Terahertz Quasi-Time Domain Spectroscopy using a 808nm multimode diode laser

We report on a terahertz quasi-time domain spectroscopy (QTDS) system based on a low-cost continuous wave multimode diode laser. Commercially available low-temperature grown gallium arsenide (LT-GaAs) based photoconductive antennas (PCAs) with spiral and dipole configurations were used as emitter and detector, respectively. Terahertz pulses spaced at approximately 55 ps with a bandwidth of 400 GHz were obtained. Parametric measurements of the terahertz peak-to-peak intensity were performed by varying the injection current and temperature while maintaining incident laser power. The highest peak-to-peak intensity was obtained at 170mA injection current and 20° C temperature settings. The change in the THz peak-to-peak intensity is attributed to the mode hopping characteristics of the device which in turn, is dependent on injection current and temperature

Cherenkov-Phase-Matched Nonlinear Optical Detection and Generation of Terahertz Radiation via GaAs With Metal-Coating

Terahertz (THz) wave detection and emission via Cherenkov-phase-matched nonlinear optical effects at 1.55-μm optical wavelength were demonstrated using a GaAs with metal-coating (M-G-M) and bare GaAs as a reference sample in conjunction with a metallic tapered parallel-plate waveguide (TPPWG). The metal-coated GaAs is superior to the bare wafer both as a THz electro-optic detector and as an emitter. Significant improvements in the detection and emission efficiency were obtained by utilizing a metal-coating due to better confinement and lower loss of the THz waves propagating in the M-G-M compared with bare GaAs

Terahertz Emission of Gallium Arsenide on Textured p-type Silicon (100) Substrates Grown via Molecular Beam Epitaxy

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Arsenide (GaAs) on surface textured p-type Silicon (p- Si) (100) substrates. Surface texturing was achieved by anisotropic wet chemical etching using 5% wt Potassium Hydroxide (KOH): Isopropyl alcohol (IPA) (50:1) solution for 15, 30, 45, and 60 minutes. Reflectivity measurements for the textured p-Si(100) substrates indicated that the overall texturing increases with longer etching times. Raman spectroscopy of the post-growth samples confirmed GaAs growth. The THz emission intensities were the same order of magnitude. The GaAs grown on p-Si(100) textured for 60 minutes exhibited the most intense THz emission attributed to the increased absorption from a larger surface-to-volume ratio due to surface texturing. All GaAs on textured p-Si(100) samples had frequency bandwidth of ~2.5 THz

Coherence length measurement for ultra-short laser pulses using digital holography and statistical fringe analysis

Various techniques in interferometry require lasers of short coherence length, taking advantage of the fact that light interferes only in the areas where the optical path difference (OPD) of two beams is shorter than its coherence length. Examples of these techniques are light-in-flight recording by holography [1], optical coherence tomography [2], 3D shape measurement [3-4], and short coherence microscopy [5]. These techniques use a priori knowledge of the laser’s coherence length in order to extract valuable information about the test object (e.g., its position, shape, or refractive index distribution)

Lens coupler and magnetic field terahertz emission enhancement in InSb and InAs under 1.55-μm excitation

Terahertz (THz) emission from InSb and InAs utilizing an MgO lens coupler in conjunction with an applied magnetic field is reported. A THz peak amplitude enhancement of 74 times in InSb compared to 26 times in InAs was observed. The considerable enhancement in InSb is attributed to its large Hall angle, small THz divergence and reduced total internal reflection. A corresponding emission shift to lower THz frequencies in both samples was observed due to the spectral dependence of the MgO lens\u27 collection efficiency. This enhancement configuration is easily applicable in the design of a compact THz time-domain spectroscopy system. © 2013 The Japan Society of Applied Physics

Modal analysis of Teflon photonic crystal fiber as a terahertz waveguide

We analyze the full vector two-dimensional modal characteristics of Teflon photonic crystal fiber (PCF) as a terahertz (THz) waveguide. Finite-difference time domain computational strategy is applied in investigating the THz waveguide characteristics of Teflon PCF. Computational results show interesting single-mode and multi-mode behavior of Teflon PCF in the THz region. © 2005 IEEE

The effect of arsenic on MBE-grown modulation-doped GaAs/AlGaAs heterostructures

In this paper, the effect of As on the mobility and junction electric field of modulation-doped GaAs/AlGaAs heterostructures were investigated by varying the As flux during the MBE growth of the samples. Hall measurements using the van der Pauw configuration determined the carrier concentration and Hall mobility. The carrier concentration was observed to increase with As flux. The room temperature mobilities of the samples tend to decrease, while the 77 K mobilities increase with increasing As flux. Photoreflectance spectroscopy was utilized to derive the junction electric field. These values were observed to be higher than the electric field values calculated from Hall measurements for the samples grown at relatively lower As fluxes. This may be the effect of electron traps due to As vacancies coupled to C impurities. A remarkable improvement in the 77 K mobility of the sample grown after baking the substrate holder was observed. The 10 K mobility however was low compared to the benchmark set by other groups

Enhanced terahertz emission and Raman signal from silicon nanopyramids

The Raman scattering and Terahertz emission of silicon nanopyramids (SiNPys) formed at different etching times were investigated. Additionally, photoluminescence spectroscopy measurements were performed to investigate the recombination properties of SiNPys. The SiNPys were fabricated via wet chemical etching of heavily doped p-type silicon (100) in potassium hydroxide (KOH) solution. The formation of nanopyramidal structures was verified using Scanning Electron Microscopy (SEM). Enhanced Raman and THz signals were observed from SiNPys compared to un etched silicon surface. The enhancement of Raman signal in SiNPys is ascribed to the enhanced photon absorption from efficient light trapping effect of the nanopyramids. Moreover, broadening of the Raman peaks was observed indicating an amorphous-like structure with prolonged etching. The enhancement of THz signal in SiNPys is ascribed to increased transient current on the nanopyramids\u27 surface. The maximum enhancement for Raman and THz signals was found for SiNPys formed after 30 mins etching. Further etching beyond 30 mins resulted in weaker Raman and THz signals. Results suggest strong correlation between the THz emission and Raman scattering of SiNPy\u27s. This correlation may be understood from the vibrational mode dependence of both Raman scattering and THz emission

Efficient electro-optic sampling detection of terahertz radiation via Cherenkov phase matching

We experimentally demonstrate an efficient electro-optic sampling scheme based on Cherenkov phase matching of broadband terahertz radiation with 800-nm femtosecond probe beam in a 0.5 mm-thick LiNbO3 crystal coupled to a Si prism. The electro-optic signal from a Cherenkov-phase-matched LiNbO3 crystal is found to be comparable to that with a 4 mm-thick ZnTe crystal using a collinear phase matching. The Cherenkov phase matching technique can be achieved with any probe wavelength and hence has an advantage over the collinear phase matching method