i) High-average power femtosecond laser source

The high-average power femtosecond laser source includes a sub-10 fs oscillator, a stretcher, a regenerative amplifier, a multi-pass amplifier, a compressor, a synchronized controller, and several sets of pump lasers. The oscillator generates high-quality seed laser pulses with a pulse duration of approximately 10 fs. Then, these pulses are stretched to 200 ps by a Martinez stretcher and injected into the regenerative laser cavity for pre-amplification.^[17] The pumping source is a commercial all-solid-state Q-switched green laser with an average power of 20 W and a repetition rate of 1 kHz. The energy of the laser pulse from the regenerative amplifier is larger than 4 mJ. Then, the laser pulse is injected into the second-stage multi-pass amplifier. The multi-pass amplification pump source comprises four commercial all-solid-state Q-switched green lasers with an average power of 25 W and a repetition rate of 1 kHz. The pump laser and the amplified laser have optimized delay, coupled matching, compensation for thermal effects in the amplification process, and other means. Eventually, energy of 30 mJ is obtained with good beam quality. Finally, to achieve compression, the amplified laser pulse is input into a compressor that is opposite to the stretcher. By optimizing the dispersion compensation, a pulse duration of approximately 40 fs and energy of over 20 mJ are obtained, corresponding to an average power of over 20 W. The design of the laser source is shown in Fig. 2.

Fig. 2.

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	Fig. 2. (color online) Design of the high-average power femtosecond light source.

ii) Few-cycle femtosecond laser pulses and CEP-locked components

Traditionally, the differentially pumped hollow fiber is effective for generating few-cycle laser pulses. Compared with the statically gas fill scheme, it has the advantages of a high transmission rate and a wide spectral-broadening range. First, the laser is focused into a waveguide fiber filled with a certain gas for broadening the spectrum. The spectrum is broader than an octave, and then a high-dispersion chirped-mirror combination compressor is used to accurately compensate the dispersion. The stretched laser pulse is compressed to few-cycle to achieve a laser-pulse output with energy greater than 5 mJ and a pulse duration shorter than 7 fs. For few-cycle laser pulses, the locking of the CEP is achieved by locking the femtosecond Ti:sapphire oscillator and the amplifier separately. In the oscillator part, the difference frequency generation (DFG) technique is used to produce 1.5-μm infrared light of the broadband femtosecond laser via a periodically poled lithium niobate crystal.^[18–20] The spectrum produced by the two parts contains the carrier-envelope offset phase signal of the output pulse obtained via the beat frequency method. The phase-locked loop is fed into the acousto-optic modulator, which can control the pump laser power to realize the CEP locking of the oscillator, yielding a “fast phase-locked loop.” The CEP locking of the amplifier uses the f–2f spectral interference technique. The interference spectrum of the f–2f spectral interference system is obtained by using the spectrum broadening of the foregoing waveguides. The phase-drift information in the amplifier is obtained by analyzing the interference spectrum, and the feedback control is obtained by changing the insertion of the prism in the compressor, eventually causing the phase locking of the amplified pulse, which is called a “slow phase-locked loop.” After the phase of the oscillator and the amplifier is locked, the CEP-locked high-energy laser pulse can be obtained.

iii) High-energy mid-infrared laser source

A multi-stage OPA is pumped by a 20-W high-average power femtosecond laser. First, the super-continuum is generated by injecting pump energy of approximately 1 mJ into a crystal. The wavelength range is obtained as 400–600 nm by controlling the power density of the crystal, and then the pump energy is injected into a multi-stage OPA pumped by 15-mJ femtosecond laser pulses. The bandwidth and gain in each stage of the OPA are controlled independently. The stability of the spectrum and energy is maintained when the parametric fluorescence is suppressed. A tunable wavelength of 1.1–2.6 μm is obtained. By using a prism pair and a chirped mirror to compensate for the dispersion, a femtosecond and widely tuned mid-infrared laser can be obtained. By optimizing the crystal angle and the pumping parameters, the maximum power of the optical parametric laser is achieved at the wavelength of 1.3 μm, with a maximum pulse energy of larger than 1 mJ and a pulse duration of approximately 100 fs.

The liquid-phase femtosecond spectroscopy experimental unit^[21] is a femtosecond spectroscopic user experimental platform that combines linear infrared and Raman spectroscopy with femtosecond pump-detection technology, including the combination of infrared spectroscopy, Raman spectroscopy, and absorption spectroscopy. This system includes an OPA unit, a pump-detection unit, a Raman spectrometer, an infrared spectrometer, a temperature-control system, and a vacuum pump unit. Thus, the platform allows the measurement of the time behavior of complex molecular dynamics and is particularly suitable for the study of condensed-phase multi-particle systems in which weak bond interaction plays an important role. The ultrafast spectroscopic and kinetic experimental platform employs OPA lasers as a pump source and super-continuum white light as a probe source. This platform can be used for the study of various types of micro/nanostructures (liquid or thin film) or complex molecular complex systems. Combined with ultrafast pulse laser shaping (amplitude and phase modulation), CPA, compression, and other technical means, it can also be used to research quantum coherence in the time domain. The overall design of the experimental unit is shown in Fig. 3.

Fig. 3.

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	Fig. 3. (color online) Diagram of the liquid femtosecond spectroscopy experimental unit.

This experimental unit combines pump-detection technology with electronic^[14] ion-momentum (energy) resolution measurement to achieve non-adiabatic dynamics of the gas-molecule excited state, quantum control, and other aspects of the study. The femtosecond lasers are divided into pump and probe lasers and focused on the molecular beam in the electron and ion velocity imaging device. Simultaneously, time-resolution and momentum-resolution measurements of the molecular fragments are performed.

The experimental unit is composed of a wavelength-tunable laser source, a pump-detection delay device, and an electron and ion velocity imager. Electron and ion velocity imagers are mainly composed of two parts: a beam source chamber and an ionization chamber. In the beam source chamber, a 1-kHz pulse valve produces the molecular beam through supersonic diffusion. The molecular beam enters the ionization chamber through a high-speed nozzle. In the ionization chamber, the molecular beam intersects the two femtosecond laser pulses perpendicularly. One of the laser beams excites the molecule to the electronic excited state (as a pump), and the other laser excites the excited state. The molecules or reaction products are ionized (as probes). The ionized electrons or ions are focused by the ion lens velocity onto the micro-channel plate detector to generate a two-dimensional electron or ion map and recorded by a charge-coupled device camera. The overall design of the experimental unit is shown in Fig. 4.

Fig. 4.

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	Fig. 4. (color online) Diagram of the gas-phase femtosecond spectroscopy experimental unit.

This experimental unit is designed as an extreme-nonlinear optical ultrafast research platform based on the CEP-locked high-energy few-cycle laser (5 mJ/7 fs/1 kHz, CEP-locked). The high-energy CEP-locked few-cycle laser interacts with a solid surface to generate high-order harmonics. Because this harmonic generation is limited to an ultrashort time, the extreme-nonlinear process can be measured by adjusting the CEP of the driving pulse.

The experimental platform includes a CEP-locked few-cycle laser source, interacting target chambers, XUV spectrometers, vacuum chambers, target components, and ultrahigh vacuum pump units. The CEP-locked few-cycle laser is focused onto a solid target by a small-F-number off-axis parabolic concave mirror. The solid target is mounted on a four-dimensional platform and turned one position at a time to ensure that each occurrence is a new interaction point. The high-order harmonics are detected via XUV spectrometry. The design of the whole system is shown in Fig. 5.

Fig. 5.

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	Fig. 5. (color online) Diagram of the extreme-nonlinear optical ultrafast experimental unit.

The femtosecond time-resolved fluorescence OPA experiment^[22–24] system is a transient fluorescence measurement technology based on the non-collinear OPA principle.^[25,26] It not only can measure the entire broadband spectrum at the same time but also has a gain of nearly 10⁶ with respect to the fluorescence signal. It can be used to achieve femtosecond time-resolved measurement of weak broadband fluorescence signals. The measurement sensitivity is 30 fluorescent photons for a 100-fs timescale.

The experimental unit mainly includes femtosecond laser sources, non-collinear OPAs, fluorescence OPA devices, pump-detection devices, and data-measurement systems. Using the high-average power femtosecond laser as a light source, the pump laser is divided into two beams by a beam splitter. One of the beams is frequency-doubled to generate a pump light with a center wavelength of 400 nm for the fluorescence parametric amplification system. The other beam is used as a collinear or non-collinear OPA pump source. The wavelength is tunable, which can be exploited to excite samples to produce fluorescence. The fluorescence is amplified in a barium-borate crystal via a non-collinear OPA process, and data acquisition of the fluorescence-amplification signal is performed using a related instrument. The entire system design is shown in Fig. 6.

Fig. 6.

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	Fig. 6. (color online) Diagram of the femtosecond time-resolved transient fluorescence optical parameter amplification spectrometer experimental unit.