Bipul Pal

On conversion of luminescence into absorption and the van Roosbroeck-Shockley relation

The problem of conversion of experimentally measured luminescence spectrum into the absorption cross section is revisited. The common practice of using the van Roosbroeck-Shockley (or Kubo-Martin-Schwinger or Kennard-Stepanov) relation in this context is incorrect because luminescence from semiconductors is essentially all due to the spontaneous emission component of the recombination of carriers distributed far-from-equilibrium. A simple, physically consistent, and practical prescription for converting the luminescence spectra into absorption is presented and its relation to the so-called nonequilibrium generalization of the van Roosbroeck-Shockley relationship is discussed.

Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states

Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

Self-assembly and nonlinear optical properties of a synthetic dipeptide

The self-assembly propensities and nonlinear optical properties of synthetic dipeptides are illustrated. The single crystal X-ray diffraction study of dipeptide 1 containing a p-nitrophenylalanine moiety reveals that the peptide adopts a supramolecular antiparallel β-sheet structure using hydrogen bonding, as well as π–π stacking interactions, in the solid state and the peptide exhibits nonlocal thermal nonlinear refraction due to the thermal lensing effect. The heat dissipation in the dipeptide 1 was a slow process with a millisecond to microsecond time scale. However the peptide 2 containing a p-nitrophenylacetic acid moiety adopts a parallel β-sheet structure and has no thermal lensing effect.

Light emission despite doubly-forbidden radiative transitions in AlP/GaP quantum wells: Role of localized states

The GaP/AlP/GaP heterostructure has an indirect gap both in real as well as momentum space, making the first order radiative recombination doubly forbidden. Nevertheless, we have observed relatively efficient emission from these structures. This paper comprehensively studies the origin of this improved light emission through a detailed analysis of the photoluminescence (PL) spectra. Our observations suggest that localized excitons within the acceptor states in GaP close to the heterostructure interface are enough for efficient light emission in these structures, doing away with the need for more complicated structures (superlattices or neighboring confinement structures). This real space localization of holes, close to the interface, apart from increasing the wave function overlap, also relaxes the delta-function momentum selection rule. Independent experimental evidence for this assertion comes from (i) the PL spectrum at high excitation power where transitions from both the localized as well as extended...

One- and Two-Dimensional Spectral Diffusions in InP/InAs/InP Core–Multishell Nanowires

We report on the photoluminescence and time-resolved photoluminescence spectroscopy of wurtzite InP/InAs/InP core–multishell nanowires. Multiple peaks appeared in the photoluminescence spectra owing to the monolayer variation of InAs layers. Each photoluminescence peak has a broad photoluminescence band coming from the inhomogeneous broadening in a core–multishell single nanowire. Inhomogeneous broadening caused time-dependent red shift (spectral diffusion) at a constant energy-loss rate. The long-time-span (~10 ns) time-resolved photoluminescence measurement clarified that spectral diffusion takes place in two stages (initially at a fast rate and later at a slow rate). The fast one is ascribed to spectral diffusion on the side of the nanowire and the slow one is ascribed to spectral diffusion at the corner of the nanowire. This suggests that photoexcited excitons in a core–multishell nanowire move from the side of the nanowire toward the corner of the nanowire.

Bulk saturable absorption in topological insulator thin films

We present nonlinear optical absorption properties of pulsed laser deposited thin films of topological insulator (TI), Bi2Se3 on a quartz substrate, using an open aperture z-scan technique. We observed saturable absorption with a low saturation intensity in as deposited thin films. Past results from the literature are inconclusive in establishing whether the saturable absorption in TI is coming from surface states or the bulk. Specifically designed experiments with magnetically doped TI samples allow us to attribute the saturable absorption characteristic of TI to the bulk states. Detailed experimental procedures and possible explanation of observed results have been discussed.

Anti-Stokes luminescence in the light of second order perturbation theory

Anti-Stokes photoluminescence is measured in high-quality GaAs quantum wells. The primary pathway for interband optical absorption and hence emission under subbandgap photoexcitation is the optical phonon-mediated second-order electric dipole transition. This conclusion is drawn from the remarkable agreement between predictions of second-order perturbation calculation and the measured intensity of anti-Stokes photoluminescence, both as function of the detuning wavelength and temperature. The results are of direct relevance to laser cooling of solids where phonon-assisted upconversion is a necessary condition.