Session A15: Focus Session: THz Devices and Materials I

Progress and Perspectives of THz Astronomical Detectors

During the past decade, THz astronomical observations from ground based, suborbital and space platforms have become a reality. The most impressive progress has been made in heterodyne spectroscopy instrumentation due to a tremendous effort in the development of solid state local oscillators and SIS and HEB superconducting mixers. Whereas the current SIS technology may have almost reached its limit, the HEB mixers expanding far into the THz frequencies have yet a large room for improvement. The progress in direct detectors has been mostly due to the development of photoconductive detectors for wavelengths shorter than 50 $\mu $m and of composite bolometers for longer wavelengths. Both technologies have pretty much reached their limitations for improvement. Future THz astronomical space missions will require both non-parallel sensitivity and large-scale arrays of both direct and heterodyne detectors. I will discuss how these objectives can be met with recently emerged types of superconducting and semiconductor sensors utilizing new physical detection mechanisms.   

Plasmon Based Grating Gate Terahertz Detector

Double quantum well grating gate detectors have recently emerged as a widely tunable detector of millimeter wave to THz radiation. A typical device consists of source and drain contacts along with a grating gate which both modulates the carrier density and couples in the free space radiation to the plasmon modes of the double quantum well heterostructure. In a resonant mode of operation, the grating period determines the frequency range of the detector while the gate bias tunes the operating frequency. When the gate is biased near pinch-off, the detector becomes bolometric and responsivity increases dramatically. This talk outlines current progress towards combining the plasmon resonant frequency selectivity with the responsivity of the bolometric regime through the application of a split grating gate that allows for more flexible biasing of the detector while still coupling free space radiation into the device. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Ballistic tranport at finite frequencies in 2DEGs and quantum point contacts

In this talk, we present our work on ballistic transport in both the spatial limit and temporal limit. In the temporal limit, we have characterized[1] the ac impedance of 2DEGs contacted with ohmic contacts at frequencies above and below the momentum scattering frequency, which is of order GHz for high mobility 2DEGs. The crossover from omega tau $<$ 1 to omega tau $>$ 1 is clearly observed. Additional non-linear effects due to plasma wave rectification in gated geometries are also under investigation, which will lead to new modes of HEMT operation not limited by transit-time effects. In order to investigate this we have fabricated devices with asymmetric ac bias conditions on the source/drain. Using an integrated, on-chip high impedance transmission line to connect the source, and a low-impedance contact to a generator on the drain, we are able to quantitatively test the dc and ac behavior of HEMT devices in the resonant limit, where standing-waves of 2d plasmons are generated in the finite length gated region (the channel). Finally, we will present data on the ac impedance of quantum point contact devices, which are ballistic in both senses: device size $<$ mean free path, and frequency $>$ momentum scattering frequency. [1] Sungmu Kang, Peter J. Burke, L. N. Pfeiffer, and K. W. West, Solid State Electronics 48, 2013-2017 (2004).   

Resonant response of a field-effect transistor to an ac signal

A theoretical investigation is made of the response of a field-effect transistor (FET) to an incoming electromagnetic radiation in the presence of a perpendicular magnetic field within the framework of hydrodynamics. The treatment is valid for a nondegenerate electron gas in which the mean free path for electron-electron scattering $\lambda_{ee}$ is much smaller than the device length $L$ and than the mean free path due to collisions with impurities and/or phonons $\lambda_{coll}$. These requirements, written as $\lambda_{ee} \ll L \ll \lambda_{coll}$, are fulfilled for magnetic fields weak enough to prevent Landau quantization. It is our general observation that the shorter device lengths, weaker magnetic fields, and lower temperatures (or higher electron mobility) are most favorable to achieve a greater resonant response of the device to an ac signal. Such resonant response makes FET a promising device for new types of sources, detectors, mixers, and multipliers in the GHz and THz frequency range.   

MBE Growth of GaAs Based THz Lasers and Detectors

Terahertz (1-10 THz, or 4-40 meV, or 30-300 $\mu$m) frequencies are among the most underdeveloped electromagnetic spectra, even though their potential applications are promising for spectroscopy in chemistry and biology, astrophysics, plasma diagnostics, remote atmospheric sensing and imaging, noninvasive inspection of semiconductor wafers, and communications. This underdevelopment is primarily due to the lack of coherent solid-state THz sources and detectors. In this talk I will discuss the important MBE growth considerations needed to make semiconductor THz devices using the GaAs-AlGaAs material system. The influence of material growth parameters on the performance of GaAs-based THz quantum cascade lasers and quantum well detectors will be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

High performance THz Quantum Cascade Lasers in very high magnetic fields

A new design for a terahertz quantum-cascade laser emitting at $\lambda \simeq 80~\mu$m is presented. A bound-to-continuum active region is coupled to an optical phonon extraction stage in order to improve the population inversion at high temperatures. The device processed with a single-plasmon waveguide shows a threshold current density in pulsed operation of J$_{thresh}=190$ A/cm$^2 $, sensibly lower of what measured in other optical-phonon based structures. Maximum operating temperature in pulsed mode is 100 K and peak powers of the order of 40 mW are observed at low temperatures. Device performances in a double-metal waveguide configuration reach 117 K in pulsed operation and 53 K in continuous-wave. This laser has been investigated also in presence of a strong magnetic field up to 27 T, in order to study the influence of an external confinement on the gain and lifetime of the electron states of a superlattice-based quantum cascade structure. Upon increasing of the magnetic field intensity, strong modulation of laser emission and threshold current are observed over the full range of magnetic field up to 27 T, with an overall reduction of the latter of a factor of two with respect to the zero field value. Spectral characterization in the presence of magnetic field has also been performed.   

Quantum Dot as High Frequency Noise Detector

We present the experimental realization of a Quantum Dot (QD) as high-frequency noise detector. Current shot noise produced by a nearby Quantum Point Contact (QPC) induce photon-assisted tunneling for the electrons in QD. Thus fluctuations generated by the QPC lead to transport through orbital excited states of the quantum dot. This allows us to measure the QPC noise modulation when we change the number of opened channels. We also investigate the dependence of detector signal on the bias voltage across the QPC. We observe and explain the saturation and quantum features in the detector signal. The measurements are consistent with previous low-frequency experiments. In our case the detection frequency is determined by the QD orbital states spacing (60 respectively 140GHz).   

Intersubband lifetime magnetophonon oscillations in AlGaAs and InGaAs:InP quantum cascade lasers

We investigate the influence of a strong magnetic field on intersubband scattering rates in AlGaAs and InGaAs quantum cascade lasers (QCL). Laser threshold, differential quantum efficiency and laser emission spectra were measured in magnetic fields up to 30T applied perpendicular to the 2D planes. Intersubband magnetophonon effect -- resonant optical phonon non-radiative relaxation, gives rise to the strong oscillations in laser emission. We derived the magnetic field dependence of intersubband lifetime and compare to the calculated dependence of electron- LO phonon scattering rates.   

1.9 THz Quantum Cascade Lasers in very high magnetic fields up to 27 T

Terahertz quantum cascade lasers operating at $\lambda = 159$ $\mu$m [1] and exploiting the in-plane confinement arising from perpendicular magnetic field are investigated in the regime of very strong magnetic confinement, with field intensities up to 27 T, corresponding to a ratio $\displaystyle\frac{\hbar \omega_c(27~T)}{h \nu}$= 5.8 of cyclotron energy $\hbar \omega_c$ over photon energy $h \nu$. Device show laser action in magnetic field starting from 2.7 T and reaches operating temperatures of 65 K in pulsed mode. As the magnetic field is increased up to 27 T, the laser intensity exhibits modulations due to the interplay between inter Landau-level scattering and e-e scattering. A strong increase of the output power is observed at the highest field values. Strong reduction of the waveguide losses and an increase in the gain attributed to carrier localization leads to a decrease of the threshold current density down to 0.6 A/cm$^2$ at B=27 T. A detailed spectral analysis, showing a progressive redshift of the spectrum in the 3-27 T magnetic field range, will also be discussed. [1] G.Scalari, S.Blaser, J.Faist, H.Beere, E.Linfield, D.Ritchie, G.Davies, Phys. Rev. Lett., in press (2004)   

High Energy Intersubband Transitions in InAs/AlSb QWs

InAs/GaSb/AlSb heterostructures are a promising material system for intersubband optically-pumped applications due to their large conduction band offsets ($\sim $2 eV in InAs/AlSb). Applications include FIR generation and ultrafast all-optical switching at the communication wavelength of 1.55 $\mu $m. We have observed intersubband absorption at E$_{12}$ up to 670 meV (1.85 $\mu $m) in 2.1 nm Si-doped InAs/AlSb QWs. We have also attempted THz generation by difference frequency mixing in resonant InAs/AlSb asymmetric double quantum wells.   

Polaritonics: bridging the gap between electronics and photonics

Between electronics and photonics there exists a frequency gap of approximately 2 octaves, i.e. the frequency range between 100~GHz and 10~THz. Here we demonstrate that phonon-polaritons in ferroelectric crystals like LiNbO$_3$ or LiTaO$_3$ may be able to bridge this gap. The ability to fabricate structures within the crystal by femtosecond laser machining facilitates all integrated signal guiding and processing. Spatiotemporal imaging is employed for direct visualization of the electromagnetic field within the crystal. Polaritonic resonators, waveguides, photonic crystals and focusing, dispersive, and diffractive elements will be demonstrated.