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SUMMARYWe have demonstrated 30-Hz three-photon imaging using a single 24-MHz mode-locked Cr:forsterite oscillator with a center wavelength at 1260 nm. By managing the dispersion distribution in the resonator using double-chirped mirrors, we have produced 32-fs pulses with 22-nJ pulse energy. Using the oscillator as a driving source, we have realized multi-color three-photon images using a GFP-labeledDrosophilabrain and an AF647-labeled mouse brain. To demonstrate the capability of deep-tissue imaging, we have obtained a 10-times higher SBR from the three-photon images than the two-photon results at different depths in a GFP-labeledDrosophilabrain dissection. Furthermore, we have shown the impact of excitation pulse width on three-photon deep-tissue imaging. Our results indicate the superiority of using shorter pulses for deeper-tissue imaging, especially in theDrosophilabrain. In addition, we have recorded the three-photon calcium imagingin vivofrom theDrosophilamushroom body in response to external electric shocks. We believe our demonstration provides a robust approach for high-speed three-photon microscopy applications, especially for intravital investigations in theDrosophilabrain.

Optics express 31(12), 19146 - 19158 (2023). doi:10.1364/OE.492228, A beamline for temporal diagnostics of extreme ultraviolet (XUV) femtosecond pulses at the free-electron laser in Hamburg (FLASH) at DESY was designed, built and put into operation. The intense ultra-short XUV pulses of FLASH fluctuate from pulse to pulse due to the underlying FEL operating principle and demand single-shot diagnostics. To cope with this, the new beamline is equipped with a terahertz field-driven streaking setup that enables the determination of single pulse duration and arrival time. The parameters of the beamline and the diagnostic setup as well as some first experimental results will be presented. In addition, concepts for parasitic operation are investigated., Published by Optica, Washington, DC

Laser & photonics reviews 17(4), 2200588 (2023). doi:10.1002/lpor.202200588 special issue: "15th Anniversary of Laser & Photonics Reviews ", The quest for ever-shorter optical pulses has been ongoing for over half a century. Although few-cycle pulses have been generatedfor nearly forty years, pulse lengths below the single-cycle limit have remained an elusive goal for a long time. For this purpose, opticalwaveform synthesizers, generating high-energy, high-average-power pulses via coherent combination of multiple pulses covering differentspectral regions, have been recently developed. They allow unprecedented control over the generated optical waveforms, spanning anextremely broad spectral range from ultraviolet to infrared. Such control allows for steering strong-field interactions with increaseddegrees of freedom. When driving high-harmonic generation, tailored waveforms can produce bright attosecond pulse trains and evenisolated attosecond pulses with tunable spectra up to the soft X-ray range. In this paper, which follows the previous review in the samejournal from Manzoni et al., recent progress on parametric and hollow-core fiber waveform synthesizers is discussed. Newly developedseeding schemes, absolute, relative and spectral phase measurement and control techniques suitable for synthesizers are described. Theprogress on serial and parallel waveform synthesis based on Ti:sapphire and Ytterbium laser systems and their latest applications inhigh-harmonic generation in gaseous and solid media, attosecond science and laser wakefield acceleration is discussed., Published by Wiley-VCH, Weinheim

Optical materials express 13(5), 1211 -1227 (2023). doi:10.1364/OME.486842, Cr:Colquiriites (Cr:LiCAF, Cr:LiSAF, and Cr:LiSGaF) are well-known for theirbroad emission bands in the near-infrared region. Unfortunately, due to their relatively weakthermomechanical strength, average powers from Cr:Colquiriite lasers have been so far limitedto sub-5 W level in continuous-wave operation at room temperature. In this study, the promiseof cryogenic operation, which shows significant power scalability in Yb-based systems, isinvestigated in detail for Cr-doped Colquiriite crystals in terms of the temperature dependence ofthe fluorescence lifetime and emission cross-section (σe) in the 78-618 K range. The lifetimemeasurements showed that the fluorescence, as well as the radiative lifetimes of Cr:Colquiriitesare temperature dependent. The emission cross-section measurements revealed that while coolingthe crystals from 300 K to 78 K, the peak σe in E||c polarization increases moderately for allcrystals: from around 1.3 × 10$^{−20}$ cm$^2$to 1.6 × 10$^{−20}$ cm$^2$in Cr:LiCAF, from around 4.5 × 10$^{−20}$cm$^2$to 6.3 × 10$^{−20}$ cm$^2$in Cr:LiSAF and from around 3.1 × 10$^{−20}$ cm$^2$to 3.95 × 10$^{−20}$ cm$^2$inCr:LiSGaF. We provide analytical formulas describing the measured temperature dependence ofall relevant quantities such as fluorescence/radiative lifetime, peak emission wavelength, peakemission cross-section, and emission full-width at half-maximum. Overall, the reported resultsconstitute a solid basis for the modeling of Cr:Colquiriite-based laser and amplifier systems,especially for the assessment of their potential at cryogenic temperatures., Published by Optica, Washington, DC

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are discussed.

We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both

Despite the popularity and ubiquity of the tilted-pulse-front technique for single-cycle terahertz (THz) pulse generation, there is a deficit of experimental studies comprehensively mapping out the dependence of the performance on key setup parameters. The most critical parameters include the pulse-front tilt, the effective length of the pump pulse propagation within the crystal as well as effective length over which the THz beam interacts with the pump before it spatially walks off. Therefore, we investigate the impact of these parameters on the conversion efficiency and the shape of the THz beam via systematically scanning the 5D parameter space spanned by pump fluence, pulse-front-tilt, crystal-position (2D), and the pump size experimentally. We verify predictions so far only made by theory regarding the optimum interaction lengths and map out the impact of cascading on the THz radiation generation process. Furthermore, distortions imposed on the spatial THz beam profile for larger than optimum interaction lengths are observed. Finally, we identify the most sensitive parameters and, based on our findings, propose a robust optimization strategy for tilted-pulse-front THz setups. These findings are relevant for all THz strong-field applications in high demand of robust high-energy table-top single-cycle THz sources such as THz plasmonics, high-harmonic generation in solids as well as novel particle accelerators and beam manipulators.

We present a diode-pumped Yb:YLF laser system generating 100-mJ sub-ps pulses at a 1-kHz repetition rate (100 W average power) by chirped-pulse amplification. The laser consists of a cryogenically cooled 78 K, regenerative, eight-pass booster amplifier seeded by an all-fiber front end. The output pulses are compressed to 980 fs in a single-grating Treacy compressor with a throughput of 89%. The laser will be applied to multi-cycle THz generation and pumping of high average power parametric amplifiers.

The generation and use of THz radiation for electron acceleration and manipulation of electron bunches has progressed over the last decade to a level where practical devices for THz guns, acceleration and a wide range of beam manipulations have become possible. Here, we present our progress on generation of single-cycle THz pulses at the two-hundred micro-Joule level to drive advanced acceleration and beam manipulation devices. Specifically, we use pulses centered at 0.3 THz to power a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches. Using this STEAM device, we demonstrate record THz-acceleration of >60 keV, streaking with s resolution, focusing with >2 kT/m strength, compression to ~100 𝑓s as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of THz-based electron accelerators, manipulators and diagnostic tools enabling science beyond current resolution frontiers with transformative impact.

We have generated pulses as short as 40 fs with an average power of 265 mW from a semiconductor saturable absorber mirror (SESAM) mode-locked Yb:YLF oscillator employing a 1% transmitting output coupler (OC). The room-temperature laser is pumped by a low-cost 960 nm single-emitter multimode diode and dispersion compensation is provided via double chirped mirrors (DCMs). The 40-fs pulses are centered around 1050 nm with a width of 34 nm at a repetition rate of 87.3 MHz. By increasing the output coupling to 5% and by using Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, we have also demonstrated 380-fs pulses with 1.85 W of average power around 1025 nm at a repetition rate of 190.4 MHz. Using an intracavity off-surface optic axis birefringent filter, the central wavelength of the pulses could be tuned in the 1020-1025 nm and 1019-1047 nm ranges for the 5% and 1% transmitting OCs, respectively. To the best of our knowledge, these are the shortest pulses and highest average and peak powers generated from room-temperature Yb:YLF lasers to date.

We show our progress towards integrated modelocked lasers producing short pulses with high peak power and low timing jitter for applications in frequency combs and optical frequency synthesizer.

We have studied thermal lensing strength of cryogenic Yb:YLF lasers in rod-geometry in detail. Overall thermal lensing is axis dependent; thermo-optic and photo-elastic effects are rather dominant while bulging and electronic lensing are quite weak.

We map out the impact of the interaction lengths on the efficiency and terahertz spatial profile in tilted-pulse-front setups via systematically scanning the 5D-parameter space spanned by fluence, pulse-front tilt, crystal-position, and beam size.

We present a compact ultrafast source tunable from 2 µm to 3.5 µm with 60 nJ pulses at 50 kHz repetition rate producing wideband spectra by adiabatic difference frequency generation supporting close to single-cycle pulses.

Laser based multi-cycle THz generation by difference frequency mixing has been pushed to optical to THz conversion efficiencies close to 1%. This opens up the possibility to implement THz driven electron and X-ray sources.

We show experimental results on large mode area waveguides in silicon photonics. Mode area up to 30 μm2 is demonstrated in a 10 cm long serpentine waveguide with 0.75 dB/cm propagation loss limited only by the aluminum oxide film loss.

We demonstrate an all-polarization-maintaining large-mode-area fiber oscillator mode-locked with a Kerr-type linear Sagnac-interferometer. The laser produces low-noise soliton-like pulses with 5.4 nJ pulse energy and 900 fs pulse duration.

We present nonlinear spectral broadening of pulses with input pulse energy of 1.74mJ at 2.05µm inside a gas-filled Herriott-type multi-pass cell with efficiency of 95%. The transform-limited pulse duration is reduced from 1.43ps to 0.27ps.

Applied physics reviews 9(3), 031407 (2022). doi:10.1063/5.0096685, Terahertz radiation (THz)-based electron acceleration has the potential as a technology for driving the next-generation, compact ultrafast and ultrabright electron and x-ray sources. Dephasing is one of the key problems that prevents long THz-electron interaction lengths in the sub- to few-MeV range where electron velocities vary significantly during high-field acceleration. Here, we present a phase-shifter design with double vacuum channels to alternate the phase velocity that effectively extends the THz-electron interaction length in THz-powered dielectrically-loaded waveguides (DLW). The electrons are swept multiple-times back and forth through the accelerating phase of the THz wave to undergo continuous acceleration along the entire interaction. In addition, the double vacuum channel design enables increases to both the phase and group velocities of the THz wave, which leads to an adaptive synchronous acceleration with extended interaction length. This method paves the way for the practical implementation of THz-powered devices for high-energy ultrafast electron sources., Published by AIP, New York, NY

We investigate simulation of complex nonlinear optical processes. A mathematical model and numerical simulation techniques are developed. They are used to elucidate THz generation in periodically poled Lithium Niobate (PPLN) including optical harmonic generation.

Our spectroscopic, lasing and thermal-lens measurements performed with Yb:YLF and Yb:YAG under almost identical conditions show that YLF host has several advantages over YAG for the development of high-power ultrafast laser/amplifier systems at cryogenic temperatures.

We report a balanced optical cross-correlator based on PPKTP waveguides with sub-attosecond noise floor and more than 100 times higher timing sensitivity when compared to similar bulk optic detectors.

We present a diode-pumped Yb:YLF oscillator with record cw (993-1110 nm) and fs (1019-1047 nm) tuning ranges, with up to 4 W power and 78% slope efficiency, generating pulses as short as 39-fs when mode-locked.

New developments regarding all-optical noise suppression in state-of-the-art Kerr-type fiber oscillators and amplifiers are presented. Optimized noise-transfer dynamics from nonlinear amplifying loop-mirror mode-locked fiber oscillators enable quantum-limited noise performance.