Phase-matching condition for THz wave generation via difference frequency generation using InxGa1-xSe mixed crystals.

Terahertz (THz) waves at 9.7, 10.1 and 10.6 THz were generated via difference frequency generation in high-quality InxG1-xaSe mixed crystals with a relatively high indium compositions (x = 0.040, 0.048, 0.074) grown from an indium flux. The phase-matching angle for THz wave generation was measured for each indium content. As a result, it is confirmed that the incident angle of the excitation light satisfying the phase-matching condition is shifted to a higher angle with an increase in the indium content.

Terahertz wave generation via difference frequency generation using 2D InxGa1-xSe crystal grown from indium flux.

We demonstrate the generation of THz waves (frequency 9.7 THz) using difference frequency generation in an InxGa1-xSe mixed crystal grown from In flux. The amount of indium and the lattice constant of the crystal were evaluated using electron micro probe analysis and X-ray diffraction, respectively. We believe that the Ga sites were substituted by In atoms in the InxGa1-xSe crystal because the In content, estimated according to the Vegard's law, was similar to that measured by EPMA. The maximum power of the generated THz wave was 39 pJ and the conversion efficiency was 1.7×10-5 J-1. This conversion efficiency was 28 times larger than that reported for undoped GaSe crystal.

Lithium intercalation into bilayer graphene.

The real capacity of graphene and the lithium-storage process in graphite are two currently perplexing problems in the field of lithium ion batteries. Here we demonstrate a three-dimensional bilayer graphene foam with few defects and a predominant Bernal stacking configuration, and systematically investigate its lithium-storage capacity, process, kinetics, and resistances. We clarify that lithium atoms can be stored only in the graphene interlayer and propose the first ever planar lithium-intercalation model for graphenic carbons. Corroborated by theoretical calculations, various physiochemical characterizations of the staged lithium bilayer graphene products further reveal the regular lithium-intercalation phenomena and thus fully illustrate this elementary lithium storage pattern of two-dimension. These findings not only make the commercial graphite the first electrode with clear lithium-storage process, but also guide the development of graphene materials in lithium ion batteries.

Measurements of glucose concentration in aqueous solutions using reflected THz radiation for applications to a novel sub-THz radiation non-invasive blood sugar measurement method.

The terahertz (THz) frequency range corresponds to molecular vibrations or relaxation modes such as those for the hydrogen bond. Most biomolecules are activated only in aqueous solutions, thus, to understand the function and structure of biomolecules, it is necessary to investigate the characteristics of electromagnetic waves in hydrated samples. THz radiation causes little damage to the human body, thus it is expected that it can be applied for noninvasive examinations. However, spectrometry of the transmitted light is difficult, since the absorption of THz radiation in water is extremely high. In this study, we used sub-THz radiation (frequencies near to 0.1 THz), where the absorption is lower than for THz radiation, to measure the reflectance of a glucose water solution. We found that the reflectance decreases in proportion with the glucose concentration. These results suggest that sub-THz radiation can be used in the noninvasive measurement of blood glucose levels.

Efficient terahertz detection in a sheet cavity using a nonlinear optical parametric process.

We designed and simulated a GaP-based sheet cavity structure for room-temperature THz wave detection via a nonlinear optical (NLO) parametric process, using photonic conversion from THz frequencies to the optical domain. The sheet cavity structure consisted of a GaP rectangular waveguide for THz waves. Pump enhancement in the cavity and strong confinement of the THz wave in the waveguide resulted in a high power conversion efficiency of 1% (detection at 5 THz). The noise equivalent power for THz wave detection using an optical single-photon detector was estimated to be in the order of a few fW Hz(-1/2), which was higher than that obtained using room-temperature bolometers, field-effect transistors, and other NLO processes.

Concept of annular vector beam generation at terahertz wavelengths via a nonlinear parametric process.

In this paper, we describe our theoretical investigation and calculations for a terahertz (THz)-wave profile generated by difference frequency mixing (DFM) of focused, cylindrically symmetric, and polarized optical vector beams. Using vector diffraction theory, the second-order nonlinear polarization was estimated from the electric field components of the optical pump beams penetrating uniaxial, birefringent nonlinear optics (NLO) crystals, GaSe and CdSe. The approximate beam patterns of the THz waves were simulated using DFM formulation. The intensity patterns of the THz waves for GaSe and CdSe showed sixfold symmetry and cylindrical symmetry, respectively, based on the nonlinear susceptibility tensor of the crystals. As the phase-matching angle θ(PM) was constant with respect to the c axis of the NLO crystals, an annular vector beam with a narrow width was expected.

Pump enhanced monochromatic terahertz-wave parametric oscillator toward megawatt peak power.

Pump enhanced optical parametric oscillation under a cavity phase matching configuration is an effective way to obtain monochromatic THz waves with high pulse energy. Numerical simulations are conducted for THz wave generations using a GaP sheet cavity. By optimizing the optical pulse duration and cavity configuration, the estimated peak power of THz waves is 4 MW at 3 THz, which corresponds to the photon conversion efficiency of η≈0.81. Our proposed scheme can generate a THz wave with high pulse energy, which is suitable for the nonlinear optical effects in the THz frequency region.

THz-wave generation via difference frequency mixing in strained silicon based waveguide utilizing its second order susceptibility χ((2)).

Terahertz (THz) wave generation via difference frequency mixing (DFM) process in strain silicon membrane waveguides by introducing the straining layer is theoretically investigated. The Si(3)N(4) straining layer induces anisotropic compressive strain in the silicon core and results in the appearance of the bulk second order nonlinear susceptibility χ((2)) by breaking the crystal symmetry. We have proposed waveguide structures for THz wave generation under the DFM process by .using the modal birefringence in the waveguide core. Our simulations show that an output power of up to 0.95 mW can be achieved at 9.09 THz. The strained silicon optical device may open a widow in the field of the silicon-based active THz photonic device applications.

Design of a GaP/Si composite waveguide for CW terahertz wave generation via difference frequency mixing.

We design a GaP/Si composite waveguide to achieve efficient terahertz (THz) wave generation under collinear phase-matched difference frequency mixing (DFM) between near-infrared light sources. This waveguide structure provides a strong mode confinement of both near-infrared sources and THz wave, resulting in an efficient mode overlapping. The numerical results show that the waveguide can produce guided THz wave (5.93 THz) with a power conversion efficiency of 6.6×10(-4) W(-1). This value is larger than previously obtained with the bulk GaP crystal: 0.5×10(-9) W(-1) [J. Lightwave Technol.27, 3057 (2009)]. Our proposed composite waveguide can be achieved by bridging the telecom wavelength and THz frequency region.

Elliptically polarized THz-wave generation from GaP-THz planar waveguide via collinear phase-matched difference frequency mixing.

We carried out terahertz (THz)-wave generation from the GaP planar waveguides under collinear phase-matched difference-frequency mixing of two near-infrared sources. TE- and TM-mode of THz-waves were generated simultaneously by adjusting the polarization direction of two incident infrared sources. The phase shift between TE- and TM-mode of THz-wave in the waveguide was dependent on the waveguide length and contributed to the generation of the elliptical polarized THz-wave. The ellipticity of generated THz-wave increased as waveguide length increased. We indicated the possibility of control of rotational direction of elliptical polarization of emitted THz wave.

Sidewall GaAs tunnel junctions fabricated using molecular layer epitaxy.

In this article we review the fundamental properties and applications of sidewall GaAs tunnel junctions. Heavily impurity-doped GaAs epitaxial layers were prepared using molecular layer epitaxy (MLE), in which intermittent injections of precursors in ultrahigh vacuum were applied, and sidewall tunnel junctions were fabricated using a combination of device mesa wet etching of the GaAs MLE layer and low-temperature area-selective regrowth. The fabricated tunnel junctions on the GaAs sidewall with normal mesa orientation showed a record peak current density of 35 000 A cm-2. They can potentially be used as terahertz devices such as a tunnel injection transit time effect diode or an ideal static induction transistor.

Continuous wave terahertz wave spectrometer based on diode laser pumping: potential applications in high resolution spectroscopy.

We constructed a high resolution terahertz (THz) spectroscopic system with an automatic scanning control using a continuous wave (cw) THz wave generator based on difference frequency generation method by excitation of phonon-polariton mode in GaP. The pump and signals lasers were compact, tunable external cavity laser, and distributed feedback (DFB) lasers, respectively. The generated THz waves were tuned automatically by changing the temperature of the DFB laser using a system control. We present the water vapor transmission characteristics of the THz wave and also absorption spectrum of a white polyethylene in the frequency range of 1.97-2.45 THz. The spectroscopic measurements performed at an output power level of 2 nW, which was obtained with a 15-mm-long GaP crystal at 2 THz. The advantage of this cw THz spectrometer is wide frequency tuning range (0.7-4.42 THz) with an estimated linewidth of full width at quarter maximum <8 MHz and this system has a potential application in high resolution spectroscopy.

Single-frequency coherent terahertz-wave generation using two Cr:forsterite lasers pumped using one Nd:YAG laser.

We have developed a compact terahertz-wave generator using two small Cr:forsterite lasers with single Nd:YAG laser pumping based on difference frequency generation in a GaP crystal. A Cr:forsterite laser was constructed with diffraction gratings, by which the pulse duration and delay time of the Cr:forsterite laser depend on the Cr:forsterite laser energy and the cavity length. The Cr:forsterite laser energy was tuned using the optical alignment and pumping energy. Temporal overlap of two Cr:forsterite laser pulses was realized at the GaP crystal. A single-frequency terahertz wave was generated at energy of 4.7 pJ around 2.95 THz using a 30-cm-long Cr:forsterite laser system.

GaP Raman Terahertz high accuracy spectrometer and its application to detect organic and inorganic crystalline defects.

One of the most important uses of THz spectrometry is to detect defects in molecular structure or in crystals efficiently. We applied GaP Raman THz (GRT) spectrometer to detect and evaluate defects in inorganic and organic materials. High THz-wave absorption due to high defect density of GaSe crystal lowered the efficiency of THz wave generation, when the crystal is used as nonlinear material for DFG (Difference Frequency Generation). Defects in organic molecules could be observed as changes in frequency, intensities of the absorption, and broadenings of the spectra.