We report extremely large field enhancement near the anode of an electrically biased metal/semi-insulator/metal structure. The large anode field results from a trap-enhanced space-charge region and is large enough to cause injection of holes at the anode. Our numerical simulations confirm this interpretation and show that for typical semi-insulating GaAs, large trap-enhanced fields (TEF) are to be expected. The TEF effect, contrary to that observed in doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.

We demonstrate near-transform-limited pulse generation through spectral compression arising from nonlinear propagation of negatively chirped pulses in optical fiber. The output pulse intensity and phase were quantified by use of second-harmonic generation frequency-resolved optical gating. Spectral compression from 8.4 to 2.4 nm was obtained. Furthermore, the phase of the spectrally compressed pulse was found to be constant over the spectral and temporal envelopes, which is indicative of a transform-limited pulse. Good agreement was found between the experimental results and numerical pulse-propagation studies.

We present a unified density-based topology-optimization framework that yields integrated photonic designs optimized for manufacturing constraints including all those of commercial semiconductor foundries. We introduce a new method to impose minimum-area and minimum-enclosed-area constraints, and simultaneously adapt previous techniques for minimum linewidth, linespacing, and curvature, all of which are implemented without any additional re-parameterizations. Furthermore, we show how differentiable morphological transforms can be used to produce devices that are robust to over/under-etching while also satisfying manufacturing constraints. We demonstrate our methodology by designing three broadband silicon-photonics devices for nine different foundry-constraint combinations.

Scanning tunneling microscopy is used to study low temperature grown (LTG) InGaAs with and without Be doping. The Be-doped material is observed to contain significantly fewer AsGa antisite defects than the undoped material, with no evidence found for Be–As complexes. Annealing of the LTG-InGaAs forms precipitates preferentially in the undoped material. The previously observed dependence of the optical response time on Be doping and annealing is attributed to changes in the As antisite concentration and the compensation effect of the Be.

Subpicosecond electron lifetimes in low-temperature-grown GaAs are unambiguously demonstrated via far infrared terahertz spectroscopy. A systematic study of low-temperature-grown GaAs, as-grown and annealed, reveal carrier lifetimes to be directly related to the excess arsenic incorporation and anneal conditions. Contrary to previous observations, electron lifetimes of 600 fs (200 fs) are found in 0.25% (0.5%) excess arsenic GaAs. We attribute the observed differences to the far infrared interaction and the use of dilute photoexcitation densities which eliminate both band-edge resonance and high carrier densities effects. A simple model is developed to determine the relative electron mobility and to interpret the results. Additionally, time resolved differential spectroscopy reveals Drude-like behavior of the free carrier conductivity within 1 ps of excitation.

We report on the transient photoconductivity of hot carriers in undoped bulklike In 0.53 Ga 0.47 As observed via time-resolved terahertz far-infrared spectroscopy. For very dilute photoexcitation densities of Ͻ1ϫ10 15 cm Ϫ3 and an initial excess carrier energy of 630 meV, we find that electrons have an effective intervalley L→⌫ return time of 3.1 ps as measured via the increased electrical conductivity associated with ⌫ electrons. In contrast, a total conductivity risetime of ϳ0.5 ps is observed for electrons with initial excess energy insufficient to cause intervalley scattering. The observed frequency dependent conductivity is analyzed via the Drude theory, allowing the determination of the temporal dynamics of the mobility at dilute excitation densities of ϳ1ϫ10 14 cm Ϫ3 .

We present a photonics topology optimization (TO) package capable of addressing a wide range of practical photonics design problems, incorporating robustness and manufacturing constraints, which can scale to large devices and massive parallelism. We employ a hybrid algorithm that builds on a mature time-domain (FDTD) package Meep to simultaneously solve multiple frequency-domain TO problems over a broad bandwidth. This time/frequency-domain approach is enhanced by new filter-design sources for the gradient calculation and new material-interpolation methods for optimizing dispersive media, as well as by multiple forms of computational parallelism. The package is available as free/open-source software with extensive tutorials and multi-platform support.

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