We present photoconductive emitters for broadband THz emission based on Ge and GeSn. Ge is attractive for this purpose due to its high carrier mobility and because of the absence of infrared-active phonons that suppress the THz emission. Emitters based on Ge, with Au trapping centers for carrier lifetime reduction, excited with 11 fs near-infrared pulses emit gapless THz spectra extending up to 70 THz. For excitation at the telecom wavelength 1550 nm a slight reduction of the bandgap of Ge is desirable. We show that this can be achieved in layers of GeSn with 2 % of Sn, grown by molecular beam epitaxy on Si. THz devices on a silicon platform offer the perspective to develop CMOS compatible THz systems with photonic integration both for the near-infrared radiation and the broadband THz radiation.
Graphene plasmonic elements enhance the light-matter interaction at resonance. Intense optical excitation results in a nonlinear response, based on two main causes: thermal effects and the Kerr effect. Here we present polarization dependent pump-probe experiments on graphene disks. Those revealed that both effects are similar in strength, though the Kerr nonlinearity is much faster, making it a candidate for efficient harmonic generation. Besides excitation with linear polarization, we investigated the impact of circularly polarized pump radiation. The circulating currents cause an effective quasi-static magnetic field perpendicular to the disks and thus Faraday rotation without the need of external magnetic field.
Germanium is a nonpolar semiconductor with missing Reststrahlen band. In spite of other promising properties including low bandgap and small effective mass, its long, µs-scale recombination time has been prohibitive for applications as photoconductive THz emitters. Using Au-implantated Ge, with recombination times reduced to sub-ns values, we have demonstrated a broadband photoconductive THz emitter compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 µm and with pulse repetition rates of 78 MHz. Reaching up to 70 THz bandwidth, this approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.
This talk advertises scattering-type scanning near-field infrared micro-spectroscopy (s-SNIM) in the spectral range of 75 to 1.3 THz [1], as provided by the free-electron laser FELBE, the narrow-band laser-light source at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. We demonstrate the -independent s-SNIM resolution of a few 10 nm by exploring structured Au samples, Graphene-transistors, meta-materials [2], and local ferroelectric phase-transitions down to LHe [3]. s-SNIM secondly was integrated into a THz pump-probe experiment for the inspection of excited states in structured SiGe samples. We developed a novel demodulation technique with high temporal resolution [4] hence achieving an excellent Signal-to-Noise Ratio. Thirdly using the super-radiant TELBE light source [5], HZDR recently extended the wavelength range down to 100 GHz radiation. We adapted our s-SNIM to this TELBE photon-source as well, achieving an equally high spatial resolution as with FELBE. Moreover, the superb 30-fs temporal resolution of TELBE will allow us to study a multitude of physical phenomena with sub-cycle resolution [5,6], such as spin-structures, magnons and phonon polaritons.
[1] F. Kuschewski et al., Appl. Phys. Lett. 108 (2016) 113102.
[2] S.C. Kehr et al., ACS Photonics 3 (2016) 20.
[3] J. Döring et al., Appl. Phys. Lett. 105 (2014) 053109.
[4] F. Kuschewski et al., Sci. Rep. 5 (2015) 12582.
[5] B. Green et al., Sci. Rep. 6 (2016) 22256.
[6] S. Kovalev et al., Struct. Dyn. 4 (2017) 024301.
A high-speed asynchronous optical sampling system (ASOPS) based on Yb:KYW oscillators with 1-GHz repetition rate is reported. Two frequency-offset-stabilized diode-pumped Yb:KYW oscillators are employed as pump and probe source, respectively. The temporal resolution of this system within 1-ns time window is limited to 500 fs and the noise floor around 10-6 (ΔR/R) close to the shot-noise level is obtained within an acquisition time of a few seconds. Coherent acoustic phonons are investigated by measuring multilayer semiconductor structures with multiple quantum wells and aluminum/silicon membranes in this ASOPS system. A wavepacket-like phonon sequence at 360 GHz range is detected in the semiconductor structures and a decaying sequence of acoustic oscillations up to 200 GHz is obtained in the aluminum/silicon membranes. Coherent acoustic phonons generated from semiconductor structures are further manipulated by a double pump scheme through pump time delay control.
We present an analytical expression for the differential transmission of a delta-shaped light field in Landau-quantized graphene. This enables a direct comparison of experimental spectra to theoretical calculations reflecting the carrier dynamics including all relevant scattering channels. In particular, the relation is used to provide evidence for strong Auger scattering in Landau-quantized graphene.
We report on Terahertz (THz) detectors based on III-V high-electron-mobility field-effect transistors (FET). The detection results from a rectification process that is still highly efficient far above frequencies where the transistor provides gain. Several detector layouts have been optimized for specific applications at room temperature: we show a broadband detector layout, where the rectifying FET is coupled to a broadband logarithmic-periodic antenna. Another layout is optimized for mixing of two orthogonal THz beams at 370 GHz or, alternatively, 570 GHz. A third version uses a large array of FETs with very low access resistance allowing for detection of very short high-power THz pulses. We reached a time resolution of 20 ps.
S. Winnerl, D. Stehr, M. Wagner, H. Schneider, M. Helm, W. Seidel, P. Michel, E. Zibik, B. Carpenter, N. Porter, M. Skolnick, L. Wilson, T. Grange, R. Ferreira, G. Bastard, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, M. Sprinkle, C. Berger, W. de Heer
The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral,
temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz
pulses in a wavelength range from 4 μm to 280 μm. Here we review the potential of the laser and focus on two highlight
pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at
energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of
14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with
our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the
second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For
excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude
longer as compared to the relaxation times observed for near infrared excitation.
We present large-area emitters based on GaInAsN which show efficient THz emission for excitation wavelengths up to 1.35 μm. The substrate material consists of a 1000 nm
Ga1-yInyAs1-xNx (y = 0.11 and x = 0.04) layer grown by molecularbeam epitaxy on semi-insulating GaAs. On top there is an additional
GaAs/Al0.3Ga0.7As heterostructure with thicknesses of 5 nm for the GaAs and 60 nm for the AlGaAs layer, respectively. Transmission measurements with a Fourier
transform spectrometer reveal a bandgap corresponding to a wavelength of 1.5 μm. The resistance of a complete device
with an active area of 1 mm2 is 0.3 MΩThis allows operation with high bias fields (30 kV/cm) without being limited by
heating.
We present photoconductive emitters and detectors for radially and azimuthally polarized terahertz beams.
Microstructured electrodes consisting of concentric rings serve as antennas for radially polarized beams. Electrode
patterns consisting of radially arranged segments are employed for the antennas for azimuthally polarized beams. A
second periodic structure is used to prevent destructive interference of THz wavelets of opposite polarization. Beam
profiles are detected for divergent beams in proximity to the emitter as well as for refocused beams. The THz beams
have a donut-like intensity distribution and the beam profiles can be quantitatively described as lowest order Bessel-
Gauss modes. Furthermore we demonstrate that detection antennas of similar patterning are selective for detection of
particular modes.
The two-photon QWIP approach involves three equidistant subbands, two of which are bound in the quantum well, and
the third state is located in the continuum. The intermediate subband induces a resonantly enhanced optical nonlinearity,
which is about six orders of magnitude stronger than in usual semiconductors. Temporal resolution is only limited by the
sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of
the intersubband polarization. Both properties make this device very promising for pulse diagnostics of pulsed midinfrared
lasers. We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL)
facility FELBE at the Forschungszentrum Dresden Rossendorf. Using a rapid-scan autocorrelation scheme at a scan
frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically
expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity
autocorrelation. Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In
addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space
charges generated in the detector.
Stephan Winnerl, Ekkehard Schomburg, S. Brandl, F. Klappenberger, Karl Renk, Alexander van der Meer, J. Hovenier, R. van Es, T. Klaasen, A. Ignatov, Nikolai Ledentsov, Victor Ustinov, Alexey Zhukov, Alexey Kovsh, Petr Kop'ev
We report on a GaAs/AlAs superlattice detector as a novel direct detector and autocorrelator for THz radiation. It is based on a doped wide-miniband GaAs/AlAs superlattice, with submonolayer AlAs barrier layers; the superlattice is operated at room temperature. THz radiation, generated by a free-electron laser and a mode locked p-Ge laser, was coupled into the superlattice via a corner cube antenna system. THz-irradiation of the biased superlattice resulted in a current reduction, which was monitored. The direct detector showed a fast response (20 ps, limited by the electronic circuit) and was robust against intense radiation pulses (peak power 10 kW). The responsivity was 100 times higher than the responsivity of detectors of comparable risetime and comparable robustness. Intense THz radiation caused a complete suppression of the current through the superlattice. This is the basis of the superlattice autocorrelator. The superlattice autocorrelator could resolve picosecond radiation pulses.
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