GaP Crystals Research Articles

Resonant Light Scattering by Optical Phonons in a Homoepitxial n-GaP Nanolayer Grown on a (001)n-GaP Substrate

The results of detection of resonant amplification of light scattering intensity by optical phonons in a homoepitaxial nanoscale 70 nm thick n-GaP layer grown by gas-phase epitaxy from organometallic compounds on a conducting strongly doped n-GaP crystal substrate oriented along the (001) axis have been presented. It has been shown that it is possible to detect rather narrow bands of second-order light scattering lines in the frequency range from 600 to 800 cm–1 at room temperature in the Raman light scattering spectrum of such a (001) n-GaP nanolayer in the n-GaP/n-GaP (001) sample in comparison with those in the spectrum of a high-resistance (001) si-GaP crystal sample. It has been established that such bands are caused by the total combinations and overtones of transverse TO(Γ) and longitudinal LO(Γ) optical phonons with wave vectors corresponding to the points Σ, K, X, L, and Γ of the Brillouin zone of a GaP crystal. It has been shown that the light scattering is resonant and caused by the presence of impurities because of the exciton-phonon interaction.

Coulomb mechanism of Raman radiation in graphene

The phenomena of single-layer graphene resonant photoluminescence and Raman radiation are discussed taking into account the photo-generated electron–hole Coulomb interaction. On the base of general principles of a many-particle interactions and the interband resonance optical transitions a photon radiation new mechanism (Coulomb mechanism) is proposed. Through Stokes 2D'-mode particular case analysis has shown that the graphene photoluminescence and the resonant Raman radiation are characterized by the same frequency shifts. Probabilities of resonance photo-radiation processes have been presented where the electron–hole Coulomb attraction has been taken into account. The probabilities are the same fourth-order small values. The weak photo-radiation Coulomb mechanism has a common character. It is applicable to both zero and nonzero band gap crystals.

Relativistic Doppler reflection of terahertz light from a moving plasma front in an optically pumped Si wafer

The relativistic Doppler reflection of terahertz (THz) light from a photoinduced moving plasma front in a silicon wafer irradiated by an optical pump light was investigated. The intensity and the phase shift spectra of the reflected THz light were investigated by THz time-domain spectroscopy with broadband electro-optic sampling in a thin GaP crystal. The frequency up-shift factor, which is a ratio of averaged frequencies from after to before Doppler reflection, was measured to be $\mathrm{\ensuremath{\Gamma}}=2.02$ with a pump energy density of $3.3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{J}/\mathrm{m}{\mathrm{m}}^{2}$ at a wavelength of 800 nm. The frequency up-shift increased as optical pump energy density increased. The frequency up-shift was reproduced by evaluating the moving plasma front whose spatial carrier density distribution depended on the pump energy density.

Efficient terahertz generation and detection in cadmium telluride using ultrafast ytterbium laser

In this work, we study THz generation and detection using cadmium telluride (CdTe) crystals pumped by amplified (1.025 μm wavelength) and oscillator (1.045 μm wavelength) ytterbium (Yb) lasers. For each laser, we compare the performances of the CdTe THz emitter and detector to those of GaP crystals. Under optimum conditions, we demonstrate that the former shows 3 and 5 times better performances compared with the latter for detection and generation, respectively. When pumped by an amplified Yb laser, we find that the CdTe crystal is more efficient than the GaP crystal for emission at optical fluences lower than 250 μJ/cm2. Although CdTe has some limitations in comparison with GaP, such as high THz absorption above 1 THz and the appearance of two-photon absorption at relatively low optical intensity, our findings demonstrate the potential of this crystal to be used as the emitter and detector in combination with the Yb laser.

First principles study of the electronic, elastic, infrared and electrical properties of LiZnPS4

LiZnPS4 is thought to be a promising mid-infrared optical crystal and lithium battery material, but its fundamental properties are still not well investigated. In this work, we performed a systematic study on its electronic, elastic, infrared and electrical properties. The results indicate that LiZnPS4 is an indirect band gap crystal. Calculated partial density of states reveals that the top of its valence band is formed by the S 3p orbital while the component of the bottom of the conduction band is complex. The Zn 4p orbital is not empty in LiZnPS4 owing to the tetrahedral coordination of the Zn atoms. Calculated elastic properties indicate that LiZnPS4 is a mechanically stable crystal with elastic anisotropy. It has a relatively low Debye temperature of 294.8 K. Its infrared absorption spectrum was simulated and the origins of the strong peaks in it were revealed. Vibrational modes at the Brillouin zone center were illustrated and Born effective charges were calculated and discussed. Computed dielectric constants uncover that electrons contribute more to the total dielectric constants than ions. Studies imply that LiZnPS4–Li+ battery has a high theoretical voltage, a low electric capacity and a slightly high activation barrier.

Highly sensitive electro-optic detection of terahertz waves in a prism-coupled thin LiNbO3 layer

Electro-optic sampling of pulsed terahertz radiation by femtosecond laser pulses in noncollinear geometry in a 40 μm thick layer of LiNbO3 attached to a Si prism is experimentally demonstrated. Terahertz radiation is prism-coupled into the layer through its surface and modulates the frequency and angular spectra of the optical radiation propagating along the layer as a fundamental mode of a dielectric slab waveguide. The modulation results in the optical intensity variations in the optical beam transmitted through the layer. By measuring the intensity variations, we achieved 38 and 216 times enhancement in the sensitivity of electro-optic detection as compared to the conventional collinear ellipsometric technique with 1 mm thick ZnTe and 0.2 mm thick GaP crystals, respectively.

Local density of optical states in the three-dimensional band gap of a finite photonic crystal

A three-dimensional (3D) photonic band gap crystal is an ideal tool to completely inhibit the local density of optical states (LDOS) at every position in the crystal throughout the band gap. This notion, however, pertains to ideal infinite crystals, whereas any real crystal device is necessarily finite. This raises the question as to how the LDOS in the gap depends on the position and orientation inside a finite-size crystal. Therefore, we employ rigorous numerical calculations using finite-difference time-domain (FDTD) simulations of 3D silicon inverse woodpile crystals filled with air or with toluene, as previously studied in experiments. We find that the LDOS versus position decreases exponentially into the bulk of the crystal. From the dependence on dipole orientation, we infer that the characteristic LDOS decay length $\ell_{\rho}$ is mostly related to far-field dipolar radiation effects, whereas the prefactor is mostly related to near-field dipolar effects. The LDOS decay length has a remarkably similar magnitude as the Bragg length for directional transport, which suggests that the LDOS in the crystal is dominated by vacuum states that tunnel from the closest interface towards the position of interest. Our work leads to design rules for applications of 3D photonic band gaps in emission control and lighting, quantum information processing, and in photovoltaics.

Reflectivity of three-dimensional GaAs photonic band-gap crystals of finite thickness

We study the optical reflectivity of three-dimensional (3D) photonic band gap crystals with increasing thickness. The crystals consist of GaAs plates with nanorod arrays that are assembled by an advanced stacking method into high-quality 3D woodpile structures. We observe intense and broad reflectivity peak with stop bands that correspond to a broad gap in the photonic band structures. The maximum reflectivity quickly reaches high values even for a few crystal layers. Remarkably, the bandwidth of the stop bands hardly decreases with increasing crystal thickness, in good agreement with FDTD simulations. This behavior differs remarkably from the large changes observed earlier in weakly interacting 3D photonic crystals. The nearly constant bandwidth and high reflectivity are rationalized by multiple Bragg interference that occurs in strongly interacting photonic band gap crystals, whereby the incident light scatters from multiple reciprocal lattice vectors simultaneously, in particular from oblique ones that are parallel to a longer crystal dimension and thus experience hardly any finite size effects. Our new insights have favorable consequences for the application of 3D photonic band gap crystals, notably since even thin structures reveal the full band gap functionality, including devices that shield quantum bits from vacuum fluctuations.

Measurement of differential cross sections of deuteron elastic scattering on 31P for EBS purposes

The 31P(d,d0)31P elastic scattering differential cross-sections were measured, for the first time, in the energy range Ed,lab=900-2400 keV, using a variable energy step, for elastic backscattering spectrometry (EBS) purposes. The measurements were performed at the 5.5 MV TN11 HV Tandem Accelerator of the N.C.S.R. “Demokritos”, implementing a high precision goniometer. The experimental setup consisted of five silicon surface barrier (SSB) detectors, placed at the laboratory scattering angles between 130° and 170° (in steps of 10°). The target used was a thin GaP layer evaporated on a self-supporting carbon foil. To validate the obtained results, benchmarking measurements were performed, using a polished GaP crystal. The determined cross-section datasets are compared with the corresponding ones using Rutherford’s formula, and similarities and discrepancies will be discussed and analyzed.

Compact single-shot electro-optic detection system for THz pulses with femtosecond time resolution at MHz repetition rates.

Electro-optical detection has proven to be a valuable technique to study temporal profiles of THz pulses with pulse durations down to femtoseconds. As the Coulomb field around a relativistic electron bunch resembles the current profile, electro-optical detection can be exploited for non-invasive bunch length measurements at accelerators. We have developed a very compact and robust electro-optical detection system based on spectral decoding for single-shot longitudinal bunch profile monitoring at the European X-ray Free Electron Laser (XFEL) for electron bunch lengths down to 200 fs (rms). Apart from the GaP crystal and the corresponding laser optics at the electron beamline, all components are housed in 19 in. chassis for rack mount and remote operation inside the accelerator tunnel. An advanced laser synchronization scheme based on radio-frequency down-conversion has been developed for locking a custom-made Yb-fiber laser to the radio-frequency of the European XFEL accelerator. In order to cope with the high bunch repetition rate of the superconducting accelerator, a novel linear array detector has been employed for spectral measurements of the Yb-fiber laser pulses at frame rates of up to 2.26 MHz. In this paper, we describe all sub-systems of the electro-optical detection system as well as the measurement procedure in detail and discuss the first measurement results of longitudinal bunch profiles of around 400 fs (rms) with an arrival-time jitter of 35 fs (rms).

A QCL-based metrological-grade source at 6 μm

In view of high-precision molecular spectroscopy in the infrared, we realized a narrow-linewidth laser source at 5.8 $$\upmu$$m traceable to the second in the International System of Units by mixing two near-infrared lasers in an orientation-patterned GaP crystal. The generated radiation is traceable to a Cs fountain through an optical fiber-delivered carrier; its estimated linewidth is 2.3 kHz at 1 s integration time. A quantum cascade laser emitting at the same wavelength is used to boost the power of the mid-IR radiation up to few tens of mW. Thanks to a phase-locking loop, about 70% of the quantum cascade laser output power is within the linewidth of the difference-frequency radiation. This apparatus is used for absolute molecular spectroscopy in a region, above 5 $$\upmu$$m, where high-precision traceability and reliable operation are still challenging. In this paper, we provide the basic information to implement a powerful and metrological grade source in the mid-infrared for precision measurements.

Ultrafast dynamics of the thin surface plasma layer and the periodic ripples formation on GaP crystal irradiated by a single femtosecond laser pulse.

Ultrafast dynamic of thin surface plasma layer plays a crucial role in the formation of periodic surface ripples after laser pulse irradiation. Using the pump-probe imaging technique, a complete scenario of the periodic ripples formation on a GaP surface is demonstrated after being irradiated by single femtosecond laser pulse. The ripples firstly emerge at delay time of several tens of picoseconds, and disappear completely at several hundreds of picoseconds, resulting in a transient overheating solid state ablation crater. It's interesting that new ripples appear and gradually become deep and clear after hundreds of picoseconds. A part of these ripples remain after the ablation crater is solidified. The period of the remained ripples is measured and approximately equal to the periods of the two transient ripples. The thin surface plasma model with multi-layer is introduced to study the formation of periodic ripples. The dynamics of the carrier excitation, carrier and lattice temperature, transient dielectric constant, and other factors are obtained by the two-temperature model and the Drude model. The results show that the periods of electric field distributions at different depths of the plasma layer are the same. The formation of the two transient ripples and the remained ripples are all related to the periodic energy deposition due to the SPP excitation at the air-plasma interface.

X-ray Imaging of Functional Three-Dimensional Nanostructures on Massive Substrates.

To investigate the performance of three-dimensional (3D) nanostructures, it is vital to study their internal structure with a methodology that keeps the device fully functional and ready for further integration. To this aim, we introduce here traceless X-ray tomography (TXT) that combines synchrotron X-ray holographic tomography with high X-ray photon energies (17 keV) in order to study nanostructures “as is” on massive silicon substrates. The combined strengths of TXT are a large total sample size to field-of-view ratio and a large penetration depth. We study exemplary 3D photonic band gap crystals made by CMOS-compatible means and obtain real space 3D density distributions with 55 nm spatial resolution. TXT identifies why nanostructures that look similar in electron microscopy have vastly different nanophotonic functionality: one “good” crystal with a broad photonic gap reveals 3D periodicity as designed; a second “bad” structure without a gap reveals a buried void, and a third “ugly” one without gap is shallow due to fabrication errors. Thus, TXT serves to nondestructively differentiate between the possible reasons of not finding the designed and expected performance and is therefore a powerful tool to critically assess 3D functional nanostructures.

Photo detector based on graded band gap perovskite crystal

In this work, millimeter sized graded band gap mixed halide hybrid perovskite crystal have been synthesized by utilizing reversible halide exchange property of hybrid perovskites. Optoelectronic performance of photo diodes fabricated using the graded band gap crystal has been studied in details. Band gap engineering has formed a smoother path for carrier transport by aligning HOMO and LUMO levels with the charge collection layers. This is reflected in good diode quality and fast photo response has been achieved. On the other hand, effect from both bulk and surface recombination of photo generated carriers are observed in the plateau type spectral response curve with a peak near band edge.

Detailed Identification of the Progression of Antiphase Boundaries in GaP/Si(001)

We present a detailed analysis of two-dimensional defects arising at the interface of the nonpolar semiconductor Si to the polar III/V-semiconductor GaP. These defects consist of so called wrong bonds, i. e. two of the same atoms bound to one another. On the two different sides of the defect, the polarity of the GaP crystal is flipped. Accordingly, the defects are called antiphase boundaries. They occur regularly during growth of polar semiconductors onto nonpolar substrates [1] and form three-dimensional antiphase domains investigated in this work.

Optical rectification of ultrafast Yb lasers: pushing power and bandwidth of terahertz generation in GaP

We demonstrate broadband high-power terahertz (THz) generation at megahertz repetition rates by optical rectification in GaP driven by an ultrafast Yb-based thin-disk laser (TDL) oscillator. We investigate the influence of pulse duration in the range of 50–220 fs and thickness of the GaP crystal on the THz generation. Optimization of these parameters with respect to the broadest spectral bandwidth yields a gap-less THz spectrum extending to nearly 7 THz. We further tailor the driving laser and the THz generation parameters for the highest average power, demonstrating 0.3 mW THz radiation with a spectrum extending to 5 THz. This was achieved using a 0.5 mm thick GaP crystal pumped with a 95 fs, 20 W TDL, operating at 48 MHz repetition rate. We also provide a simple method to estimate the THz spectrum, which can be used for design and optimization of similar THz systems.

Enhanced radiation characteristics of graphene-based patch antenna array employing photonic crystals and dielectric grating for THz applications

The Graphene patch antennas is widely used due to high electrical conductivity, high mobility, and saturation velocity in THz band. In a Terahertz (THz) band, high-gain antennas to be preferred to minimized the atmospheric attenuation and path losses. In this article, the enhanced radiation characteristics graphene-based antenna array design concept is proposed by using homogeneous substrate, photonic band gap crystal (PBG) substrate and superstrated dielectric grating. The projected graphene-based 2×1 patch array antennas works as a resonance patch with complementary split split split ring resonator (CSSSRR) for the frequency range from 0.84 THz to 0.94 THz. The tunable property of graphene material and PBG substrate are investigated by varying the chemical potential (μc) of graphene sheet and different radius of hollow cylinders respectively. The minimum return loss of -57.052 dB, peak gain of 14.69 dB and directivity of 15.5 dBi are achieved with the planned antenna employing with superstrated dielectric grating structures. The suggested 2×1 graphene patch antenna array are operating in low atmospheric attenuation window henceforth suitable for the application of medical imaging, threat detection, and THz wireless communication.

Measurement of differential cross sections of deuteron elastic scattering on 31P for EBS purposes

The differential cross sections of deuteron elastic scattering on 31P were determined in the present work for the first time, in the projectile energy range Ed,lab = 900–2400 keV (in energy steps of ~10–30 keV), suitable for analytical purposes. The measurements were performed at the 5.5 MV TN11 HV Tandem Accelerator of N.C.S.R. “Demokritos”, Athens, Greece. The experimental setup consisted of five silicon surface barrier (SSB) detectors, placed at the angles between 130° and 170° (in steps of 10°) in a high-precision goniometer. The implemented target was a thin GaP layer evaporated on top of a thin carbon foil. The obtained differential cross-section datasets were benchmarked against a polished GaP crystal and the deviations from the corresponding ones using Rutherford’s formula will be discussed and analyzed.

Ultrawide band gap design of phononic crystals based on topological optimization

In this paper, based on the combined finite element method and bidirectional evolutionary structural optimization algorithm, we perform the topological optimization of phononic crystals to obtain ultrawide band gap between two special adjacent bands for both the in-plane and out-of-plane wave modes. The influences of matrix/scatter materials, material volume fraction, and initial topological design on the unit cell optimization are discussed in detail. The results show that the strategy proposed in this paper is effective and efficient to obtain better optimization results under the similar optimization condition, solutions with ultrawide band gaps can be easily obtained within 30 iterations. Several new patterns for phononic band gap crystals with optimized band gaps are presented. This work provides useful guidance in topological optimization design of phononic crystals.

Fabrication of monodisperse ITO submicro-spheres using l-Histidine-assisted one-step solvothermal method

Abstract Indium tin oxide (ITO), a representative transparent conducting oxide (TCO), is widely applied in photoelectric devices. In this work, monodisperse ITO submicro-spheres have been prepared using one-step solvothermal method at 250 °C. l -Histidine (L-His), triethylamine (TEA) and dimethylformamide (DMF) were used as shape controller, base agent and solvent, respectively. Significantly, the effect from process variables on the size and shape of submicro-spheres was studied systematically. L-His was used to promote the formation of spherical morphologies. When reaction time was prolonged from 12 to 24 h, nano-size particles eventually grew into submicron particles. After adding L-His, the size of ITO particles became smaller and the crystal tended to sphere-shape from cubic-shape. Meaningfully, the effect from particle size and morphology plays a key role in the resistivity of resultant powders. Compared with the resistivity of larger particles with mixed morphology prepared without L-His, the one of well-crystallized sphere-shaped small particles prepared with L-His was lower. The range of resistivity was between 0.4 and 0.7 Ω cm when 0.05 M L-His was added. Moreover, uniform spheres with a diameter of 0.29 μm have been obtained by one-step solvothermal method with 1 M TEA and 0.05 M L-His, which have potential application in photonic band gap crystals.