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We show that a Bose–Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.

We study metal–insulator–semiconductor tunnel junctions where the metal electrode is a patterned gold layer, the insulator is a thin layer of Al2O3, and the semiconductor is p-type silicon. We observe light emission due to plasmon-assisted inelastic tunneling from the metal to the silicon valence band. The emission cutoff shifts to higher energies with increasing voltage, a clear signature of electrically driven plasmons. The cutoff energy exceeds the applied voltage, and a large fraction of the emission is above the threshold, ℏω > eV. We find that the emission spectrum manifests the Fermi–Dirac distribution of the electrons in the gold electrode. This distribution can be used to determine the effective electron temperature, Te, which is shown to have a linear dependence on the applied voltage. The strong correlation of Te with the plasmon energy serves as evidence that the mechanism for heating the electrons is plasmon decay at the source metal electrode.

We study the exciton gas-liquid transition in $\mathrm{GaAs}/\mathrm{AlGaAs}$ coupled quantum wells. Below a critical temperature, ${T}_{C}=4.8\text{ }\text{ }\mathrm{K}$, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density-temperature phase diagram over the temperature range 0.1--4.8 K. We find that the latent heat increases linearly with temperature at $T\ensuremath{\lesssim}1.1\text{ }\text{ }\mathrm{K}$, similarly to a Bose-Einstein condensate transition, and becomes constant at $1.1\ensuremath{\lesssim}Tl4.8\text{ }\text{ }\mathrm{K}$. Resonant Rayleigh scattering measurements reveal that the disorder in the sample is strongly suppressed and the diffusion coefficient sharply increases with decreasing temperature at $Tl{T}_{C}$, allowing the liquid to spread over large distances away from the excitation region. We suggest that our findings are manifestations of a quantum liquid behavior.

In this work, we investigate the dynamics of a single electron surface trap, embedded in a self-assembly metallic double-dot system. The charging and discharging of the trap by a single electron is manifested as a random telegraph signal of the current through the double-dot device. We find that we can control the duration time that an electron resides in the trap through the current that flows in the device, between fractions of a second to more than an hour. We suggest that the observed switching is the electrical manifestation of the optical blinking phenomenon, commonly observed in semiconductor quantum dots., Comment: 4 figures

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by T-2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton-phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.

We have improved the sensitivity and signal-to-noise ratio of a luminescence upconversion experiment, using a charge-coupled device (CCD) as the detector. We show experimentally and numerically that the bandwidth of a 1-mm-thick beta-barium berate crystal is large enough to take full advantage of the multichannel capabilities of the CCD. The improvement is significant in a standard experiment with a single laser as well as in experiments with resonant excitation that use two synchronized femtosecond pulse sources at different wavelengths. The characteristics of the two-color scheme are discussed in detail. (C) 1998 Optical Society of America.

We present an electrically driven plasmonic device consisting of a gold nanoparticle trapped in a gap between two electrodes. The tunneling current in the device generates plasmons, which decay radiatively. The emitted spectrum extends up to an energy that depends on the applied voltage. Characterization of the electrical conductance at low temperatures allows us to extract the voltage drop on each tunnel barrier and the corresponding emitted spectrum. In several devices we find a pronounced sharp asymmetrical dip in the spectrum, which we identify as a Fano resonance. Finite-difference time-domain (FDTD) calculations reveal that this resonance is due to interference between the nanoparticle and electrodes dipolar fields, and can be conveniently controlled by the structural parameters., 15 pages, 4 figures + Supporting Information file

Radio-Frequency coupling between magnetically trapped atomic states allows to create versatile adiabatic dressed state potentials for neutral atom manipulation. Most notably, a single magnetic trap can be split into a double well by controlling amplitude and frequency of an oscillating magnetic field. We use this to build an integrated matter wave interferometer on an atom chip. Transverse splitting of quasi one-dimensional Bose---Einstein condensates over a wide range from 3 to 80 ?m is demonstrated, accessing the tunnelling regime as well as completely isolated sites. By recombining the two split BECs in time of flight expansion, we realize a matter wave interferometer. The observed interference pattern exhibits a stable relative phase of the two condensates, clearly indicating a coherent splitting process. Furthermore, we measure and control the deterministic phase evolution throughout the splitting process. RF induced potentials are especially suited for integrated micro manipulation of neutral atoms on atom chips: designing appropriate wire patterns enables control over the created potentials to the (nanometer) precision of the fabrication process. Additionally, hight local RF amplitudes can be obtained with only moderate currents. This new technique can be directly implemented in many existing atom chip experiments.

The near band edge absorption spectrum of a quantum well which contains an electron gas is studied. We show that electron–hole correlations play an important role in determining this spectrum. At zero magnetic field the spectrum evolves with increasing electron density from being dominated by neutral excitons at the very dilute limit to charged exciton and then into the Fermi edge singularity. At high magnetic fields the spectrum depends on the filling factor ν. Three regimes are well distinguished: ν 2 where the electron–hole correlations do not play any important role and the spectrum is a simple band to band transition.

Matter-wave interference experiments enable us to study matter at its most basic, quantum level and form the basis of high-precision sensors for applications such as inertial and gravitational field sensing. Success in both of these pursuits requires the development of atom-optical elements that can manipulate matter waves at the same time as preserving their coherence and phase. Here, we present an integrated interferometer based on a simple, coherent matter-wave beam splitter constructed on an atom chip. Through the use of radio-frequency-induced adiabatic double-well potentials, we demonstrate the splitting of Bose-Einstein condensates into two clouds separated by distances ranging from 3 to 80 microns, enabling access to both tunnelling and isolated regimes. Moreover, by analysing the interference patterns formed by combining two clouds of ultracold atoms originating from a single condensate, we measure the deterministic phase evolution throughout the splitting process. We show that we can control the relative phase between the two fully separated samples and that our beam splitter is phase-preserving.

The emission and absorption spectra of quantum wells containing electron or hole gas are reviewed. We show that trions, also known as charged excitons, play a dominant role in determining these spectra. We discuss issues related to their behaviour at zero and high magnetic fields, their far-field and near-field spectra, and their role as a probe for delicate correlations of the surrounding electron gas.

Microscopic atom optical devices integrated on atom chips allow to precisely control and manipulate ultra-cold ( T < 1 µK) neutral atoms and Bose-Einstein condensates (BECs) close to surfaces. ...

Strong resonant enhancements of inelastic light scattering from the long wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin wave excitations are seen for the fractional quantum Hall state at $\nu = 1/3$. The energies of the sharp peaks (FWHM $\lesssim 0.2meV$) in the profiles of resonant enhancement of inelastic light scattering intensities coincide with the energies of photoluminescence bands assigned to negatively charged exciton recombination. To interpret the observed enhancement profiles, we propose three-step light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations., Comment: 5 pages, 5 figures

We study the photoluminescence (PL) spectrum of a two-dimensional electron system at the high magnetic field limit, where all electrons reside at the lowest Landau level (ν

We describe an experiment to create a sizable 87Rb Bose–Einstein condensate (BEC) in a simple magnetic microtrap, created by a current through a Z-shaped wire and a homogeneous bias field. The BEC is created close to a reflecting surface. It is an ideal coherent source for experiments with cold atoms close to surfaces, be it small-volume microtraps or directly studying the interactions between cold atoms and a warm surface.

We present a novel method for fabrication of contacts to nanosize particles. The method is based on conventional optical lithography of GaAs/AlGaAs molecular beam epitaxy grown structures. Taking advantage of the difference in etch rate between GaAs and AlGaAs a narrow gap is formed between metal contacts deposited on the side of a mesa structure. We demonstrate electrostatic trapping of charged metal clusters into these structures using alternating electric fields.

We present a microscopic understanding of the underlying physics that governs the photoluminescence spectrum at low electron densities. By performing near- and far-field measurements we show how the various characteristics of the spectrum (intensity, energy, width) are affected by the background electron density and the potential fluctuations due to the remote ionized donors.

We study the photoluminescence spectrum of a low density (ν

We determine the exciton exchange splitting in a wide GaAs quantum well. Our method is based on applying a magnetic field parallel to the layers and measuring the oscillator strength ratio of the Zeeman split lines in two linear polarizations. We develop a theoretical model to describe the effect of the magnetic field on the exciton spectrum, and use it to determine the exchange splitting in a 22-nm quantum well to be $22\ifmmode\pm\else\textpm\fi{}3 \ensuremath{\mu}\mathrm{eV}.$ These measurements also allow us to make an accurate determination of the value of $q=0.03,$ the Luttinger parameter which appears in the cubic term of the valence band Zeeman Hamiltonian.

We compare the photoluminescence spectra of the negatively and positively charged excitons in GaAs quantum wells. We use a structure which enables us to observe both complexes within the same sample. We find that their binding energy and Zeeman splitting are very similar at zero magnetic field, but evolve very differently at high fields. We discuss the implications of these observations on our understanding of the charge excitons structure in high magnetic fields.

We have studied the conductivity peak in the transition region between the two lowest integer Quantum Hall states using transmission measurements of edge magnetoplasmons. The width of the transition region is found to increase linearly with frequency but remains finite when extrapolated to zero frequency and temperature. Contrary to prevalent theoretical pictures, our data does not show the scaling characteristics of critical phenomena.These results suggest that a different mechanism governs the transition in our experiment., Comment: Minor changes and new references included

The near-field photoluminescence of a gated two-dimensional electron gas is measured. We use the negatively charged exciton, formed by binding of an electron to a photo-excited electron-hole pair, as an indicator for the local presence of charge. Large spatial fluctuations in the luminescence intensity of the negatively charged exciton are observed. These fluctuations are shown to be due to electrons localized in the random potential of the remote ionized donors. We use these fluctuations to image the electrons and donors distribution in the plane., Comment: 10 pages, 5 figures, to be published in PRL

We report the fabrication and testing of an all-GaAs/AlGaAs hybrid readout circuit operating at 77 K designated for use with an GaAs/AlGaAs background-limited quantum-well infrared photodetector focal plane array (QWIP FPA). The circuit is based on a direct injection scheme, using specially designed cryogenic GaAs/AlGaAs MODFET's and a novel n/sup +/-GaAs/AlGaAs/n/sup +/-GaAs semiconductor capacitor, which is able to store more than 15 000 electrons//spl mu/m/sup 2/ in a voltage range of /spl plusmn/0.7 V. The semiconductor capacitor shows little voltage dependence, small frequency dispersion, and no hysteresis. We have eliminated the problem of low-temperature degradation of the MODFET I-V characteristics and achieved very low gate leakage current of about 100 fA in the subthreshold regime. The MODFET electrical properties including input-referred noise voltage and subthreshold transconductance were thoroughly tested. Input-referred noise voltage as low as 0.5 /spl mu/V//spl radic/Hz at 10 Hz was measured for a 2/spl times/30 /spl mu/m/sup 2/ gate MODFET. We discuss further possibilities for monolithic integration of the developed devices.

We implement optical spectroscopy to study charged excitons in modulation-doped GaAs/AlGaAs quantum wells. We report the first observation of the positively charged excition and of the triplet state of the negatively charged excition. Applying a gate voltage at high magnetic fields, we investigate the transition between metallic and insulating states. We find that while the photoluminescence line of the metallic two-dimensional electron gas transforms smoothly into a negatively charged exciton, the Zeeman splitting of this line exhibits an abrupt change at the metal-insulator transition.

We report on shakeup processes in the luminescence spectra of a negatively charged exciton (${\mathit{X}}^{\mathrm{\ensuremath{-}}}$) at moderate magnetic fields. These processes manifest themselves as a series of low-energy satellite peaks emerging from the negatively charged exciton line. We analyze the dependence of the ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ energy on magnetic field. We conclude that at magnetic fields above \ensuremath{\sim}1 T the ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ structure can be viewed as an electron at the lowest Landau level bound to an exciton. \textcopyright{} 1996 The American Physical Society.

We implement optical spectroscopy to study charged excitons (trions) in modulation-doped GaAs/AlGaAs quantum wells. We observe for the first time several new trions: the positively charged exciton, the light-hole negatively charged exciton and the triplet state of the negatively charged exciton.

We report on the observation of Fano interference of excitons and continuum in semiconductor superlattices subjected to a magnetic field applied parallel to the layers. We demonstrate how by varying the strength of the magnetic field we are able to tune the energy of a spatially indirect exciton resonance relative to the continuum edge. We measure the transmission spectra and the four-wave-mixing decay patterns and show that both are significantly modified as the exciton crosses the continuum edge.

Time-resolved four-wave mixing is used to study electron scattering in modulation-doped GaAs quantum wells in high magnetic fields up to 8 T. A strong increase in dephasing times in strong magnetic fields is observed and attributed to a decrease in the scattering rates relative to zero field. An early time signal in high magnetic field is found, which is due to a polarization interaction process. A strong enhancement of the signal at the absorption edge is measured which is attributed to the Fermi edge singularity

We report on optical measurements of a two-dimensional electron gas near the metal-insulator transition. We observe the appearance of excitons and negatively charged excitons, ${X}^{\ensuremath{-}}$, at the onset of the transition. The fact that these excitons appear at a relatively large average electron density shows that transition is induced by localization of single electrons in the electrostatic potential fluctuations of the remote ionized donors.

We present a self-assembly method to construct CdSe/ZnS quantum dot–gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton–plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.

Resistively detected nuclear magnetic resonance is used to measure the Knight shift of the As nuclei and determine the electron spin polarization of the fractional quantum Hall states of the second Landau level. We show that the 5/2 state is fully polarized within experimental error, thus confirming a fundamental assumption of the Moore-Read theory. We measure the electron heating under radio frequency excitation, and show that we are able to detect NMR at electron temperatures down to 30 mK., Comment: 5 pages, 4 figures

Exciton Liquid Excitons, bound states of electrons and holes (states “vacated” by electrons), can be found in semiconductors and have long been predicted to form correlated phases at sufficiently large densities and low temperatures. Stern et al. (p. 55 ) studied the behavior of spatially indirect excitons, which consist of electrons and holes residing in spatially separated, but coupled, quantum wells. The excitons were created through a combination of photoexcitation and electric gating. At high enough laser power and low enough temperatures, a new phase with a distinct photoluminescence signature appeared with behavior consistent with that of a classical liquid of excitons.

Electron decay from symmetric coupled quantum wells through a potential barrier to a continuum is studied using time resolved optical spectroscopy. We find that at the limit of a thin barrier the decay rate decreases with decreasing barrier thickness and the energy splitting between the electron levels disappears. We show that this behavior is general for systems where resonant tunneling is coupled to a relaxation channel.

We report on experimental investigation of the nonlinear optical response of quantum wells containing a two-dimensional electron gas. As the magnetic field is increased, and the second Landau level is emptied of electrons, we find a sharp decrease of the four wave mixing signal at that level.

Quantum-beat spectroscopy is used to determine the exciton Zeeman splitting and Land\'e factor in narrow (30 \AA{}) and wide (120 \AA{}) GaAs quantum wells. We find that while the heavy-hole splitting decreases with well width, the light hole exhibits an opposite trend. Measurements of higher magneto-excitons of the wide well show significant increase in the Zeeman splitting with respect to the lowest heavy-hole excition.

Using time-resolved optical spectroscopy we study the decay of electrons from symmetric coupled quantum wells to a continuum through a potential barrier. We find a nonmonotonic dependence of the decay rate on the thickness of the barrier. At the limit of a thin barrier the decay rate decreases with decreasing barrier thickness and the energy splitting between the electron levels disappears. We show that this behavior is general for systems where resonant tunneling is coupled to a relaxation channel.

We demonstrate the feasibility of monolithic integration of a quantum-well infrared detector and a read-out circuit on the same GaAs/AlGaAs crystal. Charge storage capability of 2×107 electrons in a 50×50 μm2 pixel is obtained. The operation of a 5×5 test array is reported, performing all the basic functions of a practical focal plane array.

The operation of Mach–Zehnder and Michelson phase-conjugate interferometers is demonstrated. Phase conjugation is obtained by degenerate four-wave mixing in thin films of eosin. High-visibility fringes are observed both in cw and pulsed modes of operation.

We study surface-enhanced Raman scattering (SERS) of individual organic molecules embedded in dimers of two metal nanoparticles. The good control of the dimer preparation process, based on the usage of bifunctional molecules, enables us to study quantitatively the effect of the nanoparticle size on the SERS intensity and spectrum at the single molecule level. We find that as the nanoparticle size increases the total Raman intensity increases and the lower energy Raman modes become dominant. We perform an electromagnetic calculation of the Raman enhancement and show that this behavior can be understood in terms of the overlap between the plasmonic modes of the dimer structure and the Raman spectrum. As the nanoparticle size increases, the plasmonic dipolar mode shifts to longer wavelength and thereby its overlap with the Raman spectrum changes. This suggests that the dimer structure can provide an external control of the emission properties of a single molecule. Indeed, clear and systematic differences are observed between Raman spectra of individual molecules adsorbed on small versus large particles.

Optical absorption measurements are used to probe the spin polarization in the integer and fractional quantum Hall effect regimes. The system is fully spin polarized only at filling factor $\nu=1$ and at very low temperatures($\sim40$ mK). A small change in filling factor ($\delta\nu\approx\pm0.01$) leads to a significant depolarization. This suggests that the itinerant quantum Hall ferromagnet at $\nu=1$ is surprisingly fragile against increasing temperature, or against small changes in filling factor., Comment: 4 pages, 2 figures

We have used differential absorption spectroscopy to measure charge distributions in the quantum wells of several double-barrier diodes. We find that the ratio of the stored charge to the current is not equal to the coherent-state lifetime and is basically insensitive to the amount of scattering. We experimentally demonstrate that phase-breaking collisions take place in our structures by observing the energy distribution of the stored charge in both single- and double-resonance structures.

In this Letter, we study the diffusion properties of photoexcited carriers in coupled quantum wells around the Mott transition. We find that the diffusion of unbound electrons and holes is ambipolar and is characterized by a large diffusion coefficient, similar to that found in $p\mathrm{\text{\ensuremath{-}}}i\mathrm{\text{\ensuremath{-}}}n$ junctions. Correlation effects in the excitonic phase are found to significantly suppress the carriers' diffusion. We show that this difference in diffusion properties gives rise to the appearance of a photoluminescence ring pattern around the excitation spot at the Mott transition.

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.

Potential roughness has been reported to severely impair experiments in magnetic microtraps. We show that these obstacles can be overcome as we measure disorder potentials that are reduced by two orders of magnitude near lithographically patterned high-quality gold layers on semiconductor atom chip substrates. The spectrum of the remaining field variations exhibits a favorable scaling. A detailed analysis of the magnetic field roughness of a 100-mu m-wide wire shows that these potentials stem from minute variations of the current flow caused by local properties of the wire rather than merely from rough edges. A technique for further reduction of potential roughness by several orders of magnitude based on time-orbiting magnetic fields is outlined.

Shake-up processes in the photoluminescence spectra of a two-dimensional electron gas in a GaAs/AlGaAs quantum well at high magnetic fields are studied at a range of filling factors. We find that when the electrons occupy only the lowest Landau level these processes are strongly suppressed. A peculiar dependence of a giant `zeroth' shake-up line on temperature and filling factor is reported.