Raman scattering underlies a broad range of spectroscopic and light-generation techniques, yet its conventional description, based on the Raman gain spectrum, accurately describes only long-pulse, steady-state dynamics. We present a time-domain theoretical approach that provides a unified and physically-transparent description of Raman interactions across all temporal regimes. It enables direct visualization of Raman temporal dynamics and accounts for spectrotemporal aspects of Raman phenomena, which cannot be addressed by prior theories. In particular, molecules with strong Raman responses do not produce an efficient soliton self-frequency shift in gas-filled hollow-core fibers. The time-domain analysis exposes temporal and spectral distortions from the Raman response that impact frequency-shifting detrimentally, and identifies how these distortions can be suppressed by reducing the Raman interaction to a perturbation on the electronic response. Experiments that employ gas mixtures with tunable Raman fractions of the nonlinear response demonstrate up to a four-fold increase in quantum efficiency (from 20 to 80

Nonlinear amplification is a powerful technique for generating ultrashort laser pulses with high peak power in fiber systems. However, the diversity of nonlinear amplification approaches and their inherent complexities present significant challenges to achieving a unified understanding and further scaling of peak power and pulse energy while preserving ultrashort durations. Here, we report the results of a systematic optimization with respect to seed pulse duration that elucidates the dynamics of nonlinear amplification and allows identification of distinct propagation regimes. As part of this analysis, we identify a new regime, termed partial parabolic amplification, which achieves 50-fs pulse duration and yields higher peak power than any other nonlinear amplification regime. An initial experimental demonstration of partial parabolic amplification produces 50-fs and 2.2-uJ pulses with a 25-um-core Yb fiber amplifier, corresponding to a 30-MW peak power. In contrast to other nonlinear amplification techniques, practical energy scaling beyond 10 uJ and 200 MW should be achievable with available gain fibers with larger mode areas, which would fill a gap in existing fiber laser capabilities that would directly impact material processing, nonlinear bio-imaging, and other applications.

We demonstrate a simple and robust platform for femtosecond pulse compression based on large-area graded-index (GRIN) rods. Despite highly-multimode propagation, pulses accumulate a spatially uniform nonlinear phase owing to the self-imaging property of GRIN media. In initial experiments, 420 fs and ∼100 nJ pulses are compressed to 60 fs duration with excellent beam quality and spatio-spectral homogeneity. Furthermore, simultaneous beam cleaning and pulse compression are possible with complex input beams. Similar compression performance is obtained with different input beam sizes and positions, which underlie the stability and ease of use of the GRIN rod compressor.

Nonlinear amplification in large-core fibers is a potentially attractive approach to high-power short-pulse generation, but transverse mode coupling and the consequent beam degradation are concerns. Robust single-mode operation of a multimode fiber amplifier can be achieved in a regenerative amplifier, but this has only been demonstrated in linear amplification. Here, we investigate single-mode nonlinear amplification in a multimode-fiber regenerative amplifier. Pronounced mode coupling is observed when the amplified pulse accumulates a large nonlinear phase; however, cavity-assisted mode selection can suppress mode coupling and improve the spatial beam quality. By optimizing the seed pulse duration, pulse energies up to 10 μJ with good beam quality are achieved and subsequently compressed to 98 fs duration. Factors that limit the performance of this amplifier are discussed.

Raman phonons are quantized molecular motions that arise from the inelastic scattering of light and mediate a wide range of spectroscopic and nonlinear optical phenomena. These can play a major role in frequency-conversion processes, but commonly-used theoretical treatments based on the Raman gain spectrum largely neglect the phonons and their dynamical interaction with the field. In this work, we clarify the physical role of Raman phonons within a recently-developed time-domain framework based on the Raman-induced index modulation, and show that phonons correspond to the oscillatory component of the Raman-induced index modulation. The analysis further reveals a linear phonon-mediated interaction embedded within Raman scattering, in which optical fields couple through wave-vector matching with existing phonons. This mechanism underlies, but has been neglected in, coherent Stokes and anti-Stokes scattering, as well as molecular modulation. Building on this insight, we introduce a phonon-controlled approach that enables efficient conversion into a selected Stokes order by tuning the wave-vector-matching relation between the driven phonons and the targeted Raman process, and we confirm the approach by numerical simulations that consider realistic Raman dynamics. These results provide a clearer physical interpretation of Raman phonons and their dynamics, and offer new strategies for controlling Raman interactions.

Methods for simultaneously controlling the spatial, spectral, and temporal properties of ultrashort pulses after propagation through a disordered multimode fiber are presented. By leveraging the dispersion and strong mode-mixing within a disordered fiber, multidimensional control over the output pulse is achieved by tailoring only the spatial wavefront of the input pulse. The ability to generate reconfigurable spatiotemporal foci at single or multiple points in space and time is demonstrated experimentally. The control is extended to the spectral domain, as demonstrated by the successful creation of spectrally-dependent foci, with different wavelength bands focused to distinct spatial locations and times. An approach for time-averaged wavefront shaping is also introduced and is shown to enable the creation of complex spatial intensity patterns with tailored properties.

The spatial dynamics under nonlinear pulse propagation are investigated in a large–core diameter graded-index (GRIN) rod waveguide. We observe both on-axis and off-axis spatial beam cleaning, in contrast to the behavior reported in GRIN fibers.

Nonlinear pulse propagation in gas-filled waveguides has attracted substantial attention over the past decade, and a variety of capabilities have been reported. However, to our knowledge, there is no prior report of spectral compression in gas-filled waveguides or cavities, which would offer a natural route for scaling to much higher pulse energies than have been reached in solid structures. Here we report a high-energy spectral-compression technique based on nonlinear propagation in gas-filled capillaries. With 0.1- to 1-mJ pulses, compression of the spectral width by a factor up to 12 (from 60 nm to 5 nm) is demonstrated. Key to this advance is recognition that the process plays out differently in gases than in solids. In a noble gas (Ar), we find that even a small structure in the spectrum, which is mapped to the time profile, of the input pulse can degrade the compression process. We identify the delayed Raman response of molecular gases (N 2 O and N 2 ) as a mechanism that smooths and symmetrizes the nonlinear index modulation, which reduces the impact of spectral asymmetry and fine structure and enhances the fidelity of the compressed peak. The technique can be implemented with a capillary filled with ambient air, for sub-millijoule operation without a dedicated gas system. These results initiate a new direction in the optics of gas-filled waveguides and establish Raman-enhanced spectral compression as a robust route to high-energy narrowband optical sources, with potential impact in a broad range of applications.

Using spectral broadening in argon-filled fiber, we generate sub-100 fs pulses with multi-megawatt peak power, tunable from 850 to 1700 nm. Images of neurons deep in mouse brain are obtained with pulses at 1300 nm. © 2024 The Authors

While nonlinear effects in graded-index (GRIN) multimode fibers have been studied extensively, little is known about nonlinear effects in larger GRIN waveguides, where the number of modes approaches infinity and modal dispersion becomes negligible. Here we show that Gaussian beams remain nearly invariant even with large nonlinear phase accumulation and on- or off-axis trajectories in GRIN rods. In addition, spatially-complex beams can undergo self-cleaning to single-lobed profiles for both on- and off-axis trajectories. Numerical simulations exhibit the features observed in experiments, and a qualitative interpretation of these results that makes connection to beam-cleaning phenomena observed in GRIN fibers is proposed.

Recent advances in burst-mode sources of ultrashort optical pulses are enabling a range of applications. However, the growing complexity of pulse propagation in these systems now exceeds the capabilities of existing models. Critical gain dynamics with coupled spectral and temporal dependencies cannot be captured, which precludes effective analysis and design of advanced short-pulse sources. In this work, we introduce a field-based gain model (as opposed to existing power-based models) that integrates transient rate equations for the populations with the unidirectional pulse propagation equation. This model offers a comprehensive framework for the treatment of spectral and temporal dynamics that arise from ultrafast transient gain, spontaneous emission, dispersion, and Kerr and Raman nonlinearities. A frequency-scaled Fourier transform is introduced to facilitate the computation of coherent and incoherent processes that occur on disparate time scales and enhances the computational speed by orders of magnitude. The field-based model is applied to studies of (i) gain-managed nonlinear amplifiers that generate 40–fs pulses at burst rates of hundreds of gigahertz and (ii) the initiation of mode-locking from noise in a laser.

Single-transverse-mode operation of a regenerative amplifier based on highly-multimode fiber is described. This approach to short-pulse generation achieves high performance and offers opportunities for scaling to much higher peak power. In addition, it opens new opportunities to control pulse propagation and generation.

[This corrects the article on p. 415 in vol. 16, PMID: 39958867.].

This publisher's note contains a correction to Opt. Lett.50, 1593 (2025)10.1364/OL.551046.

Soliton formation and soliton self-frequency shift are investigated in a hollow-core fiber filled with N 2 gas. With 10-µJ and 80-fs input pulses at 1030 nm, solitons with greater than 500-nJ energy and duration less than 100 fs can be generated between 1090 and 1310 nm. With peak powers above 5 MW, we expect that these pulses will be useful for applications such as nonlinear microscopy.

Three-photon fluorescence microscopy (3PM) has driven rapid progress in deep-tissue imaging beyond the depth limit of two-photon microscopy, with impacts in neuroscience, immunology, and cancer biology. Three-photon excitation places a premium on ultrashort pulses with high peak power in the 1300- and 1700-nm wavelength bands, which allow deepest imaging. The inefficiency and cost of current sources of these pulses present major barriers to the use of 3PM in biomedical research labs. Fiber sources of such pulses could potentially alleviate these problems, but the peak-power limitations of optical fibers have limited their use in 3PM. Here, we describe a fiber-based source of femtosecond pulses with multi-megawatt peak power. Femtosecond pulses at 1030 nm are launched into an antiresonant hollow-core fiber filled with argon. By varying only the gas pressure, pulses with hundreds of nanojoules of energy and sub-100 fs duration are obtained at wavelengths between 850 and 1700 nm. This approach is a new route to an efficient and potentially low-cost source for deep-tissue imaging. In particular, 960-nJ and 50-fs pulses are generated at 1300 nm with a conversion efficiency of 10%. The nearly 20-MW peak power is an order of magnitude higher than the previous best from a femtosecond solid-core fiber source at 1300 nm. As an example of the capabilities of the source, these pulses are used to image structure and neuronal activity in a mouse brain as deep as 1.1 mm below the dura.

This publisher's note contains a correction to Opt. Lett.49, 5787 (2024)10.1364/OL.539381.

Electrocatalytically active titanium oxynitride (TiNO) thin films were fabricated on commercially available titanium metal plates using a pulsed laser deposition method for energy storage applications. The elemental composition and nature of bonding were analyzed using X-ray photoelectron spectroscopy (XPS) to reveal the reacting species and active sites responsible for the enhanced electrochemical performance of the TiNO electrodes. Symmetric supercapacitor devices were fabricated using two TiNO working electrodes separated by an ion-transporting layer to analyze their real-time performance. The galvanostatic charge-discharge studies on the symmetric cell have indicated that TiNO films deposited on the polycrystalline titanium plates at lower temperatures are superior to TiNO films deposited at higher temperatures in terms of storage characteristics. For example, TiNO films deposited at 300 degrees C exhibited the highest specific capacity of 69 mF/cm(2) at 0.125 mA/cm(2) with an energy density of 7.5 Wh/cm(2). The performance of this supercapacitor (300 degrees C TiNO) device is also found to be similar to 22% better compared to that of a 500 degrees C TiNO supercapacitor with a capacitance retention ability of 90% after 1000 cycles. The difference in the electrochemical storage and capacitance properties is attributed to the reduced leaching away of oxygen from the TiNO films by the Ti plate at lower deposition temperatures, leading to higher oxygen content in the TiNO films and, consequently, a high redox activity at the electrode/electrolyte interface.

Nonlinear femtosecond (fs) laser ablation enables highly localized energy deposition for cell microsurgery. Conventional systems operate at either low (∼1 kHz, amplified µJ pulse energy) or high (∼80 MHz, unamplified low nJ) repetition rates, but intermediate rates with amplified pulse energy offer a promising balance of ablation speed and thermal control. We custom-built a low-cost, 32 MHz femtosecond fiber laser system with gain-managed nonlinear amplification, boosting pulse energy from 5 to 90 nJ while compressing pulse duration to 46 fs. In this intermediate–repetition-rate regime, the use of sub-50-fs pulses enhances ablation efficiency by strengthening multiphoton absorption and lowering the effective ablation threshold, while also leveraging multi-pulse incubation effects that promote cumulative energy deposition at reduced per-pulse energies. Compared to 200–500 fs pulses typically used for ablation, shorter durations double ablation efficiency in silicon and yield a ∼10× increase in cell membrane damage. In 3D tumor models, this approach enables targeted subsurface ablation up to 400 µm depth with 6 nJ pulse energy. These results inform the design of next-generation femtosecond laser systems for microsurgery.

Pulse compression based on periodic layered Kerr media has been demonstrated as an effective technique for femtosecond pulses with energies around 100 mu J or more. We report such a compressor designed for pulses in the 10-mu J range, which is valuable for many applications. Pulses from a fiber chirpedpulse amplifier are compressed from 300 fs to as short as 60 fs, with good pulse and beam quality. The compressor is a simple and efficient way to extend the performance of common Yb-based sources of pulses with microjoule energies. (c) 2024 Optica Publishing Group. All rights, including for text lar technologies, are reserved.