We report on the observation of ultrafast impact ionization and carrier generation in high resistivity silicon induced by intense subpicosecond terahertz transients. Local terahertz peak electric fields of several MV cm −1 are obtained by field enhancement in the near field of a resonant metallic antenna array. The carrier multiplication is probed by the frequency shift of the resonance of the antenna array due to the change of the local refractive index of the substrate. Experimental results and simulations show that the carrier density in silicon increases by over seven orders of magnitude in the presence of an intense terahertz field. The enhancement of the resonance shift for illumination from the substrate side in comparison to illumination from the antenna side is consistent with our prediction that the back illumination is highly beneficial for a wide range of nonlinear processes.

A comprehensive study of surface passivation effect on porous fluorescent silicon carbide (SiC) was carried out to elucidate the luminescence properties by temperature dependent photoluminescence (PL) measurement. The porous structures were prepared using an anodic oxidation etching method and passivated by atomic layer deposited (ALD) Al2O3 films. An impressive enhancement of PL intensity was observed in porous SiC with ALD Al2O3, especially at low temperatures. At temperatures below 150 K, two prominent PL emission peaks located at 517 nm and 650 nm were observed. The broad emission peak at 517 nm was attributed to originate from the surface states in the porous structures, which was supported by X-ray photoelectron spectra characterization. The emission peak at 650 nm is due to donor-acceptor-pairs (DAP) recombination via nitrogen donors and boron-related double D-centers in fluorescent SiC substrates. The results of the present work suggest that the ALD Al2O3 films can effectively suppress the non-radiative recombination for the porous structures on fluorescent SiC. In addition, we provide the evidence based on the low-temperature time-resolved PL that the mechanism behind the PL emission in porous structures is mainly related to the transitions via surface states.

We investigate the dielectric properties of the 4H and 6H polytypes of silicon carbide in the 0.1-19 THz range, below the fundamental transverse-optical phonons. Folding of the Brillouin zone due to the specific superlattice structure of the two polytypes leads to activation of acoustic phonon modes. We use a combination of ultrabroadband terahertz time-domain spectroscopy and simulations based on density-functional perturbation theory to observe and characterize these modes, including band splitting due to the dissimilar carbon and silicon sublattices of the structures, and an indirect measurement of the anisotropic sound velocities in the two polytypes.

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Three‐dimensional photonic crystals (3D PhCs) enable light manipulations in all three spatial dimensions, however, real world applications are still faced with challenges in fabrication. Here, a facile fabrication strategy for 3D silicon PhCs with a simple cubic (SC) lattice structure is presented, which exhibits a complete photonic bandgap at near‐infrared wavelengths of around 1100 nm. The fabrication process is composed of standard deep ultra‐violet stepper lithography, followed by a single‐run modified plasma etch process. By applying a direct dry etch release step at the end of the 3D structural etch process, the fabricated 3D PhCs can be released and transferred in the form of a membrane onto other substrates such as glass, polymers, or even substrates with engineered surface. The thickness of the demonstrated membranes is around 2 µm and the size can be up to a few millimeters. A high reflectivity is observed at the stop band frequency, and a planar defect is introduced during the etching process resulting in an optical resonance mode with a small linewidth of around 30 nm. The structure constitutes an optical bandpass filter and can be used as a sensor for organic solvents.

E 1 /E 2 defects are the typical negative-U centers in n-type 6H silicon carbide (SiC). They are the main contributors to non-radiative recombination, which limits the carrier lifetime. In this study, two fluorescent 6H silicon carbide (f-SiC) samples and one bulk substrate were characterized via time-resolved photoluminescence (TRPL) and static photoluminescence (PL) measurements, where all the samples were nitrogen-boron co-doped 6H n-type. The existence of E 1 /E 2 defects, which caused the diminution of the internal quantum efficiency (IQE) and luminescence intensity of each sample, was confirmed by applying a carrier dynamics model based on negative-U centers. The carrier dynamics simulation reveals that the density of the E 1 /E 2 defects in bulk 6H SiC is two orders of magnitude higher than that of the f-SiC sample, causing much lower PL intensity in the bulk substrate compared to the two f-SiC samples. The IQE of the two f-SiC samples was extracted from the corresponding TRPL results, where the contrast between their IQE was further confirmed by the related PL measurement results. The slight difference in IQE between the two f-SiC samples was attributed to slightly different E 1 /E 2 defect concentrations. On the other hand, by implementing a steady-state donor-acceptor-pair (DAP) recombination calculation, it was found that the f-SiC sample with lower IQE had a higher DAP transition probability due to the higher doping level. This prompted further optimizations in the f-SiC crystal growth conditions in order to decrease the E 1 /E 2 defects while maintaining the correct doping parameters.

We investigate single-cycle terahertz (THz) field-induced nonlinear absorption in doped silicon carbide. We find that the nonlinear response is ultrafast, and we observe up to 20% reduction of transmission of single THz pulses at peak field strengths of 280 kV/cm. We model the field and temperature dependence of the nonlinear response by finite-difference time-domain simulation that incorporates the temporally nonlocal nonlinear conductivity of the silicon carbide. Nonlinear two-dimensional THz spectroscopy reveals that the nonlinear absorption has an ultrafast sub-picosecond recovery time, with contributions from both sumfrequency generation and four-wave mixing, in the form of a photon-echo signal. The ultrafast nonlinearity with its equally fast recovery time makes silicon carbide an interesting candidate material for extremely fast nonlinear THz modulators.

Nonlinear spectroscopic investigation in the terahertz (THz) range requires significant field strength of the light fields. It is still a challenge to obtain the required field strengths in free space from tabletop laser systems at sufficiently high repetition rates to enable quantitative nonlinear spectroscopy. It is well known that local enhancement of the THz field can be obtained for instance in narrow apertures in metallic films. Here we show by simulation, analytical modelling and experiment that the achievable field enhancement in a two-dimensional array of slits with micrometer dimensions in a metallic film can be increased by at least 60% compared to the enhancement in an isolated slit. The additional enhancement is obtained by optimized plasmonic coupling between the lattice modes and the resonance of the individual slits. Our results indicate a viable route to sensitive schemes for THz spectroscopy with slit arrays manufactured by standard UV photolithography, with local field strengths in the multi-ten-MV/cm range at kHz repetition rates, and tens of kV/cm at oscillator repetition rates.The established THz technology has proven its relevance for applications within spectroscopy and imaging by providing fundamental information about chemical composition, conductivity or composition of hidden layers 1,2 . With the recent advances of table-top THz systems offering intense ultrashort pulses with field strengths well in the MV/cm regime 3,4 , the number of studies of nonlinear effects in the THz range is steadily growing [5][6][7] , where the main focus until now has been on semiconductor systems with a strong electronic response. Still, insufficiently high field strengths seem to be the limiting factor for nonlinear studies of materials with a weaker nonlinear response, such as molecular crystals, where only a few results on the nonlinear response has been reported 5 . Recently, nanoslits were applied for THz spectroscopy studies of crystalline materials since the electric field as well as the absorption cross-section can be enhanced dramatically inside the slit 8 . Combining such field-enhancing structures with an intense THz source a field enhancement to 20 MV/cm has been achieved 9 . The linear properties of phonon modes of semiconductor quantum dots have been studied with the dots placed in a nanogap between closely spaced metallic antennas with strong, resonant field enhancement of the incoming THz field 10 , and linear optical properties of monolayers of proteins in the field-enhanced region near nano-antennas have been studied in the mid-infrared 11 .Transmission through subwavelength apertures has attracted a lot of attention since Bethe's demonstration of transmission through a subwavelength hole in 1940's 12 . Later, the field enhancement inside metallic apertures such as nano-holes 13 , rectangular apertures 14 and slits 15 has been studied extensively with emphasis on the extraordinary transmission (EOT) of light through such structures. It has been shown that field enhancements of tens of th...