Arne Thoma

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.

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.

Influence of tip-sample interaction in a time-domain terahertz scattering near field scanning microscope

Apertureless near field measurements with a metallic tip are performed in the terahertz frequency range. Lateral scans are recorded for different time delays within a terahertz pulse. The forward scattered terahertz signal strongly depends on the time delay. At larger time delays, the tip-sample interaction leads to additional structures in the scan that do not correspond to a change in topography or dielectric function.

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.

2-Dimensional mapping of frequency response of a single THz split-ring resonator probed by high speed asynchronous optical sampling

Terahertz spectroscopy is of particular interest for the investigation of low-energy elementary excitations as well as for technical purposes [1]. However, the probing of submicron-scale objects is hindered by the long wavelength of THz waves. Especially for spatially resolved spectroscopy and imaging it is highly desirable to overcome the diffraction limitation at THz frequencies. In order to achieve sub-wavelength resolution, we set up a scattering near-field microscope by combining a THz time-domain reflection spectrometer (TDS) with a scanning tunneling microscope (STM) similar to an approach in [2]. A THz pulse generated by excitation of a large-area photoconductive emitter [3] with a femtosecond laser pulse is focused on the sample underneath the STM tip. The THz electric field of the pulse scattered in the forward direction is measured by electro-optic sampling. In order to obtain the full spatially resolved frequency information for every pixel the electric field of the pulse has to be measured versus time delay. For this time consuming task when performed with a mechanical delay line we employ for the first time high-speed asynchronous optical sampling (ASOPS) where the time delay is realized through two asynchronously locked fs lasers at GHz repetition rate [4]. With this set-up one full temporal trace of 1 ns delay is scanned within 100 µs. Due to the lack of mechanical components in our THz-TDS, this scheme allows for high data acquisition rates that are necessary to scan images in reasonable data acquisition times.

Temperature controllable terahertz time-domain reflection spectroscopy of liquids

The investigation of liquids with terahertz spectroscopy is an intriguing topic. There is a variety of interacting forces between the molecules which gives rise to interesting properties like phase transitions or surface tension, these make the study of liquids by terahertz spectroscopy an intricate task.

Time-domain terahertz spectrometer based on high-speed asynchronous optical sampling

We present a terahertz time-domain spectrometer with large bandwidth, high spectral resolution, and high sensitivity. A bandwidth of larger than 3 THz, a resolution of 1 GHz and a sensitivity of larger than 60 dB dynamic range are achieved in a data acquisition time of less than 60 s. The spectrometer is based on high-speed asynchronous optical sampling which employs two femtosecond lasers of 1 GHz repetition rate stabilized to 10 kHz off-set of the repetition rates employing a 3 GHz frequency shifter. This technique opens new perspectives to perform high-resolution THz spectroscopy under rapidly varying environmental conditions.