Christof Janke

Ultrafast time-domain spectroscopy based on high-speed asynchronous optical sampling

High-speed asynchronous optical sampling (ASOPS) is a novel technique for ultrafast time-domain spectroscopy (TDS). It employs two mode-locked femtosecond oscillators operating at a fixed repetition frequency difference as sources of pump and probe pulses. We present a system where the 1 GHz pulse repetition frequencies of two Ti:sapphire oscillators are linked at an offset of Deltaf(R)=10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 micros. Mechanical delay scanners common to conventional TDS systems are eliminated, thus systematic errors due to beam pointing instabilities and spot size variations are avoided when long time delays are scanned. Owing to the multikilohertz scan-rate, high-speed ASOPS permits data acquisition speeds impossible with conventional schemes. Within only 1 s of data acquisition time, a signal resolution of 6 x 10(-7) is achieved for optical pump-probe spectroscopy over a time-delay window of 1 ns. When applied to terahertz TDS, the same acquisition time yields high-resolution terahertz spectra with 37 dB signal-to-noise ratio under nitrogen purging of the spectrometer. Spectra with 57 dB are obtained within 2 min. A new approach to perform the offset lock between the two femtosecond oscillators in a master-slave configuration using a frequency shifter at the third harmonic of the pulse repetition frequency is employed. This approach permits an unprecedented time-delay resolution of better than 160 fs. High-speed ASOPS provides the functionality of an all-optical oscilloscope with a bandwidth in excess of 3000 GHz and with 1 GHz frequency resolution.

Femtosecond time-resolved optical pump-probe spectroscopy at kilohertz-scan-rates over nanosecond-time-delays without mechanical delay line

We demonstrate a technique for femtosecond time-resolved optical pump-probe spectroscopy that allows to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with approximately 1 GHz repetition rate are linked at a fixed difference frequency of ΔfR=11kHz. One laser delivers the pump pulses, the other provides the probe pulses. The relative time delay is linearly ramped between zero and the inverse laser repetition frequency at a rate ΔfR, enabling high-speed scanning over a 1 ns time delay. The advantages of this method for all-optical pump-probe experiments become evident in an observation of coherent acoustic phonons in a semiconductor superlattice via transient reflectivity changes. A detection shot-noise limited signal resolution of 7×10−8 is obtained with a total measurement time of 250 s. The time resolution is 230 fs.

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.

Hydration dynamics of oriented DNA films investigated by time-domain terahertz spectroscopy

The B to A conformational transition of highly oriented DNA films due to a hydration change is observed with time-domain terahertz spectroscopy. Wet-spun films of calf thymus and salmon DNA are investigated for different film thicknesses and for different polarizations of the terahertz radiation relative to the DNA orientation. A clear polarization dependence is observed. Asynchronous optical sampling allows recording of terahertz absorption and background spectra in a few 10s, permitting the tracking of the dehydration dynamics on a time scale of minutes. The observation of a phase transition is corroborated by Raman spectroscopy.

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.

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring.

A high-repetition-rate pump-probe experiment is presented, based on the asynchronous sampling approach. The low-α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time-domain sampling of the coherent THz emission as well as for hard X-ray pump-probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.

Coherent acoustic phonons in phonon cavities investigated by asynchronous optical sampling

Using a recently introduced measurement technique, called asynchronous optical sampling (ASOPS), we have investigated the dynamics of coherent acoustic phonons in a semiconductor heterostructure composed of a GaAs film between two GaAs/AlAs superlattices, serving as a cavity for acoustic phonons. Measurements were performed at liquid helium temperatures. The possibility to perform two-color pump-probe spectroscopy allowed us to tune the probe pulse energy to the cavity band gap, while sweeping the pump pulse energy over the superlattice resonance. The large measurement window of 1 ns in combination with a resolution of about 150 fs made a detailed analysis of the observed phonon dynamics possible. We observed a long lived oscillation in the gap of the phonon dispersion at 466 GHz, which we attribute to a cavity mode.

High-resolution THz spectrometer with kHz scan rates

A high-resolution THz-spectrometer without mechanical delay-line is demonstrated. THz-field-transients with 1 ns duration are acquired at 9 kHz scan-rate. Spectra up to 3 THz are obtained with a 40 dB signal-to-noise ratio within 250 s of total measurement time.

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.