Thomas Dekorsy

Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers.

Investigation of field, carrier, and coherent phonon dynamics in low-temperature grown GaAs

We compare the dynamics of electric field, transport, and coherent phonons in as‐grown and annealed low‐temperature (LT) GaAs by an electro‐optic technique on a subpicosecond time scale. The buildup and decay of space‐charge fields associated with the photo‐Dember effect are investigated. The recombination dynamics of trapped carriers is monitored via the ps decay of the electro‐optic signal. Differences in annealed and as‐grown LT GaAs are related to the different microscopic form of excess arsenic and point defect density. In the coherent phonon signal a large red shift of the LO phonon and an additional local vibration below the LO phonon provides information on structural defects in as‐grown LT GaAs.

Subpicosecond electric field dynamics in low-temperature-grown GaAs observed by reflective electro-optic sampling

Abstract For the first time we investigate the subpicosecond electric field and coherent LO phonon dynamics in as-grown and post-growth annealed low-temperature (LT) GaAs by reflective electro-optic sampling (REOS) with femtosecond time-resolution. This technique allows a direct observation of the screening of built-in surface fields after pulsed optical excitation of free carriers and the built-up of an electric field due to the different mobilities of electrons and holes (photo Dember effect) in this high resistivity material. After trapping of mobile carriers the decay of the signal gives an accurate measure of the carrier lifetime in trapped states. The observed differences in annealed and as-grown LT GaAs are related to the microscopic form of excess arsenic and the density of point defects. The generation of coherent LO phonons allows the determination of the LO phonon frequency and lifetime in as-grown and annealed LT GaAs. Besides a red-shift of the LO phonon in as-grown LT GaAs we observe an additional local-mode below the LO frequency, which gives evidence for structural defects.

High-speed asynchronous optical sampling for high-sensitivity detection of coherent phonons

A new optical pump-probe technique is implemented for the investigation of coherent acoustic phonon dynamics in the GHz to THz frequency range which is based on two asynchronously linked femtosecond lasers. Asynchronous optical sampling (ASOPS) provides the performance of on all-optical oscilloscope and allows us to record optically induced lattice dynamics over nanosecond times with femtosecond resolution at scan rates of 10 kHz without any moving part in the set-up. Within 1 minute of data acquisition time signal-to-noise ratios better than 107 are achieved. We present examples of the high-sensitivity detection of coherent phonons in superlattices and of the coherent acoustic vibration of metallic nanoparticles.

Spatially resolved optical measurements of electric terahertz signals on passive devices

Monolithic mm-wave integrated circuits have experienced strong improvements in operation frequencies in the last years. However, reports of III-V semiconductor transistors to have maximum frequencies as high as 400 GHz often stem from extrapolated measurements made at lower frequencies, due to bandwidth-limitations of the electronic equipment. The extrapolation of measurements at lower frequencies is insufficient for an accurate determination of the characteristics of passive or active elements in this frequency range. Another frequent restriction of conventional measurement techniques is that the signal can only be probed at specially designed interfaces. Optical sampling techniques allow the detection of electric fields with a high temporal and spatial resolution of 150 fs and 10 micrometers , respectively, at any point within or outside the device. In addition to S-parameter measurements at passive devices we demonstrate the spatial field distribution of an ultra-short electric pulse propagating through a band-stop filter with a broad stop-band probed via electrooptic sampling. To demonstrate the potentially high bandwidth of the measurement system the geometry of the stubs has been designed to show significant attenuation around a frequency f0 equals 350 GHz.

Femtosecond Studies of Coherent LO-Phonons in GaAs

Time-resolved detection of amplitude and phase of coherent LO-phonons, which are impulsively excited in surface-space-charge regions by ultrashort laser pulses, allows detailed studies of the generation and the dynamics of coherent phonon modes in the presence of a dense nonequilibrium electron-hole-plasma.

Rapid-Scanning THz Spectrometer Based on High-Speed ASOPS with >3 THz Bandwidth

A rapid-scanning THz-spectrometer with 1GHz spectral resolution based on high-speed asynchronous optical sampling (ASOPS) is presented. Asynchronously linking two 1-GHz femtosecond-oscillators at their third repetition rate harmonic permits an improved system bandwidth of >3THz.

Coupled Bloch-phonon oscillations in GaAs/AlGaAs superlattices: theory and experiment

Abstract We report on the femtosecond dynamics of coherent Bloch oscillations in GaAs/Al x Ga 1− x As superlattices. In a superlattice with a miniband width equal to the optical phonon energy of GaAs the Bloch frequency can be tuned into resonance with the LO phonon by an applied electric field. We observe a strong coupling between Bloch oscillations and phonons leading to the coherent excitation of optical phonons when both are brought into resonance. This phenomenon is analyzed theoretically on the base of a microscopic Hamiltonian for the electron–phonon interaction in the superlattice. We show that coupled modes are described by a nonlinear pendulum coupled linearly to an oscillator representing the phonons. Under resonance condition the amplitudes of both modes increase and sidebands appear. The theoretical results are compared to the experimental observations.

Phonon-pair combination states driven by second-order Raman scattering in KTaO3

Excitation of KTa03 with ultrashort laser pulses drives phonon-pair combination states via impulsive second-order Raman-scattering. A model explains the resulting displacementamplitude covariance-oscillations of the two correlated phonon modes observed in a pump-probe experiment.

Unidirectionally mode-locked femtosecond Ti:sapphire ring laser with l.5-GHz repetition rate

We demonstrate 22 fs pulse generation from a unidirectionally modelocked Ti:sapphire ring laser at a repetition rate of 1.5 GHz. Intracavity dispersion control is accomplished by dispersive mirrors.