Wide-bandgap Crystals Research Articles

Extension of the multiple rate equation model for conduction band dynamics under near- and mid-IR femtosecond excitation of dielectrics and semiconductors

We present here investigation of theoretical models for conduction band dynamics in solids induced by near- and mid-IR femtosecond laser field. Widely spread Multiple Rate Equation model (MRE) [B. Rethfeld. Phys. Rev. Lett. 92, 187401 (2004)] is considered to be inapplicable when laser wavelength belongs to the near- and mid-IR ranges. We report on an extension of the MRE model and give comprehensive description of the new model. In particular, an additional averaged energy level above the critical one is introduced and phenomenological parameters such as one-photon absorption rate and impact ionization rate are eliminated. The range of laser fluences and wavelengths where the extended model should be used are defined both in narrow and wide bandgap crystals. Bulk micromodification of silicon using 4.5 µm 200-fs laser pulses at laser fluences higher than 0.049 J/cm2 are shown. The proposed Extended Multiple Rate Equation model (EMRE) can be considered as a basis for further theoretical investigation of laser-matter investigation.

Topological waves guided by a glide-reflection symmetric crystal interface

A domain wall separating two different topological phases of the same crystal can support the propagation of backscattering-immune guided waves. In valley-Hall and quantum-Hall crystal waveguides, this property stems from symmetry protection and results from a topological transition at a Dirac point. Since an initially closed band gap has to open, the guidance bandwidth remains limited compared to that of wide band gap crystals. When a glide-symmetric dislocation is introduced in a 2D crystal, we show that a pair of wide-bandwidth, single-mode, and symmetry-protected guided waves appear in the bulk band gap. The 2D Zak phase changes by $\pi$ on either side of the interface, providing a topological invariant protected by glide-reflection symmetry at the X point of the Brillouin zone. A demonstration experiment is performed with acoustic waves in water, at ultrasonic frequencies, and shows the continuous tuning of transmission as a function of the glide parameter. The concept further extends to other types of waves, including the case of elastic waves in solids, but also of optical and electromagnetic waves.

Commercialization of microcavity plasma devices and arrays: Systems for VUV photolithography and nanopatterning, disinfection of drinking water and air, and biofilm deactivation for medical therapeutics

AbstractA little more than two decades after the introduction of the first microcavity plasma devices, a growing body of commercial products based on the remarkable properties of these low‐temperature, atmospheric plasmas is now available. Following a brief review of early microplasma lamp development, this article describes microplasma‐based devices and systems currently being manufactured for applications in photolithography, photopatterning, and other nanofabrication processes, such as atomic layer deposition, spectroscopic identification and analysis of materials (including wide bandgap crystals), the disinfection of drinking water in off‐grid communities, deactivating COVID‐19 and other pathogens in air and on surfaces in public spaces, and vacuum ultraviolet lamps for driving atomic clocks. The microplasma‐based treatment of otitis media in the human ear will also be discussed.

Non-steady-state photo-EMF in β-Ga2O3 crystals at λ = 457 nm

The non-steady-state photoelectromotive force is excited in a monoclinic gallium oxide crystal at wavelength λ = 457 nm. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved without application of any electric field to the sample. The dependencies of the signal amplitude on the frequency of phase modulation, light intensity, spatial frequency and light polarization are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal's photoconductivity, responsivity and diffusion length of electrons are estimated for the chosen light wavelength and compared with the ones for other wide-bandgap crystals.

Quantum defects by design

Abstract Optically active point defects in wide-bandgap crystals are leading building blocks for quantum information technologies including quantum processors, repeaters, simulators, and sensors. Although defects and impurities are ubiquitous in all materials, select defect configurations in certain materials harbor coherent electronic and nuclear quantum states that can be optically and electronically addressed in solid-state devices, in some cases even at room temperature. Historically, the study of quantum point defects has been limited to a relatively small set of host materials and defect systems. In this article, we consider the potential for identifying defects in new materials, either to advance known applications in quantum science or to enable entirely new capabilities. We propose that, in principle, it should be possible to reverse the historical approach, which is partially based on accidental discovery, in order to design quantum defects with desired properties suitable for specific applications. We discuss the biggest obstacles on the road towards this goal, in particular those related to theoretical prediction, materials growth and processing, and experimental characterization.

Structural, optical, electrical and thermal characteristics of La{CS(NH2)2CO(NH2)2}Cl3 : A novel hybrid complex for optoelectronic applications

A hybrid complex of lanthanum chloride with urea and thiourea is crystallized at room temperature by conventional solution method. The structural investigations by single crystal X-ray diffraction confirmed the orthorhombic system with cell dimensions a = 7.676 Å, b = 8.564 Å, c = 5.498 Å, α = β = γ = 90°. The optical studies by UV–VIS absorption spectroscopy shows the complex exhibits transmittance about 96%. The complex is characterized as the wide band gap crystals with value greater than 4 eV. The dielectric properties were studied in frequency range 10 kHz–1 MHz in temperature range 303–453 K. The ac conductivity was found to obey the universal power law and suggests the correlated barrier hoping mechanism dominating the conduction process. The thermal stability and the decomposition mechanism were confirmed by thermo gravimetric and differential thermo analytical (TG/DTA) techniques.

Two dimensional PbMoO4: A photocatalytic material derived from a naturally non-layered crystal

Metal molybdates, such as lead molybdate (PbMoO4), are wide bandgap crystals with favourable photophysical and photocatalytic properties and also high stability. If these crystals are synthesized in two dimensional (2D) morphologies, they can offer the needed large surface-area for photo-reactions. However, this category of materials does not constitute natural stratified crystals, and hence cannot be readily formed into 2D sheets using conventional methods. Here we present a two-step synthesis approach. First, we exfoliate stratified α-MoO3 into α-MoO3-x. Subsequently, these defect rich 2D α-MoO3-x nanosheets are transformed into stable 2D PbMoO4 nanosheets by dipping-pulling. We show that the transformed 2D PbMoO4 display trap states within its bandgap, allowing its efficient performance as a photocatalyst under the visible light condition. The presented method in this work can be extended to establish a variety of highly stable defect rich 2D metal molybdates, which are otherwise challenging to achieve, for visible light region photo-reactions.

Spectral features of the Stokes part of supercontinuum generated by femtosecond light pulses in selected oxide crystals: A comparative study

The peculiarities of the Stokes part of supercontinuum (SC) generated by femtosecond light pulses at wavelength 800 nm in single crystals of Gd2SiO5(GSO), Ca4GdO(BO3)3(GCOB), Gd3Ga5O12(GGG), LiTaO3 (LTO) and LuVO4 (LVO) were investigated. Spectral bandwidth and intensity of SC were measured as a function of energy of incident 100 fs pulses employing a grating spectrograph coupled with an InGaAs detector and spatial characteristics of the beam inside crystal samples were monitored perpendicularly to the direction of propagation and recorded using an optical microscope coupled with a camera. It was found that spectral widths of the Stokes part of SC increase markedly with increasing energy of incident pulses for all crystals under study. For fixed focusing conditions the spectral widths of generated SC in GSO, GCOB and GGG wide band-gap crystals are relatively large with cut-off wavelengths close to 1500 nm. Bandwidths of SC generated in LVO and LTO crystals, characterized by band-gaps Eg inferior to three times incident photon energy, are markedly smaller with cut-off wavelengths of 1300 nm and 1150 nm, respectively. Increase of incident pulse energy affects SC spectra giving rise to plateau-like regions stretching to ca 1000 nm.

2016 International Conference on Defects in Insulating Materials (ICDIM 2016)

The 2016 edition of the International Conference on Defects in Insulating Materials (ICDIM 2016) was held in Lyon, France on July 10-15, 2016. A scientific community always originates from a common historical background and our common background is the science of color centers, which has been nicely reviewed by Prof. Heiz Pick in 1980 [1]. Because a scientific community always needs events to gather scientists from all around the world, setting up a conference series is fruitful way to regularly stimulate joint research and share recent advances. In this spirit, the first international event related to color center science was held in Argonne-USA in 1956. In 1973, a similar conference series entitled Europhysics Conference: Lattice defects in Ionic Crystals emerged in Europe (Marseille). In 1977, in Gatlingburg, the Colour Centre conference series became the first official International Conference on Defects in Insulating Materials and in 1994, in Lyon, the European meeting on Lattice Defects in Ionic Crystal merged with ICDIM and became EuroDIM. Both events occur every 4 years, leading to an event on materials defect science every 2 years. Figure 1 depicts the historical revue of the evolution of the ICDIM/EuroDIM conference series. Note that during ICDIM 2016, Dr. Martin Nikl and Prof. Rob Jackson have been elected as the chairperson of respectively ICDIM and EuroDIM conference series.Figure 1:the historical evolution of the 60 years of conferences on defects in insulating crystalsThis conference series is rather focused and aims to promote material sciences around defects, particularly in wide band gap crystals and glasses. But to adapt to the current trends, the content of the topics has evolved along the following items:-Point and extended defects-Phenomena at surfaces and interfaces-Low dimensional and nano-morphological systems-Defect modeling and computational methods-Luminescence of excitons, impurities and defects-Radiation induced defects and material damage-Scintillation, energy transfer and storage, carrier trapping-Laser actived centers and nonlinear optical phenomena-Disordered systems-Applied materials and interdisciplinary topics.This issue of the Institute Of Physics conference series is dedicated to papers submitted during the ICDIM 2016 event in Lyon, France. 209 abstracts were selected for oral or poster presentation, always in plenary session. We have also proposed 7 invited keynote speakers who presented didactical talks introducing their respective fields to young researchers. Most of the talks have been video recorded and can be seen on the following YouTube channel [2].The International Committee has selected Guangzhou (China) for ICDIM 2020, which will be chaired by Prof. Mingying Peng.[1] H.Pick 1980, J. de Physique, Colloque C6, Tome 41, C6-1, Fifty years of colour centre physics[2] https://www.youtube.com/playlist?list=PL_ZZHWrfptdQPjO8Q9H6rOBkuFS7pbTBVGuest EditorsProf. C.Dujardin, Conference Chair and Senior EditorDr. E.Homeyer, Associate EditorDr. J.Houel, Associate EditorDr. G.Ledoux, Associate Editor

Storing energy in lithium fluoride crystals irradiated with femtosecond laser pulses

The release of energy in the form of the light sum of thermally stimulated luminescence (TSL), stored under conditions of self-focusing and the multiple filamentation of femtosecond laser radiation during the interaction between model wide-bandgap dielectric crystals of lithium fluoride is studied. It is shown that F2 color centers are important centers of emission in the TSL process.

Detection of phase- and frequency-modulated optical signals using dynamic space-charge gratings in wide-bandgap crystals

The excitation of the non-steady-state photoelectromotive force and two-wave mixing signals is theoretically investigated for the case of uniformly accelerated motion of the recording light pattern. Such illumination is usually created by the linear frequency modulation of the interfering light beams. For the both effects we predict the pulsed response arising at the moments, when the velocities of the interference pattern and space-charge wave coincide. The application of the effects in laser Doppler velocimeters and accelerometers is discussed as well.We also study the excitation of the non-steady-state photoelectromotive force in semiinsulating SiC crystals illuminated by a phase-modulated light signal. Influence of an external ac electric field and shallow traps on the signal is revealed. Photoelectric parameters of the material and sensitivity of the detectors are estimated for the light wavelength λ=532nm.

Transparent wide band gap crystals follow indirect allowed transition and bipolaron hopping mechanism

Recently, we carried out structural, optical and dielectric studies on micro-crystals of Oxypeucedanin (C16H14O5), isolated from the roots of plant Prangos pabularia (Mir et al. (2014) [3,4]). The obtained trend in frequency exponent (s) with frequency (ω) indicates that the universal dynamic response is followed by this compound. From optical absorption spectroscopy, the optical band gap (Eg) was estimated around 3.76eV and system is showing indirect allowed transition. Using Eg in certain relation of s, a close value of s (as much close obtained by fitting ac conductivity) was obtained. This method was further used for other similar systems and again same trend was obtained. So a general conclusion was made that the high transmitting wide band insulators or semiconductors may follow bipolaron hopping transport mechanism.

Luminescence of impurity-bound excitons in Li6GdB3O9:Ce3+ single crystals

The anomalous (τ < 10 ns) luminescence of wide bandgap crystals of lithium–gadolinium orthoborate Li6GdB3O9 doped with trivalent cerium ions, has been revealed for the first time and investigated through the low-temperature time-resolved vacuum ultraviolet synchrotron spectroscopy. It was shown that the optical transitions at 6.2 eV are due to electron transfer from the ground 4f1 states of Ce3+ ion onto the autoionized states near the conduction band bottom of a crystal. These transitions lead to the formation of impurity-bound excitons in the form of correlated electron–hole pair, in which the hole component is localized at 4f-level of the cerium ion and an electron component is located at the conduction band bottom in the attractive potential of this hole. It is established that the direct radiative recombination of the cerium impurity-bound exciton leads to a fast broadband emission at 4.25 eV. The energy threshold for creation of the impurity-bound excitons was determined on the basis of the obtained spectroscopic data. We calculated the H(k) functions of distribution of the elementary relaxations over the reaction rate constants and explained on this basis the decay kinetics and quenching processes, not only for the anomalous emission at 4.25 eV, but for the ordinary 5d–4f luminescence at 3.0 eV in Ce3+ ions. The paper discusses the decay channels for the impurity-bound excitons and their influence on the decay kinetics and spectra of luminescence in Li6GdB3O9 crystals.

Modelling of decay kinetics of self-trapped exciton luminescence in CdWO4 under femtosecond laser excitation in absorption saturation conditions

Abstract The analytical expressions for the three-dimensional spatial distribution of excitons and luminescence decay kinetics of wide band-gap crystals were derived taking into account the saturation effect for absorption near the centre of the incident laser beam with Gaussian transversal profile. The model is verified for the excitonic emission of CdWO4 crystals under excitation by ultra-short laser pulses in the region of weak excitonic absorption.

Modeling of the luminescence-decay kinetics of self-trapped excitons at a high excitation density under conditions of absorption saturation

An analytical model for determining the spatial distribution of self-trapped excitons upon excitation at the Urbach absorption tail under conditions of absorption saturation is developed, and an expression for exciton luminescence decay kinetics in wide band-gap crystals was derived taking into account concentrational exciton self-quenching under such excitation conditions.

An STM for molecules and wide-bandgap crystal

We are all influenced by Nicolai Delone’s research. Through much of his career, his influence reached Canada through his published papers. However, Professor Delone visited the National Research Council of Canada’s laboratories at least three times from about 1989 to 1995. At that time he was primarily interested in stabilization. Stabilization refers to the fact that, as the intensity of laser light illuminating an atom is increased, the ionization rate passes through a maximum, then falls. Surprisingly the atom becomes stable against ionization at very high intensities.

Photoionization rate in wide band-gap crystals

The Keldysh model of the photoionization [L. V. Keldysh, Sov. Phys. JETP 20, 1307 (1965)] is extended by deriving a formula for the photoionization rate of crystals based on the cosine energy-momentum relation. The relation is characteristic of tight-binding approximation and directly takes into account the influence of Bragg-type reflections of oscillating electrons at the edges of the first Brillouin zone. Due to the reflections and oscillations, the dependence of the photoionization rate on laser and material parameters takes form of a multibranch function with the branches separated by singularity points with unlimited increasing of the rate. Each of the singularities is coupled to the flattening of the effective-band structure. The laser intensity corresponding to the first singularity is found to be about $10\phantom{\rule{0.3em}{0ex}}\mathrm{TW}∕{\mathrm{cm}}^{2}$ for most wide band-gap crystals. We also show that the lowest-order branch of the photoionization rate completely corresponds to the multiphoton regime, and the first singularity takes place before the tunneling regime starts to dominate. Analysis of the ionization-rate asymptotic in the vicinity of the first-singularity point suggests possibility of ionization suppression by high-intensity radiation for certain ranges of laser wavelength.

Time resolved luminescence of solids excited by femtosecond VUV pulses and synchrotron radiation

AbstractTime‐resolved luminescence spectroscopy has been widely used in studies of electronic excitations and their dynamics in insulators. Present paper discusses the potential of vacuum ultraviolet spectroscopy applied for wide band gap crystals using classical synchrotron radiation as well as more recently developed pulsed femtosecond short‐wavelength light sources. Optical spectroscopy using novel free electron lasers and laser systems based on higher harmonic generation process allow detailed investigation of the interaction between electronic excitations and behaviour of solids under extreme radiation conditions. Significant changes in the quantum yield and emission decay kinetics as well as surface damage of single crystals were observed for CdWO4 and CaWO4 crystals studied in the temperature range of 8‐300 K. However, in the range of excitation densities applied, up to ∼1012 W/cm2, the nature of luminescence centres and their characteristic emission spectra remained similar to those observed at low excitation densities. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Soft X‐ray excitation of luminescence in wide bandgap crystals doped with rare‐earth ions

AbstractLuminescence excitation spectra of wide bandgap oxide crystals were measured in the soft X‐ray range of excitation (300–750 eV) at the BEAR beamline of the electron storage ring Elettra (Trieste). We give a special attention to changes of the light yield in the vicinity of the absorption K‐edge of oxygen. The excitation spectra of two well‐known scintillators YAG‐Yb and LuAP‐Ce (chemical formulae are Y3Al5O12:Yb (15%) and LuAlO5:Ce (0.4%)) reveal a non‐proportional change of the quantum yield and a different response to the jump of absorption coefficient at the 1sO edge for certain emission bands. The analysis of experimental data shows that both the surface loss and interaction effects between secondary electronic excitations (EEs) can influence the energy conversion process in crystals. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Observation of high energy photoelectrons from solids at moderate laser intensity

We investigate the photoemission for a set of wide band-gap crystals irradiated by femtosecond Ti-Sapphire laser pulses at intensities varying from 0.5 to 6 TW/cm2 (below the optical breakdown threshold). The measured total electron yield increases linearly with the laser intensity in this intensity range. An intense and wide plateau of high energy electrons appears in the photoelectron spectra at excitation intensities larger than 1 TW/cm2. The exponential cut-off of this plateau reaches 40 eV at maximal applied intensities. In order to explain such a behavior, we propose a mechanism where the heating is due to a sequence of direct interbranch one- and multi-photon transitions in the conduction band.