J. Bhattacharyya

Optical polarization properties of interband transitions in strained group-III-nitride alloy films on GaN substrates with nonpolar orientation

The authors present results of a perturbation theory study of the combined effects of composition and anisotropic in-plane strain on the optical polarization properties of the three interband transitions in the vicinity of the fundamental energy gap of wurtzite group-III-nitride alloy films, pseudomorphically grown on GaN substrates with nonpolar orientation such as M-plane GaN(11¯00). Valence band mixing induced by the anisotropic in-plane strain is shown to have a dramatic influence on the optical polarization properties. The results indicate that an increased efficiency of light emission in the visible spectral range can be achieved with compressively strained InxGa1−xN active layers. While AlxGa1−xN layers under tensile strain will exhibit a very poor light emission efficiency in the ultraviolet (UV) spectral range, efficient emission in the UV range can instead be achieved with InxAl1−xN films. These results also hold for alloy films on A-plane GaN(112¯0) substrates.

Polarized photoluminescence and absorption in A-plane InN films

The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [112¯0] oriented A-plane wurtzite InN films grown on R-plane (11¯02) sapphire substrates using molecular beam epitaxy. For A-plane films the c axis lies in the film plane. The PL signal collected along [112¯0] with electric vector E⊥c is more than three times larger than for E‖c. Both PL signals peak around 0.67eV at 10K. The absorption edge for E‖c is shifted to higher energy by 20meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7eV at 10K.

Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy

Using scattering-type near-field infrared microscopy in combination with a free-electron laser, intersublevel transitions in buried single InAs quantum dots are investigated. The experiments are performed at room temperature on doped self-assembled quantum dots capped with a 70 nm GaAs layer. Clear near-field contrast of single dots is observed when the photon energy of the incident beam matches intersublevel transition energies, namely the p-d and s-d transition of conduction band electrons confined in the dots. The observed room-temperature line width of 5-8 meV of these resonances in the mid-infrared range is significantly below the inhomogeneously broadened spectral lines of quantum dot ensembles. The experiment highlights the strength of near-field microspectroscopy by demonstrating signals from bound-to-bound transitions of single electrons in a probe volume of the order of (100 nm)(3).

Simultaneous time and wavelength resolved spectroscopy under two-colour near infrared and terahertz excitation

Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.

Observation of Forbidden Exciton Transitions Mediated by Coulomb Interactions in Photoexcited Semiconductor Quantum Wells

W. D. Rice, J. Kono,1,2,∗ S. Zybell, S. Winnerl, J. Bhattacharyya, H. Schneider, M. Helm, B. Ewers, A. Chernikov, M. Koch, S. Chatterjee, G. Khitrova, H. M. Gibbs,†,6 L. Schneebeli, B. Breddermann, M. Kira, and S. W. Koch, Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany Technische Universität Dresden, 01062 Dresden, Germany Department of Physics, Philipps University Marburg, Renthof 5, D-35032 Marburg, Germany College of Optical Science, University of Arizona, Tucson, Arizona 85721-0094, USA ∗corresponding author: kono@rice.edu

Polarized photovoltage spectroscopy study of InAs∕GaAs(001) quantum dot ensembles

We have studied self-assembled InAs quantum dot (QD) ensembles on GaAs(001) substrate using polarized photovoltage spectroscopy. The photovoltage spectrum shows four prominent QD related features whose nature differs for probe light incident along [001] and polarized parallel to [110] and [11¯0] directions. The polarization anisotropy suggests that for the lowest energy transition there is only a change in the oscillator strength with change in polarization, while for the higher energy transitions there is also an apparent shift in the transition energy. By comparison with simulations of the absorption spectrum, we show that the main features of the observed polarization anisotropy can be understood on the basis of a model where an anisotropic two dimensional harmonic oscillator potential represents the lateral confinement of the carriers within the QD in the (001) plane.

Terahertz activated luminescence of trapped carriers in InGaAs/GaAs quantum dots

Optical properties and interdot transfer dynamics of trapped carriers in InGaAs quantum dots (QDs) are investigated. Time resolved photoluminescence (PL) was measured for time-delayed interband and intraband excitations. Terahertz activated luminescence (TAL) from trapped carriers having lifetimes of ∼250 ns at 8 K, was observed. Spectral shift of the TAL with respect to the PL showed the trionic nature of the PL in the n-doped QDs. With increasing terahertz excitation intensity, the TAL increased and reached saturation. The activation energy associated with the trapped carrier decay was quite close to the intersublevel transition energy (∼20 meV) indicating trapping in the QDs.

In-plane interdot carrier transfer in InAs/GaAs quantum dots

Using time resolved photoluminescence (PL) quenching measurements, we investigated inplane carrier transfer in InAs/GaAs self-assembled quantum dots (QDs). THz pulses from a free-electron laser tuned to the intersublevel transition energy were used to excite carriers to higher levels causing quenching in the PL. These carriers could either fall back to the lower energy states and recombine or get transferred to adjacent QDs. The relaxation of the carriers was directly reflected in the recovery of the PL signal. Comparing measurements from two samples, we found that the redistribution of carriers into the neighbouring QDs is the dominant mechanism of carrier relaxation. The data were fitted using a rate equation model to estimate the PL recovery time which we attribute to the interdot carrier transfer time.

Optical polarization properties of M-plane GaN films investigated by transmittance anisotropy spectroscopy

The authors investigate the in-plane optical polarization properties of [11¯00]-oriented (M-plane) GaN films on γ-LiAlO2(100) substrates by transmittance anisotropy spectroscopy (TAS). This technique is sensitive to the difference in the transmittance between light polarized parallel and perpendicular to the c axis of GaN, which for M-plane GaN lies in the film plane. The TAS spectrum exhibits a clear resonance in the vicinity of the fundamental bandgap. Simulations demonstrate that this resonance directly reflects the polarization-dependent shift of the bandgap. The zero crossings of the differential TAS spectrum are shown to be a measure for the polarization-dependent transition energies.

Phase sensitive monitoring of electron bunch form and arrival time in superconducting linear accelerators

In this Letter, we present a simple approach for monitoring electron bunch form and arrival time combining electro-optic sampling and phase and frequency sensitive signal detection. The sensitivity of the technique has the potential to allow online diagnostics to be performed down to bunch charges in the femto coulomb regime. The concept has high impact for the developments of the next generation of 4th generation x-ray light sources working with long pulse trains or continuous wave mode of operation.