Zeptojoule detection of terahertz pulses by parametric frequency upconversion.

We combine parametric frequency upconversion with the single-photon counting technology to achieve terahertz (THz) detection sensitivity down to the zeptojoule (zJ) pulse energy level. Our detection scheme employs a near-infrared ultrafast source, a GaP nonlinear crystal, optical filters, and a single-photon avalanche diode. This configuration is able to resolve 1.4 zJ (1.4 × 10-21 J) THz pulse energy, corresponding to 1.5 photons per pulse, when the signal is averaged within only 1 s (or 50,000 pulses). A single THz pulse can also be detected when its energy is above 1185 zJ. These numbers correspond to the noise-equivalent power and THz-to-NIR photon detection efficiency of 1.3 × 10-16 W/Hz1/2 and 5.8 × 10-2%, respectively. To test our scheme, we perform spectroscopy of the water vapor between 1 and 3.7 THz and obtain results that are in agreement with those acquired with a standard electro-optic sampling (EOS) method. Our technique provides a 0.2 THz spectral resolution offering a fast alternative to EOS THz detection for monitoring specific spectral components in spectroscopy, imaging, and communication applications.

Ultrafast photocarrier dynamics in Fe-implanted InGaAs polycrystalline photoconductive materials.

We investigate the ultrafast photoconductivity and charge-carrier transport in thermally annealed Fe-implanted InGaAs/InP films using time-resolved terahertz spectroscopy. The samples were fabricated from crystalline InGaAs films amorphized with Fe ions implantation. The rapid thermal annealing of the InGaAs layer induces solid recrystallization through the formation of polycrystalline grains whose sizes are shown to increase with increasing annealing temperature within the 300-700 °C range. Based on the influence of the laser fluence, the temporal profile of the time-resolved photoconductivity was reproduced using a system of rate equations that describe the photocarrier dynamics in terms of a capture/recombination mechanism. For annealing temperatures below 500 °C, the capture time is found to be less than 1 ps while the recombination time from the charged states did not exceed 5 ps. However, for higher annealing temperatures, the capture and the recombination times show a continuous increase, reaching 7.1 ps and 1 ns respectively, for the film annealed at 700 °C. Frequency-dependent photoconductivity curves are analyzed via a modified Drude-Smith model that considers a diffusive restoring current and the confining particles' sizes. Our results demonstrate that the localization parameter of the photocarrier transport model is correlated to the polycrystalline grain size. We also show that a relatively high effective mobility of about 2570 cm2 V-1 s-1is preserved in all these Fe-implanted InGaAs films.

Structural, optical and terahertz properties of graphene-mesoporous silicon nanocomposites.

We investigate the structural, optical and terahertz properties of graphene-mesoporous silicon nanocomposites using Raman, terahertz time-domain and photoluminescence spectroscopy. The nanocomposites consist of a free-standing mesoporous silicon membrane with its external and pore surfaces coated with few-layer graphene. Results show a stabilization of the porous silicon morphology by the graphene coating. The terahertz refractive index and absorption coefficient were found to increase with graphene deposition temperature. Four bands in the 1.79-2.2 eV range emerge from the PL spectra of the nanocomposites. The broad bands centered at 1.79 eV and 1.96 eV were demonstrated to originate from Si nanocrystallites of different sizes. The narrower bands at 2.11 eV and 2.14 eV could be related to a thin SiC film at the Si/C interface.