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Recent research activities
Investigation of optical emission from Carbon dots (CDs)
this work we investigate the emissive properties like intensity, spectrum, bandwidth of CDs. Recently, we showed enhancement in emission by proper choice of substrate enabling carrier transfer within the heterostructures. Optimization of synthesis and purification are important. We are heading towards fabrication of highly emissive thin films for optoelectronic applications.
Energy levels and mechanisms involved in emission from CDs are topics of debate and interest.  Measurements like photoluminescence excitation and transient luminescence can shed light.
CD emission

Electroabsorption measurements on organic semiconductors 
We study the effect of space charge in organic semiconductor thin film on EA signal, and the implications on carrier trapping.
EA_configuration    Fit of EA

Investigation of optical properties on organic semiconductors

Change of optical emission due to substrate patterning and blending composition in composites are being explored.

Variation of PL intensity as a function of polarization angle on the pentacene grown on the crest (triangle) and trough
(square) of the patterned substrate and on reference unpatterned (circle) substrate. (b) Schematic diagram of the sample indicating
measurement positions for anisotropy mapping (c) Spatial mapping of the polarization anisotropy of the PL emission at the positions
indicated in figure (b).

PL emission efficiency variation with the PCBMconcentration (solid dots) at different excitation intensities. Theoretical fit (dashed lines) using the mean field model. (b) Schematic of the processes considered in the mean field model proposed.

Postdoctoral work

I worked on time-resolved spectroscopy using pulsed laser excitations to study carrier dynamics in semiconductor heterostructures. One important aspect of this work was the use of a free electron laser which emits pulsed, intense and wavelength tunable mid-infrared to far-infrared radiation. This allowed the study of low energy intra-band and excitonic transitions in semiconductor quantum wells and dots. The work done can be classified into two parts.

Investigation of intersublevel carrier dynamics in quantum dots

  • Performed time-resolved photoluminescence quenching measurements using two-color pulsed laser sources and streak camera based detection
  • Demonstrated the presence of long-lived trapped carriers and their influence on photoluminescence transients
  • Studied the influence of interdot carrier transfer and the carrier lifetimes in InAs/GaAs quantum dot ensembles
  • Observed negative spin polarization in circularly polarized photoluminescence quenching measurements

Investigation of magneto-excitons in semiconductor quantum wells

  • Performed time-resolved photoluminescence quenching measurements in quantum wells in presence of external magnetic field
  • Photoluminescence quenching induced intersublevel transitions in the quantum wells showed exciton and cyclotron resonances 
  • Observed THz induced intraexcitonic carrier transfer and manipulated the transfer rates using an external magnetic field

PhD work
My PhD thesis title was “Optical Polarization Anisotropy in Semiconductor Heterostructures”  done under supervision of Prof. Sandip Ghosh at Tata Institute of Fundamental Research, Mumbai.  

 Anisotropy arising from the confinement potential in heterostructures

  • Developed a novel Transmittance Anisotropy Spectroscopy technique
  • Showed that anisotropy in A-plane and M-plane GaN and InN epitaxial films originated from in-plane anisotropy strain
  • Performed strain dependent electronic band structure calculations based on the Bir-Pikus Hamiltonian
  • From polarization properties we confirmed the long debated bandgap of InN to be 0.7 eV
  • Our calculations showed that InGaN films are more suited as lasing medium compared to AlGaN films

 Anisotropy arising from the crystal structure

  • Performed polarized photovoltage spectroscopy on InAs/GaAs quantum dots
  • Developed an empirical model for polarization dependent absorption in quantum dots  
  • Explained the anisotropy in the optical response of quantum dots by difference in the in-plane electron-hole confinement potential along the crystal axes

Anisotropy arising from electric field

  • Performed electro-reflectance measurements on GaAs/AlGaAs vertical cavity surface emitting laser structures
  • Simulated the electro-reflectance spectrum for such multilayer structure from first principles
  • Showed that polarization dependence of the electro-reflectance signal was due to linear electro-optic effect