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

Abstract

High-speed asynchronous optical sampling ͑ASOPS͒ is a novel technique for ultrafast time-domain spectroscopy ͑TDS͒. 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 ⌬f R = 10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 s. 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 ϫ 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.

Coherent dual-comb spectroscopy at high signal-to-noise ratio

“…1d and 1e). In the time domain, the pulses from each comb source overlap on the photo detector at varying time delays, essentially creating a virtual scanning interferometer as in asynchronous optical sampling [10,38]. The resulting crosscorrelation of the source and LO pulses is analogous to a conventional interferogram in DFTS.…”

“…The recent advances in spectroscopy using dual frequency combs offers an interesting new approach to FTS [1][2][3][4][5][6][7][8][9]. This dual-comb spectroscopy approach can also be viewed as a form of infrared time-domain spectroscopy (TDS) analogous to THz TDS [10][11][12] , or as a massively parallel multiheterodyne laser spectrometer [13].…”

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

“…Alloying with cobalt results in a shift of T M far above room temperature and is thus of vital importance for technological applications [20]. While STM provides direct access to static structural modulation, the bulk sensitive femtosecond optical spectroscopy is particularly useful for studying low-energy collective excitations, like Raman active phonons [21] or amplitude modes of charge-density wave (CDW) systems [22][23][24][25].In this Letter, we demonstrate the existence of collective modes in the 14M phase that stem from the coupling of the lattice to the underlying electronic instability. We show that the stacking sequence recorded by STM, which is close to the structure proposed by the adaptive martensite scenario [17,18], can be consistently explained by the higher-order coupling between the electronic order and the lattice.…”

Uncertainty and Ambiguity in Terahertz Parameter Extraction and Data Analysis

“…5. The index of refraction and absorption coefficient for anthracene were measured with an asynchronous optical sampling (ASOPS) THz time-domain spectrometer [24]. The sample was pressed into a pill and the empty pill holder was used for the reference measurement.…”

“…Whereas an increase in the number of measurements decreases the statistical error, further repetitions lead to long measurement times and might be accompanied by long-term drifts and instabilities of the signal. On the other hand, with the advent of fast-scanning THz TDS such as vibrating membrane or asynchronous optical sampling spectrometers [24], the measurement speed can be largely increased. We present a procedure that determines the confidence intervals for the complex index of refraction of the sample starting with the uncertainty of the time-domain data.…”