Ultra-broadband infrared pump-probe spectroscopy using synchrotron radiation and a tuneable pump

Abstract

Synchrotron infrared sources have become popular mainly because of their excellent broadband brilliance, which enables spectroscopically resolved spatial-mapping of stationary objects at the diffraction limit. In this article we focus on an often-neglected further advantage of such sources - their unique time-structure - to bring such broadband spectroscopy to the time domain, for studying dynamic phenomenon down to the 100 ps limit. We describe the ultra-broadband (12.5 to 1.1 μm) Fourier transform pump-probe setup, for condensed matter transmission- and reflection-spectroscopy, installed at the X01DC infrared beam-line of the Swiss Light Source (SLS). The optical pump consists of a widely tuneable 100 ps 1 kHz laser system, covering 94% of the 16 to 1.1 μm range. A thorough description of the system is given, including (i) the vector-modulator providing purely electronic tuning of the pump-probe overlap up to 1 ms with sub-ps time resolution, (ii) the 500 MHz data acquisition system interfaced with the experimental physics and industrial control system (EPICS) based SLS control system for consecutive pulse sampling, and (iii) the step-scan time-slice Fourier transform scheme for simultaneous recording of the dual-channel pumped, un-pumped, and difference spectra. The typical signal/noise ratio of a single interferogram in a 100 ps time slice is 300 (measured during one single 140 s TopUp period). This signal/noise ratio is comparable to that of existing gated Globar pump-probe Fourier transform spectroscopy, but brings up to four orders of magnitude better time resolution. To showcase the utility of broadband pump-probe spectroscopy, we investigate a Ge-on-Si material system similar to that in which optically pumped direct-gap lasing was recently reported. We show that the mid-infrared reflection-spectra can be used to determine the optically injected carrier density, while the mid- and near-infrared transmission-spectra can be used to separate the strong pump-induced absorption and inversion processes present at the direct-gap energy.

Ultrafast carrier dynamics in Ge by ultra-broadband mid-infrared probe spectroscopy

“…Additionally, the position of minimum Δ R / R (or plasma edge) gradually shifts toward a low-wavenumber region as the delay time increases; meanwhile, the negative hump (yellow area) also gradually narrow down. Similar phenomena were observed also by Carroll et al 14. in bulk Ge with 100-ps resolution.…”

Group IV Direct Band Gap Photonics: Methods, Challenges, and Opportunities

“…First, as is shown above by the gain and loss experiment (Carroll et al, 2011), the loss in Ge layers strongly exceeds the gain at all the investigated carrier densities up to 10 20 cm −3 and in all investigated cases, i.e., Ge with and without weak strain and/or n-doping. Hence, Carroll's results are apparently in conflict with the observation of lasing in similar but not identical material.…”

“…However, particularly synchrotron-based infrared pump-probe spectroscopy has been shown to offer advantageous conditions for measuring the carrier density, their lifetime as well as gain and loss due to its extended bandwidth and suitable pulse lengths (Carroll et al, 2012;Geiger et al, 2014a,b). At the infrared beamline of the SLS, 100 ps long pulses of infrared light are supplied from the synchrotron and serve as broadband probe pulses, whereas the excess charge carriers are optically excited by a 100 ps Nd:YAG laser at 1064 nm (Carroll et al, 2011). The delay time between pump and probe pulses can be varied electronically, which offers the possibility to follow the dynamics of a system over a long time period by probing at different times after excitation.…”

Ultrabroadband Midinfrared Pump-Probe Spectroscopy Using Chirped-Pulse Up-conversion in Gases

“…In contrast, the bandwidth of our probe pulse is so broad that the entire free-carrier behavior predicted by the Drude model could be experimentally visualized. Although such reflectivitychange spectra in a similar MIR range has been measured by using a synchrotron radiation source with 100 ps time resolution [26], our time resolution of 70 fs is several orders of magnitude higher.…”

“…The MIR pulse is incident at an angle of 45°with p polarization. We assumed m à ¼ 0.34m e [26] and ϵ ∞ ¼ 16 [27]. Reference reflectance is set to 0.24, which corresponds to a condition where free carriers are not present in the Ge crystal wafer.…”