Cascaded four-wave mixing for broadband tunable laser sideband generation

Mapping Structural Dynamics of Proteins with Femtosecond Stimulated Raman Spectroscopy

The structure-function relationships of biomolecules have captured the interest and imagination of the scientific community and general public since the field of structural biology emerged to enable the molecular understanding of life processes. Proteins that play numerous functional roles in cellular processes have remained in the forefront of research, inspiring new characterization techniques. In this review, we present key theoretical concepts and recent experimental strategies using femtosecond stimulated Raman spectroscopy (FSRS) to map the structural dynamics of proteins, highlighting the flexible chromophores on ultrafast timescales. In particular, wavelength-tunable FSRS exploits dynamic resonance conditions to track transient-species-dependent vibrational motions, enabling rational design to alter functions. Various ways of capturing excited-state chromophore structural snapshots in the time and/or frequency domains are discussed. Continuous development of experimental methodologies, synergistic correlation with theoretical modeling, and the expansion to other nonequilibrium, photoswitchable, and controllable protein systems will greatly advance the chemical, physical, and biological sciences.

High-performance seed pulses at 910 nm for 100 pw laser facilities by using single-stage nondegenerate four-wave mixing

As the first step in a 100 petawatt (PW) laser facility, seed pulses with high performance are important to guarantee the quality of the output laser pulse. Here we propose a novel method based on a single-stage four-wave mixing process for the generation of seed pulses with a smooth and broadband spectrum, high energy, and high temporal contrast (TC). As high as 250 μJ pulses at approximately 910 nm central wavelength with a high TC and broader than 200 nm bandwidth are obtained in a piece of transparent medium directly after a commercial Ti:sapphire amplifier. The angular dispersion of the generated seed pulse is linear to the wavelength, which can be compensated well by using angular dispersive optics, such as a prism. The extremely simple process and setup make the output seed pulses stable and reliable for 100 PW laser facilities.

Generation of high-energy clean multicolored ultrashort pulses and their application in single-shot temporal contrast measurement

We demonstrate the generation of 100-μJ-level multicolored femtosecond pulses based on a single-stage cascaded four-wave mixing (CFWM) process in a thin glass plate by using cylinder lenses. The generated high-energy CFWM signals can shift the central wavelength and have well-enhanced temporal contrast because of the third-order nonlinear process. They are innovatively used as clean sampling pulses of a cross-correlator for single-shot temporal contrast measurement. With a simple homemade setup, the proof-of-principle experimental results demonstrate the single-shot cross-correlator with dynamic range of 10¹⁰, temporal resolution of about 160 fs and temporal window of 50 ps. To the best of our knowledge, this is the first demonstration in which both the dynamic range and the temporal resolution of a single-shot temporal contrast measurement are comparable to those of a commercial delay-scanning cross-correlator.

Capturing Structural Snapshots during Photochemical Reactions with Ultrafast Raman Spectroscopy: From Materials Transformation to Biosensor Responses

Chemistry studies the composition, structure, properties, and transformation of matter. A mechanistic understanding of the pertinent processes is required to translate fundamental knowledge into practical applications. The current development of ultrafast Raman as a powerful time-resolved vibrational technique, particularly femtosecond stimulated Raman spectroscopy (FSRS), has shed light on the structure-energy-function relationships of various photosensitive systems. This Perspective reviews recent work incorporating optical innovations, including the broad-band up-converted multicolor array (BUMA) into a tunable FSRS setup, and demonstrates its resolving power to watch metal speciation and photolysis, leading to high-quality thin films, and fluorescence modulation of chimeric protein biosensors for calcium ion imaging. We discuss advantages of performing FSRS in the mixed time-frequency domain and present strategies to delineate mechanisms by tracking low-frequency modes and systematically modifying chemical structures with specific functional groups. These unique insights at the chemical-bond level have started to enable the rational design and precise control of functional molecular machines in optical, materials, energy, and life sciences.

Unraveling ultrafast photoinduced proton transfer dynamics in a fluorescent protein biosensor for Ca(2+) imaging

Imaging Ca(2+) dynamics in living systems holds great potential to advance neuroscience and cellular biology. G-GECO1.1 is an intensiometric fluorescent protein Ca(2+) biosensor with a Thr-Tyr-Gly chromophore. The protonated chromophore emits green upon photoexcitation via excited-state proton transfer (ESPT). Upon Ca(2+) binding, a significant population of the chromophores becomes deprotonated. It remains elusive how the chromophore structurally evolves prior to and during ESPT, and how it is affected by Ca(2+) . We use femtosecond stimulated Raman spectroscopy to dissect ESPT in both the Ca(2+) -free and bound states. The protein chromophores exhibit a sub-200 fs vibrational frequency shift due to coherent small-scale proton motions. After wavepackets move out of the Franck-Condon region, ESPT gets faster in the Ca(2+) -bound protein, indicative of the formation of a more hydrophilic environment. These results reveal the governing structure-function relationship of Ca(2+) -sensing protein biosensors.

Broadband cascaded four-wave mixing and supercontinuum generation in a tellurite microstructured optical fiber pumped at 2 μm

We demonstrate the broadband cascaded four-wave mixing (FWM) and supercontinuum (SC) generation in a tellurite MOF which is made from 76.5TeO(2)-6ZnO-11.5Li(2)O-6Bi(2)O(3) (TZLB, mol%) glass. By using a 2-μm picosecond laser with the center wavelength of ~1958 nm as the pump source, the broadband FWM with the frequency separation of ~1.1 THz is obtained. The bandwidth of the frequency comb spans a range of ~630 nm from ~1620 to 2250 nm at the average pump power of ~125 mW. With the average pump power increasing to ~800 mW, the broadband mid-infrared SC generation with the spectrum from ~900 to 3900 nm is observed. Changing the pump source to a femtosecond laser (optical parametric oscillator, OPO) with the center wavelength of ~2000 nm, solitons and dispersive waves (DWs) are obtained.

Excited state structural events of a dual-emission fluorescent protein biosensor for Ca²⁺ imaging studied by femtosecond stimulated Raman spectroscopy

Fluorescent proteins (FPs) are luminescent biomolecules that emit characteristic hues upon irradiation. A group of calmodulin (CaM)-green FP (GFP) chimeras have been previously engineered to enable the optical detection of calcium ions (Ca(2+)). We investigate one of these genetically encoded Ca(2+) biosensors for optical imaging (GECOs), GEM-GECO1, which fluoresces green without Ca(2+) but blue with Ca(2+), using femtosecond stimulated Raman spectroscopy (FSRS). The time-resolved FSRS data (<800 cm(-1)) reveal that initial structural evolution following 400 nm photoexcitation involves small-scale coherent proton motions on both ends of the chromophore two-ring system with a <250 fs time constant. Upon Ca(2+) binding, the chromophore adopts a more twisted conformation in the protein pocket with increased hydrophobicity, which inhibits excited-state proton transfer (ESPT) by effectively trapping the protonated chromophore in S1. Both the chromophore photoacidity and local environment form the ultrafast structural dynamics basis for the dual-emission properties of GEM-GECO1. Its photochemical transformations along multidimensional reaction coordinates are evinced by distinct stages of FSRS spectral evolution, particularly related to the ∼460 and 504 cm(-1) modes. The direct observation of lower frequency modes provides crucial information about the nuclear motions preceding ESPT, which enriches our understanding of photochemistry and enables the rational design of new biosensors.

Generation of octave-spanning supercontinuum by Raman-assisted four-wave mixing in single-crystal diamond

An octave-spanning coherent supercontinuum is generated by non-collinear Raman-assisted four-wave mixing in single-crystal diamond using 7.7 fs laser pulses that have been chirped to about 420 fs in duration. The use of ultrabroad bandwidth pulses as input results in substantial overlap of the generated spectrum of the anti-Stokes sidebands, creating a phase-locked supercontinuum when all the sidebands are combined to overlap in time and space. The overall bandwidth of the generated supercontinuum is sufficient to support its compression to isolated few-to-single cycle attosecond transients. The significant spectral overlap of adjacent anti-Stokes sidebands allows the utilization of straight-forward spectral interferometry to test the relative phase coherence of the anti-Stokes outputs and is demonstrated here for two adjacent pairs of sidebands. The method can subsequently be employed to set the relative phase of the sidebands for pulse compression and for the synthesis of arbitrary field transients.

Distinct broadband third-harmonic generation on a thin amorphous medium-air interface

We achieve a series of distinct broadband third-harmonic generation (THG) signals at the amorphous glass-air interface attributed to third-order and cascaded third-order nonlinear processes. A novel THG laser sideband arises from cascaded interaction between fundamental pulses and the cascaded four-wave mixing signals. The spatially separated THG sidebands with distinct spectral profiles manifest femtosecond quantum beats. Using a versatile setup with two crossing near-IR pulses temporally delayed to each other, we design an incident pulse polarization control method on the THG sideband lasers and also acquire the low-frequency surface Raman spectrum of the amorphous medium.