Transient transmission of THz metamaterial antennas by impact ionization in a silicon substrate.

The picosecond dynamics of excited charge carriers in the silicon substrate of THz metamaterial antennas was studied at different wavelengths. Time-resolved THz pump-THz probe spectroscopy was performed with light from a tunable free electron laser in the 9.3-16.7 THz frequency range using fluences of 2-12 J/m2. Depending on the excitation wavelength with respect to the resonance center, transient transmission increase, decrease, or a combination of both was observed. The transient transmission changes can be explained by local electric field enhancement, which induces impact ionization in the silicon substrate, increasing the local number of charge carriers by several orders of magnitude, and their subsequent diffusion and recombination. The studied metamaterials can be integrated with common semiconductor devices and can potentially be used in sensing applications and THz energy harvesting.

Strong Anisotropy in Liquid Water upon Librational Excitation Using Terahertz Laser Fields.

Tracking the excitation of water molecules in the homogeneous liquid is challenging due to the ultrafast dissipation of rotational excitation energy through the hydrogen-bonded network. Here we demonstrate strong transient anisotropy of liquid water through librational excitation using single-color pump-probe experiments at 12.3 THz. We deduce a third-order response of χ3 exceeding previously reported values in the optical range by 3 orders of magnitude. Using a theory that replaces the nonlinear response with a material property amenable to molecular dynamics simulation, we show that the rotationally damped motion of water molecules in the librational band is resonantly driven at this frequency, which could explain the enhancement of the anisotropy in the liquid by the external terahertz field. By addition of salt (MgSO4), the hydration water is instead dominated by the local electric field of the ions, resulting in reduction of water molecules that can be dynamically perturbed by THz pulses.

Optical addressing of an individual erbium ion in silicon.

The detection of electron spins associated with single defects in solids is a critical operation for a range of quantum information and measurement applications under development. So far, it has been accomplished for only two defect centres in crystalline solids: phosphorus dopants in silicon, for which electrical read-out based on a single-electron transistor is used, and nitrogen-vacancy centres in diamond, for which optical read-out is used. A spin read-out fidelity of about 90 per cent has been demonstrated with both electrical read-out and optical read-out; however, the thermal limitations of the former and the poor photon collection efficiency of the latter make it difficult to achieve the higher fidelities required for quantum information applications. Here we demonstrate a hybrid approach in which optical excitation is used to change the charge state (conditional on its spin state) of an erbium defect centre in a silicon-based single-electron transistor, and this change is then detected electrically. The high spectral resolution of the optical frequency-addressing step overcomes the thermal broadening limitation of the previous electrical read-out scheme, and the charge-sensing step avoids the difficulties of efficient photon collection. This approach could lead to new architectures for quantum information processing devices and could drastically increase the range of defect centres that can be exploited. Furthermore, the efficient electrical detection of the optical excitation of single sites in silicon represents a significant step towards developing interconnects between optical-based quantum computing and silicon technologies.