Showing papers by "L. N. Pfeiffer published in 1997"

Controlling the sign of quantum interference by tunnelling from quantum wells

Abstract: The sign of the interference (constructive or destructive) between quantum-mechanical paths depends on the phase difference between the paths. In the Fano effect1 two optical paths from the ground state of a system — one direct and one mediated by a resonance — to a state in an energy continuum interfere to produce an asymmetric absorption spectrum that falls to zero near the absorption maximum. Zero absorption occurs as the wavelength is scanned across the resonance, at a photon energy corresponding to a 180 ° phase difference between the paths. Similar interference effects occur when two absorption paths are mediated by two different states, and they provide the basis for lasers that operate without a population inversion2,3,4,5,6,7. Here we report the control, by quantum mechanical tunnelling, of interference in optical absorption. The two intermediate states are resonances that arise from the mixing of the states in two adjacent semiconductors quantum wells, which are broadened by tunnelling into the same energy continuum through an ultra-thin potential-energy barrier. Inverting the direction of tunnelling by reversing the position of the barrier with respect to the two quantum wells changes the interference from destructive to constructive, as predicted theoretically. This effect might provide a way to make semiconductor lasers without population inversion8.

Periodic and Aperiodic Bunching in the Addition Spectra of Quantum Dots

Abstract: We study electron addition spectra of quantum dots in a broad range of electron occupancies starting from the first electron. Spectra for dots containing <200 electrons reveal a surprising feature. Electron additions are not evenly spaced in gate voltage. Rather, they group into bunches. With increasing electron number the bunching evolves from occurring randomly to periodically at about every fifth electron. The periodicity of the bunching and features in electron tunneling rates suggest that the bunching is associated with electron additions into spatially distinct regions within the dots.

Intersubband emission in double-well structures with quantum interference in absorption

Abstract: Intersubband electroluminescence is investigated in structures where two excited states are coupled by tunneling to a common continuum, and exhibit a clear sign of destructive interference in absorption spectroscopy. We show that, in addition to the nonreciprocity between the matrix elements for absorption and emission, the difference between the absorption and emission profiles has its origin also in the electron distribution of the injector.

g factor of composite fermions around ν=2 from angular-dependent activation-energy measurements

Abstract: We have determined the g factor of composite fermions (CF's) around Landau-level filling factor \ensuremath{ u}= through angular-dependent thermal-activation energy measurements at several fractional quantum Hall states. The g factors agree to within 10% with the results of earlier coincidence measurements, supporting the concept of a CF with a spin. Deviations from a simple level-crossing scheme cannot be accounted for solely by a broadening of the CF levels.

Magnetic breakdown and Landau-level spectra of a tunable double-quantum-well Fermi surface

Abstract: By measuring longitudinal resistance, the authors map the Landau level spectra of double quantum wells as a function of both parallel (B{sub {parallel}}) and perpendicular (B{sub {perpendicular}}) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B{sub {parallel}}. This causes the two corresponding ladders of Landau levels formed at finite B{sub {perpendicular}} to exhibit multiple crossings. They also observe a third set of landau levels, independent of B{sub {parallel}}, which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full quantum mechanical calculations show good agreement with the data.

Carrier-lifetime enhancement and mass discontinuity inferred from transport in a parabolic quantum well during subband depopulation

Abstract: In ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ parabolic quantum wells, subbands are depopulated by a magnetic field in the well plane. A small additional perpendicular field induces Shubnikov--de Haas (SdH) oscillations, which we have used to determine the carrier density, effective mass, and lifetime, throughout the two-subband-to-one-subband transition. The masses of carriers in the first and second subbands differ by 50% near the threshold of population of the second subband. The second subband SdH lifetime is enhanced near the threshold, even though the onset of two-subband transport increases the sample resistance.

Design And Performance Of Singular Electric-field Terahertz Emitters And Detectors

Abstract: Free-space terahertz (THz) electromagnetic waves are generated when a short laser pulse excites a semiconductor. We have recently shown that when electric fields in biased semiconductors are enhanced by geometric effects, the THz generation is significantly enhanced.' In this workwe describe new THz emitters and detectors based on this phenomena by using new electrode structures and smaller gaps. The samples are fabricated by conventional lithography on annealed low-temperature GaAs (LTG). The metal patterns are deposited by conventional lithography. Figure 1 shows a schematic diagram of the metal electrodes we used. We vary both the dipole length d and the gap distance g. Two-dimensional electrostatic calculations show that the strongest fringing electric fields are found when the electrodes have triangular shapes (i.e., structure TTO in Fig. 1). These results are verified experimentally as will be discussed later. We measure the THz emission intensity of such structures by using a standard coherent THz detection system with scanning capabilities on the emitter side. A short pulse (780 nm, 130 fs) excites the sample through a 50X objective. The generated THz radiation is then collected with a pair of off-axis paraboloids and focused on a THz dipole antenna fabricated on LTG. This antenna is excited by another 780-nm short laser pulse. The emitter is then scanned two dimensionally. Figure 2a shows the measured THz signal distribution for a fixed bias of 20V for a 60-pm dipole with a constant gap of 5 pm, when exciting a sharp singularity. This is an important point as regular dipoles that were previously used also have areas of singular electric

Coherent terahertz detection: free space electro-optic sampling versus antenna detection

Abstract: The recent surge of interest in ytterbium-based lasers is focused on high-power diode-pumped all-solid-state laser sources. One of the advantages of the Yb3+ ion are the relatively broad absorption and emission bands. They are much smoother and display less structure in glass host materials than those of %:YAG, a promising feature for tunable operation and generation of ultrashort laser pulses. Efficient diode pumping of %:glass has already been demonstrated under continuous-wave conditions.'32 The first attempt to modelock the %:YAG laser by a passive method (A-FPSA) resulted in 540-fs-long pulses at 1.03 I J . ~ . ~ Here we demonstrate cw passive modelocking of an %:glass laser for the first time to our knowledge, based entirely on the Kerr effect in the active medium in a resonator with dispersion compensation. The glass sample used was doped with 6 X 10' cm-3 Yb3+ and exhibits a fluorescence lifetime of 1.2 ms. In our experiments a cw Tksapphire laser acts as the pump source. The output of the pump laser (3 W at 940 nm) was focused into the 4-mm-thick %:glass sample by a 60 mm lens. The active material was positioned at the Brewster angle between two folding mirrors in a standard astigmatically compensated type-Z laser configuration (Fig. 1). The cavity could be operated as a free-running cw laser by use of M, (Fig. 1) yielding >350 mW ofoutput power near 1035 nm at 1.2 W of absorbed pump radiation with a threshold of -150 mW. Essential for the achievement of pure passive modelocking was the minimization of all losses at the maximum pump power available. After optimization in the cw free-running regime modelocked operation was achieved then by precise adjustment of the two prisms, the %:glass sample and the 3-mm-wide vertical slit employed as an aperture near the output coupler Mi. Starting of the laser was accomplished through slow translation of one of the

Singular Electric Field Terahertz Emitters and Detectors.

Abstract: We present a new family of more efficient terahertz (THz) radiation emitters and detectors enhanced by electric field singularities near sharp electrode features. The structures are fabricated on low temperature GaAs and with small dipole gaps (<10μm). The THz emission is maximal when fringing fields are enhanced by laterally offset sharp electrodes. We also find that the asymmetric THz emission near the anode gradually disappears as the dipole gap shrinks. The THz polarization properties of these new emitters will also be discussed.