Quantum Correlated Polariton Modes in a Semiconductor Vertical Triple Microcavity

Three different kinds of quantum wells (QWs) were grown at the same growth temperature by molecular beam epitaxy (MBE): InGaAs/GaAs QW without any N2 background; InGaAs(N2)/GaAs double QWs with N2 flow to the sample but without plasma; and InGaAsN/GaAs QW with N-plasma. The latter two types of QW were grown on one sample. Post-growth rapid thermal annealing (RTA) was applied to them at 700 °C. After 31.5 min of RTA, it was observed that the photoluminescence blue shift of the InGaAs(N2) QW was 40 meV, which was 9 meV more than that for the InGaAsN QW, and the InGaAs QW had only about 13 meV blue shift. However, for the as-grown case, the PL peak position of the InGaAs QW and InGaAs(N2) QWs were the same (1148 nm), and for the InGaAsN QW it was 220 nm longer. This indicates that there is N2 incorporation during InGaAsN growth, the N2 incorporation does not affect the band structure and the N2 incorporation dominates the blue shift during RTA.

In order to study the luminescent efficiency of InGaAs quantum wells on Si via SiGe interlayers, identical In0.2Ga0.8As quantum well structures with GaAs and Al0.25Ga0.75As cladding layers were grown on several substrates by an atmospheric metalorganic vapor deposition system. The substrates used include GaAs, Si, Ge, and SiGe virtual substrates. The SiGe virtual substrates were graded from Si substrates to 100% Ge content. Because of the small lattice mismatch between GaAs and Ge (0.07%), high-quality GaAs-based thin films with threading dislocation densities <3×106 cm−2 were realized on these SiGe substrates. Quantitative cathodoluminescence was used to compare the luminescent efficiency of the quantum well structure on the different substrates and cross-sectional transmission electron microscopy was used to characterize dislocation densities. Our results show that the InGaAs quantum wells grown on the GaAs substrates have the highest luminescent efficiencies due to the lowest dislocation densities. Interestingly, InGaAs quantum wells grown on the SiGe virtual substrates outperform those on Ge substrates, both in terms of luminescent efficiency and dislocation density. This difference is attributed to the variation in thermal expansion coefficient (α) and its impact on defect structure during the process cycle. The SiGe virtual substrate has a smaller α compared to a Ge substrate because of the smaller α of the Si substrate, which helps minimize compressive strain in the quantum well layer during the temperature decrease from the growth temperature. Consequently, fewer misfit dislocations are created between the quantum well and cladding interfaces. These misfits can greatly affect the luminescent efficiency since they can act as recombination sites. In general, the efficiencies of the quantum wells on the SiGe and Ge substrates were affected only by higher misfit dislocation densities, whereas the quantum wells on the Si substrate had low efficiency due to high threading dislocation density.

Introducing spatial autocorrelation analysis of nanophotoluminescence images of atomically rough single GaAs quantum wells, we show that the bright spots in the images are not randomly distributed but rather tend to “repel each other” along certain directions. Simple computer simulations on the anisotropic transport of excitons in the disorder potential and on the resulting images can reproduce this surprising behavior.

Luminescence properties of doped and undoped GaAs/(AlGa)As quantum wells, wires, dots, and antidots are discussed. The complex lineshape of high quality modulation doped single quantum wells can be understood in terms of wave vector conserving and nonconserving transitions. A shallow etching technique together with holographic optical lithography results in a spatial separation of electrons and photogenerated holes. The luminescence features of wires, dots, and antidots show both direct and indirect character in real space. Isolated single quantum dots have been fabricated by local interdiffusion of undoped GaAs/AlGaAs quantum wells. Due to the inherent exclusion of inhomogeneous broadening in single dot structures very sharp luminescence lines are observed in originally 3 nm wide quantum wells.

Resonant tunneling through a single quantum well of GaAs has been observed. The current singularity and negative resistance region are dramatically improved over previous results, and detecting and mixing have been carried out at frequencies as high as 2.5 THz. Resonant tunneling features are visible in the conductance-voltage curve at room temperature and become quite pronounced in the I-V curves at low temperature. The high-frequency results, measured with far IR lasers, prove that the charge transport is faster than about 10−13 s. It may now be possible to construct practical nonlinear devices using quantum wells at millimeter and submillimeter wavelengths.

Spectroscopy of intersubband transitions in Si–Si 1− xGe x quantum wells

Quantum well nonlinear microcavities

In recent years, the control of charge and spin in semiconductors has experienced outstanding progress. The extended spin coherence times established in high quality materials together with the advance in elaborate fabrication methods enable coherent spin control, ultimately leading to new spintronic devices. In this thesis, we investigate various aspects of quantum transport with respect to potential spintronic applications in two different kind of 2-dimensional electron gases (2DEGs) in GaAs/AlGaAs crystals. First, we try to combine the advantages of optically active InAs quantum dots (QDs) and the versatile possibilities of gate tunable 2DEGs in a novel hybrid heterostructure. We have characterized a series of hybrid wafers with different spacings between the inverted 2DEG and Stranski-Krastanov grown self-assembled InAs QDs. Depending on this distance and actual QD nucleation – verified with atomic force microscopy on the wafer surface – the 2DEG mobility is reduced due to scattering induced by the QDs and the InAs wetting layer. For a tunnel barrier of 45 nm, the 2DEG exhibits mobilities exceeding 500′000 cm2/(Vs) despite the presence of InAs QDs, while coherent tunneling between 2DEG and QDs is still permitted. Using a top down approach, lateral quantum point contact gates are aligned precisely to a single, specifically chosen InAs QD. The 1D conductance is not sensitive to QD charging events, but is dominated by a disorder potential, even suppressing quantization. In the second part of the thesis, we study the control of spin-orbit (SO) interaction. This relativistic coupling of the electron spin to its momentum can be used for coherent spin manipulation, but at the same time also causes spin relaxation. Theory predicts a special symmetry, protecting spin from relaxation in diffusive transport, when the two main contributions of SO interaction in GaAs quantum wells (QW) – the Rashba and the Dresselhaus effect – are of equal strength. We demonstrate broad, independent control of all relevant SO fields, allowing us to tune into this regime. By electrically locking the Rashba and Dresselhaus SO fields via top and back gate, we achieve spin protection for a wide range of voltages on a single QW. We use quantum corrections to 2D conductivity as a sensitive probe of SO coupling. The combination of transport data and numerical calculations allows us to quantify the relevant SO coefficients.

We investigate the optical interband properties of GaAs quantum wells driven by a terahertz field. The application of this field shifts the exciton absorption energy. Additionally, terahertz optical sidebands are observed up to 140 K.

(110)-oriented GaAs-based quantum wells (QWs) are promising active layers of spin lasers because of long spin relaxation time at room temperature. So far, (110)GaAs/AlGaAs QWs have been investigated intensively and long carrier lifetime of several ns has been reported. In contrast, there have been few reports on the crystal growth of (110)InGaAs/(Al)GaAs QWs despite its great potential to the active layer, resulting in rather poor crystal quality with short carrier lifetime. In this study, we tried to grow high-quality (110)InGaAs/(Al)GaAs QWs by optimizing MBE growth conditions and obtained long carrier and spin lifetime (>1 ns) in the QWs grown with relatively high temperature and As/Ga flux ratio. This reveals that the (110)InGaAs/(Al)GaAs QWs are applicable to the active layer of spin lasers. In addition, we propose a novel structure for a spin transport layer between the ferromagnetic electrode (spin injector) and the active layer of spin laser.

Microwave quantum illumination with entangled pairs of microwave signal and optical idler modes can achieve sub‐optimal performance with joint measurement of the signal and idler modes. Here, a testbed for microwave quantum illumination is proposed with an optical memory simulated with a delay line in the idler mode. It provides the amount of input two‐mode squeezing necessary to compensate for the loss of optical memory while maintaining a quantum advantage over a coherent state. When the memory is lossy, the input two‐mode squeezing has to be higher through high cooperativity in the optical mode. Under the testbed, a single‐mode phase conjugate receiver is proposed consisting of a low‐reflectivity beam splitter, an electro‐optomechanical phase conjugator, and a photon number‐resolving detector. The performance of the newly proposed receiver approaches the sub‐optimal quantum advantage of 3 dB. Furthermore, the receiver achieves the quantum advantage even with an on‐off detection while being robust against the loss of the memory.

The polariton dynamics in a microcavity in the parametric oscillator mode is studied when two pump polaritons turn into polaritons of signal and idler modes and vice versa. The pumping is performed by two laser pulses with close frequencies. Analytical solutions to a system of nonlinear differential equations are obtained. Periodic and aperiodic regimes of transformation of a pair of pump polaritons into polaritons of signal and idler modes, as well as the quiescent mode of the system, are obtained.

Quantum illumination uses entangled light that consists of signal and idler modes to achieve higher detection rate of a low-reflective object in noisy environments. The best performance of quantum illumination can be achieved by measuring the returned signal mode together with the idler mode. Thus, it is necessary to prepare a quantum memory that can keep the idler mode ideal. To send a signal towards a long-distance target, entangled light in the microwave regime is used. There was a recent demonstration of a microwave quantum memory using microwave cavities coupled with a transmon qubit. We propose an ordering of bosonic operators to efficiently compute the Schrieffer–Wolff transformation generator to analyze the quantum memory. Our proposed method is applicable to a wide class of systems described by bosonic operators whose interaction part represents a definite number of transfer in quanta.

Thomas–Fermi–Dirac theory of the hole gas of a double p-type δ-doped GaAs quantum wells

We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\ensuremath{\sim}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\ensuremath{\sim}{10}^{7}$ and $\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\text{V}\phantom{\rule{0.16em}{0ex}}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.