J. N. Hovenier

Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer

We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.

Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions

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.

Phase locking of a 2.7 THz quantum cascade laser to a microwave reference

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 (x12) from a microwave synthesizer at approximately 15 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.

Hot electron bolometer heterodyne receiver with a 4.7-THz quantum cascade laser as a local oscillator

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-rec(DSB)) of 815 K, which is similar to 7 times the quantum noise limit (hv/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

Quasiparticle relaxation in optically excited high-Q superconducting resonators.

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.

Stimulated terahertz emission from group-V donors in silicon under intracenter photoexcitation

Frequency-tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in bulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature while resonant pumping of odd parity impurity states. The threshold was about two orders of magnitude below the value for photoionization pumping. The influence of nonequilibrium intervalley TO phonons on the population of excited Bi impurity states is discussed.

High-resolution heterodyne spectroscopy using a tunable quantum cascade laser around 3.5 THz

A frequency tunable terahertz heterodyne spectrometer, based on a third-order distributed feedback quantum cascade laser as a local oscillator, has been demonstrated by measuring molecular spectral lines of methanol (CH3OH) gas at 3.5 THz. By varying the bias voltage of the laser, we achieved a tuning range of ? 1?GHz of the lasing frequency, within which the molecular spectral lines were recorded. The measured spectra show excellent agreement with modeled ones. By fitting we derived the lasing frequency for each bias voltage accurately. The ultimate performance of the receiver including the resolution of noise temperature and frequency is also addressed.

Surface plasmon quantum cascade lasers as terahertz local oscillators

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs. Compared to the latter, a more directional beam allows for better coupling of the radiation power to the mixer. We obtain a receiver noise temperature of 1050 K when the mixer is at 2 K, which, to our knowledge, is the highest sensitivity reported at frequencies beyond 2.5 THz.

Active mode locking of a p-Ge hot hole laser

The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm−1) was achieved due to active mode locking by electrical intracavity modulation of the gain.

Phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser using a room-temperature superlattice harmonic mixer

We report on the phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser (QCL) using a room temperature GaAs/AlAs superlattice diode as both a frequency multiplier and an internal harmonic mixer. A signal-to-noise level of 60 dB is observed in the intermediate frequency signal between the 18th harmonic of a 190.7 GHz reference source and the 3433 GHz QCL. A phase-lock loop with 7 MHz bandwidth results in QCL emission that is 96% locked to the reference source. We characterize the QCL temperature and electrical tuning mechanisms and show that frequency dependence of these mechanisms can prevent phase-locking under certain QCL bias conditions.