Jayeeta Bhattacharyya

Bio: Jayeeta Bhattacharyya is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topic(s): Photoluminescence & Exciton. The author has an hindex of 9, co-authored 34 publication(s) receiving 283 citation(s). Previous affiliations of Jayeeta Bhattacharyya include Helmholtz-Zentrum Dresden-Rossendorf & Tata Institute of Fundamental Research.

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

Abstract: 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.

Polarized photoluminescence and absorption in A-plane InN films

Abstract: 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.

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

Abstract: 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.

Observation of forbidden exciton transitions mediated by Coulomb interactions in photoexcited semiconductor quantum wells.

Abstract: We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the $1s$ exciton emission, which we attribute to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced enhancement of the $2s$ exciton emission is observed, despite the $1s$-to-$2s$ transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the $2s$ and $2p$ states, yielding an effective terahertz transition between the $1s$ and $2s$ populations.

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

Abstract: 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.

When group-III nitrides go infrared: New properties and perspectives

Abstract: Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN

Abstract: We have derived consistent sets of band parameters band gaps, crystal field splittings, band-gap deformation potentials, effective masses, and Luttinger and EP parameters for AlN, GaN, and InN in the zinc-blende and wurtzite phases employing many-body perturbation theory in the G0W0 approximation. The G0W0 method has been combined with density-functional theory DFT calculations in the exact-exchange optimized effective potential approach to overcome the limitations of local-density or gradient-corrected DFT functionals. The band structures in the vicinity of the point have been used to directly parametrize a 44 k·p Hamiltonian to capture nonparabolicities in the conduction bands and the more complex valence-band structure of the wurtzite phases. We demonstrate that the band parameters derived in this fashion are in very good agreement with the available experimental data and provide reliable predictions for all parameters, which have not been determined experimentally so far.

Ultrafast multi-terahertz nano-spectroscopy with sub-cycle temporal resolution

Abstract: The authors demonstrate ultrabroadband time-resolved THz spectroscopy on a single InAs nanowire with 10 nm spatial resolution and sub-100 fs time resolution. Phase-locked ultrashort pulses in the rich terahertz spectral range1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8,12, high-temperature superconductivity11 and carrier transport in nanomaterials1,6,13,14,15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4,8,10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16,17,18,19,20,21,22,23,24,25,26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.

Ultrafast multi-terahertz nano-spectroscopywith sub-cycle temporal resolution

Abstract: This thesis has introduced a novel microscope which combines the outstanding properties of ultrafast multi-terahertz spectroscopy and scattering-type scanning near-field optical microscopy to observe the femtosecond dynamics of low-energy elementary excitations, e.g. phonons, plasmons and excitons, at the surface of solid-state systems. Ultrabroadband phase-locked mid-infrared light pulses are focused onto a nanometer-sized metal tip of the atomic force microscope. When approached to a sample surface, the tip acts as a local optical probe, confining the light pulses at its apex. Detection of the scattered radiation reveals information about the optical properties of the sample with a spatial resolution of 10nm, ultimately limited by the tip radius of curvature. Electro-optic sampling, one of the most notable techniques of terahertz spectroscopy, is employed to detect the scattered oscillating electric near field directly in the time domain with a temporal resolution of 10fs. Together with pump-probe experiments this technique enables the observation of the dynamics of low-energy elementary excitations on sub-cycle timescales, faster than a single oscillation cycle of the multi-terahertz probe pulses. The combination of ultrafast multi-terahertz spectroscopy and near-field microscopy culminates in a unique microscope that achieves an unprecedented combined temporal (10fs) and spatial (10nm) resolution in the mid-infrared wavelength region.%%%%In dieser Dissertation wird ein neuartiges Mikroskop beschrieben, welches die herausragenden Eigenschaften der ultraschnellen multi-Terahertz Spektroskopie und der Nahfeldmikroskopie verbindet. Damit ist es zum ersten Mal moglich die Femtosekunden-Dynamik von niederenergetischen Elementaranregungen, wie zum Beispiel Phononen, Plasmonen oder Exzitonen, direkt an der Oberflache von Festkorpern zu untersuchen. Ultrabreitbandige, phasen-stabile, mittelinfrarote Lichtimpulse werden dazu auf die Spitze eines Rasterkraftmikroskops fokussiert. Angenahert an die Oberflache einer Probe, fungiert diese Spitze als Nanometer grose Sonde, die das Licht an ihrem Ende bundelt. Die Detektion des gestreuten Lichts ermoglicht es dann Informationen uber die lokalen optischen Eigenschaften der Probe direkt unterhalb der Spitze zu erhalten. Die raumliche Auflosung betragt hierbei 10 Nanometer und ist grundsatzlich nur durch den Radius der Spitze limitiert. In diesem Experiment wird das gestreute elektrische Nahfeld direkt mit Hilfe eines Elektro-optischen Detektors gemessen. Diese Methode gewahrleistet eine ultimative Zeitauflosung von 10fs und erlaubt in Kombination mit Anrege-Abtast Experimenten eine direkte Beobachtung der Dynamik von niederenergetischen Elementaranregungen auf einer Sub-Zyklen Zeitskala, schneller als ein einzelner Oszillationszyklus des multi-Terahertz Abtastimpulses. Die Kombination von ultraschneller multi-Terahertz Spektroskopie mit der Nahfeldmikroskopie gipfelt in einem einzigartigen Mikroskop mit einer bisher nicht dagewesenen kombinierten zeitlichen (10fs) und raumlichen…