Raphael Gebs

Terahertz emission from lateral photo-Dember currents.

The photo-Dember effect is known as source of impulsive THz radiation after excitation with femtosecond optical pulses. The origin of the emission is the ultrafast separation of electron and holes in strong carrier gradients due to different diffusion coefficients. For a simple semiconductor surface the time dependent polarization is oriented perpendicular to the excited surface which complicates efficient out coupling of THz radiation. We investigate a new scheme for generating strong carrier gradients parallel to the surface. In that case the photo-Dember currents are oriented parallel to the surface and the generated THz radiation can be easily out coupled. This concept can be scaled up so that multiple phase coherent photo-Dember currents contribute to the THz emission. These passive THz emitters reach electric field amplitudes comparable to high-efficiency externally biased photoconductive emitters.

Rapid-scanning terahertz precision spectrometer with more than 6 THz spectral coverage

We report a terahertz time-domain spectrometer with more than 6 THz spectral coverage and 1 GHz resolution based on high-speed asynchronous optical sampling. It operates at 2 kHz scan rate without mechanical delay stage. The frequency error of the system at 60 s acquisition time is determined by comparing a measured water vapor absorption spectrum to data reported in the HITRAN database. The mean error of 87 evaluated absorption lines is 142 MHz.

High-speed asynchronous optical sampling with sub-50fs time resolution

We report an ultrafast time-domain spectroscopy system based on high-speed asynchronous optical sampling operating without mechanical scanner. The system uses two 1 GHz femtosecond oscillators that are offset-stabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the time-delay scan rate, in the range of a few kilohertz is accomplished using direct-digital-synthesis electronics for the first time. The time-resolution of the system over the full available 1 ns time-delay window is determined by the laser pulse duration and is 45 fs. This represents a three-fold improvement compared to previous approaches where timing jitter was the limiting factor. Two showcase experiments are presented to verify the high time-resolution and sensitivity of the system.

Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser

We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the systems quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

Spectrally resolved optical frequency comb from a self-referenced 5 GHz femtosecond laser

In this paper, a self-referenced Ti:sapphire femtosecond laser frequency combs (FLFC) with 5 GHz repetition rate and 230 pJ of pulse energy is reported. The individual modes of the 5 GHz FLFC are dispersed and recorded in a 2D image using a high-resolution spectrometer. This experiment is an important milestone within ongoing efforts to investigate applications in spectroscopy and arbitrary waveform generation with well-stabilized mode-locked femtosecond lasers.

1-GHz repetition rate femtosecond OPO with stabilized offset between signal and idler frequency combs

We report an optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN) that is synchronously pumped by a femtosecond Ti:sapphire laser at 1 GHz repetition rate. The signal output has a center wavelength of 1558 nm and its spectral bandwidth amounts to 40 nm. The OPO operates in a regime where the signal- and idler frequency combs exhibit a partial overlap around 1600 nm. In this near-degeneracy region, a beat at the offset between the signal and idler frequency combs is detected. Phase-locking this beat to an external reference stabilizes the spectral envelopes of the signal- and idler output. At the same time, the underlying frequency combs are stabilized relative to each other with an instability of 1.5x10(-17) at 1 s gate time.

High-Resolution Terahertz Spectrometer

Terahertz time-domain spectroscopy (THz-TDS) based on high-speed asynchronous optical sampling (ASOPS) with two offset-locked GHz femtosecond lasers requires no mechanical time-delay scanner. Consequently, measurements with 1-GHz frequency resolution are performed at intrinsically high scan rates in the multikilohertz range. This is at least one order, in most cases several orders of magnitude faster than conventional approaches employing mechanical time-delay scanners. We report a system offering a unique combination of high-frequency resolution (1 GHz) and high scan rate (2 kHz) with a spectral coverage of more than 6 THz. Its capabilities for high-precision spectroscopy are demonstrated by measuring the absorption spectrum of a mixture of H2O, D2O, and hydrogen deuterium oxide (HDO) vapor. H2O and HDO vapor absorption spectra are accurately tabulated in databases. However, D2 O absorption data are rare, because of residual H2O and HDO often present when measuring pure D2O. Here, we present a high-resolution absorption spectrum of D2O vapor numerically extracted from the absorption spectrum of the three-component mixture. In addition, we show that the high spectral resolution of the ASOPS THz-TDS system provides benefits in the analysis of frequency-selective surface sensors, which are promising candidates for biosensing applications in the THz regime.

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities.

Intense terahertz generation based on the photo-Dember effect

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters.

Asynchronous optical sampling with GHz repetition rate femtosecond lasers for high precision terahertz spectroscopy

We report a terahertz time-domain spectrometer with 6 THz spectral coverage and 1 GHz resolution which is based on high-speed asynchronous optical sampling. High-speed asynchronous optical sampling is based on two femtosecond lasers with approximately 1 GHz repetition rate. The two lasers are stabilized in their repetition rate to an off-set frequency of approximately 2 kHz. The time delay between a pump pulse exciting a photoconductive THz emitter and the probe pulses is hence scanned at 2 kHz scan rate without employing a mechanical delay stage. The timing jitter between the pump and the probe pulses is reduced to 40 fs over the full scan range giving a high time-resolution. We demonstrate the capabilities of this system for gas spectroscopy and the characterization of frequency selective surfaces with characteristic frequencies in the THz range.