We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2 THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8 THz as local oscillator. We measure a double sideband receiver noise temperature of 1400 K at 2.8 THz and 4.2 K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1949724͔ Present day heterodyne receivers use a combination of an electronically tunable solid-state local oscillator ͑LO͒ source based on multiplier chains, 1 with either a superconductor-insulator-superconductor 2 mixer or a hotelectron bolometer ͑HEB͒ mixer. 3,4 The heterodyne instrument for the far infrared on the Herschel Space Observatory, 5 to be launched in 2007, is the first instrument to perform very high-resolution spectroscopy using such receivers from 480 GHz to 1.9 THz in space. Future space missions require improved angular resolution, improved sensitivity, and, most important, an increase in frequency from 2 to 6 THz. 6 The development of new receivers operating at such high frequencies is limited by the availability of suitable LO sources. The existing solid state LOs are unlikely to generate sufficient output power at such high frequencies since the power falls off rapidly with increasing frequency due to reduced multiplication efficiency. 1 Optically pumped gas lasers can operate at higher frequencies but are in general massive, bulky, and power hungry. Very recently, a new type of solidstate THz source was developed based on quantum cascade laser ͑QCL͒ structures. 7 This new source holds great promise for LO applications because of its compactness and high power efficiency. Here we report the first demonstration of a fully operational heterodyne receiver at 2.8 THz based on such a THz QCL as LO source and a hot-electron bolometer as mixing element.The concept of a QCL was first demonstrated in the midinfrared ͑ Х 4 m;75 THz͒ by Faist et al. 8 Photons are created via electronic intersubband transitions in semiconductor heterostructures that take place entirely within the conduction band. Furthermore, in a QCL the heterostructure active region consists of a stack of repeated identical quantum well modules ͑typically 20-200͒, which enables a single electron to cascade down and emit a photon in each module. Due to this cascading effect, QCLs have large quantum efficiency and high output power. The QCL frequency range is determined by the energy spacing of the subbands, which is set by the design and growth of the quantum-well structure. The precise operating frequency is determined by the waveguide cavity of the laser. While the development of a THz QCL has proven to be more challenging than for mid-infrared QCLs because of the difficulty of achieving population inversion for small...
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...
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...
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