1,721,005 research outputs found
Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation
No full textThe photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed
Optical second-harmonic generation selection rules and resonances in buried oxide interfaces: the case of LaAlO3/SrTiO3
No full textDespite an intense research effort, the physical mechanism underlying the formation of a quasi-two-dimensional electron gas at the interface between the band insulators LaAlO3 and SrTiO3 is still not fully understood. Interface- sensitive optical second-harmonic spectroscopy can shed light on this mechanism, by accessing specific information on the orbital and structural reconstruction taking place at the interface that is not accessible by other techniques, and in particular by transport measurements. Here we present a detailed theoretical analysis of the spectral transitions that are most relevant in the second-order nonlinear optical response of oxide interfaces with a square symmetry, in general. In particular, we discuss the case of LaAlO3∕SrTiO3 interfaces, using symmetry arguments to derive specific selection rules, which have strong consequences on the second-harmonic spectra recorded with different input/output polarization combinations of light. These selection rules may in particular explain recent experimental findings
Synthesis, characterization and field emission studies of NiO-rGO nanocomposite
No full textIn this work, NiO-rGO nanocomposite was synthesized by hydrothermal/solvothermal method on nickel
foil. The as-synthesized product was subjected to various characterizations such as X-ray Diffraction
(XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy, in order to reveal its phase, morphology
and structure. The SEM analysis reveals formation of nearly vertically oriented and randomly
distributed nanoflakes of NiO-rGO nanocomposite on entire substrate surface. The Raman spectrum of
the NiO-rGO composite showed characteristic D and G bands of rGO, confirming its presence in the composite.
The NiO-rGO composite emitter showed promising field emission behaviour. The values of turnon
and threshold fields, defined at emission current density of 1 and 10 mA/cm2, were observed to be 1.8
and 2.25 V/mm, respectively, which are comparable to alike composite emitters. Furthermore, from the
NiO-rGO composite emitter, maximum current density of 719 mA/cm2 has been extracted at an applied
field of 4.5 V/mm. The observed results imply potential of NiO-rGO composite emitter for application in
vacuum microelectronic devices
Optical second harmonic on oxide interfaces
No full textDiscontinuities are the fundamentals of any interface-based device. The rich physics of oxide interfaces lies in the electronic correlations at the boundaries. There, the intrinsic symmetry of the bulk constituent materials is inherently broken. Optical second harmonic generation is, thus, an ideal tool to investigate the manifestations of several effects related to the presence of the interface. Two examples, addressing different issues in different systems, are reviewed here in order to highlight the potential richness of this technique when applied to interface-related phenomena in oxides
Optical Second Harmonic Generation on LaAlO3/SrTiO3 Interfaces: A Review
No full textAs we approach the limits of semiconductor technology, the development of new materials and technologies for the new era in electronics is compelling. Among others, perovskite oxide hetero-structures are anticipated to be the best candidates. As in the case of semiconductors, the interface between two given materials can have, and often has, very different properties, compared to the corresponding bulk compounds. Perovskite oxides show spectacular interfacial properties due to the the rearrangement of charges, spins, orbitals and the lattice structure itself, at the interface. Lanthanum aluminate and Strontium titanate hetero-structures (LaAlO3/SrTiO3) can be regarded as a prototype of this wider class of interfaces. Both bulk compounds are plain and (relatively) simple wide-bandgap insulators. Despite this, a conductive two-dimensional electron gas (2DEG) is formed right at the interface when a LaAlO3 thickness of n≥4 unit cells is deposited on a SrTiO3 substrate. The 2DEG is quite thin, being confined in only one or at least very few mono-layers at the interface, on the SrTiO3 side. A very intense and long-lasting study was triggered by this surprising discovery. Many questions regarding the origin and characteristics of the two-dimensional electron gas have been (partially) addressed, others are still open. In particular, this includes the interfacial electronic band structure, the transverse plane spatial homogeneity of the samples and the ultrafast dynamics of the confined carriers. Among a very long list of experimental techniques which have been exploited to study these types of interfaces (ARPES, XPS, AFM, PFM, …and many others), optical Second Harmonic Generation (SHG) was found to be suitable for investigating these types of buried interfaces, thanks to its extreme and selective interface-only sensitivity. The SHG technique has made its contribution to the research in this field in a variety of different and important aspects. In this work we will give a bird’s eye view of the currently available research on this topic and try to sketch out its future perspectives
Q-plate technology: a progress review
Full text linkSince their first introduction in 2006, q-plates have found a constantly increasing number of uses in diverse contexts, ranging from fundamental research on complex structured light fields to more applicative innovations of established experimental techniques, passing through a variety of other emerging topics, such as, for instance, quantum information protocols based on the angular momentum of light. In this paper, we present a bird’s-eye view of the progress of this technology in recent years and offer some educated guesses on the most likely future developments
Complex Permittivity of Ionic Liquid Mixtures Investigated by Terahertz Time-Domain Spectroscopy
No full textIonic liquids are salts found in their liquid state at ambient temperature. The physicochemical properties of ionic liquids can be tailored by selecting constituent cation and anion from numerous available ions. The physicochemical properties can be further tuned by mixing different neat ionic liquids. Reported data of ionic liquid mixtures reveal that frequently investigated properties such as density, viscosity, and thermal stability follow corresponding mixing laws. Complex permittivity in the interval of terahertz frequencies is of great importance to understand the molecular interactions and the solvation dynamics which drive the macroscopic properties of ionic liquids; however, to the best of our knowledge, there are few reports about the mixing behavior of complex permittivity in ionic liquid mixtures. In this contribution, binary mixtures of 1-butyl-3-methylimidazoulium iodide ([C4C1im]I) and 1-butyl-3- methylimidazoulium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) are investigated in the terahertz spectral range, and the resulting low-energy spectra are analyzed in order to clarify the mixing laws at play. The results show that the complex permittivity of mixtures of [C4C1im]I and ([C4C1im][NTf2] obeys a linear mixing law
Monitoring the interfacial electric field in pure and doped SrTiO3 surfaces by means of phase-resolved optical second harmonic generation
No full textOxides and new functional materials such as oxide-based hetero-structures are very good candidates to achieve the goal of the next generation electronics. One of the main features that rules the electronic behavior of these compounds is the interfacial electric field which confines the charge carriers to a quasi-two-dimensional space region. The sign of the confined charge clearly depends on the electric field direction, which is however a very elusive quantity, as most techniques can only detect its absolute value. Even more valuable would be to access the sign of the interfacial electric field directly during the sample growth, being thus able to optimize the growth conditions directly looking at the feature of interest. For this aim, solid and reliable sensors are needed for monitoring the thin films while grown. Recently optical second harmonic generation has been proposed by us as a tool for non-invasive, non-destructive, real-time, in-situ imaging of oxide epitaxial film growth. The spatial resolution of this technique has been exploited to obtain real-time images of the sample under investigation. Here we propose to exploit another very important physical property of the second harmonic wave: its phase, which is directly coupled with the electric field direction, as shown by our measurements
Two-Dimensional Ferroelectrics: A Review on Applications and Devices
Full text linkOver the last few years, research activities have seen two-dimensional (2D) materials become protagonists in the field of nanotechnology. In particular, 2D materials characterized by ferroelectric properties are extremely interesting, as they are better suited for the development of miniaturized and high-performing devices. Here, we summarize the recent advances in this field, reviewing the realization of devices based on 2D ferroelectric materials, like FeFET, FTJ, and optoelectronics. The devices are realized with a wide range of material systems, from oxide materials at low dimensions to 2D materials exhibiting van der Waals interactions. We conclude by presenting how these materials could be useful in the field of devices based on magnons or surface acoustic waves
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