Abebe Tilahun Tarekegne

Impact ionization in high resistivity silicon induced by an intense terahertz field enhanced by an antenna array

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.

Amplification of resonant field enhancement by plasmonic lattice coupling in metallic slit arrays

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 table-top 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.

Influence of negative-U centers related carrier dynamics on donor-acceptor-pair emission in fluorescent SiC

E1/E2 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 E1/E2 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 E1/E2 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 E1/E2 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 E1/E2 defects while maintaining the correct doping parameters.E1/E2 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 E1/E2 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 E1/E2 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 be...

High impact ionization rate in silicon by sub-picosecond THz electric field pulses (Conference Presentation)

Metallic antenna arrays fabricated on high resistivity silicon are used to localize and enhance the incident THz field resulting in high electric field pulses with peak electric field strength reaching several MV/cm on the silicon surface near the antenna tips. In such high electric field strengths high density of carriers are generated in silicon through impact ionization process. The high density of generated carriers induces a change of refractive index in silicon. By measuring the change of reflectivity of tightly focused 800 nm light, the local density of free carriers near the antenna tips is measured. Using the NIR probing technique, we observed that the density of carriers increases by over 8 orders of magnitude in a time duration of approximately 500 fs with an incident THz pulse of peak electric field strength 700 kV/cm. This shows that a single impact ionization process is happening in a time duration of less than 20 fs. The measurement is repeated by exciting the sample with an optical pump beam at a wavelength of 400 nm. The optical pump sets the initial free carrier density before the THz-induced impact ionization. The measurements show that the carrier generation mechanism depends on the initial free carrier density which confirms that the carrier generation mechanism is impact ionization, rather than the alternative carrier generation mechanism in high electric field, i.e. Zener tunneling. Finally this technique can be extended to investigate carrier dynamics in other semiconductors.

Ultrafast nonlinear response of silicon carbide to intense THz fields

We demonstrate ultrafast nonlinear absorption induced by strong, single-cycle THz fields in bulk, lightly doped 4H silicon carbide. A combination of Zener tunneling and intraband transitions makes the effect as at least as fast as the excitation pulse. The sub-picosecond recovery time makes the observed response the fastest nonlinear modulation scheme for THz signals reported so far.