Projects per year

### Abstract

In this thesis, we investigate nonlinear interactions of an intense terahertz (THz) field with semiconductors, in particular the technologically relevant materials silicon and silicon carbide. We reveal the time-resolved dynamics of the nonlinear processes by pump-probe experiments that involve weak THz and near infrared pulses as probes. Firstly, an intense THz pulse is used to study THz-induced impact ionization (IMI) dynamics in silicon. Local field enhancement by metallic dipole antenna arrays has been used to generate strong electric fields of several MV/cm in the hot spots near the antenna tips. For the first time, this enables investigation of field-induced IMI in silicon under very strong fields and at very low initial carrier concentrations. These regimes have previously been inaccessible in conventional transport measurements, due to avalanche breakdown. Using field enhancement technique we investigated the timeresolved dynamics of the IMI process by optical/THz pump-optical probe experiments. Our experimental results, in combination with Monte Carlo simulations, clarify that carrier multiplication dynamics depends strongly on the initial densities of carriers. In the limit of low initial carrier density (1.5×1010 cm−3), a single electron is multiplied in a cascade of IMI events to generate more than 108 electrons within a few hundred femtoseconds. At high initial densities of carriers, the impact ionization rate reduces to values known from the literature due to Auger recombination, field screening and electron-hole scattering effects. Silicon carbide (SiC) stands out as a promising alternative material platform for high power THz applications due to its high radiation resistance. Linear spectroscopy with broadband THz light reveals very sharp and strong resonant absorption lines due to folded zone lattice vibrations. These folded zone acoustic phonon modes can be seen as Si-C atomic planes moving with respect to each other within the unit cell in a pattern characteristic to each polytype of SiC. Their specificity to the polytype is an ideal tag to identify polytypes uniquely. Finally it is demonstrated for the first time that SiC can be tailored to have extremely fast THz-induced nonlinear behavior in moderate THz electric fields by addition of appropriate dopants. A 4H-SiC sample with high concentrations of nitrogen and boron dopants shows a nonlinear THz transmission attributed to THz-induced dopant state ionization and scattering of hot electrons to a lower-mass conduction band. THz pump-THz probe experiments show that the nonlinear process has an ultrafast sub-picosecond recovery time. This demonstrates that the nonlinear response of doped SiC is among the fastest nonlinear modulation schemes for THz signals that can be applicable over wide ranges of operating temperatures.

Original language | English |
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Publisher | DTU Fotonik |
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Number of pages | 171 |

Publication status | Published - 2017 |

## Projects

- 1 Finished

## Extreme nonlinear THz optics of metals

Tarekegne, A. T., Jepsen, P. U., Iwaszczuk, K., Willatzen, M., Johnston, M. B. & Kampfrath, T.

Technical University of Denmark

15/03/2014 → 23/08/2017

Project: PhD

## Cite this

Tarekegne, A. T. (2017).

*Nonlinear dynamics of semiconductors in strong THz electric fields*. DTU Fotonik.