Terahertz quantum cascade laser as local oscillator in a heterodyne receiver

Hübers, Heinz‐Wilhelm; Pavlov, S. G.; Semenov, A. D.; Köhler, R.; Mahler, L.; Tredicucci, Alessandro; Beere, Harvey E.; Ritchie, D. A.; Linfield, E. H.

doi:10.1364/opex.13.005890

Cited by 150 publications

(74 citation statements)

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“…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.…”

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confidence: 99%

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

Adam

Kašalynas

Hovenier

et al. 2006

Applied Physics Letters

113

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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...

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confidence: 99%

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

Adam

Kašalynas

Hovenier

et al. 2006

Applied Physics Letters

113

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“…The THz QCLs have been developed as the local oscillators (Hübers et al 2005) for heterodyne spectroscopy, using the GaAs/AlGaAs superlattice structure. The lasers oscillates in the frequency range from 1.9 to 4.8 THz and can be operated at the temperatures lower than 160 K. The output power of THz QCLs is very high, ∼90 mW in single mode operation.…”

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confidence: 99%

THz and Submillimeter‐wave Spectroscopy of Molecular Complexes

Tanaka

Harada

Yamada

2011

Handbook of High‐resolution Spectroscopy

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This article reviews the recent progress of terahertz (THz) and submillimter‐wave (SMMW) spectroscopy in the frequency range of 10–300 cm −1 and its application to the study of weakly bound molecular complexes. The first section describes the light sources used for generating THz radiation, which includes the backward‐wave oscillators (BWO), the multipliers with the GaAs monolithic membrane diode (MOMED), and the supperlattice electronic devices (SLED), as well as the tunable far infrared (TuFIR) lasers. In the following section, we introduce extremely sensitive spectrometers developed for observing very weakly bound molecular complexes, such as the intra‐cavity OROTRON spectrometer and the millimeterwave‐Fourier transform microwave (MMW‐FTMW) double resonance spectrometer. We also review the traditional millimeter‐wave spectrometers, electric‐resonance optothermal spectrometers (EROS), and the tunable far‐infrared (TuFIR) laser spectrometers, which have also been applied successfully to study weakly bound molecular complexes. In the last section, the recent topics of the THz and millimeterwave spectroscopy of molecular complexes are reviewed in the four subsections; (i) Rare gas‐molecule complexes: Rg‐HX, Rg‐CO, and Rg‐HCN, (ii) dimers: (CO) 2 , CO‐N 2 , HCN‐H 2 , and (H 2 O) 2 , (iii) water complexes (H 2 O) n , with n up to 6, and (iv) ion and radical complexes: ArSH, ArH 2 O, H 2 OHO 2 , and ArH 3 + .

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“…They were measured using roomtemperature Schottky diodes to mix signals from a terahertz QCL and a FIR gas laser, 6 two terahertz QCLs, 7 or two longitudinal emission modes of a single QCL. 3 A LW as small as 20 kHz was observed. 7 When averaged for a longer time period, however, the single LWs in those experiments could exceed 1 MHz due to fluctuations of temperature and bias current, which affect the refractive index of the laser gain medium.…”

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confidence: 92%

“…As demonstrated at several frequencies, a terahertz QCL can be used as a local oscillator ͑LO͒ for a heterodyne receiver 2,3 which is a crucial instrument for astronomical and atmospheric high-resolution spectroscopy. For those applications a narrow emission linewidth ͑LW͒ from a QCL under frequency stabilization is essential.…”

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Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser

Baryshev

Hovenier

Adam

et al. 2006

Applied Physics Letters

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We have studied the phase locking and spectral linewidth of an ϳ2.7 THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8 GHz is compared with a microwave reference by applying conventional phase lock loop circuitry with feedback to the laser bias current. Phase locking has been demonstrated, resulting in a narrow beat linewidth of less than 10 Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ϳ6.3 kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2227624͔ Significant progress has made terahertz quantum cascade lasers 1 ͑QCLs͒ promising coherent solid-state sources for various applications in spectroscopy, sensing, and imaging. As demonstrated at several frequencies, a terahertz QCL can be used as a local oscillator ͑LO͒ for a heterodyne receiver 2,3 which is a crucial instrument for astronomical and atmospheric high-resolution spectroscopy. For those applications a narrow emission linewidth ͑LW͒ from a QCL under frequency stabilization is essential. In the case of a heterodyne space interferometer, 4 phase locking to an external reference is also required.Ideally, phase locking of the terahertz QCL would take place with respect to a harmonic of a microwave reference signal; however, it has not yet been demonstrated. Recent work has demonstrated frequency locking of a QCL to a far-infrared ͑FIR͒ gas laser line at 3.105 THz. 5 This same work demonstrated a lasing LW of 65 kHz, which could be maintained indefinitely as a result of the frequency stabilization. The LWs of QCLs that were reported earlier than Ref. 5 were unstabilized and could be measured only for a short sweep time of ϳ3 ms. They were measured using roomtemperature Schottky diodes to mix signals from a terahertz QCL and a FIR gas laser, 6 two terahertz QCLs, 7 or two longitudinal emission modes of a single QCL. 3 A LW as small as 20 kHz was observed. 7 When averaged for a longer time period, however, the single LWs in those experiments could exceed 1 MHz due to fluctuations of temperature and bias current, which affect the refractive index of the laser gain medium. Here we report the demonstration of phase locking of the beat signal of a two lateral-mode terahertz QCL to a microwave reference. Additionally, under frequency stabilization conditions we are able to study the emission spectrum of the terahertz QCL as a function of the laser power, in order to investigate the nature of the limit to its LW.We use a terahertz QCL based on the resonant phonon design 8 with an active region containing 176 GaAs/ Al 0.15 Ga 0.85 As quantum-well modules and having a total thickness of 10 m. The cavity of the QCL is a doublesided metal waveguide, which is 40 m wide and 1 mm long. In order to facilitate the experiment described in this letter, we specifically selected a laser with two closely spaced lasing modes. When operated in cw mode ...

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Terahertz quantum cascade laser as local oscillator in a heterodyne receiver

Cited by 150 publications

References 21 publications

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

THz and Submillimeter‐wave Spectroscopy of Molecular Complexes

Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser

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