Antireflection coating formed by plasma-enhanced chemical-vapor deposition for terahertz-frequency germanium optics

A method of manufacturing optical coatings for germanium optics used at terahertz frequencies has been developed. The various optical coatings used at terahertz frequencies are difficult to manufacture conventionally because these coatings must be as thick as several tens of micrometers, which is far thicker than those used in the optical region. One way to overcome this problem is to form a silicon oxide layer through plasma-enhanced chemical-vapor deposition, with silane (SiH4) as a source gas. Using this method, I formed 21-microm-thick silicon oxide films as antireflection (AR) layers for germanium optics and obtained low reflection at 1.7 THz (wavelength, lambda = 175 microm). This method is easily applied to large-aperture optics and micro-optics as well as to optics with a complex surface form. The AR coatings can also be formed for photoconductive detectors made from germanium doped with gallium at a low temperature (160 degrees C); this low temperature ensures that the doped impurities in the germanium do not diffuse. Fabrication of optical coatings upon substrates that have refractive indices of 3.84-11.7 may also be possible by control of the refractive indices of the deposited layers.     

The Need of Terahertz Cameras for Standardizing Sensitivity Measurements

The applicability of a blackbody source to sensitivity measurements and calibration of an uncooled Terahertz (THz) focal plane array (FPA) is discussed, but the combination of a blackbody source and a band-pass filter may not be suitable for these purposes. Two ways to measure the minimum detectable power (MDP) of uncooled THz-FPAs are described and compared when used with strong THz sources such as a quantum cascade laser (QCL). The MDP is defined as the radiant power that produces a signal-to-noise ratio of unity in the output of a THz imager that uses a THz-FPA operating at a TV frame rate. One method (method 1) is based on the hypothesis that the beam pattern of the THz source is close to a Gaussian pattern. In another method (method 2), the signal level of the background in an image that does not contain a THz source is subtracted from the signal of the image. While method 2 is more flexible, how large the signals coming from THz source should be, as compared with the noise level, remains to be defined. Finally, based on issues with the current non-uniformity correction (NUC) technique, specifications are proposed for THz source power and wobbling technique to obtain uniform illumination for an ideal NUC technique.     

Terahertz electroluminescence from GaSb/AlSb quantum cascade laser

A phonon depopulation GaSb/AlSb quantum cascade laser was fabricated for the first time. The low longitudinal-optical phonon energy and small effective electron mass of GaSb enables a low electric field at the lasing threshold and a wide design range. The frequency of oscillation was designed to be 2.6 THz. With an increase of injection current, an abrupt increase of luminescence was observed.     

Multilayer optical thin films for use at terahertz frequencies: method of fabrication

A new method of fabricating multilayer optical coatings used at terahertz frequencies has been developed. Using plasma-enhanced chemical-vapor deposition, a multilayer antireflection coating for germanium optics at terahertz frequencies was fabricated. The coating consists of amorphous silicon and silicon-oxide layers. The transmittance and structure of the coating were experimentally investigated. The transmittance spectrum of the coating on the Ge substrate shows a wideband antireflection behavior in the 40-120 cm(-1) region.     

Numerical study of terahertz quantum cascade lasers subjected to near-infrared optical pulse injection

We propose a global simulation scheme for studying the output characteristics of quantum cascade lasers subjected to near-infrared optical pulses with a wide range of injection powers. We treat nonresonant excitations where the photon energy of the injected pulse is far from that of QCLs. To maintain consistency even at large injection powers wherein thermal effects become non-negligible, internal parameters, e.g., electron temperatures, scattering times, gains, and waveguide loss, are treated as functions of space and time and evaluated based on the temperature and carrier distribution in the device. The number of photons in the cavity is converted into output power using the time-dependent reflectivity at the cavity facet. This all-in-one scheme well reproduces light output-to-current characteristics observed in a relevant experiment and is able to relate the induced changes in the output powers with the relevant intersubband gain and cavity loss components over a wide range of time scales after the pulse injection.     

Terahertz Technology Research at NICT

The National Institute of Information and Communications Technology (NICT) has been researching the use of terahertz (THz) band from its source to industrial applications. Major accomplishments to date include the development of THz devices such as a quantum cascade laser source, a portable stand-off imaging system, an establishment of propagation model, and a spectral database. We also encourage applications in a range of research fields, such as art conservation science. Because standardisation will be an important step in making the THz wave a common tool, NICT will be pursuing THz methodological activities.     

Millimeter Wave Imaging System Using Monopole Antenna with Cylindrical Reflector and Silicon Lens

We built a reflection imaging system that uses a monopole antenna with a cylindrical reflector and silicon semi-spherical lens for millimeter waves to identify detachments of alabaster from support material such as wood and stone, which can be subject to painting deterioration. Based on the electric field property near the monopole antenna in the system and the lens effect, the system was able to clearly image a test sample made of 2-mm width aluminium tape, which was placed within a range of approximately 10 mm from the lens. In practical imaging testing using a detachment model, which consists of alabaster and wood plating, the result also showed the possibility of observing slight detachment of the alabaster from the wood more easily than an imaging with large numerical aperture.     

At the Dawn of a New Era in Terahertz Technology

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.