Optimisation of GaAs nanocrystals synthesis by laser ablation in water

Laser ablation of a gallium arsenide (GaAs) wafer immersed in distilled water was carried out using the fundamental wavelength of a high frequency Nd:YAG laser with 240?ns pulse duration. Rate of nanoparticles generation through laser ablation for various amounts of laser pulse energies (0.4–0.94?mJ) and repetition rates (400–2000?Hz) were studied and a maximum ablation rate of 19.6?µgr/s was obtained. Formation of the pure GaAs nanocrystals (NCs) is confirmed using TEM micrograph and X-ray diffraction analysis. Band-gap energy of generated GaAs NCs is calculated by Tauc method to be between 2.48 and 2.60?eV which is larger than the band-gap energy of bulk GaAs. The band-gap energy of NCs is increased by increasing the energy of laser pulses and is decreased by increasing the pulse repetition rate.     

Theoretical study and 2D computational modeling of middle-infrared radiation generation in the field of few-cycle laser pulse propagating in GaSe slab waveguide

We have performed the theoretical study and 2D finite-difference time-domain-based computational modeling of middle-infrared radiation generation in the field of few-cycle laser pulse propagating in GaSe slab waveguide. The interaction of linearly polarized pulse with 1.98 um central wavelength, 22.44 fs duration, and electric field amplitudes 100 MV/m, propagated along the [010] crystalline direction of GaSe crystal with polarization aligned along [100] is considered. The crystal length chosen is equal to 21.78 um. Symmetric GaSe slab waveguides with 6.336 um, 7.92 um, 11.88 um, and 15.84 um thicknesses excited by the Gaussian pump beams with 3.96 um, 5.94 um, 8.91 um, and 11.88 um diameters, respectively, are considered. The 2D distributions of the electric fields are obtained by numerical integration of Maxwell’s equations’ systems. The spatially spectral normalized distributions of the electric fields are obtained. The efficiency of generated infrared (IR) radiation vs. spatial transverse coordinate is obtained.     

Stable supercontinuum generation in short lengths of conventional dispersion-shifted fiber

By propagating 500-fs pulses through 2.5 m of standard fiber followed by 2 m of dispersion-shifted fiber, we generated >200 nm of spectral continuum between 1430 and 1630 nm, which is flat to less than ?0.5 dB over more than 60 nm. Pulses obtained by filtering the continuum show no increase in timing jitter over the source laser and are pedestal free to >28 dB, indicating excellent stability and coherence. We show that the second- and third-order dispersions of the continuum fiber and self-phase modulation are primarily responsible for the continuum generation and spectral shaping and found close agreement between simulations and experiments.     

An Experimental Study of a Synchronously Pumped Fibre Raman Oscillator

Abstract

A synchronously pumped fibre Raman oscillator has been constructed employing a mode-locked c.w. Nd:YAG laser as a pump source and 150 m of single-mode optical fibre as the Raman-active medium. A detailed spectral and temporal study of the laser has been undertaken. Time-dispersion tuning offered an operating spectral range of 1·0725–1·1220 µm for the first Stokes oscillation and 1·149–1·179 µm for the associated second Stokes component. Pulse durations ～ 100 ps were generated with average output powers of about 40 and 9 mW for the first and second Stokes Raman pulses respectively.     

Propagation effects in electro-optic sampling of terahertz pulses in GaAs

Freely propagating terahertz pulses are detected in the time domain by electro-optic sampling in bulk GaAs. We investigate the influence of dispersion of the near-infrared sampling pulse on the transients by varying the thickness of the GaAs crystal. Pronounced propagation effects are identified that originate from the frequency dependence of the phase-matching condition between the terahertz and the sampling pulse.     

Acoustostimulated change in the optical absorption coefficient of gallium arsenide in the 0.81–1.77 μm wavelength range

The effect of ultrasonic waves on the spectral absorption coefficient of gallium arsenide (GaAs) in the 0.81–1.77 μm wavelength range. The treatment of GaAs single crystals with ultrasonic waves leads to a change in their coefficients of absorption of electromagnetic radiation. This phenomenon is related to the fact that an acoustic wave, being an energy carrier, modified a defect system of the crystal and produces a redistribution of impurities in the crystal lattice. The interaction of photons with acoustogenerated defects changes the optical absorption coefficient near the edge of the fundamental absorption band of GaAs.     

Direct quasi-phase-matched fourth-harmonic generation

We have theoretically demonstrated direct fourth-harmonic generation (FHG) based on a quasi-phase-matching (QPM) configuration in periodically poled lithium niobate. The wavelength dependence of period of FHG QPM gratings is calculated. Bandwidths of fundamental wavelength, temperature, and incident angle are also studied. We find a very wide bandwidth, as large as 115 nm, of fundamental wavelength near the wavelength of 3797 nm with the QPM period of 9.73 microm. The numerical calculation shows that the conversion efficiency for QPM FHG and cascading QPM by two-step second-harmonic generation is almost identical.     

An investigation of a solid–liquid phase-change-cooled mirror

This article details a new design for a laser-reflecting mirror utilizing phase-change-cooling technology, which is innovative in using spiral cooling ducts filled with solid-liquid phase-change material to maintain a constant temperature inside the reflecting mirror in a positive-branch telescopic, virtually confocal unstable resonator. The structure no longer needs the 45° scraper plate that is always utilized in a positive-branch telescopic, virtually confocal unstable resonator for coupling the output and provides a high-quality coaxial laser output. The numerical calculation results show, that when phase-change cooling is utilized, the localized maximum temperature dropped from 44 to 33°C. Experiments show that the total surface deformation of a copper mirror dropped from 1.4?µm to 0.27?µm when using phase-change cooling, after continued laser exposure for 10?s and, under the same conditions, that of a silicon mirror dropped from 1.1?µm to 0.4?µm.     

Third-order dispersion in a pair of prisms

The general equation for third-order group velocity dispersion is derived in this paper, which contains all the terms of equation (11) in Arissian and Diels [1], as well as one extra term, which we have verified is not negligible. To verify our equation we have modeled the sum of Gaussian modulated frequencies. We show the electric field of bandwidth limited incident ultrashort pulses with a time duration of: 5 fs, 10 fs, 15 fs, 20 fs, 30 fs and 50 fs after propagating through a pair of identical isosceles prisms for a central frequency of 0.8 µm and 0.620 µm. The model is applied by using (a) the finite phase, (b) the phase approximated up to the second-order dispersion and (c) up to the third-order dispersion for each frequency component of the pulse. Graphs are presented for prisms made with fused silica and SF14.     

Application of diamond-like carbon films to increase transmission of semi-insulating GaAs crystals in the IR spectral range

The effect of treatments in plasmas of different gases and subsequent deposition of diamond-like carbon films (DCFs) on the transmission of semi-insulating GaAs crystals in the IR spectral range has been analyzed. It is shown that deposition of 1- to 1.5-??m DCFs makes it possible to increase the GaAs transmission in the range of 4?C15 ??m and that preliminary treatment in H⁺ or Ar⁺ plasma increases the optical transmission of the DCF-GaAs structure. A mechanism is proposed to explain the effect of plasma treatment on the optical transmission of semi-insulating GaAs in the IR spectral range.     

High characteristic temperature 1.5 µm wavelength laser diode via Sb-based quantum dots in quantum wells

The advent of laser diodes in the 1.55?µm wavelength region is becoming a hot topic in the field of telecommunications. The growth of strain-engineered Sb-based multi-stacks quantum dots (QDs) on GaAs by molecular beam epitaxy (MBE) is advantageous to restrain its drawback of self-absorption and thus beneficial for preparing efficient laser diodes (LDs). Moreover, owing to a strong electronic coupling between the QDs layers and the quantum wells (QWs), strain-engineered QDs introducing antimony (Sb) with emission wavelength up to 1.5?µm were achieved at room temperature. The p-type doping can substantially increase the QD laser’s ground state gain at room temperature. Based on this simple process, high efficient LD was obtained. The LD was fabricated with a cavity length of 1000?μm and a stripe width of 100?μm. The output performance was achieved with threshold current densities of the device as low as 135?A/cm², and with high Characteristic Temperatures of 118?K or higher in the temperature range between 20°C and 80°C. The continuous wave operating up to 32?mW were achieved at room temperature (RT).     

Acoustostimulated expansion of the short-wavelength sensitivity range of AlGaAs/GaAs Solar cells

The effect of ultrasonic waves on the spectral sensitivity of solar energy converters based on AlGaAs/GaAs heterostructures has been studied. Ultrasonic treatment of a zinc-doped graded-gap Al_xGa_1−x As film leads to the formation of a surface layer sensitive to electromagnetic radiation in the wavelength range λ < 0.551 μm. It is established that this layer is formed as a result of the acoustostimulated inward diffusion of zinc from the surface to the bulk of the graded-gap layer. The observed expansion of the short-wavelength sensitivity range and an increase in the efficiency of nonequilibrium charge carrier collection in AlGaAs/GaAs solar cells are due to improvement of the crystal defect structure and the dopant redistribution under the action of ultrasound.     

Self-mode-locking effects in heavily doped single-clad Tm3+-doped silica fibre lasers

Self-mode-locking in single-clad Tm-doped silica fibre lasers operating at 1.92?µm is reported with 100% modulated mode-locked pulses observed for longer fibre lengths up to 6?m. The self-mode-locking was accompanied by self Q-switching. A mode-locked pulse duration of 191?ps has been measured using an autocorrelator based on a two-photon absorption detector for operation at 2?µm assembled for the purpose of this experiment. The experimental pulse train characteristics are found to be consistent with mode-locking theoretical analysis. The origin and properties of self-mode-locking in heavily Tm-doped fibre lasers are discussed.     

Hafnium Diboride as a saturable absorber for Q-switched lasers

Hafnium diboride (HfB₂) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB₂ is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB₂ as a saturable absorber in Q-switched lasers. HfB₂ has a non-saturable absorptance of 2.5?dB/µm, a saturable absorptance of 0.8?dB/µm and a saturation fluence of 12?µJ/cm². In addition, the saturation lifetime is estimated as 989.3?ps, while the recovery lifetimes are estimated as 254.9 and 22.1?ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000?ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250?ns. HfB₂ can potentially work at very high power, since its damage fluence is no less than 361?mJ/cm².     

Increasing the degradation resistance of semi-insulating gallium arsenide crystals by plasma processing

The effect of processing in hydrogen plasma on the resistance of semi-insulating GaAs crystals to degradation under the action of high-frequency (HF) electromagnetic fields and heat treatments has been studied by measuring the room-temperature IR transmission spectra of samples in a 5?C15 ??m wavelength range. It is established that the transmission of plasma-treated crystals, in contrast to untreated samples, does not decrease under subsequent HF irradiation and even increases in comparison to the initial transmission. A mechanism is proposed that explains the influence of processing in hydrogen plasma on the degradation resistance and optical transmission of semi-insulating GaAs crystals in the IR spectral range. This mechanism takes into account the relaxation of internal mechanical stresses in a near-surface layer of a crystal as a result of the plasma processing.     

Second-order optical effects in seed-mediated grown palladium nanoparticles

Palladium nanoparticles deposited on SnO₂-doped In₂O₃ substrate show substantial optical second harmonic generation (SHG) in the spectral range (λ?=?120–160?nm) which is a part of the vacuum ultraviolet (VUV) spectrum. A single crystalline Li₂ B₄ O₇ optical parametric generator (OPG) pumped by nanosecond xenon–fluorine excimer laser (EMG 500/218 (Lambda Physics)) with the wavelength 218?nm, pulse duration about 6–8?ns; pulse rate about 80?hz, average pulse power about 0.2?MW and beam diameter varying within the 1.3–7.5?nm was used to form the fundamental beam. The OPG Li₂ B₄ O₇ single crystal was cut in the XZ optical plane. We have tuned the fundamental wavelengths within the 250–320?nm spectral range varying the angle of the plane with respect to the incident pumping beam. Maximal SHG output (in the reflected SHG geometry) is observed for the incident angles 75^○–80^○ with respect to the surface normal and p-polarized incident fundamental ultraviolet beams. Spectral separation between the vacuum ultraviolet (VUV) SHG intensities and the fundamental beams was performed using a VUV Seya-Numioka vacuum monochromator with spectral resolution 6?nm in the investigated spectral range. We have found that decreasing mean average palladium nanoparticle sizes favour substantial enhancement of the output SHG within the 120–160?nm spectral range. A layer of platinum nanoparticles coated on a layer of palladium nanoparticles suppresses the SHG effect indicating a quenching of the surface plasmon excitation originating from the palladium nanoparticles. The observed effect allows utilizing the palladium nanoparticles as an efficient material for frequency transformation of the UV nanosecond pulses (spectral range 240–310?nm) into the nanosecond laser pulses with wavelengths 120–160?nm.     

Field emission properties of Fe70Pt30 catalysed multiwalled carbon nanotubes

Multi-walled carbon nanotubes were grown on nanocrystalline Fe₇₀Pt₃₀ film using low-pressure chemical vapour deposition (LPCVD) method. The growth time was varied between 5?min to 30?min. SEM micrograph of this film revealed that the size of nanoparticles varied from 5?nm to 10?nm. The diameter of the carbon nanotubes varied from 20?nm to 50?nm as verified by TEM. HRTEM image confirmed that the carbon nanotubes are bamboo-shaped multiwalled carbon nanotubes (MWNTs). Field emission characteristics of MWNTs at various growth times (5?min, 15?min and 30?min) with working distances (50?µm, 100?µm and 150?µm) were also studied. The carbon nanotubes grown for 30?min with working distance 150?µm exhibited the lowest turn-on field of 2.45?V/µm. The turn-on field increases from 2.45?V/µm to 6.21?V/µm as the growth time decreases from 30?min to 5?min whereas for lower working distances (100?µm and 50?µm), the turn-on field increases from 4.74?V/µm to 6.74?V/µm and from 8.79?V/µm to 14.49?V/µm respectively. The turn-on field (E _to) and field enhancement factor (β) were studied as a function working distance (d) and growth time respectively to see the effect of these parameters on field emission properties. The field enhancement factor (β) was also studied as a function of radius of apex curvature (r) . It was found the field enhancement factor (β) decreases with the increase in radius of apex curvature (r) and growth time whereas the value of field enhancement factor (β) increases as working distance (d) increases. On the basis of the dependence of β on radius of apex curvature (r) a relationship of β?∝?r ^?1/2 is fitted.     

Surface integrity and removal mechanism in grinding sapphire wafers with novel vitrified bond diamond plates

In order to improve machining efficiency of sapphire wafer machining using the conventional loose abrasive process, fixed-abrasive diamond plates are investigated in this study for sapphire wafer grinding. Four vitrified bond diamond plates of different grain sizes (40?µm, 20?µm, 7?µm, and 2.5?µm) are developed and evaluated for grinding performance including surface roughness, surface topography, surface and subsurface damage, and material removal rate (MRR) of sapphire wafers. The material removal mechanisms, wafer surface finish, and quality of the diamond plates are also compared and discussed. The experiment results demonstrate that the surface material is removed in brittle mode when sapphire wafers are ground by the diamond plates with a grain size of 40?µm and 20?µm, and in ductile mode when that are ground by the diamond plates of grain sizes of 7?µm and 2.5?µm. The highest MRR value of 145.7?µm/min is acquired with the diamond plate with an abrasive size of 40?µm and the lowest surface roughness values of 3.5?nm in Ra is achieved with the 2.5?µm size.     

Ultralong-Range Energy Transport in a Disordered Organic Semiconductor at Room Temperature Via Coherent Exciton-Polariton Propagation

Amorphous molecular solids are inherently disordered, exhibiting strong exciton localization. Optical microcavities containing such disordered excitonic materials have been theoretically shown to support both propagating and localized exciton-polariton modes. Here, the ultrastrong coupling of a Bloch surface wave photon and molecular excitons in a disordered organic thin film at room temperature is demonstrated, where the major fraction of the polaritons are propagating states. The delocalized exciton-polariton has a group velocity as high as 3 × 10⁷ m s^–1 and a lifetime of 500 fs, leading to propagation distances of over 100 µm from the excitation source. The polariton intensity shows a halo-like pattern that is due to self-interference of the polariton mode, from which a coherence length of 20 µm is derived and is correlated with phase breaking by polariton scattering. The demonstration of ultralong-range exciton-polariton transport at room temperature promises new photonic and optoelectronic applications such as efficient energy transfer in disordered condensed matter systems.