H.-W. Hübers

Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler

A liquid cryogen-free terahertz heterodyne receiver in a pulse tube cooler has been realized. The receiver operates at 2.5 THz. It is based on a quantum cascade laser (QCL) as local oscillator and a hot electron bolometric mixer. A detailed study of the QCL beam quality yielded a beam propagation factor of 1.1–1.2. The double sideband noise temperature of the system is 2000 K and when corrected for optical losses in the signal path it is ∼800 K.

Terahertz lasers based on germanium and silicon

Recent experimental and theoretical results of impurity doped germanium and silicon terahertz lasers are reviewed. Three different laser mechanisms exist in p-type germanium. Depending on the operating conditions and the properties of the crystal, laser transitions can occur between light- and heavy-hole subbands, between particular light-hole Landau levels or between impurity states. Electric and magnetic fields are required for laser operation. In n-type silicon lasing originates solely from impurity transitions of group-V donors, which are optically excited. The properties of these lasers depend upon the chemical nature of the impurity centre and the properties of the host material. The principles of operation are discussed in terms of their basic physical concepts. The state-of-the-art performance of these lasers is summarized.

THz near-field imaging employing synchrotron radiation

Terahertz scanning near-field infrared microscopy below 1 THz is demonstrated at an electron storage ring using coherent synchrotron radiation. Spatial resolution below the diffraction limit down to about λ/40 at 2 cm−1 is derived from the transmittance spectra of a conical aperture probe. The potential of the technique is exemplified by imaging wet biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 cm−1. The THz near-field images reveal distinct structural differences in the mesophytic and xerophytic leaves investigated.

Submegahertz frequency stabilization of a terahertz quantum cascade laser to a molecular absorption line

The frequency of a terahertz quantum-cascade laser is stabilized to the absorption line of methanol gas at a frequency of 2.55 THz. The method is based on frequency modulation of the laser emission across the absorption line. The resulting derivativelike signal is used as an error signal for a control loop that keeps the laser frequency at maximum absorption. The unstabilized laser that is operated in a pulse tube cooler has frequency fluctuations of 15 MHz, which are reduced to 300 kHz with the control loop in action. The line shape of the locked signal is Gaussian.

Towards traceable radiometry in the terahertz region

PTB and DLR join their expertise and experience in optical radiometry and in THz techniques to perform what is to our knowledge the first traceable measurement of radiant power of a THz quantum cascade laser and the first absolute calibration of a THz radiation detector against a cryogenic radiometer (CR). A total standard uncertainty of 7.3% was achieved at a frequency of 2.5?THz corresponding to a wavelength of 120??m. This uncertainty is dominated by the limited knowledge of the absorptance of the CR cavity. All other uncertainty contributions including those arising from diffraction are only 2%.

Far-infrared stimulated emission from optically excited bismuth donors in silicon

Far-infrared stimulated emission from optically pumped neutral Bi donors in silicon has been obtained. Lasing with wavelengths of 52.2 and 48.6 μm from the intra-center 2p±→1s(E:Γ8),1s(T2:Γ8) transitions has been realized under CO2 laser pumping. The population inversion mechanism is based on fast optical-phonon-assisted relaxation from the 2p0 and 2s excited states directly to the ground 1s(A) state leading to relatively small population in the intermediate 1s(E), 1s(T2) excited states.

Tantalum nitride superconducting single-photon detectors with low cut-off energy

Materials with a small superconducting energy gap are expected to favor a high detection efficiency of low-energy photons in superconducting nanowire single-photon detectors. We developed a TaN detector with smaller gap and lower density of states at the Fermi energy than in comparable NbN devices, while other relevant parameters remain essentially unchanged. The observed reduction of the minimum photon energy required for direct detection is in line with model predictions of ≈1/3 as compared to NbN.

Fluctuation effects in superconducting nanostrips

Superconducting fluctuations in long and narrow strips made from ultrathin NbN films, have been investigated. For large bias currents close to the critical current fluctuations led to localized, temporary transitions into the normal conducting state, which were detected as voltage transients developing between the strip ends. We present models based on fluctuations in the Cooper pair density and current-assisted thermal-unbinding of vortex-antivortex pairs, which explain the current and temperature dependence of the experimental fluctuation rates.

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.

Stimulated terahertz emission from arsenic donors in silicon

Stimulated emission has been obtained from intra-center donor transitions in silicon monocrystals doped by arsenic. The Si:As laser was optically excited by radiation from a CO2 laser. The emission spectrum consists of two lines corresponding to the 2p±→1s(E) and 2p±→1s(T2) intra-center arcenic transitions. The population inversion is formed due to fast 2s→1s(A1) electron relaxation assisted by intervalley longitudinal acoustic f-phonon emission. This keeps the excited donor states below the 2p± state unpopulated. Thus population inversion occurs between the 2p± state and the 1s(E), 1s(T2) states.