Prashanth C. Upadhya

Subpicosecond Optical Switching with a Negative Index Metamaterial

We demonstrate a nanoscale, subpicosecond (ps) metamaterial device capable of terabit/second all-optical communication in the near-IR. The 600 fs response, 2 orders of magnitude faster than previously reported, is achieved by accessing a previously unused regime of high-injection level, subpicosecond carrier dynamics in the alpha-Si dielectric layer of the metamaterial. Further, we utilize a previously unrecognized, higher-order, shorter-wavelength negative-index resonance in the fishnet structure, thereby extending device functionality (via structural tuning of device dimensions) over 1.0-2.0 microm. The pump energy required to modulate a single bit is only 3 nJ over our current 700 microm(2) area device and can be easily scaled into the picoJoule regime with smaller cross sectional areas.

Understanding ultrafast carrier dynamics in single quasi-one-dimensional Si nanowires

We use femtosecond optical pump-probe spectroscopy to study ultrafast carrier dynamics in single quasi-one-dimensional silicon nanowires. By isolating individual nanowires, we avoid complications resulting from the broad size and alignment distribution in nanowire ensembles, allowing us to directly probe ultrafast carrier dynamics. Spatially-resolved experiments demonstrate the influence of surface-mediated mechanisms on carrier dynamics in a single NW, while polarization-resolved experiments reveal a clear anisotropy in carrier lifetimes measured parallel and perpendicular to the long axis of the NW, due to density-dependent Auger recombination. These results suggest the possibility of tailoring carrier relaxation in a single nanowire for a desired application.

The influence of defect states on non-equilibrium carrier dynamics in GaN nanowires

Semiconductor nanowires have recently attracted much attention for their unique properties and potential applications in a number of areas, most notably in nanophotonics. However, the presence of defect states in these quasi-one-dimensional nanostructures can significantly detract from nanophotonic device performance. Here, we use ultrafast optical pump-probe spectroscopy to study the influence of defect states on carrier dynamics in GaN nanowires by probing carrier relaxation through the states responsible for yellow luminescence, an undesirable effect that plagues many GaN-based photonic devices. Faster relaxation is seen in nanowires grown at lower temperatures, which also exhibits higher lasing thresholds. We attribute this to rapid trapping of photoexcited carriers into additional impurity sites that are present at lower growth temperatures. In addition, excitation density-dependent measurements reveal a decrease in carrier lifetimes with increasing pump fluence. These results demonstrate the influence of both radiative and non-radiative defect states on carrier dynamics in GaN nanowires and indicate that relaxation rates can be controlled by varying the growth temperature, which should enable researchers to optimize nanowire properties for a given application.

Ultrafast nonlinear optical spectroscopy of a dual-band negative index metamaterial all-optical switching device.

We study the nonlinear optical response of a fishnet structure-metamaterial all-optical switching device that exhibits two near-infrared negative-index resonances. We study and compare the nonlinear optical response at both resonances and identify transient spectral features associated with the negative index resonance. We see a significantly stronger response at the longer wavelength resonance, but identical temporal dynamics at both resonances, providing insight into separately engineering the switching time and switching ratio of such a fishnet structure metamaterial all-optical switch. We also numerically reproduce the nonlinear behavior of our device using the Drude conductivity model and a finite integration technique over wide spectral and pump fluence ranges. Thereby, we show that beyond the linear properties of the device, the magnitude of the pump-probe response is completely described by only two material parameters. These results provide insight into engineering various aspects of the nonlinear response of fishnet structure metamaterials.

Probing Ultrafast Carrier Dynamics in Silicon Nanowires

We present the first ultrafast optical pump-probe spectroscopic measurements, to the best of our knowledge, on silicon nanowires (SiNWs). In this study, we performed femtosecond pump-probe measurements on vapor-liquid-solid-grown SiNWs to investigate the influence of the NW diameter, pump and probe polarizations, and pump fluence on the observed dynamics while tuning the probe wavelength below and above the indirect bandgap in Si. For smaller NW diameters, carriers were found to relax more rapidly into both extended and localized states, indicating that a surface-mediated mechanism governs the observed dynamics. The magnitude of the photoinduced transmission change exhibited strong polarization dependence, showing that optical transitions in these quasi-1D systems are highly polarized along the NW axis. Finally, density-dependent experiments revealed that the relaxation time decreases with increasing photoexcited carrier density for an above bandgap probe; however, no significant density-dependent changes in the relaxation dynamics were observed when probed below the bandgap. In short, our experiments reveal the influence of diameter, polarization, and carrier density on carrier dynamics in SiNWs, shedding light on the phenomena that govern carrier relaxation in these important nanosystems and giving insight on their future use in nanophotonic applications.

Bianisotropic Negative-Index Metamaterial Embedded in a Symmetric Medium

In order to more clearly observe the bianisotropic effects due to fabrication-induced structural asymmetries in negative-index metamaterials based on a fishnet structure, it is necessary to measure the optical properties with symmetric substrate and superstrate bounding layers. This is accomplished in this report using an index-matching fluid and identical substrate and superstrate glass materials.

The influence of radial heterostructuring on carrier dynamics in gallium nitride nanowires

Ultrafast optical pump-probe spectroscopy is used to study the influence of aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) shells on carrier dynamics in radially heterostructured GaN nanowires (NWs). Our experiments reveal longer carrier relaxation times and lower lasing thresholds in NWs passivated with a higher bandgap shell, which can be attributed to a reduction in surface defect state density. We observe that carrier relaxation times vary with the nitride shell material and thickness, likely due to strain from the lattice mismatch between the core and shell materials. Our results demonstrate that radial heterostructuring is a promising route to controlling carrier dynamics in III-nitride NWs.

Space-and-time-resolved spectroscopy of single GaN nanowires

Spatially-resolved ultrafast transient absorption measurements on a single GaN nanowire give insight into carrier relaxation dynamics as a function of the laser polarization and position on the nanowire on a femtosecond timescale.

Terahertz magneto-optical spectroscopy of a two-dimensional hole gas

Terahertz magneto-optical spectroscopy on a two-dimensional hole gas reveal a nonlinear dependence on the applied magnetic field. This is due to its complex non-parabolic valence band structure, as verified by multiband Landau level theoretical calculations.

III-nitride nanowires: novel materials for solid-state lighting

Although planar heterostructures dominate current solid-state lighting architectures (SSL), 1D nanowires have distinct and advantageous properties that may eventually enable higher efficiency, longer wavelength, and cheaper devices. However, in order to fully realize the potential of nanowire-based SSL, several challenges exist in the areas of controlled nanowire synthesis, nanowire device integration, and understanding and controlling the nanowire electrical, optical, and thermal properties. Here recent results are reported regarding the aligned growth of GaN and III-nitride core-shell nanowires, along with extensive results providing insights into the nanowire properties obtained using cutting-edge structural, electrical, thermal, and optical nanocharacterization techniques. A new top-down fabrication method for fabricating periodic arrays of GaN nanorods and subsequent nanorod LED fabrication is also presented.