Extremely non-perturbative terahertz nonlinearities in GaAs metamaterials

erahertz near fields of gold metamaterials resonant at a frequency of 0.88 THz allow us to enter an extreme limit of nonperturbative ultrafast terahertz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasistatic interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of 1019  cm−3. This process causes bright luminescence at energies up to 0.5 eV above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude-modulation of transmitted few-cycle terahertz transients. Our results pave the way towards highly nonlinear terahertz optics and optoelectronic nanocircuitry with subpicosecond switching times

Nonlinear spin control by terahertz-driven anisotropy fields

Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light. Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations. Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field. Using the prototypical antiferromagnet thulium orthoferrite (TmFeO3), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe3+ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials

Use of SU8 as a stable and biocompatible adhesion layer for gold bioelectrodes.

Gold is the most widely used electrode material for bioelectronic applications due to its high electrical conductivity, good chemical stability and proven biocompatibility. However, it adheres only weakly to widely used substrate materials such as glass and silicon oxide, typically requiring the use of a thin layer of chromium between the substrate and the metal to achieve adequate adhesion. Unfortunately, this approach can reduce biocompatibility relative to pure gold films due to the risk of the underlying layer of chromium becoming exposed. Here we report on an alternative adhesion layer for gold and other metals formed from a thin layer of the negative-tone photoresist SU-8, which we find to be significantly less cytotoxic than chromium, being broadly comparable to bare glass in terms of its biocompatibility. Various treatment protocols for SU-8 were investigated, with a view to attaining high transparency and good mechanical and biochemical stability. Thermal annealing to induce partial cross-linking of the SU-8 film prior to gold deposition, with further annealing after deposition to complete cross-linking, was found to yield the best electrode properties. The optimized glass/SU8-Au electrodes were highly transparent, resilient to delamination, stable in biological culture medium, and exhibited similar biocompatibility to glass

Efficient nonlinear control of spins by ultrashort THz-fields

Ultrashort pulses of intense THz radiation have been shown to represent a powerful and versatile tool for spin control. Here, we employ intense THz pulses in two ways to enter the regime of nonlinear THz-spin interaction. In the first approach, we use the magnetic field of intense THz pulses with amplitudes of up to 0.4 T and frequencies between 0.3 and 2 THz to exert a resonant Zeeman torque onto the spins of nickel oxide (NiO). THz-induced magnetic dynamics is monitored by magneto-optical effects that act on the polarization of ultrashort near-infrared laser pulses. In the second approach, we demonstrate this novel concept of spin excitation in the weak ferromagnet thulium orthoferrite (TmFeO3). In this material, the magnetic anisotropy for the Fe spins is set by electronic orbitals of the Tm ions. THz transients can resonantly excite transitions between these orbital states and thus modify the anisotropy field. We expose a single crystal of TmFeO3 to ultrashort THz pulses of variable peak amplitudes and trace the THz-induced magnetization. The oscillatory magnon traces originate from the quasi-ferromagnetic (q-FM, 0.1 THz) and the quasi-antiferromagnetic (q-AFM, 0.8 THz) mode of TmFeO3. Surprisingly, the relative strength of the q-FM mode is rastically enhanced when the THz amplitude increases