Stephan Winnerl

High-intensity terahertz radiation from a microstructured large-area photoconductor

We present a planar large-area photoconducting emitter for impulsive generation of terahertz (THz) radiation. The device consists of an interdigitated electrode metal-semiconductor-metal (MSM) structure which is masked by a second metallization layer isolated from the MSM electrodes. The second layer blocks optical excitation in every second period of the MSM finger structure. Hence charge carriers are excited only in those periods of the MSM structure which exhibit a unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to THz electric field amplitudes up to 85V∕cm when excited with fs optical pulses from a Ti:sapphire oscillator with an average power of 100mW at a bias voltage of 65V applied to the MSM structure. The proposed device structure has a large potential for large-area high-power THz emitters.

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point.

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.

Long lifetimes of quantum-dot intersublevel transitions in the terahertz range

Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to approximately 2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.

Impulsive terahertz radiation with high electric fields from an amplifier-driven large-area photoconductive antenna

We report on the generation of impulsive terahertz (THz) radiation with 36 kV/cm vacuum electric field (1.5 mW average thermal power) at 250 kHz repetition rate and a high NIR-to-THz conversion efficiency of 2 x 10(-3). This is achieved by photoexciting biased large-area photoconductive emitter with NIR fs pulses of microJ pulse energy. We demonstrate focussing of the THz beam by tailoring the pulse front of the exciting laser beam without any focussing element for the THz beam. A high dynamic range of 10(4) signal-to-noise is obtained with an amplifier based system.

Room-temperature operation of a unipolar nanodiode at terahertz frequencies

We report on the room-temperature electrical rectification at 1.5 THz of a unipolar nanodiode based on symmetry breaking in a nanochannel. The exploitation of its nonlinear diodelike characteristic and intrinsically low parasitic capacitance enables rectification at ultrahigh speed. The zero-voltage threshold and unique planar layout make the nanodiode suitable for building large arrays. This is the highest speed reported in nanorectifiers to date.

Ultrafast graphene-based broadband THz detector

We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 μm to 220 μm. The detector rise time is 50 ps in the wavelength range from 30 μm to 220 μm. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e.g., in two-color pump-probe experiments.

Spectroscopic THz near-field microscope

We demonstrate a scattering-type scanning near-field optical microscope (s-SNOM) with broadband THz illumination. A cantilevered W tip is used in tapping AFM mode. The direct scattering spectrum is obtained and optimized by asynchronous optical sampling (ASOPS), while near-field scattering is observed by using a space-domain delay stage and harmonic demodulation of the detector signal. True near-field interaction is determined from the approach behavior of the tip to Au samples. Scattering spectra of differently doped Si are presented.

Optimum excitation conditions for the generation of high-electric-field terahertz radiation from an oscillator-driven photoconductive device

We report the impulsive generation of terahertz (THz) radiation with a field amplitude of more than 1.5 kV/cm at megahertz repetition rates, using an interdigitated photoconducting device. The approach provides an average THz power of 190 microW, corresponding to an optical-to-THz conversion efficiency of 2.5 x 10(-4). Optimum conditions are achieved when the excitation spot size is of the order of the THz wavelength.

1550 nm ErAs:In(Al)GaAs large area photoconductive emitters

We report on high power terahertz (THz) emission from ErAs-enhanced In0.52Al0.48As-In0.53Ga0.47As superlattices for operation at 1550 nm. ErAs clusters act as efficient recombination centers. The optical power is distributed among a large, microstructured area in order to reduce the local optical intensity. A THz field strength of 0.7 V/cm (1 V/cm peak-to-peak) at 100 mW average optical power has been obtained, with emission up to about 4 THz in air, limited by the detection crystal used in the system.

Absorption saturation in optically excited graphene

We investigate the saturation of the optical absorption in graphene induced by ultrafast optical pulses. Within a microscopic theory, we study the momentum-, angle-, and time-resolved interplay of anisotropic excitation, carrier-carrier, and carrier-phonon scattering, and its influence on the saturation of absorption and transmission. In agreement with performed experiments, we observe a linear regime for the intensity-dependence of the transmission at low pump fluences and a nonlinear saturation in the high excitation regime. Applying 10 fs-pulses, we obtain a saturation fluence of approximately 0.65 mJ/cm2. We demonstrate how the interplay of Pauli-blocking and intensity-dependent relaxation determines the saturation behavior.