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 quasiparticle relaxation time in superconducting films has been measured as a function of temperature using the response of the complex conductivity to photon flux. For tantalum and aluminum, chosen for their difference in electron-phonon coupling strength, we find that at high temperatures the relaxation time increases with decreasing temperature, as expected for electron-phonon interaction. At low temperatures we find in both superconducting materials a saturation of the relaxation time, suggesting the presence of a second relaxation channel not due to electron-phonon interaction. DOI: 10.1103/PhysRevLett.100.257002 PACS numbers: 74.25.Nf, 74.40.+k The equilibrium state of a superconductor at finite temperatures consists of the Cooper pair condensate and thermally excited quasiparticles. The quasiparticle density n qp decreases exponentially with decreasing temperature. These charge carriers control the high frequency (!) response of the superconductor through the complex conductivity 1 ÿ i 2 . At nonzero frequencies, the real part 1 denotes the conductivity by quasiparticles, and the imaginary part 2 is due to the superconducting condensate [1,2]. When the superconductor is driven out of equilibrium, it relaxes back to the equilibrium state by the redistribution of quasiparticles over energy and by recombination of quasiparticles to Cooper pairs. The recombination is a binary reaction, quasiparticles with opposite wave vector and spin combine, and the remaining energy is transferred to another excitation. The latter process is usually controlled by the material-dependent electronphonon interaction [3,4]. With decreasing temperatures, the recombination time increases exponentially, reflecting the reduced availability of quasiparticles. Here we report relaxation time measurements in superconducting films far below the critical temperature T c . We find strong deviations from exponentially rising behavior, which we attribute to the emergence of an additional relaxation channel in the superconducting films.We have measured the time dependence of the complex conductivity of superconducting films after applying an optical photon pulse. In addition, the noise spectrum is measured in the presence of a continuous photon flux [5]. The superconducting film is patterned as a planar microwave resonator. The resonator is formed by a meandering coplanar waveguide (CPW), with the central line 3 m and the slits 2 m wide, and is coupled to a feed line; see Fig. 1(a) [6]. The complex conductivity results in a kinetic inductance L k / 1=d! 2 , for thin films with thickness d, which is due to the inertia of the Cooper pair condensate. It sets together with the length of the central line the reso-, with l the length of a quarter-wave resonator, L g the geometric inductance, and C the capacitance, both per unit length. The variation in kinetic inductance due to photons is connected to the quasiparticle density n qp by L k =L k 1 2 n qp =n cp , with n cp the Cooper pair density (n qp n cp ). Resonance

We demonstrate the phase locking of a 2.7 THz metal-metal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier chain ͑ϫ12͒ from a microwave synthesizer at ϳ15 GHz. Both laser and reference radiations are coupled into a bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. The spectral analysis of the beat signal confirms that the QCL is phase locked. This result opens the possibility to extend heterodyne interferometers into the far-infrared range. To proceed with the applications of terahertz QCLs as LOs in a heterodyne spectrometer, the stabilization of the frequency or phase is required to either eliminate the frequency jitter or to reduce the phase noise. For a heterodyne interferometer either on the earth [4] or in space [5], the phase locking of multiple LOs to a common reference at low frequency is essential. Phase locking [6] a laser to a reference means to control the phase of the laser radiation field precisely. This serves not only to stabilize the frequency but also to transfer the line profile of the reference to the laser. In the case of frequency locking, the laser's average frequency is fixed, but its linewidth remains equal to the laser's intrinsic linewidth. Several experiments to stabilize a terahertz QCL have been published. They are the frequency locking of a 3.1 THz QCL to a far-infrared (FIR) gas laser [7], the phase locking of the beat signal of two lateral modes of a terahertz QCL to a microwave reference [8], and the phase locking of a 1.5 THz QCL to a multiplier chain LO source [9]. Since the multiplier chain source has been demonstrated only up to about 2 THz, for a practical solution for the use of a QCL as LO beyond this frequency, the phase needs to be locked to an external reference that can be generated conveniently and should preferably be far below the QCL frequency. Therefore, an important challenge is the demonstration of the phase locking of a singlemode terahertz QCL to a microwave reference signal (MRS), which is the scheme commonly used in existing solid-state LOs. The MRS should be multiplied to a terahertz frequency in the vicinity of the laser frequency to obtain a beat note or an intermediate frequency (IF). As demonstrated in the measurements of FIR gas laser frequency, the harmonics of MRS at 3.8 THz [10] and at 4.3 THz [11] can be generated by Josephson junction harmonic mixers, resulting in a beat between the laser and the upconverted frequencies. Another commonly used harmonic generator is a Schottky diode [12].In this Letter, we report the phase locking of a 2.7 THz QCL to a harmonic generated from an MRS by a semiconductor superlattice (SL) nonlinear device in combination with a multiplier chain. We demonstrate a practical phase-locking scheme that is extendable to a much higher frequency and quantify the phaselocked power.The QCL used is based on the doubl...

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 report on a heterodyne receiver designed to observe the astrophysically important neutral atomic oxygen [OI] line at 4.7448 THz. The local oscillator is a third-order distributed feedback Quantum Cascade Laser operating in continuous wave mode at 4.741 THz. A quasi-optical, superconducting NbN hot electron bolometer is used as the mixer. We recorded a double sideband receiver noise temperature (T DSB rec ) of 815 K, which is ∼7 times the quantum noise limit ( hν 2k B ) and an Allan variance time of 15 s at an effective noise fluctuation bandwidth of 18 MHz. Heterodyne performance was confirmed by measuring a methanol line spectrum. a)