Klatt, Gregor
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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 system’s quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.
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 $hbox{H}_{2}hbox{O}$, D$_2$O, and hydrogen deuterium oxide (HDO) vapor. $hbox{H}_{2}hbox{O}$ and HDO vapor absorption spectra are accurately tabulated in databases. However, D$_2$ O absorption data are rare, because of residual $hbox{H}_{2}hbox{O}$ and HDO often present when measuring pure D$_2$O. Here, we present a high-resolution absorption spectrum of D$_2$O 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.
Rapid and precise read-out of terahertz sensor by high-speed asynchronous optical sampling
High-speed asynchronous optical sampling is used for the rapid and precise determination of transmission resonances of terahertz surface sensors based on asymmetric double split ring resonator arrays. The sensor response represented by a characteristic resonant frequency is determined to 0.66113 THz with an accuracy of +325 MHz within 2 seconds of read-out time.
Terahertz emission from lateral photo-Dember currents
The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In0.53Ga0.47As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.
Time-domain terahertz spectroscopy based on asynchronous optical sampling with femtosecond semiconductor disk laser
A terahertz time-domain spectroscopy (THz-TDS) system using an optically pumped semiconductor disk laser and high-speed asynchronous optical sampling is reported. The combination of these techniques holds considerable promise towards a compact and low-cost implementation of THz-TDS. The spectrometer offers a 333 ps time delay window scanned at 10kHz and delivers more than 1.2 THz spectral coverage at a data acquisition time of one minute. As a proof-ofprinciple experiment, a 0.8 THz resonance of a frequency-selective surface structure was measured.
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 offsetstabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the timedelay scan rate, in the range of a few kilohertz is accomplished using directdigital-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.
Frequency selective surface sensor for terahertz bio-sensing applications
Using high-speed asynchronous optical sampling, read-out of novel terahertz surface sensors directed at bio-sensing applications is presented. The surface sensor is based on periodically arranged metallic THz split ring resonators on a 27-ìm-thin polymer membrane.