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Abstract
We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6-18 μm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 μm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-μm pulses in GaSe.
Highlights
Coherent mid-infrared (MIR) light sources have shown promise in the study of a wide range of resonant light–matter interactions, most notably in the study of ro-vibrational molecular transitions and dynamics [1], as well as the investigation of low-energy phonon dynamics [2], charge transport in semiconductors [3], and non-linear and high-field interactions [4]
Recent advances in nearinfrared (NIR) femtosecond laser technology have provided a viable route towards table–top broadband ultrafast MIR sources via nonlinear processes such as parametric amplification or difference frequency generation (DFG) [6,7,8,9,10,11,12]
intra-pulse DFG (IPDFG), which generates a MIR idler signal through phase matched mixing of spectral components within a broadband driving pulse, ensures a passively waveform-stable output. This enables electro-optic sampling (EOS) to detect the generated electric field, transfer the detection to the NIR spectral region where low-noise semiconductor photodetectors offer increased performance over cooled MIR detectors [13] used in conventional Fourier transform infrared (FTIR) spectroscopy, and allows for ultrasensitive [14] and broadband [15,16] detection across the entire molecular fingerprint region
Summary
Coherent mid-infrared (MIR) light sources have shown promise in the study of a wide range of resonant light–matter interactions, most notably in the study of ro-vibrational molecular transitions and dynamics [1], as well as the investigation of low-energy phonon dynamics [2], charge transport in semiconductors [3], and non-linear and high-field interactions [4]. Our novel MIR source is based on 2-μm fiber laser technology, enabling both generation and field-sensitive detection of phase-stable MIR pulses These MIR transients, generated via IPDFG of 30-W average power, 32-fs pulses in a GaSe crystal, have been characterized using EOS with few-cycle, watt-level 2-μm sampling pulses. For accurate temporal measurement of MIR waveforms, a second fiber self-compression stage was implemented in parallel for use as the EOS sampling pulse train In this case, a smaller core PCF fiber (13-μm MFD and 2.3 cm long) was used to spectrally broaden the CPA driving pulses to a width of 700 nm at −20 dB with >60% efficiency while simultaneously temporally compressing the pulse to ∼2 optical cycles (13-fs FWHM), see Figs. Dispersion compensation of pulses in this spectral range can be difficult due to a relatively small
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