In the past decade, sound amplification by the stimulated emission of (acoustic phonon) radiation (saser) devices for generating coherent terahertz (THz) acoustic waves have been demonstrated [1 – 3]. The devices exploit the electron-phonon interactions in periodic semiconductor nanostructures known as superlattices (SLs) to amplify acoustic phonons. In addition, the particular acoustic properties of SLs can be exploited to make mirrors and cavities for THz phonons. Thus SLs can provide the two essential elements of a saser: the acoustic gain medium and the acoustic cavity.
In this presentation I will describe experimental studies of the THz phonon dynamics in a weakly-coupled GaAs/AlAs saser SL, which is DC electrically biased into the Wannier-Stark regime. Picoseconds-duration pulses of coherent THz acoustic phonons were generated using pump light pulses from a femtosecond laser and injected into the SL device. These phonon pulses seeded the saser cavity modes at about 220 and 440 GHz, which were amplified within the device. The phonons were detected using two methods: reflection of femtosecond probe light pulses, in a conventional pump-probe arrangement, and through the transient electrical response of the device itself.
When the DC bias conditions for saser were achieved in the device, the amplitude and lifetime of the seeded modes were both increased, analogous to the threshold and spectral line narrowing effects seen in laser devices.
[1] R P Beardsley et al., Phys. Rev. Lett. 104, 085501 (2010).
[2] W Maryam et al., Nature Communications 4:2184 (2013).
[3] K Shinokita et al., Phys. Rev. Lett. 116, 075504 (2016).
We report a continuous wave 1 μm laser based on InAs Stranski-Krastanov quantum dots (SK-QD) which is optically
pumped on a wetting layer absorption band at 915 nm. The slope efficiency of this laser relative to absorbed pump
power was measured to be 56% with wetting layer pumping, 1.75 times larger than when pumped with 830 nm light
absorbed into the barriers between the SK-QD layers. Compared to barrier pumping, wetting layer pumping benefits
from a smaller quantum defect, with less heat deposited in the active region, at the expense of weaker pump absorption
in the thin (~1 nm) wetting layer.
When a 50 μm thick intracavity diamond heatspreader was contacted to the optically pumped gain structure, a 10-fold
increase in output power, up to 2.25W, was obtained in the barrier pumped case. A much smaller 2-fold increase in
power, to a maximum of 0.35 W, was seen for the wetting layer pumped case. The diamond heatspreader is more
effective in removing heat from the active region, where it is deposited by barrier pumping, than from the substrate,
which absorbs residual pump radiation in the barrier pumping case.
A gain sample with a doubly periodic DBR to back reflect pump radiation, will allow the full potential of wetting layer
pumping to be realised, both by increasing pump absorption due to the double pass through the active region, and by
localising heat generation in the active region.
The energy resolution of a detector is related to the figure of merit NEP×√τ which is proportional to the heat
capacity of the detector. Hot electron (cold electron) devices have much lower heat capacity than bolometers with
silicon nitride based thermal isolation. Traditional hot electron bolometers (HEB) require sub-micron fabrication
for use at submm wavelengths and it is difficult to simultaneously couple radiation and read out these devices.
The 2D electron gas (2DEG) in a semiconductor heterojunction effectively acts as a metal film with a thickness
of a few angstroms and a tunable density and electron mobility. We describe a HEB that uses a 2DEG as an
absorber and present simulations of optical coupling schemes for this type of detector including an antenna
coupled to a coplanar waveguide with distributed 2DEG absorbers.
There is a demand for more efficient sources of electromagnetic radiation in the terahertz (THz, 1012 Hz)
frequency region. One common method of generating THz-frequency radiation is to direct fs pulses of near-infrared
laser radiation onto a material which then re-radiates. This approach permits coherent pulses of THz
radiation to be produced which, for example, may be used for time-domain spectroscopy (TDS). There are three
principal mechanisms by which THz radiation is generated under the stimulus of ultra-short pulses: optical
rectification (OR) in electro-optic materials, photoconductivity (PC) from materials with suitable electrodes,
and surface-field (SF) effects in other cases. The III-V compound semiconductor GaAs doped with the acceptor
impurity Be produces relatively small amounts of THz radiation via the OR and SF mechanisms, but relatively
large amounts via the PC mechanism. We have studied the PC emission of THz radiation from layers of GaAs(Be)
grown epitaxially on GaAs substrates. The THz power generated depends on the bias applied to the electrodes
approximately quadratically. This is typical of the PC mechanism. The dependence of the THz power on the
power of the pump beam is approximately linear. In general, the THz generated tends to decrease as the doping
level increases. If the doping level exceeds the Mott limit and the material becomes highly conductive then the
photoconductivity and consequently the THz production are quenched.
Low dimensional structures (LDS) form a major new branch of physics research. They are semiconductor structures, which have such a small scale in one or two spatial dimensions that their
electronic properties are significantly different from the same material in bulk form. These properties are changed by quantum effects. Throughout the world there is increasing interest in the preparation, study and application of LDS. Their investigation has revitalised condensed matter science, in particular semiconductor materials. These complex LDS offer device engineers new design
opportunities for tailor-made new generation electronic and photonic devices. New crystal growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapour (MOCVD) deposition have made it possible to produce such LDS in practice. These sophisticated technologies for the growth of high quality epitaxial layers of compound semiconductor materials on single crystal semiconductor substrates are becoming increasingly important for the development of the semiconductor electronics industry. This article is intended to convey the flavour of the subject by focussing on the technology and
applications of self-assembled quantum dots and to give an elementary introduction to some of the essential characteristics.
We describe a new type of terahertz (THz) detector for astronomical observation using a two-dimensional electron gas (2DEG) as the absorbing medium. The detection principle is based on the hot electron effect in 2DEGs. Electrons are heated by THz radiation and the electron temperature is read out by two symmetrical superconductor - 2DEG tunnelling junctions. Hot electrons are removed via tunnelling through a barrier into the superconducting contacts. The energy gap in the superconducting contacts prevents the escape of the colder, non-photoexcited electrons from the 2DEG. The high mobility 2DEG itself is created within AlGaAs/GaAs heterostructure with a single quantum well. In this paper we present low temperature DC measurements of 2DEG detectors, and measurements of the electron-phonon thermal conductivity of a 2DEG at 4.2 K and 300 mK as a function of electron temperature and magnetic field (in the 4.2 K case). From these measurements we estimate the noise equivalent power (NEP) of an element in a filled array of S-2DEG-S detectors at 4.2 K to be on the order of ≈
10-14W/√Hz with a response time of ≈ 1ns; at 300 mK, an NEP on the order of ≈
10-19W/√Hz and a response time of ≈ 0.1μs. Using measured parameters for the normal resistance of the S-2DEG-S contacts, we calculate the effect of using a voltage bias to cool the electrons in the absorber to significantly below a 300 mK base temperature. In this configuration, S-2DEG-S detectors can achieve sufficient sensitivity to detect individual THz photons.
We have generated pulsed beams of longitudinal and transverse polarized acoustic phonons by ultra fast optical excitation of gallium arsenide/aluminium arsenide superlattice structures. The phonons propagated ballistically over macroscopic (~ mm) distances at low temperatures and were detected using superconducting bolometers. We used superlattice phonon filters and the frequency-dependent phonon scattering in gallium arsenide to analyse the phonon spectrum. The phonons were found to be monochromatic, with a centre frequency given by υ = cs/dSL, where cs is the phonon speed and dSL is the superlattice period, and having a spectral line width (full width at half maximum) of less than 50 GHz. We measured a mean free path of 0.8 mm for both the longitudinal and transverse modes, consistent with point defect scattering in the GaAs substrate. Such phonons, with frequencies in the THz range, have potential applications in a number of areas, e.g. acoustic microscopy of solid-state nanostructures.
The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B = 0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. Vor I fluctuations. The normalized noise magnitude SR/R2 increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, SR/R2 ~ R2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density pc.
We have studied magneto-oscillations of the tunnelling current through a quantum well (QW) incorporating InAs self-assembled quantum dots in magnetic fields up to 28 T applied normal to the QW plane. We find evidence for the strong modification of the Landau levels in the host GaAs quantum well in the presence of dots embedded at the center of the well, which we attribute to electron-electron interactions.
We obtain inelastic electronic light scattering for interband transitions between valence-band states of GaAs layer embedded by self-assembled InAs quantum dots (QDs). Under a low-power selective cw excitation above the InAs band gap but below that of GaAs at a lattice temperature Tl = 5.1 K we find anomalous photoexcitation of carriers in the InAs QDs. Unusual photoinjection of the carriers to the GaAs barrier via strong Coulomb interactions results in creation of the nonequilibrium electron-hole plasma in the GaAs layer with density of n = p = 2.5 x 1018 cm-3 and an electron temperature Te = 25 K. Observed spectra reflects the band anisotropy and extends from zero to rather large frequency shifts with a long tail with a peak at about 300 - 400 cm-1 in good agreement with theoretical prediction.
We use resonant magnetotunneling spectroscopy, with the magnetic field applied parallel to the interfaces, to investigate the local band structure in the quantum well (QW) of a resonant tunneling diode. By rotating the magnetic field in the plane of the interfaces, we investigate the energy surface at constant k||. Using this technique, we have studied two different types of double barrier structures. We obtain different results depending on whether or not the QW contains a narrow InAs layer.
We have studied the near-equilibrium tunnelling between identical two-dimensional electron systems at 0.3 K over a wide range of magnetic field applied normal to the electron layers. The magnetic field suppresses the electron tunnelling. Our results are consistent with the co-existence of two types of tunneling gap. Moreover, at ν < 2 additional features arise in the tunneling spectra, which we interpret in terms of the emission of some as yet unidentified quasiparticle.
In this communication, we report on the design and the fabrication of quantum well barrier varactor structures with state of the art results in terms of capacitance ratio over a narrow voltage range. Basically, the fact to consider is a barrier cladded by two quantum wells with respect to a single barrier heterostructure. It has several consequences for the non linear character of the device. The capacitance mechanism is governed at low voltage by the electron population rates off the quantum well rather than the conventional depletion mode process. A true band gap capacitance engineering is here demonstrated with thee kinds of structures either in the InP material system with a InGaAs/InAs/AlAs heterostructure or in the GaAs material system with GaAs/InGaAs/AlAs pseudomorphic epilayers and lattice matched AlGaAs/GaAs/AlAs heterojunctions. Self- consistent simulations, based on the solution of Poisson and Schroedinger coupled equations system, were performed in order to calculate the electron wave function and the conduction band bending. High capacitance ratios can be predicted depending on material parameters and structure geometry. Test samples were then fabricated and rf tested. The devices very high capacitance ratios is excess of 5 to 1 over a 1 Volt range.
We have studied the influence of silicon doping on the electric-field-induced Stark shifts of the subband states for AlGaAs/GaAs/AlGaAs multiple quantum well structures. The investigations were performed applying electroreflectance (ER), photoreflectance (PR), and for comparison photoluminescence (PL) spectroscopy. The analysis of the PR spectra yields the transition energies at approximately zero electric field, which are in good agreement with PL peaks. The shift of the Er ground state heavy hole transition with respect t the PR data and accompanying self-consistent calculations allow the determination of the field strength F at each gate voltage with high accuracy. On this bases, the other transitions can be identified by their expected shift. It will be shown that all transitions involving the first heavy hole subband show a strong increase of their signals with increasing field strength. The plot of F2 versus gate voltage for both samples fits a straight line, slope of which provides the corresponding concentrations of the ionized donors.
We report on the influence of a 2-D electron gas (2-D EG) on the electroreflectance (ER) spectra of an AlGaAs/GaAs/AlGaAs single quantum well. The ER and, for comparison, photoluminescence (PL) measurements were performed as a function of temperature and carrier density in a gated 2-D EG. For negative gate voltages the observed ER signals are due to the quantum confined Stark effect (QCSE). For forward biases and at a temperature of 10 K, the broadening of the PL line and the increase of the Stokes shift between the corresponding ER and PL transitions give clear evidence for the population of the first electron subband. Moreover, with the formation of the 2-D EG we found a strong enhancement of the magnitude as well as two additional phase inversions for the ER transitions between the ground heavy and light hole states and the first electron subband. These effects were seen to decrease with increasing temperature. The results are explained in terms of transformation of the excitonic absorption mechanism from a QCSE-related mechanism to one determined by the `Mahan exciton.'
Very large fractional reflectivity changes (30%) under optical pump densities of only 3 mW/cm2 are found in a novel hetero-nipi structure, the Bragg Hetero-nipi Doubly Resonant Optical Modulator (BH-DROM). The high sensitivity of this optical nonlinearity stems from the strong interaction between the excitonic resonances in the quantum well regions of the heteronipi, and an optical Bragg resonance designed into the structure through its optical periodicity. Device response time has been controlled by etching the surface into pixels of various sizes. The measured switching times decrease with decreasing pixel size, with a 50 micrometers by 50 micrometers pixel switching in 150 microsecond(s) . Uniform device switching is ensured by the rapid in-plane transport of carriers through the structure due to the 'Giant Ambipolar Diffusion' mechanism, which has also been studied. A detailed numerical model of this device has been developed which shows good qualitative agreement with the experimental results. Predictions of purely intrinsic bistable behavior and also greatly enhanced contrast ratios and sensitivities are made for similar structures.
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