ABSTRACT
We demonstrate a dynamic light-induced resonator for terahertz (THz) frequency light created on ultrashort time scales inside a planar waveguide. The resonator is created by patterned femtosecond photoexcitation of a one-dimensional array of photoconductive regions inside a silicon-filled parallel plate waveguide. The metal-dielectric photonic crystal is created on a 2 ps time scale, ten times faster than the 20 ps transit time of the THz light through the array. The resonance reveals itself through narrowband THz transmission enhancement with accompanying phase modulation producing an induced group delay of up to 10.8 ps near resonance.
ABSTRACT
We demonstrate all-optical control of terahertz (THz) wavemode coupling in a silicon-filled parallel plate waveguide. Using an asymmetric photoexcitation of charge carriers on the surface of the silicon slab within the waveguide, the symmetry is broken, and THz light is partially coupled from TEM to higher-order TM modes. The resulting interference between these modes and the residual TEM mode leads to a strong frequency-dependent transmission modulation. This frequency-selective modulation is widely tunable by adjusting the relative modal phases by translating the excitation along the propagation direction. The experimental observations are well described by a numerical and analytic model of modal interference.
ABSTRACT
The small thiol ß-mercaptoethanol (BME) has been used as an anti-blinking reagent for CdSe/ZnS quantum dots (QDs), although its effects on QD photoluminescence are complex. It acts as an antioxidant as well as a hole scavenger on both CdSe and CdTe, which leads to changes in emission intensity and lifetime that vary qualitatively according to BME concentration, time of incubation, and pH of the solution. Because the band edge energies of InP/ZnS are shifted from those of CdTe and CdSe, it may be expected that thiols including BME might be unable to trap holes from these QDs. In this study, we use steady-state and time-resolved emission spectroscopy with physical fitting models combined with blinking analysis to compare the effects of different concentrations of BME on CdSe/ZnS vs. InP/ZnS QDs over time. We also find excellent correspondence between simple physical model parameters and blinking off times, a finding that will be useful for all blinking studies involving semiconductor nanoparticles. BME alters blinking in InP/ZnS QDs with a single ZnS shell, but not those with double thickness shells. The effects are similar to those seen with CdSe/ZnS, despite very different effects of BME on steady-state spectra, and highly pH-dependent.
