We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, T-C = 4.8 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 less than or similar to 1.1 K, similarly to a Bose-Einstein condensate transition, and becomes constant at 1.1 less than or similar to T <4.8 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 T <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.
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 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.
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.
Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenide-coupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electron-hole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a short-range order, which is manifested in the photoluminescence line shape.
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-75 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.
We present an approach that allows forming a nanometric double dot single electron device. It uses chemical synthesis of metallic nanoparticles to form dimeric structures, e-beam lithography to define electrodes and gates, and electrostatic trapping to place the dimers in between the electrodes. We demonstrate a control of its charge configuration and conductance properties over a wide range of external voltages. This approach can be straightforwardly generalized to other material systems and may allow realizing quantum information devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624899]
We apply polarization resolved photoluminescence spectroscopy to measure the spin polarization of a two dimensional electron gas in perpendicular magnetic field. We find that the splitting between the sigma(+) and sigma(-) polarizations exhibits a sharp drop at v = 5/2 and is equal to the bare Zeeman energy, which resembles the behavior at even filling factors. We show that this behavior is consistent with filling factor v = 5/2 being unpolarized.
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 (similar to 40 mK). A small change in filling factor (delta nu approximate to +/- 0.01) 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.
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-i-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.
We employ a combination of optical and electron-beam lithography to create an atom chip combining submicron wire structures with larger conventional wires on a single substrate. The multilayer fabrication enables crossed wire configurations, greatly enhancing the flexibility in designing potentials for ultracold quantum gases and Bose-Einstein condensates. Large current densities of > 6x10(7) A/cm(2) and high voltages of up to 65 V across 0.3 mu m gaps are supported by even the smallest wire structures. We experimentally demonstrate the flexibility of the next generation atom chip by producing Bose-Einstein condensates in magnetic traps created by a combination of wires involving all different fabrication methods and structure sizes. (c) 2008 American Institute of Physics.
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.
Magnetic trapping potentials for atoms on atom chips are determined by the current flow in the chip wires. By modifying the shape of the conductor we can realize specialized current flow patterns and therefore microdesign the trapping potentials. We have demonstrated this by nano-machining an atom chip using the focused ion beam technique. We built a trap, a barrier, and using a Bose-Einstein Condensate as a probe we showed that by polishing the conductor edge the potential roughness on the selected wire can be reduced. Furthermore, we give different other designs and discuss the creation of a one-dimensional magnetic lattice on an atom chip.
We study the absorption spectrum of a two-dimensional electron gas (2DEG) in a magnetic field. We find that at low temperatures, when the 2DEG is spin polarized, the absorption spectra, which correspond to the creation of spin up or spin down electrons, differ in magnitude, linewidth, and filling factor dependence. We show that these differences can be explained as resulting from the creation of a Mahan exciton in one case, and of a power law Fermi-edge singularity in the other.
We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0 <nu
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 mu 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.
We experimentally demonstrate that one-dimensional Bose-Einstein condensates brought close to microfabricated wires on an atom chip are a very sensitive sensor for magnetic and electric fields reaching a sensitivity to potential variations of similar to 10(-14) eV at 3 mu m spatial resolution. We measure a two-dimensional magnetic field map 10 mu m above a 100-mu m-wide wire and show how the transverse current-density component inside the wire can be reconstructed. The relation between the field sensitivity and the spatial resolution is discussed and further improvements utilizing Feshbach-resonances are outlined.
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 v. Three regimes are well distinguished: v <I where the spectrum consists of neutral and charged excitons, 1 <v <2 where the neutral exciton disappears, and v > 2 where the electron-hole correlations do not play any important role and the spectrum is a simple band to band transition. (c) 2005 Elsevier B.V. All rights reserved.
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 mu m, 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.
We present atom chip traps and guides created by a combination of two current-carrying wires and a bias field pointing perpendicular to the chip surface. These elements can be arranged in any orientation on the chip surface. We study loading schemes for the traps and present a detailed study of the guiding of thermal atomic clouds in an omnidirectional matter waveguide along a 25-mm-long curved path at various atom-surface distances (35-450 mu m). An extension of the scheme for the guiding of Bose-Einstein condensates is outlined. Such a concept enables utilizing the full two-dimensional surface of the chip.
Electrical conduction through molecules depends critically on the delocalization of the molecular electronic orbitals and their connection to the metallic contacts. Thiolated (-SH) conjugated organic molecules are therefore considered good candidates for molecular conductors(1,2): in such molecules, the orbitals are delocalized throughout the molecular backbone, with substantial weight on the sulphur - metal bonds(1-4). However, their relatively small size, typically similar to 1 nm, calls for innovative approaches to realize a functioning single-molecule device(5-11). Here we report an approach for contacting a single molecule, and use it to study the effect of localizing groups within a conjugated molecule on the electrical conduction. Our method is based on synthesizing a dimer structure, consisting of two colloidal gold particles connected by a dithiolated short organic molecule(12,13), and electrostatically trapping it between two metal electrodes. We study the electrical conduction through three short organic molecules: 4,4' - biphenyldithiol (BPD), a fully conjugated molecule; bis( 4-mercaptophenyl)- ether (BPE)(14), in which the conjugation is broken at the centre by an oxygen atom; and 1,4-benzenedimethanethiol (BDMT), in which the conjugation is broken near the contacts by a methylene group. We find that the oxygen in BPE and the methylene groups in BDMT both suppress the electrical conduction relative to that in BPD.
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.
Neutral atoms can be trapped and manipulated with surface mounted microscopic current carrying and charged structures. We present a lithographic fabrication process for such atom chips based on evaporated metal films. The size limit of this process is below 1 mum. At room temperature, thin wires can carry current densities of more than 10(7) A/cm(2) and voltages of more than 500 V. Extensive test measurements for different substrates and metal thicknesses (up to 5 mum) are compared to models for the heating characteristics of the microscopic wires. Among the materials tested, we find that Si is the best suited substrate for atom chips. (C) 2004 American Institute of Physics.
We present an omnidirectional matter waveguide on an atom chip. The guide is based on a combination of two current-carrying wires and a bias field pointing perpendicular to the chip surface. Thermal atoms are guided for more than two complete turns along a 25-mm-long spiral path (with curve radii as short as 200 mum) at various atom-surface distances (35-450 mum). An extension of the scheme for the guiding of Bose-Einstein condensates is outlined. (C) 2004 Optical Society of America.
We measure the absorption spectrum of a two-dimensional electron system (2DES) in a GaAs quantum well in the presence of a perpendicular magnetic field. We focus on the absorption spectrum into the lowest Landau level around nu=1. We find that the spectrum consists of bound electron-hole complexes, trionlike and excitonlike. We show that their oscillator strength is a powerful probe of the 2DES spatial correlations. We find that near nu=1 the 2DES ground state consists of Skyrmions of small size (a few magnetic lengths).
Microscopic flows are almost universally linear, laminar, and stationary because the Reynolds number, Re, is usually very small. That impedes mixing in microfluidic devices, which sometimes limits their performance. Here, we show that truly chaotic flow can be generated in a smooth microchannel of a uniform width at arbitrarily low Re, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating three-dimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable to the microchannel width, the flow behavior is fully compatible with that in a macroscopic channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.
We report on experiments with cold thermal (7)Li atoms confined in combined magnetic and electric potentials. A novel type of three-dimensional trap was formed by modulating a magnetic guide using electrostatic fields. We observed atoms trapped in a string of up to six individual such traps, a controlled transport of an atomic cloud over a distance of 400 mum, and a dynamic splitting of a single trap into a double well potential. Applications for quantum information processing are discussed.
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 (nu <2). Using a gated structure we tune the electron density from the dilute limit to a dense electron gas, and follow the changes in the emission spectrum. We find that the spectrum at the dilute limit consists of two bound triplets, whose behavior is consistent with that of the dark and bright triplets. We show that the spectrum undergoes critical changes at nu = 1/3, from an isolated charged exciton-like spectrum at nu <1/3, to a spectrum that reflects the interactions with the surrounding electrons above this filling factor, This behavior is found to be robust, independent of the electron density and magnetic field. We compare our observations with other recent low temperature PL measurements of a two-dimensional electron gas at high magnetic field and find good agreement and consistency. (C) 2003 Published by Elsevier Ltd.
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 0.2 meV) 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. (C) 2003 Elsevier Ltd. All rights reserved.
We describe an experiment to create a sizable Rb-87 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 ad vantage 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. (C) 2002 Elsevier Science B.V. All rights reserved.
A Bose-Einstein condensate is created in a simple and robust miniature Ioffe-Pritchard trap, the so-called Z trap. This trap results from the mere combination of a Z-shaped current-carrying wire and a homogeneous bias field. The experimental procedure allows condensation of typically 3x10(5) (87)Rb atoms in the \F=2, m(F)=2> state close to any mirroring surface, irrespective of the surface structure. Thus it is ideally suited as a simple coherent source for miniature surface traps or for cold atom physics near surfaces.
We present an experimental and theoretical microscopic view on the optical trion spectrum in the presence of disorder. Although strong spatial fluctuations in near-field spectra point to strongly localized trion states, the far-field spectrum reveals the contribution of weakly localized trion states in addition. It is shown, that the underlying physics involves the optical transition between two disorder eigenstates of different localization length.
Variations in the width of a quantum well (QW) are known to be a source of broadening of the exciton line. Using low temperature near-field optical microscopy, we have exploited the dependence of exciton energy on well width to show that in GaAs QWs, these seemingly random well-width fluctuations actually exhibit well-defined order-strong long-range correlations appearing laterally, in the plane of the QW, as well as vertically, between QWs grown one on top of the other. We show that these fluctuations are correlated with the commonly found mound structure on the surface. This is an intrinsic property of molecular beam epitaxial growth.
We use low temperature near-field optical spectroscopy to image the electron density distribution in the plane of a high mobility GaAs quantum well. We find that the electrons are not randomly distributed in the plane, but rather form narrow stripes (width smaller than 150 nm) of higher electron density. The stripes are oriented along the [1 (1) over bar0] crystal direction, and are arranged in a quasi-periodic structure. We show that elongated structural mounds, which are intrinsic to molecular beam epitaxy, are responsible for the creation of this electron density texture.
Nanosize objects such as metal clusters present an ideal system for the study of quantum phenomena and for the construction of practical quantum devices. Integrating these small objects in a macroscopic circuit is, however, a difficult task. So far, nanoparticles have been contacted and addressed by highly sophisticated techniques not suitable for large-scale integration in macroscopic circuits. We present an optical lithography method that allows for the fabrication of a network of electrodes separated by gaps of controlled nanometer size. The main idea is to control the gap size with subnanometer precision using a structure grown by molecular-beam epitaxy. (C) 2002 American Institute of Physics.
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 (v <1) two-dimensional electron gas at high magnetic fields and low temperatures. We find that the spectrum in the fractional quantum Hall regime can be understood in terms of singlet and triplet charged-excitons. We identify the dark triplet charged-exciton and show that it is visible in the spectrum at T <2 K. We find that its binding energy scales like e(2)/l, where l is the magnetic length. and it crossed, the singlet slightly above 15 T. (C) 2002 Elsevier Science B.V. All rights reserved.
We study the photoluminescence spectrum of a low-density (v <1) two-dimensional electron gas at high magnetic fields and low temperatures. We find that the spectrum in the fractional quantum Hall regime can be understood in terms of singlet and triplet charged excitons. We show that these spectral Lines are sensitive probes for the electron compressibility. We identify the dark triplet charged exciton and show that it is visible at the spectrum at T <2 K. We find that its binding energy scales as 0.1e(2)/l, where l is the magnetic length, and it crosses the singlet slightly above 15 T.
The near- and far-field photoluminescence (PL) spectra of a gated two-dimensional electron gas have been measured in a GaAs quantum well. Scanning near-field measurements reveal the microscopic origin of the different line shapes of the neutral (X) and negatively charged (X-) exciton. We find a new broadening mechanism of the exciton: local density fluctuations give rise to spatial fluctuations of the local X peak energy, and hence to inhomogeneous broadening of the far-field X line. The X linewidth is therefore proportional to the width of the electron density distribution. On the other hand, we find that the X- is homogeneously broadened, and the numerator of its Lorentzian line shape is linearly proportional to the electron density. We present a simple method to determine low electron densities from the PL spectrum.
We study the low-energy tail of the photoluminescence spectrum of a low-density two-dimensional hole gas in a magnetic field in a GaAs quantum well. A rich spectrum of lines is observed, and we show that it can be classified into two groups: the shake-up lines of the positively charged exciton (X+), and the recombination lines of a free hole with an electron bound to a donor (D(0)h). An analysis of these transitions reveals a simple picture of equidistant hole Landau levels, with a cyclotron mass of 0.6m(0).
We show that optical excitation of a wide GaAs quantum well, which is located close to the sample surface, can give rise to the creation of a high-density two-dimensional hole gas in the well. Based on this mechanism, we present a double quantum well structure in which spatially separated electron and hole gases are optically created at close proximity (similar to 20 nm). We demonstrate how the density of each gas can be independently controlled by the intensity of the exciting lasers.
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 2x10(7) electrons in a 50x50 mum(2) pixel is obtained. The operation of a 5x5 test array is reported, performing all the basic functions of a practical focal plane array. (C) 2001 American Institute of Physics.
We study the evolution of the absorption spectrum of a modulation-doped GaAs/AlxGa1-xAs semiconductor quantum well with decreasing the carrier density. We find that at some critical electron density there is a sharp change in the line shape and the transitions energies of the exciton peaks. We show that this critical density marks an abrupt transition from a simple excitonic behavior to a Fermi edge singularity.
A near-field scanning optical microscope for operation within a storage Dewar is described. It was designed for studies of opaque samples and operates in the collection mode. Illumination can be either through the tip or from the side via a separate fiber. Scans can be begun within 2 h after start of cooldown. Its rigid design allows high resolution and long scans with no additional vibration isolation. To illustrate its performance, measurements of photoluminescence in GaAs/AlGaAs heterostructures are presented. The signal and noise levels for the two illumination modes are examined. (C) 2000 Elsevier Science B.V. All rights reserved.
We study the spatial distribution of the photoluminescence of a gated two-dimensional electron gas with sub-wavelength resolution. This is done by scanning a tapered optical fibre tip with an aperture of 250 nm in the near field region of the sample surface, and collecting the photoluminescence. The spectral line of the negatively charged exciton, formed by binding of a photo-excited electron-hole pair to an electron, serves as an indicator for the local presence of charge. The local luminescence intensity of this line is directly proportional to the number of electrons under the tip. We observe large spatial fluctuations in this intensity in the gate voltage range, where the electron conductivity exhibits a sharp drop. The amplitude of these fluctuations increases and the Fourier spectrum extends to lower spatial frequencies as the gate voltage becomes more negative. We show that the fluctuations are due to the statistical distribution of localised electrons in the random potential of the remote ionised donors. We use these fluctuations to image the electron and donor distribution in the plane.
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 +/- 3 mu 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. [S0163-1829(99)51348-9].
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. [S0163-1829(99)51516-6].
We report on time-resolved photoluminescence studies of charged and neutral excitons in a modulation doped GaAs quantum well under resonant excitation and high magnetic field. The radiative lifetime of the charged exciton is rather short, 60 ps at zero held, and is found to increase by a factor of similar to 2 up to 7 T. The short lifetimes suggest that, under magnetic field, the exciton bound in the trion is delocalized. (C) 1998 Elsevier Science B.V. All rights reserved.
We study the dynamics of the charged and neutral excitons in a modulation-doped GaAs quantum well by time-resolved photoluminescence under a resonant excitation. The radiative lifetime of the charged exciton is found to be surprisingly short, 60 ps. This time is temperature independent between 2 and 10 K, and increases by a factor of 2 at 6 T. We discuss our findings in view of present theories of exciton radiative decay. [S0163-1829(98)03143-9].
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 do not show the scaling characteristics of critical phenomena. These results suggest that a different mechanism governs the transition in our experiment. [S0031-9007(98)07743-6].
The near-field photoluminescence of a gated two-dimensional electron gas is measured. Mie use the negatively charged exciton, formed by binding an electron to a photoexcited electron-hole pair, as an indicator fur 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 electron and the donor distribution in the plane. [S0031-9007(98)06953-1].
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. (C) 1998 Elsevier Science B.V. All rights reserved.
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.
Keywords: RECOMBINATION SPECTRA; LUMINESCENCE SPECTRA; EXCITATIONS; PHOTOLUMINESCENCE; EXCITONS; PHONON
We report the fabrication and testing of 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(+)-GaAs/AlGaAs/n(+)-GaAs semiconductor capacitor, which is able to store more than 15 000 electrons/mu m(2) in a voltage range of +/-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 mu V/root Hz at 10 Hz was measured for a 2 x 30 mu m(2) gate MODFET. We discuss further possibilities for monolithic integration of the developed devices.
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 observe shakeup processes in the photoluminescence spectra of a two-dimensional electron gas in a GaAs/AlxGa1-xAs quantum well at high magnetic fields. We find that when the electrons occupy only the lowest Landau level these processes are strongly suppressed. We show that this behavior can be accounted for by first-principles calculations. We use the same considerations to explain the giant intensity of the shakeup line, which appears just below the main luminescence line.
We study the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radiation. At high exciton densities, the emission is due to incoherent luminescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger than their diameter, the temporal shape is independent of density and rises quadratically, in excellent agreement with recent theories for resonant Rayleigh scattering.
We implement optical spectroscopy to study charged excitons (trions) in modulation doped GaAs/AlGaAs quantum wells. We observe several types of trions: the heavy and light-hole negatively charged exciton, the positively charged exciton and the triplet state of the negatively charged exciton. We unambiguously prove the three-body nature of the negatively charged exciton by studying its shakeup processes. We discuss the structure of different trions at zero and high magnetic fields.
We report on shakeup processes in the luminescence spectra of a negatively charged exciton (X(-)) 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 X(-) energy on magnetic field. We conclude that at magnetic fields above similar to 1 T the X(-) structure can be viewed as an electron at the lowest Landau level bound to an exciton.
We report the observation of the positively charged exciton and of the triplet state of the negatively charged exciton in modulation doped GaAs quantum wells. Applying a gate voltage at high magnetic fields we find that the photoluminescence line of the two-dimensional electron gas smoothly transforms into a negatively charged exciton and not into a neutral exciton. The Zeeman splitting of this line exhibits an abrupt change at the metal-insulator transition.
We implement optical spectroscopy to study charged excitons in modulation-doped GaAs/AlGaAs quantum wells. We report the first observation of the positively charged exciton and of the triplet state of the negatively charged exciton. 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 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 bath 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−, 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.
Keywords: EXCITONS; COHERENT; BEATS; HETEROSTRUCTURES; GENERATION; DYNAMICS; LIGHT
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.
We use time resolved four wave mixing to study electron scattering in modulation doped GaAs quantum wells in high magnetic fields up to 8 T. We find a decrease in scattering rates relative to the zero field behavior. The four wave mixing signal shows a strong enhancement at the lowest Landau level which we attribute to the Fermi edge singularity.
Quantum-beat spectroscopy is used to determine the exciton Zeeman splitting and Lande factor in narrow (30 angstrom) and wide (120 angstrom) 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 exciton.
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 present a model for the nonlinear optical response of quantum wells, which includes biexcitons. We show that within this model, the interaction of two laser pulses, mediated by the nonlinear susceptibility, results in oscillations and in coupling between sigma+ and sigma- excitons. This explains the temporal behavior of the differential absorption and four-wave mixing in recent experiments [Phys. Rev. Lett. 68, 349 (1992); 68, 1880 (1992)]. The oscillations have a frequency equal to the biexciton binding energy, and are different from known interference and quantum beat phenomena.
We report the results of four-wave-mixing experiments in GaAs quantum wells in a magnetic field. We find that the time evolution of the excitonic signal is strongly modified when a magnetic field is applied. Quantum beats, rotation of the echo polarization, recovery of the echo for cross-polarized exciting beams, and an increase in the dephasing times are observed.
Keywords: Optics; Physics, Condensed Matter
Using a picosecond optical pump and probe technique we monitor the time it takes for carriers to leave a GaAs/AlxGa1-xAs superlattice at various electric fields. We find that when a Stark ladder is optically observed, carriers escape more slowly out of the superlattice than when the absorption spectrum of a miniband is seen.
We report the experimental observation of exciton spin relaxation in GaAs quantum wells in moderate magnetic fields. We resolve the electron and hole contributions and discuss the large sensitivity of the spin-relaxation time to exciton localization and quantum well width. We use the long duration of spin orientation to demonstrate deep transient oscillations, resulting from biexcitonic effects.
We use differential absorption spectroscopy to study experimentally the temperature dependence of the accumulated charge and transit time in a double-barrier resonant-tunneling diode. We find that both are approximately constant over a broad temperature range (10 K <T <300 K). We show that the electrons experience inelastic-scattering events while they cross the structure, and that they are thermalized with the lattice at that temperature range. We discuss these results in the context of present models of resonant tunneling.
A time-dependent approach is applied to the resonant-tunneling problem. The time dependence of the accumulated charge and the tunneling current are calculated. Special attention is given to the effect of inelastic scattering on the resonant-tunneling process. The coherent and incoherent contributions to the total current are found, and are shown to modify it, sometimes in an unexpected manner.
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.
We analyze the dynamics of resonant tunneling in an asymmetric double-well potential when it is combined with a relaxation process. We show that quantum-interference effects could appear in the relaxation process, resulting in an unexpected dependence of the tunneling time on the relaxation rate. These effects prevail even when the system is prepared in an incoherent initial state. We compare our analysis with recent experimental results and show a very good agreement.
We report the experimental observation of a transient oscillatory behavior of magnetoexciton absorption in GaAs/AlGaAs heterostructures. We interpret these oscillations as absorption quantum beats of a coherent superposition of two excitonic spin states. Measurements of the oscillations period may be used as a new technique for accurately measuring the Lande g factor of excitons in semiconductors.