22 pages, 12 figures, review paperInternational audienceResonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging

No abstract

No abstract

No abstract

The need to reach single-mode lasing and minimize at the same time the electrical dissipation of cryogenically operated terahertz quantum cascade lasers may result in small and subwavelength cavity dimensions. To assess the influence of such dimensions on the shape of the laser emission, we have measured the beam pattern of two metal-metal cavity quantum cascade lasers. The patterns show regular angular intensity variations which depend on the length of the laser cavity. The physical origin of these features is discussed in terms of interference of the coherent radiation emitted by end and side facets of the laser bar. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2194889͔The quest for terahertz sources has resulted recently in the development of the terahertz quantum cascade laser ͑QCL͒. 1 At present, continuous-wave ͑cw͒ QCLs in the terahertz range have been demonstrated for frequencies as low as 2.0 THz ͑Ref. 2͒ and 1.9 THz ͑Ref. 3͒ ͑ Ϸ 160 m͒ and for temperatures up to 117 K. 4 These sources are very promising as local oscillators for heterodyne detection 5,6 and for general terahertz imaging applications. 7 The terahertz QCLs which have achieved the highest temperature performance are based on the so-called "metal-metal waveguides" of subwavelength dimensions. 8,9 Such waveguides minimize lasing threshold current densities due to their strong confinement of the mode to the gain region, their low losses, and their enhanced facet reflectivities. 10 Furthermore, the strong confinement has allowed the fabrication of structures with small lateral and transverse dimensions which minimizes electrical power dissipation; this is critical for their cryogenic operation and leads to improved cw performance. It is expected that the emitted beam from a cavity with subwavelength dimensions would be strongly divergent. 10 Study of the beam profile therefore is important to characterize this type of terahertz source.The heterostructure design employed for the terahertz QCLs used in this research is based on resonant longitudinaloptical-phonon scattering to selectively depopulate the lower radiation level. 11,12 The metal-metal waveguide was fabricated using a copper-to-copper thermocompression bonding technique. 8 We will report here results of beam profile measurements on two laser samples with subwavelength dimensions, fabricated from the same wafer. The metal-metal cavities are bonded to an n + GaAs substrate. The front and back facets of the cavities are uncoated. The cavity dimensions and the free space wavelengths are given in Table I, together with the relation between geometry and the Cartesian coordinate system, used to present the experimental data. Figure 1 shows our experimental setup used to measure the beam patterns. The n + GaAs substrate is indium soldered to a copper sample holder, which in turn is attached to the copper cold plate of a helium flow cryostat. The laser bar can be mounted in various orientations with respect to the 50 mm diameter window at a minimum distance of about 10 mm. It has bee...

We demonstrate for the first time the applicability of antenna-coupled field-effect transistors for the detection of terahertz radiation (TeraFETs) for multi-spectral imaging from 0.76 to 4.25 THz. TeraFETs were fabricated in a commercial 90-nm CMOS process and noise-equivalent powers of 59, 20, 63, 85 and 110 pW/√(Hz) at 0.216, 0.59, 2,52, 3.11 and 4.25 THz, respectively, have been achieved. A set of TeraFETs has been applied in raster-scan transmission and reflection imaging of pellets of sucrose and tartaric acid simulating common plastic explosives. Transmittance values are in good agreement with Fourier-transform infrared spectroscopy data. The spatial distribution of the components in the samples has been determined from the transmission data using principal component analysis.

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.

An antenna model is proposed for long (L ) lasers with subwavelength cross sections (wire lasers). It is shown that the far-field pattern of the wire lasers is determined by the ratio of the wavelength to the length. The radiation of the wire laser is predicted to be concentrated in a narrow beam ' 2=L p for laser modes where the longitudinal phase velocity is in synchronism with the velocity of light in air. Experimental results obtained using a terahertz quantum cascade wire laser are in agreement with the model. DOI: 10.1103/PhysRevLett.96.173904 PACS numbers: 42.60.Jf, 42.25.ÿp, 78.67.-n Is it possible to concentrate radiation from a laser with a subwavelength cross section into a narrow beam? The combination of a small laser aperture with a low beam divergence would open the perspective of local laser excitation of individual nano-objects, which would be of interest in numerous applications such as optical communication, high-density magneto-optic data storage, biological studies, and quantum information. However, the reduction of a laser aperture is known to cause an increase of the beam divergence due to diffraction. The diffraction limited minimum angular size of the beam for radiation with a wavelength for an aperture of size a is determined by ' =a for sources with a > [1]. Thus, for example, in diode lasers, the localization of the optical mode in a thin active region (with the width of the order of several wavelengths) leads to a high beam divergence in the plane perpendicular to the active layer [2]. High efficiency and gain achieved in nanostructure lasers permits laser dimensions comparable to or smaller than the wavelength [3][4][5]. Highly divergent radiation is expected from lasers with subwavelength apertures [4]. The methods used to improve the directivity of laser radiation, using, for example, surface emission [2], an array of lasers [6], or using external optical elements, are all based on the increase of the effective size of an aperture. The recently discovered effects of surface plasmons on the transmission of light through subwavelength apertures [7] also imply an increase of the effective aperture size due to the formation of plasmon-polaron excitations at the surface of a metallic screen. The development of nanotechnologies has brought along new methods to manipulate light on the scales comparable to the wavelength using the concepts of photonic crystals [8], left-handed synthetic materials [9], and onedimensional plasmonic waveguides [10]. These methods however do not solve the problem of the guidance of light in the air outside the artificially fabricated media.In this Letter we propose a method to achieve a high directivity for lasers with subwavelength apertures (wire lasers). The idea is based on an antenna approach to analyze laser modes. Since individual sources in the laser medium emit coherently with their phases determined by the cavity modes, each laser mode can be thought of as a continuous phased array. The different laser modes do not interfere because of their slig...