Nikesh Valappil

Lasing from InGaP quantum dots in a spin-coated flexible microcavity

We report the realization of a mechanically flexible microcavity laser emitting at 657 nm using spin coating. These optically pumped vertical cavity surface emitting lasers use InGaP colloidal quantum dots as the active medium and alternating polymer layers of different refractive indices as the Bragg mirrors. Results of photoluminescence measurements indicating enhancement in spontaneous emission are presented. We also demonstrate the possibility of peeling the device off the substrate yielding a flexible structure that can conform to any shape and whose emission spectra can be mechanically tuned. This new class of hybrid lasers combines advantages of organic and inorganic materials.

Investigating the distance limit of a metal nanoparticle based spectroscopic ruler

Conventional Förster resonance energy transfer (FRET) processes involving a pair of fluorophore and organic quencher are restricted to an upper distance limit of ~10 nm. The application of a metal nanoparticle as a quencher can overcome the distance barrier of the traditional FRET technique. However, no standard distance dependence of this resonance energy transfer (RET) process has been firmly established. We have investigated the nonradiative energy transfer process between an organic donor (fluorescein) and gold nanoparticle quencher connected by double stranded (ds) DNA. The quenching efficiency of the gold nanoparticle as a function of distance between the donor and acceptor was determined by time-resolved lifetime analyses of the donor. Our results showed a 1/d4 distance dependence for the RET process for longer distances (>10 nm) and 1/d6 distance dependence for shorter distances (<10 nm). Our results clearly indicate the applicability of metal nanoparticle based quenchers for studying systems that exceed the 10 nm FRET barrier.

Luminescence properties of a Fibonacci photonic quasicrystal

An active one-dimensional Fibonacci photonic quasi-crystal is realized via spin coating. Luminescence properties of an organic dye embedded in the quasi-crystal are studied experimentally and compared to theoretical simulations. The luminescence occurs via the pseudo-bandedge mode and follows the dispersion properties of the Fibonacci crystal. Time resolved luminescence measurement of the active structure shows faster spontaneous emission rate, indicating the effect of the large photon densities available at the bandedge due to the presence of critically localized states. The experimental results are in good agreement with the theoretical calculations for steady-state luminescence spectra.

Photoluminescence enhancement from colloidal quantum dots in a flexible microcavity

We report enhanced photoluminescence from InGaP/ZnS core/shell quantum dots embedded in a one dimensional flexible microcavity realized via spin coating alternating layers of polymers. Tunability of emission wavelength by bending the microcavity is also demonstrated.

Investigation of Steady-State and Time-dependent Luminescence Properties of Colloidal InGaP Quantum Dots.

Quantum dots play a promising role in the development of novel optical and biosensing devices. In this study, we investigated steady-state and time-dependent luminescence properties of InGaP/ZnS core/shell colloidal quantum dots in a solution phase at room temperature. The steady state experiments exhibited an emission maximum at 650 nm with full width at half maximum of ~ 85 nm, and strong first-excitonic absorption peak at 600 nm. The time-resolved luminescence measurements depicted a bi-exponential decay profile with lifetimes of {tau}1sim 47 ns and {tau} 2sim 142 ns at the emission maximum. Additionally, luminescence quenching and lifetime reduction due to resonance energy transfer between the quantum dot and an absorber are demonstrated. Our results support the plausibility of using these InGaP quantum dots as an effective alternative to highly toxic conventional Cd or Pb based colloidal quantum dots for biological applications.

Luminescence from a Fibonacci Photonic Crystal

We report the realization of an active Fibonacci photonic quasi-crystal via spin coating. Alternation of the luminescence properties of rhodhamine dye embedded in the quasi-crystal is presented and compared to theoretical simulations.

Photonic Devices Using Colloidal Quantum Dot Composites

We report We report the development of waveguides and microresonators using colloidal CdSe quantum dot composites. Results of steady state and time resolved photoluminescence performed on the dots in various hosts are also presented.

Organic Photonic Crystal Microcavities for a Room-Temperature Single-Photon Source on Demand

A single-photon source based on single CdSe quantum-dot fluorescence in a chiral-photonic-bandgap liquid-crystal host manifests itself in observed fluorescence antibunching. Single quantum-dot fluorescence imaging inside different types of organic photonic bandgap microcavities is also presented.