We present terahertz (THz) emission of optically pumped 5- and 12.5-nm GaAs/AlGaAs multiple quantum wells (MQWs) even at excitation energies below the bandgap. The effect of the externally applied magnetic field parallel to the MQW plain is to enhance or reduce the THz emission intensity for the up or down direction, respectively. For both MQW samples, the excitation energy corresponding to the peak THz emission is red-shifted with respect to the photoluminescence (PL) and PL excitation peak. This is attributed to an instantaneous bandgap renormalization that occurs at the same time scale as the generation of the THz transients. Moreover, an emission shoulder at similar to 40 meV below the THz emission peak of the 5-nm MQW was observed. Deep level transient spectroscopy results do not indicate this to be due to electron traps. However, an indistinct 1.0 phonon-related, below-bandgap, PL feature was seen at low temperature, which agrees well with the observed THz radiation feature. These results suggest that the THz action spectrum is more sensitive to phonon-mediated processes as compared to more conventional optical spectroscopy techniques.

Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.

The azimuthal angle dependence of the terahertz (THz) radiation power of (100) p-type In As under 1-T magnetic field is presented. Results exhibited four-fold symmetry for the s-polarized THz radiation power. Moreover, the two-fold symmetry for the p-polarized THz radiation was modified to four-fold symmetry for a 1-T applied magnetic field. A tentative explanation regarding a magnetic field-enhanced L-valley carrier scattering is proposed. The actual physical origin of these results is currently under investigation.

A low-loss CYTOP planar photonic crystal THz waveguide with single-mode propagation is realized. The highly transparent nature of CYTOP from deep ultraviolet to the THz region indicates its potential usage as component of hybrid optics.

The generation of terahertz (THz) pulses from photoconductive switch was demonstrated for the first time using silicon as substrate [1]. Subsequently, various materials have been used for photoconductive antennas, one of which is low-temperature grown gallium arsenide. These antenna devices have the advantage of acquiring both amplitude and phase information, which can be used to determine vital electronic parameters in the characterization of various materials. Aside from this, the output power from photoconductive switch can be easily improved by simply increasing the bias voltage. This is because an increase in the applied bias voltage would enhance the electric field acting on the photoexcited carriers. However, the maximum bias voltage that can be applied on an antenna device is limited by the material’s dielectric breakdown. It is therefore advantageous to obtain materials for photoconductive antenna devices with high breakdown voltage. This task would entail looking at wide band gap materials since most materials with large breakdown voltage have wide band gaps. Recently, high-power THz radiation emitted from diamond photoconductive antenna array is demonstrated [2]. This study is significant since diamond with a relatively high breakdown electric field of 2 MV/cm ushered the search for other wide band gap materials for THz radiation. Moreover, the excitation source for wide gap materials is not anymore a drawback due to the development of ultraviolet gain mediums and nonlinear crystals [3-5]. As such, other wide band gap materials are obtained and are reported to be good candidates for photoconductive antenna with high THz emission power [6]. In this letter, we report the generation of THz radiation from a photoconductive switch fabricated on a wide band gap material, ZnO single crystal. Additionally, since ZnO possesses highly transparent nature in the visible, near-infrared, mid-infrared and THz frequency regions, the possibility of using ZnO as integrated active optics is mentioned. The ZnO crystal is of interest as photoconductive material for THz generation because of its ease in fabrication, wide band gap character and rather high mobility and resistivity. The relative ease of fabricating single crystal ZnO is due to the recently reported hydrothermal method. Using such technique, the growth of large-sized ZnO single crystal can be readily facilitated [7]. In terms of mobility, ZnO (200 cm/Vs) is one order larger than that of polycrystalline diamond (1-70 cm/Vs). Aside from these, ZnO has the capacity to be excited by second harmonics of Ti:sapphire laser since its transmission edge is located at 390 nm (3.2-eV band gap). All these qualities mentioned make ZnO another good candidate as a high power THz emitter. In our experiment, frequency tripling of the Ti:sapphire regenerative amplifier output were used as the excitation source at a wavelength of 290 nm. High peak power laser pulses can be easily obtained in the region between 280 nm 330 nm using Ce:LiCaAlF6 crystal as gain medium [4,5]. The femtosecond pulses with 1-kHz repetition rate had an average output power of 25 mW. The ZnO single crystals as substrates of photoconductive antennas had dimensions of 10 x 10 x 0.5 mm and the crystallographic c-axis of the crystals were oriented parallel to the excitation laser. Two parallel 5-mm long and 1-mm wide coplanar silver strip lines were fabricated onto these substrates. The gap between the transmission JFH4-6

A lens duct is designed to facilitate the launching of terahertz radiation from an InAs emitter into a photonic crystal fiber waveguide. The assembly comprising of emitter, lens duct and waveguide is called "terahertz pigtail" and is found to be a potential means of effectively channelling and directing terahertz radiation.

The design principle for fluoride-containing optical devices for applications in the deep ultraviolet range is discussed. Variations in band gap energy, band structure and lattice constant of LiBa x Ca y Sr (1- x - y ) F 3 and Li (1- x ) K x Ba (1- y ) Mg y F 3 have been studied. The band structure and transition type of these fluorides are predicted by ab initio band calculations based on the local density approximation. The lattice-matched double-hetero structure of direct-band-gap compounds LiBa x Ca y Sr (1- x - y ) F 3 on LiSrF 3 and Li (1- x ) K x Ba (1- y ) Mg y F 3 on either LiBaF 3 or KMgF 3 is sufficiently feasible to fabricate.

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.

The design and fabrication of an integrated optics called “terahertz pigtail” is presented. The terahertz pigtail is an assembly comprising of an InAs emitter, a TPX lens duct and a plastic photonic crystal fiber waveguide.

This work reports on the feasibility of a design principle formulated using Li(1-x)KxBa(1-y)MgyF3 double-hetero structure laser diode (LD) based on either KMgF3 or LiBaF3 substrate and operating at a wavelength in the deep ultraviolet region.

The construction of long and non-polarization changing photonic fiber waveguide was demonstrated using highly flexible plastic materials. Due to its relatively low-loss coefficient, the possibility of preparing longer photonic fiber waveguide can be easily attained.

It has been known that the surface-field THz emission in a semiconductor and in a semiconductor quantum well occurs when a short laser pulse creates electron-hole (e-h) dipoles that are accelerated in opposite directions by the surface electric field. The accelerating charges emit a THz transient signal [1]. Other mechanisms include intersubband spontaneous emissions and the excitation of quantum beats in the quantum well subbands [2]. In these scenarios, it is always surmised that the energy of the excitation laser pulse is higher than the bandgap of the semiconductor for it to be able to create photogenerated e-h pairs. In this work, we present initial results showing surface field THz radiation of a narrow and wide uncoupled GaAs/AlGaAs multiple quantum well (MQW) heterostructures for excitation energies lower than the bandgap. Furthermore, the emission enhancement and suppression effects of an externally applied magnetic field (B-field) are also investigated.

An integrated optics called terahertz (THz) pigtail, which is comprised of an emitter, an optically transparent launching media, and a waveguide, is devised and fabricated. The InAs emitter under a 1T magnetic field is coupled to the launching media using silicone grease, an index matching liquid. The launching media, a lens duct made from a polymer based on poly 4-methyl pentene-1 (commonly known as TPX), is designed based on the concept of guiding THz radiation into Teflon photonic crystal fiber (PCF) waveguide by means of total internal reflection. It is found that the constructed THz lens duct is able to channel and couple the THz radiation into the PCF waveguide with a loss of <1dB. The results here show that the idea of using the THz pigtail can be a potential means of effectively directing THz radiation.

We present terahertz emission of optically pumped GaAs/AlGaAs multiple quantum wells (MQWs). The action spectra exhibit emission even at excitation energies below the bandgap. Results show that the radiation could not have come from the bulk layers and is inferred to be due to shallow states under the band edge.

The excitation-fluence and magnetic-field dependence of terahertz (THz) radiation power from InAs is investigated. For magnetic-field dependence, two completely different behaviors were observed depending on the excitation fluence. At low excitation fluence, the enhanced THz-radiation mainly originates from the carrier acceleration by the surface electric field. The Lorentz force changes the direction of carrier acceleration toward surface parallel, and the THz-radiation power is enhanced regardless of magnetic field direction. In contrast, at high excitation fluence, the surface electric field is almost screened out and the diffusion process becomes significant. By applying a magnetic field, the dipole is rotated to the direction in which the THz-radiation is efficiently or inefficiently extracted from the surface, and the radiation power is either enhanced or reduced depending on the magnetic-field direction. Additionally, from the magnetic-field dependence up to 27 T, it is found that THz-radiation power saturates at approximately 3 T and also at 13 T, and that the THz-radiation power at 3 T is much higher than that at 13 T. For the generation of broadband THz-radiation, focus is made on n-type InAs irradiated by ultrafast optical pulses. From the magnetic-field dependence of THz-radiation power, using n-type InAs under magnetic field is found to be the practical method to generate broadband THz-radiation, and the origin of higher-frequency component is identified to the hybrid modes.

In recent years, the terahertz (THz) frequency region has become readily accessible and since then the development of THz technology has been enormous. Numerous reports have shown that biochemicals exhibit specific frequency-dependent absorption in the THz regime. Among these biochemicals, the 1,4-naphthol, exhibited a broad absorption peak at around 0.4 THz in room temperature. Such peak was ascribed to some interaction between THz radiation and hydrogen bonds that link into chains the adjacent molecules in the isomer [1]. Temperature dependence measurements of this isomer revealed that such absorption peak is due to intermolecular hydrogen bonding and that this low-energy vibrational mode is strongly coupled to the lattice [2].

Conductive polypyrroles are characterized using temperature-dependent terahertz (THz) spectroscopy. At 100K, THz radiation power started to level off and an absorption peak is manifested. This is attributed to the disorder generated during synthesis and doping processes, which plays a critical role in the microscopic disorder and/or structural amorphous regions of the polymer.

Terahertz (THz) radiation emission from photoconductive antenna fabricated on single crystal ZnO is presented. Results reveal that ZnO is highly transparent in the visible, near-infrared, mid-infrared and THz frequency regions. These characteristics will open up the possibility of using ZnO as material for the realization of integrated active optics.

The progress of terahertz (THz) technology has revealed that spectroscopic measurements of a wide variety substances using THz radiation are possible. Recently, the onset of solid-state phase transition has been detected using THz transmission spectroscopy in an estrogen-like chemical [1]. Aside from this, THz spectroscopic studies of biomolecules, illicit chemicals and conducting polymers have been conducted [2-4]. In particular, polypyrrole (Ppy) is a highly conductive polymer that can be readily prepared electrochemically and by solution polymerization. Its stability in air makes it a possible prototype material for a wide range of applications such as lightweight batteries, electromagnetic shielding materials and chemical sensors [5]. The electrical conductivity and physical properties of Ppy, including stability, are very dependent upon synthesis conditions and the choice of dopant. In a recent work, electrical characterization of conducting Ppy using THz-time domain spectroscopy has been studied [6]. In such work, it was found that the carrier scattering time of Ppy is 1/10 that of semiconductors thereby illustrating the disorder and low mobility of such polymer. However, such report only showed the property of Ppy at room temperature.

The variation of band gap energy and band structure with respect to composition of Li/sub (1-x)/K/sub x/Ba/sub (1-y)/Mg/sub y/F/sub 3/ perovskite-like fluorides is investigated. Analysis of energy chart shows the feasibility of fabricating Li/sub (1-x)/K/sub x/Ba/sub (1-y)/Mg/sub y/F/sub 3/ on either LiBaF/sub 3/ or KMgF/sub 3/.