336 research outputs found
Conical refraction Nd:KGd(WO<sub>4</sub>)<sub>2</sub> laser
No full textIn 1832 Hamilton predicted conical refraction, concluding that if a beam propagates along an optic axis of a biaxial crystal, a hollow cone of light will emerge. Nearly two centuries on, cascade conical refraction involving multiple crystals has not been investigated. We empirically investigate a unique two-crystal configuration, and use this to demonstrate an ultra-efficient conical refraction Nd:KGd(WO4)2 laser providing multi-watt output with excellent beam quality independent of resonator design with a slope efficiency close to the theoretical maximum, offering a new route for power and brightness-scaling in solid-state bulk lasers
Subpicosecond quantum dot saturable absorber mode-locked semiconductor disk laser
No full textWe report the generation of subpicosecond pulses from a passively mode locked, optically pumped quantum well semiconductor disk laser using a quantum dot semiconductor saturable absorber mirror (SESAM). We obtained 870 fs pulses at a repetition rate of 895 MHz with average output power of 45 mW at 1027.5 nm. The mode locking operation was insensitive to SESAM temperature over the range of ?10 to 85 °C, with the pulse duration variation thought to be dominated by the temperature dependence of the group delay dispersio
Novel Mid-IR Light Sources for Emerging Applications
No full textThe short-wave mid-infrared, i.e., the wavelength band within 2.5–5 μm has been of keen interest in recent research due to the presence of the molecular fingerprinting region exhibiting strong absorption and resonance of bonds of organic molecules like H2O, CO2, CO and CH4 and a highly efficient water absorption maxima within this spectral range, making this wavelength window exceedingly viable for spectroscopy and material processing applications, that can be extended to medical diagnostics and surgery amongst others. Such high precision applications require the development of novel sources in the mid-IR that can deliver sufficient output power or high pulse energies, along with reasonably broad tunability and good beam quality to cover the region of interest. Additionally, it necessitates the prospect of using ns- and ps-laser sources to treat, and ablate materials with high accuracy, particularly ones containing O-H molecular bonds, usually present as contaminants in glasses, and as natural component of organic materials, and biological tissues. As tunable sources, optical parametric oscillator (OPO) can generate tailored output wavelengths by the process of nonlinear frequency conversion, with reasonable output powers in CW and pulsed mode operations. Selecting suitable nonlinear materials like MgO:PPLN, that can be quasiphasematched for nonlinear conversion, provides adequate transparency to generate mid-IR wavelengths up to 5 μm. For applications that require delivery of power with high precision and good beam quality, fibre amplifiers are excellent choices. For mid-IR, currently, fluoride-based fibres, like ZBLAN, with rare earth doping have been able to deliver sufficiently high pulse energies for several applications with Er3+ dopant capable of offering the highest output powers. From the analysis of energy levels and lifetimes of Erbium doping in ZBLAN, these fibres can also allow sufficiently broad tunability, covering the water absorption window. The PhD work presented here investigates the amplification, gain-bandwidth and wavelength tuning of a 2.1 m long, single stage, double-clad, single mode, 7 mol% Erbium-doped ZBLAN fibre amplifier, pumped by a high-power multimode diode near 980 nm, and seeded by nanosecond pulses of 5.2 ns pulse-width, derived from a broadly tunable MgO-PPLN based OPO, operating at a repetition rate of 10 kHz. At the most efficient wavelength of 2790 ± 1 nm, the highest gain of up to 20 dB with 52.7 μJ pulse energy and up to 8 kWpeak power, using 0.5 μJ seed pulses, were obtained. Over hundred nanometre continuous wavelength tuning between 2712-2818 nm was achieved at a slightly lower seed pulse energy of 0.27 μJ, and a lower pump power, recording a maximum gain of 26 dB at 2790 ± 1 nm, corresponding to 37.5 μJ peak pulse energy. The investigation also sheds light on practical limitations to power scaling and wavelength tuning, arising from self-phase modulation, self-lasing and fibre degradation due to back reflections
Realisation of an efficient Terahertz source using Quantum dot devices
No full textThe development of a compact, tunable, room-temperature operating THz source remains one of the key unsolved tasks in the scientific community to unlock the numerous advantages and applications of THz radiation in spectroscopy, communication, sensing, and imaging among others. Pulsed terahertz systems requires femtosecond optical pumping from mainly bulky lasers, while earlier approaches of continuous-wave(CW) THz generation involved using pumped gas lasers which were very bulky and barely tunable. However, more recent sources of CW THz mostly use Quantum Cascade lasers (QCL) and Photoconductive antennas (PCA); The QCL suffers from cryogenic operating conditions and production complexity that keeps it out of commercial reach while conventional Photoconductive antennas are mainly limited by low optical-terahertz conversion efficiency. The use of quantum dots (QD) in PCA substrate material for THz generation has been implemented by research in this thesis to achieve both pulsed and continuous wave terahertz radiation, to provide access to optical pumping from compact semiconductor lasers and more importantly to reduce the carrier lifetime in PCAs to enable more efficient and optimised photoconductive antenna for THz generation. This research has demonstrated a tunable continuous-wave Quantum Dot external cavity laser emitting at two frequencies as an optical pump for continuous wave terahertz generation. The external cavity QD Laser has been characterised with tunability of 152nm and a tuning range from 1143nm-1295nm that lies within the THz difference frequency for the generation of THz radiation from PCAs. This research work also presents the enhancement of THz PCA’s power output with Quantum dots at pump powers and operating conditions that are analogous to that of semiconductor lasers for a compact THz system. The generation of pulsed THz radiation from the designed quantum dot photoconductive antennas (PCAs) pumped at 800nm and 700nm and optical pump power of 1mW-10mW with an applied bias voltage of 2V-20V has been recorded and presented in chapter 4. This PhD project investigates the output and characteristics of the generated THz from the QD PCAs alongside a comparison with a commercial antenna from Teravil with low-temperature grown GaAs substrate. The QD PCAs outputs significantly higher THz power than the commercial PCA at low pump powers that are representative of semiconductor lasers. This provides a significant step towards the realisation of an efficient compact THz system
New generation light emitting diodes:fundamentals and applications
No full textLight emitting diodes (LEDs) have made tremendous progress in last 15 years andhave reached to a point where they are reinventing and redefining artificial lighting.The efficiency and better control over light quality parameters have been the keyattributes of LEDs that makes them better than the existing lighting solutions.Nevertheless, in their own realm they suffer from decrease in efficiency at highercurrents, i.e. the “efficiency droop” phenomenon. Thus, a better understandingof the mechanisms leading to droop is of utmost importance. Moreover, the fullpotential in terms of light quality, i.e. colour rendering index (CRI) and correlatedcolour temperature (CCT) that can be offered by these devices can be furtherimproved with existing or alternative schemes and device configurations.In this thesis, a novel phosphor covered approach is investigated towards improvingthe CRI for indoor lighting applications. A monolithic di-chromatic LEDemitting at blue and cyan wavelengths is used to pump a green-red phosphor mixtureand a warm (CCT ∼ 3400 K) white light with a superior CRI of 98.6 is achieved.An alternate phosphor free solution to achieve warm white light emission is alsostudied. These monolithic di-chromatic QW devices emitting at blue and greenwavelengths under electrical pumping demonstrated tuneable emission from cool(CCT ∼ 22000 k) to warm (CCT ∼ 5500 K) white light. A maximum CRI of 67,which is the highest value demonstrated for such devices till date to the best of myknowledge, is also achieved.On the subject of efficiency of LEDs, temperature dependence of LEE andIQE of commercial InGaN/GaN based blue LED is studied in light of a step-wiseprocessing procedure based on the ABC-model to determine these quantities. Adecrease in both IQE and LEE with temperature is noted. On the other hand,efficiency decrease in the investigated AlGaInP based red LEDs under pulsed currentshows a shift in the onset of efficiency decrease towards higher current values withdecreasing pulse width with < 1% duty cycle. For sub-nanosecond pulses a linearrelation between applied peak current and peak output power is obtained. Theseobservations indicate device self-heatin
In vivo fluorescence measurements of biological tissue viability
No full textAbstract
Modern preclinical and clinical studies show that the use of diagnostic methods based on the registration of fluorescent radiation can improve the early diagnosis of cancer and other destructive processes of various organs. Currently, many studies are being conducted aimed at studying the processes of oxidative metabolism using methods that register the fluorescence of various fluorophores. New instrument designs and analytical tools for the analysis of diagnostic information are being intensively developed. This chapter provides an overview of the main problems of fluorescence detection methods, experimental studies aimed at studying the effect of the main absorbing tissue chromophores on the fluorescence intensity, examples of applications in various fields of medicine and test objects for device calibration and measurement verification. This chapter does not seek to review all the existing problems of the method and present all possible areas of application, but only focuses on a number of important aspects.Abstract
Modern preclinical and clinical studies show that the use of diagnostic methods based on the registration of fluorescent radiation can improve the early diagnosis of cancer and other destructive processes of various organs. Currently, many studies are being conducted aimed at studying the processes of oxidative metabolism using methods that register the fluorescence of various fluorophores. New instrument designs and analytical tools for the analysis of diagnostic information are being intensively developed. This chapter provides an overview of the main problems of fluorescence detection methods, experimental studies aimed at studying the effect of the main absorbing tissue chromophores on the fluorescence intensity, examples of applications in various fields of medicine and test objects for device calibration and measurement verification. This chapter does not seek to review all the existing problems of the method and present all possible areas of application, but only focuses on a number of important aspects
Development of novel compact ultrashort pulse lasers for biomedical applications
No full textLasers have established their ubiquity across a broad spectrum of applications, ranging from manufacturing and communication to entertainment, and the medical field is no exception. However, biomedical applications place unique demands on laser parameters such as operating mode, wavelength, and output power. Furthermore, the physical characteristics of the laser must include reliable operating stability, high resistance to fluctuations of environmental conditions, and compact size. The requirements for these laser parameters strongly depend on the specific application, as different laser modes, irradiation levels, and exposure durations cause diverse effects on various tissue types. Therefore, a comprehensive understanding of light-tissue interaction with the target tissues is essential before designing lasers for biomedical purposes. This thesis provides experimental and computational research, shedding light on the interaction of post-mortem mouse head tissues with continuous-wave light and ultrashort pulses. The study reveals the tissue penetration depth of single- and multi-layers of skin, skull, and brain in visible and near-infrared ranges, providing valuable information about their optical properties and required laser parameters for non- or minimal invasive neurostimulation. The dissertation is devoted to the improvement of neurostimulation methods, including a comprehensive study of the optical properties of light-sensitive proteins applicable as optogenetic tools and fluorescent biomarkers and the development of a compact ultrashort-pulse high-peak-power laser system for optogenetic research on in vivo animal samples. In addition, the work presents a developed tunable fibre laser operating at wavelengths of 850 nm and 1700 nm that can be used as a versatile light source in the multimodal cancer detection system. The work presented in the dissertation includes the development of different laser sources intended for applications in biomedical research, neurophotonics, and tumour diagnostics
The Development of 3D Tissue and Novel Photonics Technologies for Bio-Medical Theragnostic
No full text3D in-vitro models have emerged as substitutes of complex tissue structures of human body. These models can depict in-vivo conditions of real tissues and can be utilized as valuable tools for per-clinical applications. This work presented development of 3D in-vitro tissue models of full-thickness skin equivalents (FSE) and Melanoma full-thickness skin equivalents (Melanoma-FSE). The development of 3D architecture of FSE and Melanoma-FSE are crucial requirement for investigation of skin pathologies, personalized skin disease treatments, disease modelling, drug testing. We designed high-resolution 3D scaffolds to support the growth and maturation of these skin models. Additionally, we developed and validated a cost-effective, custom-built system combining fluorescence spectroscopy (FS) and optical coherence tomography (OCT) for non-destructive analysis of the metabolism and morphology of 3D FSEs. This system proved highly sensitive in detecting fluorescence from key metabolic co-enzymes (NADH/NADPH and FAD) in solutions and cell suspensions, while OCT provided adequate resolution to observe the morphology of FSEs. As a result, both the 3D FSE model and the dual-mode optical system hold significant potential for use in 3D bioprinting of biological tissues, as well as in the development of cosmetics, drugs, and in monitoring their maturation over time. In scaffold fabrication, we explored two latest emerging bioprinting technologies: Digital Light Processing (DLP) and Two-Photon Polymerization (2PP). The difference between 3D tissue engineered constructs by using these two methods are presented to demonstrate the selection of technology considering to the specific application and tissue construction. For microscale functional tissue structures, 2PP is unique and precise method but having some limitations as compared to DLP. Furthermore, due to therapeutic potential of 1267nm laser irradiation, it is used for generation of singlet oxygen without using photosensitizer (PS). 1267nm laser-induced 1O2 production can cause massive oxidative effect on cellular functions, e.g., mitochondrial dysfunction with mtDNA degradation, cancer cell death. In this study, we investigated real time monitoring of production and role of 1O2 in apoptosis with the aim of optimizing therapeutic outcomes. We found that 1O2 caused the initiation of apoptosis in melanoma cancer cell line more effectively as compared to the primary human fibroblasts and HaCaT cells. Collectively, this work demonstrates the integration of tissue engineering, optical diagnostics and phototherapy for advancing diagnostics and treatment
Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures
No full textAdvanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications
Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 126 µm
No full textWe experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 μm without pulse compression, external cavity, gain-or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5 W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 p
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