
In Chapter 6, we further extend our studies as presented in the previous chapter. By performing bulk-sensitive HAXPES measurements, we show the presence of two characteristics peaks, V2p3/2, and V2p1/2, at ~515.8 eV and ~523.1 eV, respectively, for the 7.5 nm VO2 film in the insulating phase. Importantly, a systematic shift of the main component of the V2p3/2 peak to higher binding energies is observed for the VO2 heterostructures, with the highest increase in binding energy (~250 meV) observed for the heterostructure with the thinnest VO2 layer (1.5 nm) suggesting the signature of band bending of an electron-doped VO2 in the insulating phase. However, these V2p spectra show two remarkable features. The first one is the gradual increase in lower binding energy shoulder around ~514.5 eV in the insulating phase with the increase in carrier densities in VO2 suggesting that the additional charge transferred to the VO2 channel layer enables non-local screening previously observed only in the metallic phase of VO2. The second is the emergence of an additional peak with a larger binding energy at around 517.5 eV. This higher binding cannot be a trivial higher valence peak, such as V5+. This is because a more oxidative state for vanadium ions is inconsistent with electron doping implied by Hall measurements. Furthermore, the intensity of this additional higher binding energy shoulder is directly correlated with carrier density. Based on the LDA+DMFT Anderson impurity model calculation of an electron-doped VO2, we tentatively assign the 517.5 eV peak to a satellite peak induced by electron doping. Remarkably, across the MIT, HAXPES spectra show a clear change of V3d spectral weight at the fermi level even for the heterostructure with the thinnest VO2 layer (1.5 nm) and the highest carrier density suggesting that the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium.
In Chapter 7, we conclude by pointing out the future direction of this study in general and more specifically application of modulation-doped heterostructures which is a powerful technique for achieving high carrier densities, close to those possible with elemental doping. Since our approach does not need any epitaxially matched spacer and dopant layers, it expands the library of materials that can be utilized to explore ‘pure’ electronic effects in correlated oxides and related systems
Automatic speech recognition for low-resource Indian languages
No full textBuilding good models for automatic speech recognition (ASR) requires large amounts of annotated speech data. Recent advancements in end-to-end speech recognition have aggravated the need for data. However, most Indian languages are low-resourced and lack enough training data to build robust and efficient ASR systems. Despite the challenges associated with the scarcity of data, Indian languages offer some unique characteristics that can be utilized to improve speech recognition in low-resource settings. Most languages have an overlapping phoneme set and a strong correspondence between their character sets and pronunciations. Though the writing systems are different, the Unicode tables are organized so that similar-sounding characters occur at the same offset in the range assigned for each language.
In the first part of the thesis, we try to exploit the pronunciation similarities among multiple Indian languages by using a shared set of pronunciation-based tokens. We evaluate the ASR performance for four choices of tokens, namely Epitran, Indian language speech sound label set (ILSL12), Sanskrit phonetic library encoding (SLP1), and SLP1-M (SLP1 modified to include some contextual pronunciation rules). Using Sanskrit as a representative Indian language, we conduct monolingual experiments to evaluate their ASR performance. Conventional Gaussian mixture model (GMM) - hidden Markov model (HMM) approaches, and neural network models leveraging on the alignments from the conventional models benefit from the stringent pronunciation modeling in SLP1-M. However, end-to-end (E2E) trained time-delay neural networks (TDNN) yield the best results with SLP1.
Most Indian languages are spoken in units of syllables. However, syllables have never been used for E2E speech recognition in the Indian language, to the best of our knowledge. So we compare token units like native script characters, SLP1, and syllables in the monolingual settings for multiple Indian languages. We also evaluate the performance of sub-word units generated with the byte pair encoding (BPE) and unigram language model (ULM) algorithms on these basic units. We find that syllable-based sub-word units are promising alternatives to graphemes in monolingual speech recognition if the dataset fairly covers the syllables in the language. The benefits of syllable sub-words in E2E speech recognition may be attributed to the reduced effective length of the token sequences. We also investigate if the models trained on different token units can complement each other in a pretraining-fine-tuning setup. However, the performance improvements in such a setup with syllable-BPE and SLP1 character tokens are minor compared to the syllable-BPE trained model. We also investigate the suitability of syllable-based units in a cross-lingual training setup for a low-resource target language. However, the model faces convergence issues. SLP1 characters are a better choice in crosslingual transfer learning than the syllable sub-words.
In the first part, we also verify the effectiveness of SpecAugment in an extremely low-resource setting. We apply SpecAugment on the log-mel spectrogram for data augmentation in a limited dataset of just 5.5 hours. The assumption is that the target language has no closely related high-resource source language, and only very limited data is available. SpecAugment provides an absolute improvement of 13.86% in WER on a connectionist temporal classification (CTC) based E2E system with weighted finite-state transducer (WFST) decoding. Based on this result, we extensively use SpecAugment in our experiments with E2E models.
In the second part of the thesis, we address the strategies for improving the performance of ASR systems in low-resource scenarios (target languages), exploiting the annotated data from high-resource languages (source languages). Based on the results in the first part of the thesis, we extensively use SLP1 tokens in multilingual experiments on E2E networks. We specifically explore the following settings:
(a) Labeled audio data is not available in the target language. Only a limited amount of unlabeled data is available. We propose using unsupervised domain adaptation (UDA) approaches in a hybrid DNN(deep neural network)-HMM setting to build ASR systems for low-resource languages sharing a common acoustic space with high-resource languages. We explore two architectures: i) domain adversarial training using gradient reversal layer (GRL) and ii) domain separation network (DSN). The GRL and DSN architectures give absolute improvements of 6.71% and 7.32%, respectively, in word error rate (WER) over the baseline DNN with Hindi in the source domain and Sanskrit in the target domain. We also find that a judicious selection of the source language yields further improvements.
(b) Target language has only a small amount of labeled data and has some amount of text data to build language models. We try to benefit from the available data in high-resource languages through a common shared label set to build unified acoustic (AM) and language models (LM). We study and compare the performance of these unified models with that of the monolingual model in low-resource conditions. The unified language-agnostic AM + LM performs better than monolingual AM + LM in cases where (a) only limited speech data is available for training the acoustic models and (b) the speech data is from domains different from that used in training. Multilingual AM + monolingual LM performs the best in general. However, from the results, applying unified models directly (without fine-tuning) to unseen languages does not seem to be a good choice.
(c) There are N target languages with limited training data and several source languages with large training sets. We explore the usefulness of model-agnostic meta-learning (MAML) pre-training for Indian languages and study the importance of selection of the source languages. We find that MAML beats joint multilingual pretraining by an average of 5.4% in CER and 20.3% in WER with just five epochs of fine-tuning. Moreover, MAML achieves performances similar to joint multilingual training with just 25% of the training data. Similarity with the source languages impacts the target language’s ASR performance. We propose a text-similarity-based loss-weighting scheme to exploit this artifact. We find absolute improvements of 1% (on average) in WER with the loss-weighing scheme.
The main contributions of the thesis are:
1. Finding that the use of SLP1 tokens as a common label set for Indian languages helps to remove the redundancy involved in pooling the characters from multiple languages.
2. Exploring for the first time (to the best of our knowledge) syllable-based token units for E2E speech recognition in Indian languages. We find that they are suitable only for monolingual ASR systems.
3. Formulating the ASR in a low-resource language lacking labeled data (for the first time) as an unsupervised domain adaptation problem from a related high-resource language.
4. Exploring for the first time both unified acoustic and language models in a multilingual ASR for Indian languages. The scheme has shown success in cases where the data for acoustic modeling is limited and in settings where the test data is out-of-domain.
5. Proposing a textual similarity-based loss-weighing scheme for MAML pretraining which improves the performance of vanilla MAML models
Potential Applications of Snail Mucus in Cartilage Repair and Fabrication of Porous 3d Scaffolds
No full textArticular cartilage damage is a major challenge in orthopedics. This medical condition is exacerbated by the cartilage's inability to self-repair due to the non-vascularization of the tissue and many other factors. Several treatment strategies for cartilage repair have been explored, including autologous chondrocyte implantation, microfracture, osteochondral plug transplantation, pridie perforation, spongilization and debridement. However, these strategies require complex surgery and have limited success in fully regenerating functional cartilage. In this regard, tissue engineering approaches are promising alternatives to the existing approaches for cartilage tissue repair and regeneration. The success of this approach heavily depends on three major factors, usually referred to as the triad of tissue engineering, which includes appropriate cells of interest, scaffolds, and bioactive factors. The cartilage tissue mainly contains chondrocyte cells, which are the cells of interest explored for cartilage tissue engineering applications. On the other hand, agarose is one of the biopolymers explored for cartilage regeneration due to its unique thermal and mechanical properties, biocompatibility, biodegradability and ease of fabrication. However, this biopolymer lacks cell adhesion property, a major requirement for tissue engineering scaffolds. Therefore, using agarose for tissue engineering applications requires modification of the polymer to achieve the desired properties that will enhance cell attachment and proliferation on the scaffolds.
Snail mucus is a complex viscoelastic bioactive substance containing 90 – 97% water by weight, a mixture of glycosaminoglycans (GAGs), proteoglycan, enzymes, metal ions, copper, and antimicrobial peptides, collagen, glycolic acid, allantoin, and elastin. The GAGs reduce cartilage deterioration and inflammation, while other molecules exhibit wound healing, anti-inflammatory, and antibacterial properties, making the gastropod mucus a very rich and suitable material for biomedical applications. Despite these facts, the application of snail mucus in cartilage regeneration has not been explored.
Therefore, this thesis reports the application of Achatina fulica (A. fulica) mucus, a novel and cheaper substance, as a therapeutic repair agent in cartilage damage and its use to enhance the bioactivity and cell adhesion properties of bioinert agarose scaffolds.
Firstly, the use of A. fulica mucus as a novel potential therapeutic repair agent for osteoarthritis and cartilage tissue repair in vitro was explored. The snail mucus was isolated, sterilized, and characterized using FTIR, XPS, Rheology, and LC-MS/MS. The GAGs, sugar, phenol, and protein contents were estimated using standard assays. The LC-MS/MS detected 6-gingerol and some other small molecules in the snail mucus for the first time. The effects of the sterilized mucus were studied on human chondrocytes using the C28/I2 cells as a model for the in vitro assays. The MTT assay indicates that mucus extracted from the pedal of A. fulica is biocompatible with the cells up to a concentration of 50 µg/mL. The mucus promoted cell migration and proliferation and completely closed the scratch wound within 72 h compared to the control, as indicated in the in vitro scratch assay. In addition, the snail mucus reduced apoptosis significantly (p < 0.05) in the treated cells by 74.5% and preserved the cells’ cytoskeletal integrity, attributed mainly to GAGs and 6-gingerol present in the SMu. Having established the therapeutic effects of the snail mucus and its biocompatibility on the human chondrocytes, C28/I2 cells, the biomaterial was then explored for designing and fabricating porous 3D scaffolds that can be explored for cartilage tissue engineering.
Next, porous 3D blend scaffolds containing agarose polymer (AG) and sterilized snail mucus (SMu) were prepared by the freeze-drying method. The scaffolds were characterized by FTIR, FESEM, DMA and other parameters such as open porosity, pore size, swelling capacity, and biodegradability. The scaffolds' bioactivity and cell adhesion effects were determined on the C28/I2 cells. FTIR showed that SMu was successfully incorporated into the scaffolds, and the FESEM result revealed the microporous morphology of the scaffolds with an average pore size of 245 µm. The compression test showed that SMu significantly (p < 0.05) increased the mechanical strength of the composite scaffolds by more than 80% compared to the pristine AG scaffold. The degradation study indicated that SMu enhanced the scaffolds' degradation properties. The MTT assay (24 h and 48 h) and FACS established the biocompatibility of all the scaffolds, while the hemolysis assay with porcine RBCs confirmed the hemocompatibility of the scaffolds having hemolysis levels below 10%. At the same time, CLSM revealed that the cells presented spheroidal morphology with filopodia and lamellipodia protrusions of the actin filaments, confirming cell attachment on the AGSMu blend scaffolds compared to AG scaffolds where no cytoskeletal protrusions were observed for the days 1, 3 and 7 study.
Finally, the scaffold’s properties were further enhanced, especially to maintain the structural integrity of the scaffolds and reduce the degradation rate long enough to allow cell proliferation and extracellular matrix (ECM) deposition. In order to achieve this, reduced graphene oxide (rGO) was used as a filler to prepare AG-rGO-SMu composite scaffolds. Scaffolds containing agarose with and without rGO and SMu were designed and fabricated via freeze-drying. The scaffolds were also characterized by FTIR, FESEM, DMA and other parameters such as open porosity, pore size, swelling capacity, and biodegradability. The bioactivity of the scaffolds for cell adhesion properties was also investigated on the C28/I2 cells. The FTIR data showed that SMu and rGO were successfully incorporated into the composite scaffolds and significantly increased the mechanical properties of the composite scaffolds, AG-rGO (0.69 MPa), AG-SMu (0.73 MPa), AG-rGO-SMu-0.5 (0.98 MPa), AG-rGO-SMu-1 (1.09 MPa), AG-rGO-SMu-2 (1.22 MPa) compared to the AG scaffold (0.43 MPa) as revealed by the compressive mechanical analysis results. Incorporating the duo, rGO and SMu, also improved the porosity of the scaffolds from 58% in AG to 74% in AG-rGO-SMu-2 but synergistically reduced the pore sizes based on rGO content in the composite scaffolds. The swelling capacity reduced as the rGO content in the composite scaffolds increased. In addition, rGO significantly reduced the enzymatic and hydrolytic degradation of the composite scaffolds from 40% in AG to 15% in AG-rGO-SMu-2 after 28 days, indicating that the composite scaffolds degraded slowly and could maintain structural integrity long enough to support the deposition of cellular ECM which is required for cartilage regeneration. The MTT assay results showed that all the scaffolds are biocompatible with the C28/I2 cells, as cell viability on all the scaffolds was about 100% and higher for the 24 and 48 h studies. Likewise, all the composite scaffolds incubated with the porcine RBCs demonstrated a high level of hemocompatibility as hemolysis was less than 5%, which is a significant improvement from the less than 10% and 20% hemolysis levels recorded from the AGSMu scaffolds.
Furthermore, the CLSM micrographs showed that only the scaffolds containing rGO, SMu or both showed spheroids with filopodia and lamellipodia protrusions of the actin filaments, while pristine AG scaffolds presented cells with only spheroidal morphologies without any observable cytoskeletal protrusions. Therefore, it can be concluded that SMu is a suitable therapeutic repair agent for cartilage repair, and it improved the bioactivity of agarose polymer by enhancing the attachment of C28/I2 cells on the modified AGSMu blend scaffolds. In addition, rGO acted synergistically with SMu to further modify the agarose polymer to enhance its mechanical and cell adhesion properties and, more importantly, reduce the degradation rate of the AG-rGO-SMu composite scaffolds. The scaffolds can, therefore, be explored for cartilage tissue engineering
Many-body classical chaos: A case study across thermal phase transitions and connection to quantum measurements
No full textThere has been enormous interest in understanding the emergence of thermal behavior in systems isolated from the environment, be it classical or quantum. For classical systems, it has been argued that chaos plays a vital role in attaining the ‘ergodicity’ and ‘mixing,’ two essential ingredients behind thermalization. Therefore, it becomes indispensable to understand the chaos for many particles (or many-body) with interactions, where the role of chaos in thermalization has long been studied. In the first part of this thesis, we discuss our findings on the behavior of many-body chaos across thermal phase transitions in a classical spin system. We have studied a well-known paradigmatic model: a two-dimensional XXZ Model in two universality classes, namely the Kosterlitz-Thouless (KT) and Ising. After suitably defining a recently introduced classical out-of-time-ordered correlator (cOTOC) for our model, we calculate the Lyapunov exponent and butterfly velocity, diagnostics for a chaotic system, across the aforementioned phase transitions. We, thereafter, discuss how many-body chaos can be an additional tool to characterize different thermal phases. For quantum systems, in addition to a remarkable bound in chaos, there exists a phenomenological relation between short- or intermediate-time chaos and long-time hydrodynamic transport. We explore such a connection in classical systems where no such bound in chaos exists. After discussing the dynamical structure factor for conserved modes, we comment on the connection between chaos and transport across the KT and Ising phase transition in the same XXZ model. To extend our discussion further, we discuss the fate of many-body chaos when randomness (or noise) is added to the dynamics. As we see, using a modified cOTOC with noise, there exists a critical noise strength, after which a chaotic system becomes non-chaotic and vice versa. After suitably defining a chaotic model of coupled anharmonic oscillators, such transition in the classical systems can be liked with quantum measurement-induced transitions via a nontrivial Schwinger-Keldysh path-integral. Finally, we discuss the many-body chaotic behavior in a ‘frustrated’ spin system, where we can access a low-temperature classical spin-liquid phase. After introducing the two-dimensional Kagome Heisenberg anti-ferromagnetic model, We discuss the equilibration at low temperatures using Monte Carlo and show the results of the Lyapunov exponent and butterfly velocity across the crossover from the paramagnetic to the spin-liquid phase. We parallelly explore the connection between chaos and transport. In the end, I conclude with the future outlook
Probing the inflationary universe and reheating with primordial black holes and induced gravitational wave background
No full textThe direct observational test for the earliest phase of our universe comes from cosmic microwave background (CMB) radiation, which can only probe up to the recombination era. The only definitive event during the pre-recombination era is the Big Bang nucleosynthesis (BBN), which is highly sensitive to any change in the constituents of the universe. This leaves a gap in understanding the expansion history between the end of inflation and BBN, which is not accessible from any direct observations at our disposal.
Our study shall provide a unique way to connect the non-trivial expansion history with future gravitational wave (GW) observations in the context of primordial black holes (PBHs). PBHs have emerged as a promising candidate to explain the existence of cold dark matter (DM) in our universe. PBHs can form due to small-scale amplification in inflationary scalar perturbations when these perturbation modes re-enter the horizon in the post-inflationary universe. Thus, the existence of PBHs in the early universe opens up the possibility to probe our universe in the last forty folds of inflation and the unknown phase between the end of inflation and the BBN. We study the PBH-forming models and different expansion histories for this intermediate phase, and we connect our results with observations; one is through the detection of second-order induced stochastic gravitational wave background (ISGWB) generated during PBH formation, other one is the GW signal coming from PBH density fluctuations in the post-inflationary universe, which can be potentially probed in different future gravitational wave observatories. Many crucial phenomena of our universe occur during this pre-BBN era, like Baryogenesis, EW phase transition, QCD phase transition, Dark matter production and freeze-out, Neutrino decoupling, etc. Thus, our study, determining the equation of the state of the pre-BBN universe, shall have enormous indirect implications for each of these events. On the other hand, the PBHs produced during this era can also directly contribute to baryogenesis, dark matter, and dark radiation production, which we can constrain and probe through future GW observatories. With the recent advances in GW detection techniques, breaking the degeneracies of GW background sources with other astrophysical sources has also become very important. The study of ISGWB, modified due to the non-trivial pre-BBN universe, is a crucial step toward identifying and classifying ISGWBs from PBH-forming models.
We study the generation of PBHs in a single field inflection point inflationary model wherein the effective potential is expanded up to the sextic order, and the inversion symmetry is imposed such that only even powers are retained in the potential. Such a potential allows an inflection point that leads to a dynamical phase of ultra-slow roll evolution, thereby causing an enhancement of the primordial perturbation spectrum at smaller scales. Working with a quasi-inflection point in the potential, we find that PBHs can be produced in our scenario in the asteroid-mass window with a nearly monochromatic mass fraction which can account for the whole dark matter in the universe. For different choices of quasi-inflection points and other parameters of our model, we can also generate PBHs in higher mass windows. Still, the primordial spectrum of curvature perturbations becomes strongly tilted at the CMB scales. Moreover, we study the effects of a reheating epoch after the end of inflation on the PBHs mass fraction and find that an epoch of a matter-dominated reheating can shift the mass fraction to a larger mass window as well as increase their fractional contribution to the total dark matter even for the case of a monochromatic mass fraction.
In all inflationary scenarios of PBH formation, amplified scalar perturbations inevitably accompany an ISGWB at smaller scales. We study the ISGWB originating from the inflection point inflationary model wherein PBHs can be produced with a nearly monochromatic mass fraction in the asteroid mass window accounting for the total dark matter in the universe. In our scenario, we numerically calculate the ISGWB for frequencies ranging from nanoHz to KHz that cover the observational scales corresponding to future space-based GW observatories such as IPTA, LISA, DECIGO, and ET. Interestingly, we find that ultralight PBHs (), which shall completely evaporate by today with an exceedingly small contribution to dark matter, would still generate an ISGWB that may be detected by a future design of the ground-based Advanced LIGO detector. Using a model-independent approach, we obtain a stringent lower mass limit for ultralight PBHs, which would be valid for a large class of ultra-slow roll inflationary models.
Further, we extend our formalism to study the imprints of a reheating epoch on both the ISGWB and the derived lower mass bound. We find that any non-instantaneous reheating leads to an even stronger lower bound on PBHs mass and an epoch of prolonged matter-dominated reheating shifts the ISGWB spectrum to smaller frequencies. In particular, we show that an epoch of an early matter-dominated phase leads to a secondary amplification of ISGWB at much smaller scales corresponding to the smallest comoving scale leaving the horizon during inflation or the end of inflation scale.
Ultra-low mass spinning and non-spinning PBHs, which may briefly dominate the energy density of the universe but completely evaporate before the BBN, can lead to interesting observable signatures. Here the first-order adiabatic perturbation from inflation and from the isocurvature perturbations due to PBH distribution source tensor perturbations in the second-order lead to two peaks in the induced GW background. These resonant peaks are generated at the beginning of standard radiation domination in the presence of a prior PBH-dominated era. We studied this generation of the doubly peaked spectrum of induced ISGWB for such a scenario and explored the possibility of probing PBH evaporation-induced baryogenesis, dark matter, and dark radiation. Hawking evaporation of PBHs can lead to a class of baryogenesis models wherein the emission of massive unstable particles from the PBH evaporation and their subsequent decay contributes to the matter-antimatter asymmetry.
The emission of light relativistic dark sector particles can contribute to the dark radiation and massive stable dark sector particles accounting for the dark matter component of our universe. The ISGWB can probe or constrain such scenarios. For the case of dark radiation, we find a novel complementarity between the measurements of from these emitted particles and the ISGWB from PBH domination.
Our results indicate that the ISGWB weakly depends on the initial PBH spin. However, for gravitons as the dark radiation particles, the initial PBH spin plays a significant role, and only above a critical value of the initial spin parameter , which depends only on initial PBH mass, the graviton emission can be probed in the CMB-HD experiment. Upcoming CMB experiments such as CMB-HD and CMB-Bharat, together with future GW detectors like LISA and ET, open up an exciting possibility of constraining the PBHs parameter space providing deeper insights into the expansion history of the universe between the end of inflation and BBN
Genetic and environmental determinants of swarming in Pseudomonas aeruginosa
No full textSwarming, in bacteria, is a type of group translocation phenomenon observed in several flagellated species characterized by their ability to spread across solid or semi-solid surfaces rapidly – often with a distinct colony pattern. For instance, swarming observed in Pseudomonas aeruginosa, an opportunistic human pathogen widely known for its ability to engage in various forms of bacterial sociality, entails a characteristic pattern composed of five to eight branches called tendrils or dendrites, which radiate outwards from the point of inoculum. While some species can swarm over a wide range of surface conditions, swarming in many species, like P. aeruginosa, is observed exclusively under a narrow range of semi-solid agar (0.4% – 0.7%) surface conditions. Interestingly, as described in species like E. coli, whether swarming is a chemotactically driven motility is often contested with a few lines of opposing evidence. Nonetheless, in P. aeruginosa, as well as in other species, swarming is strictly observed under a limited set of conditions suggesting specific extrinsic cues could influence swarming. Even though swarming has been reported in several bacterial lineages, including numerous Gram-positive and Gram-negative species, the ecological and evolutionary relevance of the phenomenon is still unclear.
In this study, using PA14, a laboratory strain of P. aeruginosa as a model, we investigate the relevance of swarming as an important social attribute of bacteria through a reductionist approach. This study is designed to experiment, at least partially, with the following running hypotheses in the lab:
• Swarming is a population-wide response to ecologically relevant external cues.
• Multiple, yet specific, external cues can influence swarming either independently or in a combinatorial fashion.
• The bacterium must encode and utilize dedicated signal perception and response units to perceive and differentiate such varied signals.
In the first part of my thesis, I describe the details of the experiments we performed to understand the swarming attributes of PA14 across four swarming-supporting formulations. This chapter of my work primarily aims to uncover various genetic determinants which might be critical in modulating this behaviour. To address this, we set out to answer whether molecular sensors of the bacterium are essential for swarming. By screening a targeted subset of a transposon mutant library of P. aeruginosa, we found that multiple genes encoding sensor kinases (SKs), or response regulators (RRs), of the two-component signalling systems (TCSs) are required in a nutrient-specific manner.
In the second part of the work, we attempted to identify cues sensed by the TCS system by a transcriptomic approach. For this, we performed an RNA-seq analysis by comparing the transcriptome of PA14 in two distinct phases of swarming, i.e., early swarm lag (3 hours) and late swarm lag (12 hours). In addition to numerous stationary phase and quorum sensing associated transcripts, we found that a cluster of contiguously placed seventeen genes, including those encoding a periplasmic ethanol dehydrogenase (ExaA) and its regulators, are significantly upregulated in late swarm-lag compared to early swarm-lag. Consistently, we found that ethanol, when provided in low concentrations, can serve as an extrinsic cue for swarming in P. aeruginosa. Subsequently, we further confirmed that this ethanol oxidation cluster is downstream of ErdR, an RR identified as a regulator of swarming in the screen described in the first part of the thesis.
In the third part of the work, we further examined functional aspects of ErdR through molecular biology approaches. ErdR belongs to the NarL family of RRs without a known SK partner. To understand whether ErdR is activated via phosphorylation, a common feature of all canonical RRs, we mutated a conserved phosphorylatable-Asp residue to alanine (D58A) in ErdR. This mutation abolished ethanol-induced swarming in P. aeruginosa. We found that the DNA binding domain of ErdR is also essential for its swarm-promoting function. To identify sensor kinases, we screened a deletion mutant library comprising all the 62 SKs and found that two SK loci (ercS and PA14_21700) were necessary for ethanol-induced swarming. Although the location of both ercS and PA14_21700 are outside the erdR operon, their phenotypes were similar to erdR. To confirm this further, we created a transcriptional reporter of ethanol dehydrogenase (ExaA), which is downstream to ErdR. We found that both ErcS and PA14_21700 regulate ExaA expression. Thus, these two SKs appear to work in the same pathway as ErdR in promoting swarming and utilization of ethanol as a carbon source.
Overall, this work has shown that P. aeruginosa utilizes ethanol, an ecologically relevant cue, to facilitate swarming. The bacterium uses a cluster of seventeen genes, well conserved in the Pseudomonas genus, to utilize ethanol as a signal for swarming. Our study provides a framework for the identification of other environmental triggers which may regulate swarming in P. aeruginosa and other bacteria
Tailored Synthesis of Hexagonal Boron Nitride: Chemical Vapor Deposition and Next Generation Devices
No full textThe introduction of two dimensional (2D) materials in electronic device applications
has provided a platform for several new device concepts and
architectures. Hexagonal boron nitride (hBN) is the only insulator in the family of
2D materials. It has thus been widely regarded as the preferred substrate for 2D
nanodevices and its heterostructures due to its electrically insulating nature, excellent
thermal conductivity, suitable dielectric characteristics, and very high optical phonon
energy. Owing to such characteristics, it also has several applications as a gate
dielectric, tunnel barrier layer, and resistive switching memory, among others. To
better exploit its capabilities, hBN must be synthesized over large areas with good
crystalline quality and uniformity, along with control over the number of layers and
grain size.
In this work, by physico-chemical modelling of the ammonia borane (BH3NH3)
route, currently the most popular chemistry for hBN deposition, a predictive CVD
process is established to grow high crystalline quality uniform hBN over large areas,
6”x6”. Most other approaches have been empirical. Another key distinguishing
factor in this work is the fact that the BH3NH3 precursor is placed outside the
growth chamber which allows regulation of the precursor flux. This approach allows
for control over the formation of the impurity known as nanocrystalline-BN (nano BN) which is observed with ammonia borane as the precursor. Eliminating nano BN is very important to obtain clean surfaces/interfaces and to integrate hBN with
other materials for device implementation. The CVD process parameter window is
identified that allows for uniform deposition of n-layered hBN, n = 1, 2… > 10 with
grain sizes approaching 5 microns. To demonstrate that the hBN growth method
reported in this paper yields layers of the required quality despite its polycrystalline
nature, graphene on hBN FETs have been fabricated with record - when compared
with existing literature reports for CVD graphene/CVD hBN devices - room
temperature mobilities.
Ammonia borane (BH3NH3) as a precursor contains both boron and nitrogen which
therefore fixes the B/N ratio. In deposition of compound semiconductor films the
ability to vary this ratio has been found to be crucial to exercise better control over
the fundamental aspects of growth. Also, in the CVD hBN process using ammonia
borane, due to the relatively larger solubility of B in Cu which then acts as a
secondary source of B, lateral growth of the islands, is limited by availability of
nitrogen on the surface. Hence, to further exercise control over the growth process,
an ammonia-assisted chemical vapor deposition method is explored to synthesize
larger grain sizes, in excess of 50 microns of hexagonal boron nitride (hBN) for the
very first time to the best of our knowledge. The mechanistic understanding of the
growth is established by correlating the effect of ammonia on the nucleation density
and growth rates of the hBN grains. To validate the effect of larger grain sizes on
device performance, resistive random-access memories (RRAMs) are fabricated
based on hBN which exhibit non-volatile bipolar resistive switching (RS). The
fabricated RRAMs establish improved switching performance due to larger grain
sizes of the hBN. This result is due to the reduction of grain boundaries which
facilitate lower cycle-to-cycle variability, better endurance and higher current on/off
ratio as the grain boundaries are known to assist ion migration. A 4 order of
magnitude improvement in retention capability of the memory and a 5-order
improvement in on/off ratio is observed.
The hBN deposited here is poly-domain with grain boundaries. It has been shown
in graphene that by defect engineering of grain boundaries, properties of CVD
graphene can approach that of single crystal material. It has been established that
the grain boundaries or defect sites in polycrystalline CVD grown hBN provide
leakage routes. These paths in turn result in local generation of percolation paths
that lower the dielectric breakdown (BD) of multi-layered hBN. To verify if whether
the same ammonia annealing route can also heal defects in BN as in graphene, a post
growth annealing technique involving ammonia was employed. Structural and
Raman characterization of the obtained films has been correlated to the partial
pressure of ammonia during annealing to demonstrate reduced defect density. The
post growth annealed hBN films exhibited a high breakdown field strength of ∼13.1
MV cm−1
, which is the highest reported breakdown fields of CVD grown hBN films.
Using the understanding of resistive switching, neuromorphic behavior in a scalable
two-dimensional material structure is demonstrated consisting of CVD-hBN grown
on copper (Cu) and contacted with silver (Ag). In this system, avalanche dynamics
is examined such as those seen in cortical tissue structures which exhibit critical
neuromorphic network dynamics due to presence of atomic-scale networks which
develop as a result of diffusion of Ag inside the hBN matrix. The development of
Ag filaments by application of persistent I-V sweeps also gives rise to a resistive
switching memory device that has two states, a low resistance (LRS) and a high
resistance (HR) state. The avalanche dynamics are observed in the HRS due to the
intercalation of Ag inside the hBN matrix which results in formation of a percolation
network when Ag clusters are within the tunneling distance. In the LRS state, the
filamentary networks of Ag are formed which exhibit avalanche behavior under the
application of a constant electric field. Thus, a first of its kind brain-like avalanche
behavior is reported in a 2D material system comprising of Ag-hBN. This kind of
system can be scaled up to form large-area devices and with hBN being a 2D
material, it allows for engineered heterostructures for future applications in
neuromorphic computing.
In summary, large area deposition of hexagonal boron nitride (hBN) is enabled by
controlling the nucleation density, grain size, layer thickness and defect density. The
quality of the films grown are demonstrated to be state of the art by various
standards. The highest values of mobility of CVD Graphene on CVD hBN and
highest breakdown field of CVD hBN are reported. The improved resistive
switching characteristics and the enablement of novel neuromorphic architecture
shows the potential of CVD grown hBN in next generation electronic application
Microstructure – texture - mechanical property correlation in additively manufactured stainless Steel 316L and Cu-Ni-Sn
No full textMetal additive manufacturing (AM) has gained considerable interest in recent years because of its ability of near net shape building of the components, one-go production, ability to make intricate designs and minimized wastage. Further, the mechanical properties of the manufactured alloys are often superior and governing factors for this comes because of excellent metallurgical benefits that are attributed to high cooling rate and steep thermal gradient. The notable metallurgical benefits offered are highly refined microstructure, increased dislocation density, enhanced solid solubility, presence of crystallographic texture. Even though there are notable achievements reported as mentioned, certain mysteries are yet to get resolved. The evolution of microstructure and texture and their correlation to important mechanical performance is yet to be understood. The role of heat treatment in altering the microstructure – mechanical property correlation also needs to be re-visited. The present thesis primarily focusses on the microstructure - crystallographic texture - mechanical property correlation in additively manufactured materials prepared through selective laser melting and wire arc additive manufacturing. While most of the investigations were carried out on stainless steel 316L, a heat treatable alloy Cu-15Ni-8Sn is also investigated.
Chapter 1 deals with the general introduction on metal additive manufacturing, its benefits and various types. A brief overview of available literature is presented for SLM and WAAM process related to the evolution mechanism for microstructural features such as solidification structure, dislocation density, crystallographic phases, texture and their overall consequence on mechanical properties. Chapter 2 details about the materials and method used in carrying out the investigation.
Chapter 3 addresses the evolution mechanism behind microstructure, increased dislocation density, formation of residual stresses in additive manufacturing. This investigation has been conducted through a careful experimentation where deconvolution is done for the primary contributors such as thermal gradient and cooling rate in microstructure. It has been observed that repeated heating/cooling leads to accumulation of dislocation density and residual stress. It has been established that difference of melt droplet transfer mode in SLM and WAAM generates textures with extra orientation in WAAM in addition to usual and orientation developed during SLM.
Chapter 4 deals about the effect of SLM manufacturing variables hatch style (dividing individual layers into sub-parts and manufacture these in a sequential manner) and interlayer hatch rotation. The role of these variables on controlling the microstructure – texture – mechanical property is discussed. It has been observed that texture control can be enacted by varying interlayer hatch rotation or hatch style.
In chapter 5, a comparison of tensile properties as well as fracture toughness is done between SLM and WAAM processed sample in the as built state and the mechanism has been established. A comparison of thermal stability of as-built microstructure is also done between these two routes. The effect of heat treatment on microstructure - mechanical property relationship is also explored. It has been observed that increased propensity of twin formation, presence of melt pool boundaries whereas presence of δ-ferrite enhances the fracture toughness in SLM and WAAM processed stainless steel 316L, respectively.
In chapter 6, the benefits of AM in enhancing the mechanical properties is studied for a case when a different hardening mechanism become active, other than what has been observed for stainless steel 316L, that is, slip or twinning. The response of heat treatment in one of the popularly spinodal and age hardened Cu-15Ni-8Sn alloy manufactured through SLM has been explored and compared with solutionised and heat-treated sample. It has been observed that heat treatment in additively manufactured Cu-15Ni-8Sn gives superior mechanical properties and it is associated with highly refined spinodal decomposed structure, fine precipitates in addition to refined as built microstructure.
Chapter 7 lists the overall conclusions of the present thesis explored through experimental investigations. The suggestions for future work have also been given
Electronic behavior of epitaxial thin films of doped rare-earth nickelates
No full textRare-earth nickelates (RENiO3), a family of transition metal oxides, exhibit a complex phase diagram involving electronic, magnetic, and structural phase transitions. While LaNiO3 remains paramagnetic, metallic down to very low temperature, RENiO3 members with RE=Nd, Pr exhibit simultaneous metal-insulator transition (MIT), paramagnetic to antiferromagnetic transition, structural phase transition and a bond disproportionation (BD) transition as a function of temperature. The other members of the series such as EuNiO3, SmNiO3, etc. first undergo simultaneous MIT, BD, and structural phase transition and further becomes antiferromagnetic upon lowering the temperature. Understanding the origin of the MIT in this family remains a challenging problem and has attracted a lot of attention in recent times. The MIT temperature can be tuned by a variety of parameters such as chemical doping, pressure, epitaxial strain, light, etc. In this thesis, we have grown epitaxial thin films of doped rare-earth nickelates and investigated their electronic and magnetic behavior using several experimental techniques, including synchrotron-based measurements.
In the first part, we have investigated Ca2+ (divalent) and Ce4+ (tetravalent) doped NdNiO3 thin films. Doping with divalent ions at the Nd sites introduces holes, whereas doping with tetravalent ions introduces electrons, resulting in a change in the formal valence of Ni. Both electron and hole doping suppress the insulating phases with asymmetric suppression rates for the metal-insulator phase transition. We have shown that the effective charge transfer energy changes with carrier doping and the formation of the BD phase is not favored above a critical doping, suppressing the insulating phase. Our research clearly shows that the appearance of BD mode is critical for the appearance of MIT in RENiO3 family.
In the second part, we have investigated rare-earth nickelate in high entropy oxide (HEO) form. HEOs are defined as a class of materials containing equimolar or nearly equimolar portions of five or more elements stabilizing in a single phase. HEOs have been explored in recent years to achieve tunable properties in unexplored parts of the complex phase diagram. However, epitaxial stabilization of such multi-element systems is challenging, and it is unknown how epitaxial strain will affect the electronic and magnetic behavior of HEO. We have been able to grow (LaPrNdSmEu)0.2NiO3 [(LPNSE)NO] thin films on different substrates having different epitaxial strains. We have shown that, in spite of having multi-element and strong disorder at the RE site, the average tolerance factor determines the electronic and magnetic properties. We further studied the strain effect on MIT of those HEO thin films. We have observed that (LPNSE)NO film grown under tensile strain (substrates: NdGaO3 and SrTiO3) exhibits a metal-insulator transition. We have found that this transition can be completely suppressed by compressive strain exerted by SrLaAlO4 substrate. Surprisingly, HEO film, grown on SrPrGaO4 substrate, where the strain is almost negligible, does not exhibit any MIT. We have further demonstrated that the octahedral rotation pattern of the substrate governs the octahedral rotation and Ni-O-Ni bond angle of the epitaxial thin films, which in turn controls the MIT.
In the third part, we have explored (LPNSE)NO thin films as electrocatalysts. Oxygen evolution reaction (OER) is a key process in several alternative energy generation platforms such as solar and electric driven water splitting, fuel cells, rechargeable metal-air batteries, etc. We have investigated the thickness dependent OER of (LPNSE)NO thin films and found that the increase of film thickness results in higher OER activity. X-ray absorption spectroscopy measurements find an increase in Ni d-O p covalency and a decrease in charge transfer energy with the increase in film thickness. These facilitate higher charge transfer between Ni and surface adsorbates, resulting in higher OER activity
Resonant Metasurfaces for Mid-Infrared Photonics
No full textThe field of photonics offers a robust platform for efficient light-based technologies,
directly impacting or serving as competent alternatives to address scientific problems. With the
advent and advancement of micro-nano fabrication technologies, sub-domains of photonics like
integrated optics (nanophotonics, guided wave optics), optoelectronics etc. are vastly benefited.
These technologies resulted in noteworthy contributions in the fields of communication,
networking, sensing, and biomedical applications, to name a few. Apart from these general areas
of research, photonics played a catalytic role in the rapid growth of two-dimensional material
integrated platforms and highly sought after field of quantum technologies. One important aspect
in emerging technologies is to identify the limitations and scientific challenges associated with the
application domain. The frequency bands (S, C, Ku etc.) of operation for communication and
different wavelength windows (visible, near-infrared, mid-infrared etc.) for integrated optics
contain useful information which could help the scientific community in filling the gaps in terms
of essential technologies or supporting devices. From an integrated optics point of view, the
wavelength range spanning 3 to 8 micrometers, referred to as the mid-infrared region stands out
as a unique region contributing to a plethora of scientific applications. The wavelength range
spanning 3 to 8 micrometers, referred to as the mid-infrared region stands out as a unique region
contributing to a plethora of scientific applications. The presence of absorption signatures of most
of the molecules (CH4, NO2, N2O, CO, H2O, etc.) or molecular fingerprints ignites a lot of
applications including (infrared) spectroscopy, and nanoscale imaging as well as optics and
sources. Furthermore, the atmospheric transmission window centered around 3 and 5 μm
respectively has direct applications in defense and remote sensing. Conventional and state of the
art devices (like photodetectors, sensors), in general, are bulk in nature. From an integrated optics
perspective, this would imply larger footprint and power consumption. Existing technologies often
require larger interaction volumes to effectively transduce the targeted signals to useful
information. We explore the field of resonant metasurfaces for addressing the issues and challenges
associated with such devices and eventually realize power-efficient nanoscale devices. By scaling
down the device dimensions, interaction volume is largely reduced, but with efficient transduction
properties (for e.g. by integrating transition metal dichalcogenide materials (TMDCs)). State of
the art devices are based on conventional optical phenomena like refraction, total internal
reflection (TIR) etc., which in turn depend on the material properties and bulkiness of the material
platform. However, metasurfaces (two-dimensional equivalents of metamaterials) are artificially
engineered structures for realizing unconventional photonic applications like optical cloaking,
negative refraction, etc. Another challenge is in the choice of material. For conventional devices
material properties like index of refraction, absorption or transmission characteristics are of great
importance in selecting a compatible platform. Metasurfaces on the other hand depend on the
periodic modulation of the refractive indices and most of the functionalities are attributed to the
effective index of the structure. Transmission or reflection properties can thus be modified and
scaled by using index engineering. We explore the field of resonant metasurfaces for addressing
the issues and challenges associated with such devices and eventually realize power-efficient
nanoscale devices. Metasurfaces (two-dimensional equivalents of metamaterials) are artificially
engineered structures for realizing unconventional photonic applications like optical cloaking,
negative refraction, etc. When combined with the well-established CMOS-based fabrication
techniques, we can realize ultra-compact, power-efficient devices with metasurfaces as the
underlying platform. In our research, we study the scope of such devices in enhancing localized electric fields by energy trapping at the nanoscale via resonance harvesting.
Spectral resonances in the mid-infrared region with polarization-independence and angle-tolerance are useful for filtering applications in infrared spectroscopy and imaging systems. Metasurfaces designed to support a special class of resonances known as guided mode resonances (GMRs) using amorphous-Germanium (a-Ge) two-dimensional fully-etched high index contrast gratings (HCGs) on Calcium Fluoride substrate are presented. The resonance centered at 7.42 μm wavelength, exhibits polarization-independent, notch-type characteristics with minimal change across 0 to 30 degrees incidence angle with measured spectral width of 0.56 μm and extinction of 8 dB. These filters find potential applications in imaging applications utilizing un-polarized incident light, while at the same time having moderate field-of-view imaging capability.
Next, we experimentally demonstrated a novel quasi-bound state in the continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot-mode within a low refractive index medium (silicon nitride) sandwiched between high-index layers (a-Ge). Applications including active photonic functionalities (like non-linear responses) and sensing can greatly get benefited from energy trapping at the nanoscale. The slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved. Major observations from this study were excitation of quasi-BIC resonance at normal incidence under realistic excitation conditions and subwavelength scale light trapping as small as 10 nm. The highest quality factor of ~400 is experimentally extracted at normal incidence under classical mounting conditions with a resonance peak at 3.41 μm wavelength.
We further explored the manifestation of GMRs in metasurfaces as a function of varying device parameters and excitation conditions. We observed that with increasing the duty-cycle of
the partially etched amorphous silicon (a-Si) subwavelength gratings, the avoided crossing
between the coupled GMR branches underwent band-closure and subsequent band-flip, resulting
in the Friedrich-Wintgen type bound-states-in-the-continuum (FW-BICs) transitioning from the
upper to the lower GMR branch. An exciting observation here was an interesting
electromagnetically induced transparency (EIT)-like resonance. These devices find applications in
resonantly enhancing linear absorption from analytes which we demonstrated with the differential
enhancement of infrared absorption from a polymethyl methacrylate (PMMA) layer.
We also built a simulation model that compares ideal conditions (infinitely periodic
resonant metasurfaces under plane-wave excitation) and practical scenarios (finite-size devices
under Gaussian-like excitation). Such a model is useful especially when non-standard, off-axis
excitation (e.g. Cassegrain-type reflective objective with central obscuration) is employed. The
model is not limited to linear responses but also can be extended to non-linear studies. We
employed a plane wave expansion (PWE) method here and found good agreement with
experimental results of linear (transmission spectra) and non-linear (third-order sum-frequency
generation (TSFG)) nature.
Apart from these moderately extensive studies, a few devices were also designed and
fabricated to experimentally demonstrate enhanced non-linear responses like third harmonic
generation (THG) and TSFG. Such devices along with conventional silicon or germanium-based
(visible-near IR) photodetectors help in detecting weak mid-IR photons. The incident mid-infrared
light is up-converted to the visible wavelength either with harmonic generation (THG) or by
mixing (TSFG) with a pump beam (1040 nm here). THG and TSFG processes respectively
explored GMRs and quasi-BICs for resonantly enhancing the up-conversion process. THG process
up-converted 2.4 μm to 800 nm with an enhancement factor of 900 times whereas TSFG resulted in approximately 300 times enhancement for a 3 μm to 650 nm up-conversion