On optical characterization of CdSe/CdS dot/rod heterostructures.

Colloidal semiconductor nanocrystals (NCs) hold great promise as optical gain media due to their inexpensive fabrication methods, tunable emission wavelengths and high photoluminescence quantum yield. In particular, core-shell nanorods like CdSe/CdS have attracted a large research interest due to improved optical performance over traditional colloidal quantum dots. This is due to spatial separation of electron and hole wavefunctions in these kind of quasi-Type II nanocrystals. This charge separation can lead to a repulsive excition-exciton interaction and suppressed Auger recombination rates, allowing a longer optical gain lifetime. A key experimental technique widely used in measuring the optical gain of semiconductor materials is the variable stripe length (VSL) method. The thin film sample is optically pumped with a stripe shaped beam of variable length, and the intensity of the edge emitted amplified spontaneous emission (ASE) is measured as a function of the stripe length. By fitting an appropriate equation, the optical gain can be obtained. However, there are some crucial issues in applying this method due to the assumptions in the underlying one dimensional optical amplifier model.In our work, the new method of measuring optical gain is proposed and experimentally tested. The same sample is pumped using a focused beam and the lens is moved parallel to the beam propagation to change the spot size on the sample. By measuring the pump fluence at ASE threshold as a function of the spot size, and fitting an appropriate equation, we are able to obtain unambiguous gain values and avoid the problems associated with the traditional VSL method.Bachelor of Science in Physic

Wavelength Tunable Single Nanowire Lasers Based on Surface Plasmon Polariton Enhanced Burstein–Moss Effect

Wavelength tunable semiconductor nanowire (NW) lasers are promising for multifunctional applications ranging from optical communication to spectroscopy analysis. Here, we present a demonstration of utilizing the surface plasmon polariton (SPP) enhanced Burstein–Moss (BM) effect to tune the lasing wavelength of a single semiconductor NW. The photonic lasing mode of the CdS NW (with length ∼10 μm and diameter ∼220 nm) significantly blue shifts from 504 to 483 nm at room temperature when the NW is in close proximity to the Au film. Systematic steady state power dependent photoluminescence (PL) and time-resolved PL studies validate that the BM effect in the hybrid CdS NW devices is greatly enhanced as a consequence of the strong coupling between the SPP and CdS excitons. With decreasing dielectric layer thickness <i>h</i> from 100 to 5 nm, the enhancement of the BM effect becomes stronger, leading to a larger blue shift of the lasing wavelength. Measurements of enhanced exciton emission intensities and recombination rates in the presence of Au film further support the strong interaction between SPP and excitons, which is consistent with the simulation results

Cooperative Enhancement of Second-Harmonic Generation from a Single CdS Nanobelt-Hybrid Plasmonic Structure

Semiconductor nanostructures (<i>e</i>.<i>g</i>., nanowires and nanobelts) hold great promise as subwavelength coherent light sources, nonlinear optical frequency converters, and all-optical signal processors for optoelectronic applications. However, at such small scales, optical second-harmonic generation (SHG) is generally inefficient. Herein, we report on a straightforward strategy using a thin Au layer to enhance the SHG from a single CdS nanobelt by 3 orders of magnitude. Through detailed experimental and theoretical analysis, we validate that the augmented SHG originates from the mutual intensification of the local fields induced by the plasmonic nanocavity and by the reflections within the CdS Fabry–Pérot resonant cavity in this hybrid semiconductor–metal system. Polarization-dependent SHG measurements can be employed to determine and distinguish the contributions of SH signals from the CdS nanobelt and gold film, respectively. When the thickness of gold film becomes comparable to the skin depth, SHG from the gold film can be clearly observed. Our work demonstrates a facile approach for tuning the nonlinear optical properties of mesoscopic, nanostructured, and layered semiconductor materials

Elucidating the localized plasmonic enhancement effects from a single Ag nanowire in organic solar cells

The origins of performance enhancement in hybrid plasmonic organic photovoltaic devices are often embroiled in a complex interaction of light scattering, localized surface plasmon resonances, exciton–plasmon energy transfer and even nonplasmonic effects. To clearly deconvolve the plasmonic contributions from a single nanostructure, we herein investigate the influence of a single silver nanowire (NW) on the charge carriers in bulk heterojunction polymer solar cells using spatially resolved optical spectroscopy, and correlate to electrical device characterization. Polarization-dependent photocurrent enhancements with a maximum of ∼36% over the reference are observed when the transverse mode of the plasmonic excitations in the Ag NW is activated. The ensuing higher absorbance and light scattering induced by the electronic motion perpendicular to the NW long axis lead to increased exciton and polaron densities instead of direct surface plasmon-exciton energy transfer. Finite-difference time-domain simulations also validate these findings. Importantly, our study at the single nanostructure level explores the fundamental limits of plasmonic enhancement achievable in organic solar cells with a single plasmonic nanostructure.Accepted versio