Abstract
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.
Highlights
Ultrafast time-domain spectroscopy (TDS) with femtosecond lasers is one of the pivotal techniques to elucidate dynamic processes in the natural sciences occurring on time scales between a few tens of femtoseconds to a few nanoseconds
The full width at half maximum (FWHM) of the averaged “cross-correlation” signal divided by a deconvolution factor 1.54 for the squaredhyperbolic secant pulses from the lasers is a measure of the time resolution of the high-speed asynchronous optical sampling (ASOPS) system at a given time-delay position
We have demonstrated a high-speed ASOPS TDS system with 45 fs time resolution over a 1 ns time-delay window based on two 1-GHz repetition rate femtosecond Ti:sapphire lasers
Summary
Ultrafast time-domain spectroscopy (TDS) with femtosecond lasers is one of the pivotal techniques to elucidate dynamic processes in the natural sciences occurring on time scales between a few tens of femtoseconds to a few nanoseconds. High-speed asynchronous optical sampling (ASOPS) is an ultrafast TDS method that operates without mechanical scanner, permits multi-kHz scan rates, allows fast measurements directly at the shot-noise limit, and offers the straightforward possibility to perform two-color pump-probe measurements [15,16,17]. We demonstrate for the first time a repetition rate offset-locking method operating at the tenth harmonics of the repetition rates combined with an offset-frequency definition based on direct-digital-synthesis (DDS) electronics This method permits a laser-pulse-duration-limited time resolution of 45 fs over the full 1 ns measurement window, i.e. an improvement by a factor of three compared to our previous approach [16]. In a more application-oriented example, we use our system for rapid and sensitive mapping of the layer period and total stack thickness of soft-X-ray multilayer mirrors via detection of acoustic phonons and acoustic echoes following optical excitation
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