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 α-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 μm. The pump energy required to modulate a single bit is only 3 nJ over our current 700 μm2 area device and can be easily scaled into the picoJoule regime with smaller cross sectional areas.

Terahertz time-domain spectroscopy has been used to measure the vibrational spectra of polycrystalline purine and adenine over the temperature range 4–290 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–3.0 THz, which are interpreted as originating from intermolecular vibrational modes mediated by hydrogen bonds. We find that as the temperature is reduced, the observed absorption bands resolve into narrower peaks and some shift towards higher frequencies. We explain the temperature dependence of the spectra by the anharmonicity of the vibrational potentials and give an empirical expression to describe the frequency shift.

Terahertz radiation was generated with a biased and asymmetrically excited low-temperature-grown GaAs photoconductive emitter, and characterized with a 20-μm-thick ZnTe crystal using free-space electro-optic sampling. Using a backward collection scheme, we obtained terahertz radiation with frequency components over 30 THz, the highest ever observed for photoconductive emitters. We present spectra over the whole frequency range between 0.3 and 20 THz, demonstrating the use of this source for ultrabroadband THz spectroscopy.

Semiconductor nanowires (NWs) are nanostructures with a number of novel optical and electronic properties that offer great promise for applications in areas including nanoelectronics, thermoelectrics, sensing, and nanophotonics. To realize the full potential of these unique nanosystems, however, a deep understanding of their response to optical excitation on a sub-picosecond time scale is required. Here, we review recent ultrafast optical studies of carrier dynamics in semiconductor NWs. These experiments have been performed on different materials as a function of both intrinsic NW parameters such as diameter and doping as well as experimental parameters including photoexcited carrier density and wavelength. A variety of phenomena, including one-dimensional (1D) exciton dynamics, rapid carrier trapping at surface and bulk defects, and lasing from an electron-hole plasma (EHP) have been observed. These first measurements of ultrafast carrier dynamics are a tantalizing hint of the rich physics yet to be discovered in these quasi-1D systems.Ultrafast optical spectroscopy can track the temporal evolution of carrier populations in semiconductor NWs with femtosecond time resolution.

Broadband terahertz time-domain spectroscopy (THz-TDS) has been used to measure the far-infrared (FIR) vibrational spectra of several commonly used pure explosives, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3-dinitrato-2,2-bis(nitratomethyl)propane (PETN), and two types of plastic explosive, SEMTEX and SX2. A number of distinct absorption peaks, originating from FIR-active vibrational modes of these polycrystalline energetic materials, were observed in the frequency range 0.3-7.5 THz (10-250 cm(-1)). In addition, the temperature-dependent FIR vibrational spectra of PETN were measured between 4 K and 296 K with several well-resolved absorption peaks observed across this temperature range. We find that as the temperature is reduced, the observed absorption peaks resolve into narrower features and shift towards higher frequencies. The temperature dependence of the spectra is explained in terms of the anharmonicity of the vibrational potentials of crystalline compounds, and an empirical fit is given to describe the peak shift with temperature.

We report the coherent generation and detection of ultrabroadband terahertz (THz) radiation using low-temperature-grown GaAs photoconductive antennas as both emitters and receivers. THz radiation with frequency components over 15 THz was obtained, the highest reported for a THz time-domain system based on photoconductive antennas. Such a system has a smooth spectral distribution between 0.3 and 7.5 THz, ideal for spectroscopic applications. In addition, sharp spectral features at 8.0 and 8.8 THz were observed, and explained in terms of optical phonon resonances in the photoconductive antennas.

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.

No abstract