J. Kuhl

Velocity matching by pulse front tilting for large-area THz-pulse generation

We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8- 0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.

XFROG-A New Method for Amplitude and Phase Characterization of Weak Ultrashort Pulses

We present a new method to characterize the amplitude and phase of weak ultrashort pulses. Our method is based on the spetrally resolved crosscorrelation signal of a weak test pulse with a fully characterized intensive reference pulse and requires no spectral overlap between the signal and the reference. To retrieve the amplitude and phase of the test pulse, we use an iterative Fourier transform algorithm with generalized projections.

Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range

The absorption coefficient and the index of refraction of Mg-doped LiNbO/sub 3/ crystals with different compositions are determined in the 30 - 200 cm-1 frequency range. We find that stoichiometric LiNbO/sub 3/ has smaller absorption and index of refraction than congruent samples.

Scaling up the energy of THz pulses created by optical rectification

The possibility for up-scaling the energy of sub-ps THz pulses generated by tilted pulse front excitation is demonstrated. Using 150-fs-long 500 muJ optical pump pulses at 800 nm up to 240 nJ THz pulse energy has been achieved. For a 1.2 mm2 pump spot area, the energy conversion efficiency of pump energy to THz pulse energy had a maximum of 5 x 10-4 at 300 muJ pump pulse energy. The corresponding photon conversion efficiency amounts to 10 %. For comparison, the maximum attainable THz pulse energy was limited to 3.1 nJ if a line focusing excitation geometry was utilized. This limit was reached at 32 muJ pump energy. For the latter configuration the THz energy dropped for larger pump energies. The tilted pulse front excitation allows further up-scaling of the THz pulse energy by using a larger pump spot size and still stronger pump pulses.

Fabrication of photonic crystals by deep x-ray lithography

We have developed a new microfabrication technique for the construction of three-dimensional photonic crystals. In particular, we used multiple tilted x-ray lithography exposures in order to construct structures with photonic band gaps in the infrared region. First polymethylmethacrylate (PMMA) resist layers with a thickness of 500 μm were irradiated, then the holes in the resist structure were filled with preceramic polymer and subsequent pyrolysis converts the preceramic polymer into a SiCN ceramic. Theoretical results with fitted values of the dielectric constant are in good agreement with the transmission measurements.We have developed a new microfabrication technique for the construction of three-dimensional photonic crystals. In particular, we used multiple tilted x-ray lithography exposures in order to construct structures with photonic band gaps in the infrared region. First polymethylmethacrylate (PMMA) resist layers with a thickness of 500 μm were irradiated, then the holes in the resist structure were filled with preceramic polymer and subsequent pyrolysis converts the preceramic polymer into a SiCN ceramic. Theoretical results with fitted values of the dielectric constant are in good agreement with the transmission measurements.

Efficient generation of subpicosecond terahertz radiation by phase-matched optical rectification using ultrashort laser pulses with tilted pulse fronts

We report on the generation of subpicosecond terahertz (THz) electromagnetic radiation in a nonlinear crystal by ultrashort laser pulses with a tilted pulse front. Free-space THz pulses with an energy of 98 pJ and a repetition rate of 200 kHz have been obtained by optical rectification using 2.3 μJ near-infrared laser pulses in a LiNbO3 crystal cooled down to 77 K. The emitted THz beam has a divergence smaller than 40 mrad, which is close to the diffraction limit. Spectral analysis of the generated THz beam with a Michelson interferometer reveals an approximately 1.5 THz broad asymmetric peak centered at 2 THz and a pulse duration of less than 500 fs.

Subpicosecond carrier lifetimes in radiation‐damaged GaAs

We investigate the dependence of carrier lifetimes in radiation‐damaged, GaAs on proton implantation dose by means of time‐resolved reflectivity and photoconductivity experiments with subpicosecond resolution. The carrier lifetimes decrease with increasing implantation dose at low implantation levels whereas beyond the ‘‘amorphization dose’’ a saturation at 0.5 ps can be observed due to a saturation of the defect density.

Amplitude and phase characterization of weak blue ultrashort pulses by downconversion

We present difference-frequency generation cross-correlation frequency-resolved optical gating, a new method of characterizing the amplitude and phase of weak ultrashort pulses in the blue spectral region. The method uses the spectrally resolved downconversion signal of the blue pulse and a fully characterized reference pulse with a lower center frequency. The amplitude and phase of the blue test pulse are retrieved from the corresponding spectrogram and electric field of the reference pulse with an interactive Fourier-transform algorithm.

Picosecond pulse response characteristics of GaAs metal‐semiconductor‐metal photodetectors

We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal‐semiconductor‐metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self‐consistent two‐dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5 μm, an electric‐field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a subsequent slower tail attributed to the hole current. For weaker electric fields and/or higher light intensities a significant slowing down of the detector speed is predicted because of effective screening of the electric field by the photoexcited carriers. Heterostructure layer‐based devices are shown to provide superior performance compared to diodes manufactured on bulk substrates. Experimental data obtained by photoconductive or electro‐optic sampling on diodes with electrode separation between 0.5 and 1.2 μm agree fairly well with the theoretical predictions.We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal‐semiconductor‐metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self‐consistent two‐dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5 μm, an electric‐field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a s...

Coherent nonlinear pulse propagation on a free-exciton resonance in a semiconductor

Publisher Summary This chapter discusses the coherent nonlinear pulse propagation. It identifies coherent exciton light coupling over a broad intensity range and permits comparison with numerical calculations based on the semiconductor Maxwell–Bloch equations. At low light intensities, polariton propagation beats owing to the interference between excited states on both polariton branches. In an intermediate intensity regime, the temporal polariton beating is suppressed in consequence of exciton–exciton interaction. At the highest light intensities, self-induced transmission and multiple pulse breakup are identified as a signature for carrier density Rabi flopping. Exciton–phonon scattering is shown to gradually eliminate coherent nonlinear propagation effects due to enhanced dephasing of the excitonic polarization. The experiments can be described theoretically using the semiconductor Maxwell–Bloch equations, which accomplish the transition from linear to nonlinear optics by taking into account many-body interactions consisting of mean-field and correlation effects. The chapter, in addition, discusses the intensity to pulse area relation, pulse delays, and effective propagation velocities in dependence on the pulse intensity yield quantitative agreement between the experiment and the semiconductor Maxwell–Bloch theory.