Gallium Phosphide Crystals Research Articles

High-field THz source centered at 2.6 THz.

We demonstrate a table-top high-field terahertz (THz) source based on optical rectification of a collimated near-infrared pulse in gallium phosphide (GaP) to produce peak fields above 300 kV/cm with a spectrum centered at 2.6 THz. The experimental configuration, based on tilted-pulse-front phase matching, is implemented with a phase grating etched directly onto the front surface of the GaP crystal. Although the THz generation efficiency starts showing a saturation onset as the near-infrared pulse energy reaches 0.57 mJ, we can expect our configuration to yield THz peak fields up to 866 kV/cm when a 5 mJ generation NIR pulse is used. This work paves the way towards broadband, high-field THz sources able to access a new class of THz coherent control and nonlinear phenomena driven at frequencies above 2 THz.

Lifetime visualization of femtosecond laser-induced plasma on GaP crystal.

Gallium phosphide (GaP) is a widely used and promising semiconductor material for photonics devices and we suppose the ultrafast laser can be a competitive tool for GaP processing. We used an 800 nm centered femtosecond (fs) laser with a pulse duration of 50 fs to irradiate the GaP crystal. The ablation threshold was first determined, and then the ultrafast dynamics including plasma expansion, shockwave formation and propagation, and spectral evolution were acquired and analyzed. The evolution of ejected plasma in the initial stage changed from cylindrical to planar propagation with the augment of laser fluence. The study on the propagation properties of shockwaves showed that the energy of propelling shockwaves accounted for 12% to 18% of the laser pulse energy at all fluences above the ablation threshold. A prominent plasma splitting was observed at a fluence slightly higher than the threshold, and a phenomenon that the plasma protruded out of the shockwaves was also found. Finally, the transient temperature and density of electron at different fluences were calculated. The temperature difference between the plasma and the shockwave proved the heating effect of the plasma during ablation.

Scalable microstructured semiconductor THz pulse sources.

In recent years several microstructured lithium niobate THz pulse source were suggested for high-energy applications. Two types of those, the reflective and the transmissive nonlinear slab are adopted here for semiconductors. These new sources are scalable both in THz energy and size. Furthermore, they can outperform the already demonstrated contact grating source in diffraction and THz generation efficiency. Compared to the lithium niobate sources, they are more feasible, thanks to the easier manufacturing and the longer pump wavelength. They can produce intense, nearly single-cycle THz pulses at higher frequencies. With 20 mJ pumping at 1.8 µm wavelength, 45 µJ THz energy, and 17 MV/cm focused peak electric field can be expected at 3 THz phase matching frequency from the transmissive nonlinear echelon slab setup consisting of a 4 mm thick structured plan-parallel gallium phosphide crystal.

Broadband terahertz time-domain polarimetry based on air plasma filament emissions and spinning electro-optic sampling in GaP.

We report on a time-domain polarimetry (TDP) system for generating and detecting broadband terahertz (THz) waves of different polarization angles. We generate THz waves from two-color laser filaments and determine their polarization states with a detection bandwidth of up to 8 THz using a spinning gallium phosphide crystal. The polarization of THz emission can be controlled by adjusting the position and tilt angle of the β-barium borate crystal. We characterize the precision of this system for polarimetric measurements at fixed time delay to be and for complete time-domain waveforms. We also demonstrate the feasibility of our TDP system by measuring broadband optical properties of anisotropic samples in both transmission and reflection geometries. The THz-TDP technique can be easily integrated in conventional THz time-domain spectroscopy setups using nonlinear crystal detectors.

Simple approach to broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser.

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. Although IDFG systems pumped with 1.5- or 2-µm ultrashort pulsed lasers have been extensively developed, few systems have been demonstrated with 1-µm lasers, which use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-µm-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm-1 in the fingerprint region (760-1240 cm-1, 8.1-13.1 µm). The 1-µm-based IDFG system allows for additional generations of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 50-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

Intra-oscillator broadband THz generation in a compact ultrafast diode-pumped solid-state laser.

We demonstrate broadband and powerful terahertz (THz) generation at megahertz repetition rate based on intra-oscillator optical rectification (OR) in gallium phosphide (GaP). By placing the nonlinear crystal directly inside the cavity of a Kerr-lens mode-locked ultrafast diode-pumped solid-state laser (DPSSL) oscillator, we demonstrate a compact and single-stage THz source. Using only 7 W of diode-pump power, we drive OR in a GaP crystal with 22 W of average power at ∼80 MHz repetition rate. In a first configuration, using a 0.3-mm-thick GaP and 105 fs driving pulses, we generate up to 150 µW of THz radiation with a spectrum extending to 5.5 THz. In a second configuration allowing for sub-50-fs pulse duration, we generate up to 7 THz inside a 0.1-mm-thick GaP crystal. This performance is well suited for THz time-domain spectroscopy and THz imaging. Intra-oscillator THz generation in sub-100-fs DPSSLs is a promising way to scale down footprint, complexity and cost of powerful broadband THz sources.

Generation of even and odd high harmonics in resonant metasurfaces using single and multiple ultra-intense laser pulses

High harmonic generation (HHG) opens a window on the fundamental science of strong-field light-mater interaction and serves as a key building block for attosecond optics and metrology. Resonantly enhanced HHG from hot spots in nanostructures is an attractive route to overcoming the well-known limitations of gases and bulk solids. Here, we demonstrate a nanoscale platform for highly efficient HHG driven by intense mid-infrared laser pulses: an ultra-thin resonant gallium phosphide (GaP) metasurface. The wide bandgap and the lack of inversion symmetry of the GaP crystal enable the generation of even and odd harmonics covering a wide range of photon energies between 1.3 and 3 eV with minimal reabsorption. The resonantly enhanced conversion efficiency facilitates single-shot measurements that avoid material damage and pave the way to study the controllable transition between perturbative and non-perturbative regimes of light-matter interactions at the nanoscale.

Cryogenically cooled GaP for optical rectification at high excitation average powers

We present a detailed exploration of the behavior of gallium phosphide (GaP) crystals used for optical rectification (OR) of high average power (> 100 W), MHz repetition rate ultrafast lasers. We measure thermal load, Terahertz (THz) refractive index and THz yield over a wide temperature range (77 K to 500 K) in this unusual excitation regime. Our thermal load measurements indicate that nonlinear absorption remains the main contribution to crystal heating and thus the main limitation to scaling the conversion efficiency and show that cryogenic cooling can partly relax these limitations. Furthermore, we present first temperature-dependent refractive index measurements of GaP for frequencies up to 4 THz, showing only minor deviation from room temperature values and no significant degradation of coherence length. Last but not least, we present first experiments of OR in GaP at cryogenic temperatures and observe an increase in THz yield (30%) at cryogenic temperatures when using short pulse duration excitation, due to reduced THz absorption at broad THz bandwidth. Our results indicate that OR in cryogenically cooled GaP is a promising approach for achieving broadband, high-average power THz radiation using short-pulse (< 50 fs) excitation at even higher average power (>> 100 W) - performance that is readily available from state-of-the-art ultrafast Yb-doped solid-state lasers.

Broadband terahertz solid-state emitter driven by Yb:YAG thin-disk oscillator

We report on a table-top, high-power, terahertz (THz) solid-state emitter driven by few-cycle near-infrared pulses at 16 MHz repetition rate in gallium phosphide (GaP) crystals. Two external nonlinear multi-pass cells are used to shorten the output of a home-built, 100 W, 265 fs, 6.2 μJ Yb:YAG thin-disk oscillator, operating at 1030 nm, to 18 fs with 3.78 μJ pulse energy. The broadband spectrum of the THz driver allowed for the extension of the THz cutoff frequency to 5.7 THz at the dynamic range of 104. By employing the high-power Yb:YAG thin-disk oscillator, the low efficiency of the THz generation is circumvented, resulting in the generation of up to 100 μW, multi-octave THz pulses at 5 THz cutoff frequency in a 2 mm thick GaP crystal.

Angular Dependences of the Intensity of the Raman Light Scattering on Polaritons in a Gallium Phosphide Crystal

Angular Dependences of the Intensity of the Raman Light Scattering on Polaritons in a Gallium Phosphide Crystal

Molecular fingerprinting with bright, broadband infrared frequency combs

Spectroscopy in the molecular fingerprint spectral region (6.7–20 µm) yields critical information on material structure for physical, chemical, and biological sciences. Despite decades of interest and effort, this portion of the electromagnetic spectrum remains challenging to cover with conventional laser technologies. In this paper, we present a simple and robust method for generating super-octave, optical frequency combs in the fingerprint region through intra-pulse difference frequency generation in an orientation-patterned gallium phosphide crystal. The attainable brightness from this tabletop source reaches the same level achievable by infrared synchrotron radiation with a bandwidth spanning from 4 to 12 µm. We demonstrate the utility of this unique coherent light source for high-precision, dual-frequency-comb spectroscopy of methanol and ethanol vapor. These results highlight the potential of laser frequency combs for a wide range of infrared molecular sensing applications from basic molecular spectroscopy to nanoscopic imaging.

All-fiber mid-infrared source tunable from 6 to 9 μm based on difference frequency generation in OP-GaP crystal

We report the first fully fiberized difference frequency generation (DFG) source, delivering a broadly tunable idler in the 6 to 9 μm spectral range, using an orientation-patterned gallium phosphide (OP-GaP) crystals with different quasi-phase matching periods (QPM). The mid-infrared radiation (MIR) is obtained via mixing of the output of a graphene-based Er-doped fiber laser at 1.55 μm with coherent frequency-shifted solitons at 1.9 μm generated in a highly nonlinear fiber using the same seed. The presented setup is the first truly all-fiber, all-polarization maintaining, alignment-free DFG source reported so far. Its application to laser spectroscopy was demonstrated by the absorption spectrum measurement of ν4 band of methane in 7.5 - 8.3 µm spectral range. The system simplicity and compactness paves the way for applications in field-deployable optical frequency comb spectroscopy systems for gas sensing.

Mid-infrared tunable, narrow-linewidth difference-frequency laser based on orientation-patterned gallium phosphide

We report on the first characterization of orientation-patterned gallium phosphide (OP-GaP) crystals used to generate narrow-linewidth, coherent mid-infrared (MIR) radiation at 5.85 μm by difference frequency generation (DFG) of continuous-wave (cw) Nd:YAG laser at 1064nm and diode-laser at 1301nm. By comparison of the experimental absolute MIR efficiency versus focusing to Gaussian beam DFG theory, we derive an effective nonlinear coefficient for first-order quasi-phase-matched OP-GaP at the generated DFG wavelength. Using d = (2/π)d14 and taking into account Miller’s delta rule, we retrieve an absolute value of the d14 quadratic nonlinear susceptibility coefficient of GaP of d14 = 27.2(3) pm/V at 5.85 μm, in good agreement with the latest absolute measurement of this nonlinear coefficient from non-phase-matched second-harmonic generation (1.32 μm → 0.66 μm) taking into account multiple reflection effects [Shoji et al 1997 J. Opt. Soc. Am. B 14 2268]. The temperature and signal-wave tuning curves are also in qualitative agreement with a recently proposed temperature-dependent Sellmeier equation for OP-GaP when focusing effects are taken into account.

Difference frequency generation in the mid-infrared with orientation-patterned gallium phosphide crystals.

We report on the generation of coherent mid-infrared radiation around 5.85 μm by difference frequency generation (DFG) of a continuous-wave Nd:YAG laser at 1064 nm and a diode laser at 1301 nm in an orientation-patterned gallium phosphide (OP-GaP) crystal. We provide the first characterization of the linear, thermo-optic, and nonlinear properties of OP-GaP in a DFG configuration. Moreover, by comparing the experimental efficiency to Gaussian beam DFG theory, we derive an effective nonlinear coefficient d=17(3) pm/V for first-order quasi-phase-matched OP-GaP. The temperature and signal wavelength tuning curves are in qualitative agreement with theoretical modeling.

Numerical analysis of second harmonic generation for THz-wave in a photonic crystal waveguide using a nonlinear FDTD algorithm

We have presented a numerical analysis to describe the behavior of a second harmonic generation (SHG) in THz regime by taking into account for both linear and nonlinear optical susceptibility. We employed a nonlinear finite-difference-time-domain (nonlinear FDTD) method to simulate SHG output characteristics in THz photonic crystal waveguide based on semi insulating gallium phosphide crystal. Unique phase matching conditions originated from photonic band dispersions with low group velocity are appeared, resulting in SHG output characteristics. This numerical study provides spectral information of SHG output in THz PC waveguide. THz PC waveguides is one of the active nonlinear optical devices in THz regime, and nonlinear FDTD method is a powerful tool to design photonic nonlinear THz devices.

Deep Level Studies in High-Resistive Gallium Phosphide Single Crystals

In an attempt to explore the possibility of using wide bandgap gallium phosphide (GaP) single crystals as radiation detectors, high resistivity iron (Fe)-doped GaP single crystal was grown using a solid state diffusion (SSD) technique. The grown ingot showed a resistivity in the order of 108 Ω-cm with an optical bandgap of 2.27 eV measured at room temperature. Schottky barrier diodes were fabricated by sputtering nickel (Ni) contacts on GaP wafers to study the deep level defects and junction properties. An effective donor concentration, Nd = 2.6 × 1016 cm−3 was calculated from the capacitance-voltage measurements. Deep level transient spectroscopy (DLTS) and current transient spectroscopy (CTS) were performed on the Schottky diodes to identify the electrically active defect levels in the grown high-resistive GaP crystals. DLTS measurements revealed the presence of several vacancies, impurities, and antisite defects centers with concentrations ranging from 1014 - 1016 cm−3. CTS measurements identified a new defect level in GaP crystals that could not be detected by the conventional DLTS scan due to very fast capacitance transients at temperatures below 100 K.

A flexible master oscillator for a pulse-burst laser system

A new master oscillator is being installed in the pulse-burst laser system used for high-rep-rate Thomson scattering on the MST experiment. This new master oscillator will enable pulse repetition rates up to 1 MHz, with the ability to program a burst of pulses with arbitrary and varying time separation between each pulse. In addition, the energy of each master oscillator pulse can be adjusted to compensate for gain variations in the power amplifier section of the laser system. This flexibility is accomplished by chopping a CW laser source with a high-bandwidth acousto-optic modulator (AOM). The laser source is a Laser Quantum ventus 1064 diode-pumped solid-state laser with continuous output power variable from 100 to 500 mW. The 1064 nm, 2.7 mm diameter polarized beam is focused into the gallium phosphide crystal of a Brimrose AOM, which deflects the beam by approximately 60 mR when driven by the 400 MHz fixed frequency driver. Beam deflection is controlled by a simple digital input pulse, and is capable of producing deflected pulses of less than 20 ns width at repetition rates much greater than 1 MHz. These deflected pulses from the output of the AOM are collimated and propagated into the laser amplifier system, where they will be amplified to ∼ 2 J/pulse and injected into the MST plasma.

Elasto-optic effect anisotropy in gallium phosphide crystals

Elasto-optic coefficients of gallium phosphide (GaP) crystals were calculated on the basis of their piezo-optic and elastic coefficients. Surfaces of the spatial distribution of piezo- and elasto-optic effects in these crystals were built. The maxima of the surfaces of the elasto-optic effect and the geometries of acousto-optic interaction that correspond to these maxima were found. Ratios that describe the rotation of optical indicatrix, depending on direction of the action of uniaxial pressure or deformation on cubic crystal, were recorded. It was shown that such rotations induced by mechanical stress do not exceed 1.5° in GaP, but in some cubic crystals they can reach tens of degrees.

Vibration–rotation absorption spectrum of water vapor molecular in frequency selector at 0.5–2.5 THz range

Water vapor in the atmosphere is the main absorber for terahertz electromagnetic waves. The power absorption of water vapor had been measured in 0.5–2.5THz by using terahertz time-domain spectroscopy. In time-domain waveforms, the main-peak of water vapor delayed 0.3ps with very large attenuation compared with reference. In the transmittance spectra, seven atmospheric windows obtained indicate that terahertz wave will be able to propagate in quite a long distance at atmosphere. Experiment date and first principles calculation results described gallium phosphide crystal could be useful for terahertz frequency selector based on acousto-optic interaction. We obtained 30 absorption lines in the absorption spectra and the four locations of radical groups are characteristic absorption peaks.

Excitonic Crystal, Nanotechnology and New Prospect for Optoelectronics

The review demonstrates the results of development of growth technology for perfect and free of contamination gallium phosphide (GaP) crystals and investigation of influence of crystallization conditions on quality and properties of the crystals. The long-term ordered and therefore close to ideal crystals repeat behavior of the best nanoparticles with pro- nounced quantum confinement effect. These perfect crystals are useful for application in top-quality optoelectronic devic- es as well as they are a new object for development of fundamentals of solid state physics. Since the time of original preparation by the author in the 1960s of gallium phosphide crystals doped by nitrogen (GaP:N), followed by the introduction of the excitonic crystal concept in the 1970s, the best methods of bulk, film and na- noparticle crystal growth were elaborated. The results of semi centennial evolution of GaP:N properties are compiled in the paper. Novel and useful properties of perfect GaP including its stimulated emission, very bright and broadband lumi- nescence at room temperature were observed. These results provide a new approach to selection and preparation of perfect materials for optoelectronics and a unique opportunity to realize a new form of solid-state host - the excitonic crystal as high intensity light source with expected low threshold for generation of non-linear optical effects. Using the example of GaP, here is proposed the cheap, resource-saving and impactful way for development of optoelec- tronics with the help of a special transformation of an ordinary semiconductor into the base material for various device structures.