The detection of far-infrared (far-IR) and sub-mm-wave radiation is resistant to the commonly employed techniques in the neighbouring microwave and IR frequency bands. In this wavelength detection range the use of solid state detectors has been hampered for the reasons of transit time of charge carriers being larger than the time of one oscillation period of radiation. Also the energy of radiation quanta is substantially smaller than the thermal energy at room temperature and even liquid nitrogen temperature. The realization of terahertz (THz) emitters and receivers is a challenge because the frequencies are too high for conventional electronics and the photon energies are too small for classical optics.

Semiconductor detectors and focal plane arrays for far-infrared imaging

We report on the proposal and the theoretical and experimental studies of the terahertz hot-electron bolometer (THz HEB) based on a gated GaAs structure like the field-effect transistor with the array of parallel Sn nanothreads (Sn-NTs). The operation of the HEB is associated with an increase in the density of the delocalized electrons due to their heating by the incoming THz radiation. The quantum and the classical device models were developed, the quantum one was based on the self-consistent solution of the Poisson and Schrodinger equations, the classical model involved the Poisson equation and density of states omitting quantization. We calculated the electron energy distributions in the channels formed around the Sn-NTs for different gate voltages and found the fraction of the delocalized electrons propagating across the energy barriers between the NTs. Since the fraction of the delocalized electrons strongly depends on the average electron energy (effective temperature), the proposed THz HEB can exhibit an elevated responsivity compared with the HEBs based on more standard heterostructures. Due to a substantial anisotropy of the device structure, the THz HEB may demonstrate a noticeable polarization selectivity of the response to the in-plane polarized THz radiation. The features of the THz HEB might be useful in their practical applications in biology, medicine and material science.

Lateral terahertz hot-electron bolometer based on an array of Sn nanothreads in GaAs

A short survey is presented on the recent development of optically pumped pulsed far-infrared (FIR) or THz gas lasers. This includes the new distributed-feedback (DFB) and helical-feedback (HFB) FIR gas lasers as well as the subnanosecond-single-pulse FIR gas lasers pumped by 50 ns 10 μm pulses of hybrid CO2 lasers truncated within ca. 10 ps at their maximum by a novel ultrafast laser-triggered plasma shutter. These lasers show interesting phenomena such as standard and swept-gain superradiance as well as anticorrelated oscillations of the far-infrared emission and the 10 μm pump radiation in the Raman regime. In this context it was discovered that FIR laser gases, e.g., CH3F, NH3, D2O, CH3CN, can be used successfully as spectral filters in “Optical-Free-Induction-Decay” 10 μm CO2 laser systems instead of the standard hot CO2 gas. Finally, the single ultrashort FIR pulses are presently applied to FIR or THz quantumwell detectors and to high-T

 c
 superconductors to gain further information on their exciting and complicated features.

Modern aspects of far-infrared gas lasers

An instrument able to measure electromagnetic radiation, in its different forms and spectral ranges, is called a radiometer. This chapter focuses on the radiometers used for sensing solar radiation and on the measurements of different components and types of solar irradiance. As a simple classification, we will distinguish between broadband and spectral (narrowband) sensors. First, the fundamentals of physical sensors used to measure solar radiation are briefly described. Then, importance about calibration methods and uncertainty, as well as the structure of the traceability chain in the magnitude of solar irradiance, are presented. Next, solar radiometers and measurement techniques are described, starting from direct radiation in Earth’s surface, global irradiance in horizontal and tilted surfaces, diffuse irradiance, and finally another kind of radiation sensor. This structure is not casual but follows a path similar to that of the traceability chain, starting from the more accurate to the less accurate instruments. There are two additional sections devoted to the measurement of the spectral distribution of irradiance and to the measurement of aerosol contents in the atmosphere by using filter radiometers.

Solar Radiation Measurement and Solar Radiometers

Conventional Ge:Ga photoconductors presents a residual responsivity at millimetric wavelengths, well below the cut-off wavelength due to the gallium impurity ionization energy. At constant bias voltage, this millimetric responsivity is greatly enhanced when a strong uniaxial stress is applied. We suggest that this response is mainly due to a change in the free hole lifetime when the hole gas is heated by the absorbed microwave photons. This photoconductive process has a shorter response time than the InSb Putley mechanism. Thus optimization of this millimetric Ge:Ga detector could find an application for example for heterodyne detection with larger spectral width than InSb.

Millimetric response of stressed gallium doped germanium photoconductor

Publisher Summary The basic characteristics of the submillimeter detector do not change as they depend critically upon the purity of the available InSb, which is not improved upon. More attention is given to heterodyne techniques where it is shown that the insertion loss of the InSb mixer can be as low as 7 dB, which is substantially less than that of other readily available millimeter wave mixers. The performance is improved by a careful design of the element mounting to optimize the absorption of the radiation. In applications where the short response time is not required (as in most spectroscopy) an alternative method of obtaining a highly effective responsivity is to step up the impedance of the detector by means of a transformer cooled in the liquid helium. It is cooled to eliminate noise associated with its loss components.

E.H. Putley

Papers

Chapter 3 InSb Submillimeter Photoconductive Detectors