Ultrafast Research Articles

All-optical digital-to-analog conversion based on pulse walk-off effect

A novel approach for all-optical digital-to-analog conversion (DAC) based on the pulse walk-off effect is proposed and experimentally demonstrated. In this approach, an ultrafast optical pulse train is firstly spectrum-tailored by a spectrum-shaper to achieve the multi-wavelength pulses, serving as the sampling source for carrying serial input digital signals via intensity modulation. These modulated optical signals are then injected into a dispersion medium to introduce multi-wave pulse walk-off. Consequently, the modulated optical signals partially overlap in the time domain and the incoherent weighted intensity summation can be realized. The following intensity modulator driven by the selecting codes extracts the target signals in the right time slots. The proposed approach features all-optical structure, inter-channel error avoidance, and high conversion rate, offering a promising solution for achieving high-performance photonic DAC. A proof-of-concept experiment of a 3-bit DAC system with a conversion rate of 5 Gbps based on the pulse walk-off effect has been successfully carried out. Additionally, the system’s performance in terms of Effective number of bits (ENOB) versus modulator extinction ratio and dispersion fluctuations is also discussed in detail.

Acquiring sequentially time-resolved two-dimensional images with parallel multi-group sampling strategy

A method of acquiring sequentially time-resolved two-dimensional images with parallel multi-group sampling strategy has been developed. In this technique, the two-dimensional image was sampled and transferred by a fiber bundle, the fibers of which were arranged in two-dimensional arrays in the input end and rearranged in four one dimensional lines in the output end. The output four groups of one-dimensional images were parallelly detected by a streak camera. The recorded images of the streak camera were then reconstructed according to the spatial relationship of the fibers between the input and output ends to obtain sequentially time-resolved two-dimensional images. The fiber bundle was composed of 64 × 64 pixels in the input end and four columns of 1 × 1024 pixels in the output end. Each pixel consisted of 4 × 4 fibers. The imaging system with about 10 ps temporal resolution has been applied for observing the flight of light, indicating that this technique can be a promising tool for investigating ultrafast phenomena.

Laser interaction with undercritical foams of different spatial structures

The interaction of high-power laser pulses with undercritical foams produced by different techniques but with the same average density is studied at the PALS laser facility. The spatial–temporal evolution of X-ray emission is observed using an X-ray streak camera, electron and ion temperatures are measured by X-ray spectroscopy, and hot-electron production is characterized by monochromatic X-ray imaging. Transmission of a femtosecond laser probe pulse through foams is observed in the near and far fields. In spite of large differences in pore size and foam structure, the velocity of ionization front propagation is quite similar for all the foams studied and is slower than that in a homogeneous material of the same average density. The ion temperature in the plasma behind the ionization front is a few times higher than the electron temperature. Hot-electron production in plastic foams with small pores is strongly suppressed compared with that in solid targets, whereas in foams produced by additive manufacturing, it is significantly increased to the level observed in bare copper foil targets.

Strong-Field Theory of Attosecond Tunneling Microscopy

Attosecond observations of coherent electron dynamics in molecules and nanostructures can be achieved by combining conventional scanning tunneling microscopy (STM) with ultrashort femtosecond laser pulses. While experimental studies in the subcycle regime are under way, a robust strong-field theory description has remained elusive. Here we devise a model based on the strong-field approximation. Valid in all regimes, it provides a surprising analogy to the standard model of STM. We show that the intuitive three-step model of attosecond science directly emerges from our model and we also describe the optimal conditions for attosecond STM experiments. Published by the American Physical Society 2024

Impact of repetitive, ultra-short soft X-ray pulses from processing of steel with ultrafast lasers on human cell cultures

Ultrafast lasers, with pulse durations below a few picoseconds, are of significant interest to the industry, offering a cutting-edge approach to enhancing manufacturing processes and enabling the fabrication of intricate components with unparalleled accuracy. When processing metals at irradiances exceeding the evaporation threshold of about 1010 W/cm² these processes can generate ultra-short, soft X-ray pulses with photon energies above 5 keV. This has prompted extensive discussions and regulatory measures on radiation safety. However, the impact of these ultra-short X-ray pulses on molecular pathways in the context of living cells, has not been investigated so far. This paper presents the first molecular characterization of epithelial cell responses to ultra-short soft X-ray pulses, generated during processing of steel with an ultrafast laser. The laser provided pulses of 6.7 ps with a pulse repetition rate of 300 kHz and an average power of 500 W. The irradiance was 1.95 ×1013 W/cm2. Ambient exposure of vitro human cell cultures, followed by imaging of the DNA damage response and fitting of the data to a calibrated model for the absorbed dose, revealed a linear increase in the DNA damage response relative to the exposure dose. This is in line with findings from work using continuous wave soft X-ray sources and suggests that the ultra-short X-ray pulses do not generate additional hazard. This research contributes valuable insights into the biological effects of ultrafast laser processes and their potential implications for user safety.

Switching, amplifying, and chirping diode lasers with current pulses for high bandwidth quantum technologies.

A series of simple and low-cost devices for switching, amplifying, and chirping diode lasers based on current modulation are presented. Direct modulation of diode laser currents is rarely sufficient to establish precise amplitude and phase control over light, as its effects on these parameters are not independent. These devices overcome this limitation by exploiting amplifier saturation and dramatically outperform commonly used external modulators in key figures of merit for quantum technological applications. Semiconductor optical amplifiers operated on either rubidium D line are recast as intensity switches and shown to achieve ON:OFF ratios >106 in as little as 50ns. Current is switched to a 795nm wavelength (Rb D1) tapered amplifier to produce optical pulses of few nanosecond duration and peak powers of 3 W at a similar extinction ratio. Fast rf pulses are applied directly to a laser diode to shift its emission frequency by up to 300MHz in either direction and at a maximum chirp rate of 150MHzns-1. Finally, the latter components are combined, yielding a system that produces watt-level optical pulses with arbitrary frequency chirps in the given range and <2% residual intensity variation, all within 65ns upon asynchronous demand. Such systems have broad application in atomic, molecular, and optical physics and are of particular interest to fast experiments simultaneously requiring high power and low noise, for example, quantum memory experiments with atomic vapors.

In situ characterization of two unknown ultrashort laser pulses using four-wave mixing in gas

In situ characterization of two unknown ultrashort laser pulses using four-wave mixing in gas

A simple graphics processing unit-accelerated propagation routine for laser pulses in the strong-field regime.

We present a simple and easy-to-implement Graphics Processing Unit (GPU)-accelerated routine to numerically simulate the propagation of ultrashort and intense laser pulses as they interact with a medium. The routine is based on the solution of Maxwell's wave equation in the frequency domain with an extended Crank-Nicolson algorithm implemented in the Nvidia CUDA C++ programming language. The main advantages of our method are its significant speed-up factor and its ease of implementation, requiring only basic knowledge of CUDA and C++. In this article, we review the strong-field wave equations to be solved and their discretization and demonstrate how to implement a numerical solver for them on an Nvidia GPU. We show the results of the simulation of a near-infrared laser pulse propagating through a partially ionized atomic gas and discuss the performance of our GPU-accelerated scheme. Compared to a naïve central processing unit implementation of the same routine, our GPU-accelerated version is up to 198 times faster in standard regimes.

Femtosecond fiber laser with hybrid organic small molecule material as saturable absorber

This study presents the development of an ultrafast mode-locked fiber laser utilizing a hybrid organic small molecule (HOSM) based on Tris-(8-hydroxyquinoline) aluminum (Alq3) and N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), as a passive saturable absorber (SA). The SA thin-film was integrated into the laser cavity to serve as a mode-locker. Through a series of experiments varying the cavity length, the efficacy and stability of the developed SA were examined. In these experiments, with cavity lengths of 112 m, 61.5 m, and 22.5 m, accompanied by group velocity dispersions (GVDs) of −2.365, −1.249 ps2, and −0.4 ps2 respectively, we observed a consistent and singular soliton mode-locking state. Notably, we achieved a remarkable pulse width tunability ranging from 1.98 ps to 712 fs by adjusting the cavity length. Operating in the 1560 nm region, this femtosecond soliton fiber laser holds significant promise for various applications, including high-precision optical metrology, frequency-comb generation, and broadband absorption spectroscopy.

Switchable Dual-Band Generation of Femtosecond Pulses in a Mode-Locked Erbium-Doped Fiber Laser Based on Monolayer Graphene

Switchable Dual-Band Generation of Femtosecond Pulses in a Mode-Locked Erbium-Doped Fiber Laser Based on Monolayer Graphene

Quick fabrication method of a thermally expanded core in polarization-maintaining fibers using CO[formula omitted] laser and fiber rotation

Maximizing the transmission of high-peak-power ultrashort pulses through joints between polarization-maintaining optical fibers with different mode field diameters is crucial for improving the efficiency of all-fiber laser systems. Here, we propose a method of fabricating a thermally expanded core by using a CO2 laser as a heating source that does not require a priori splicing of fibers. Unlike standard methods utilizing continuous heating of the fiber, usually on the time scale of minutes, we present the pulsed approach, which can reduce the duration of the whole process to below 30 s. Via fiber rotation, we tailor our method to polarization-maintaining optical fibers. Furthermore, we apply the thermally expanded core method to manufacturing mode field adapters between commercially available polarization-maintaining optical fibers. Exemplary splices reveal an increase in transmission of 30 %, enabling insertion losses lower than 0.4 dB. The presented methods of fabricating a thermally expanded core and mode field adapters show high potential for large-volume production, especially for high-power applications, as the splices can withstand peak powers as high as 50 kW.

Ultrafast adaptive optics for imaging the living human eye

Adaptive optics (AO) is a powerful method for correcting dynamic aberrations in numerous applications. When applied to the eye, it enables cellular-resolution retinal imaging and enhanced visual performance and stimulation. Most ophthalmic AO systems correct dynamic aberrations up to 1−2 Hz, the commonly-known cutoff frequency for correcting ocular aberrations. However, this frequency may be grossly underestimated for more clinically relevant scenarios where the medical impact of AO will be greatest. Unfortunately, little is known about the aberration dynamics in these scenarios. A major bottleneck has been the lack of sufficiently fast AO systems to measure and correct them. We develop an ultrafast ophthalmic AO system that increases AO bandwidth by ~30× and improves aberration power rejection magnitude by 500×. We demonstrate that this much faster ophthalmic AO is possible without sacrificing other system performances. We find that the discontinuous-exposure AO-control scheme runs 32% slower yet achieves 53% larger AO bandwidth than the commonly used continuous-exposure scheme. Using the ultrafast system, we characterize ocular aberration dynamics in six clinically-relevant scenarios and find their power spectra to be 10−100× larger than normal. We show that ultrafast AO substantially improves aberration correction and retinal imaging performance in these scenarios compared with conventional AO.

Crack Formation Detection during Ultra-Fast Laser Beam Machining of Alumina Ceramic Based on Acoustic Emission Signals

Crack Formation Detection during Ultra-Fast Laser Beam Machining of Alumina Ceramic Based on Acoustic Emission Signals

Tunable nonlinear optical properties in polyaniline-multiwalled carbon nanotube (PANI-MWCNT) system probed under pulsed Nd:YAG laser

The investigation of the nonlinear optical (NLO) properties of polyaniline (PANI) and polyaniline doped with multi-walled carbon nanotube (PANI-MWCNT) in both solution and film forms was conducted using the open-aperture z-scan approach. A Q-switched resonant Nd: YAG laser with a wavelength of 532nm and two different fluences was utilized in this study. Based on the z-scan experiments, it was observed that the NLO behaviour of PANI and PANI-MWCNT composites exhibited a transition from reverse saturable absorption (RSA) to saturable absorption (SA) when the samples changed from a liquid to a film state. Two photon absorption (TPA) and thermally induced nonlinear scattering are the main mechanisms behind the RSA behaviour of the materials in solution form. The reason for SA behaviour of these materials in film form is attributed as the ground-state free electron bleaching in the conduction band due to the increased laser intensity. Due to increased structural disorder and the defect states or trap levels created by the dopants in the PANI system, the NLO properties of the PANI-MWCNT composite in both solution and film forms have significantly improved compared to those of pure PANI. Urbach tail analysis and carbon cluster determination revealed the presence of defect states in the composites system. The composite between PANI and MWCNT was verified using Fourier transform infrared (FTIR) spectroscopy. The functional elements present in the composites are verified by X-ray photoelectron spectroscopy. The NLO parameters of the samples, viz., the nonlinear absorption coefficient (β), the imaginary part of the nonlinear, and susceptibility χi(3) the figure of merit (FOM) of PANI-MWCNT, exhibited a notable enhancement in their values over PANI. Moreover, the PANI-MWCNT film demonstrated superior SA performance and the PANI-MWCNT solution demonstrated better OL performance compared to that of the PANI film and solution respectively. Also the effects of dopant induced structural modifications and lattice disorder on the NLO behaviour of these materials are correlated with the obtained NLO parameters. The tunable NLO properties accomplished by controlling the structural phase and dopant-induced defects enhance the possibility of these nanostructures for applications in optical limiting, optical switching, optical modulation, optical pulse compression and laser pulse narrowing.

2.6 GW, mJ-class high-energy femtosecond laser system based on Yb:YAG single-crystal fiber amplifier

High-peak-intensity ultrafast fiber lasers show excellent prospect for ultrafast science and industrial applications. For simplicity as well as efficiency, chirped-pulse amplification (CPA) is an effective technique for the generation of high-energy sources and single crystal fiber (SCF) also shows great potential due to its convenient configuration. In this work, a high-peak-power hybrid CPA pulsed laser system based on a three-stage single-pass end-pumped Yb:YAG SCF amplifier is experimentally demonstrated. The amplification system emitted pulses with the maximum power of 103.2 W at 100 kHz repetition rate and we obtained the compressed output power of 84.2 W, corresponding to the pulse energy of 0.84 mJ. Considering the third order dispersion that induced by the stretcher and the accurate tuning effect for higher-order dispersion compensation of chirped fiber Bragg grating, we have demonstrated a nearly transform limited output pulse duration of 323 fs with the peak power exceeding 2.6 GW. It can be said that we present the results for the first implementation of the shortest pulse duration and highest peak power in such multi-stage Yb:YAG SCF amplifier. The well-preserved beam quality with the measured M2 value of 1.22 and 1.29 for the horizontal and vertical directions at the maximum achieved average power. With such outstanding combined features, the demonstrated high-energy ultrafast fiber lasers would enable broad applications.

41 W, 206 fs regenerative amplifier based on CPA-free amplification of femtosecond pulse

41 W, 206 fs regenerative amplifier based on CPA-free amplification of femtosecond pulse

Nanometric probing with a femtosecond, intra-cavity standing wave

Abstract Optical standing waves are intrinsically nanometric, spatially fixed interference-field patterns. At a commensurate scale, metallic nanotips serve as coherent, atomic-sized electron sources. Here, we explore the localized photofield emission from a tungsten nanotip with a transient standing wave. It is generated within an optical cavity with counter-propagating femtosecond pulses from a near-infrared, 100-MHz frequency comb. Shifting the phase of the standing wave at the tip reveals its nodes and anti-nodes through a strong periodic modulation of the emission current. We find the emission angles to be distinct from those of a traveling wave, and attribute this to the ensuing localization of emission from different crystallographic planes. Supported by a simulation, we find that the angle of maximum field enhancement is controlled by the phase of the standing wave. Intra-cavity nanotip interaction not only provides higher intensities than in free-space propagation, but also allows for structuring the light field even in the transverse direction by selection of high-order cavity modes.

Persistent photoconductivity and Emulating Ebbinghaus forgetting curve via characterization of excitatory synaptic transmission in a ZnO-based optoelectronic synapse with ultra-low power ([formula omitted] fJ) consumption

Optical synapses provide high bandwidth operation with low power consumption for neuromorphic computing. ZnO is established as a potential semiconductor for optoelectronic synapses with its high photosensitivity in the ultraviolet (UV) region. In this work, we focus on emulating Ebbinghaus forgetting curves via conductance decay and memory retention in a lateral synaptic device based on ZnO nanoparticles. We use optical stimulation using UV light of wavelength 375 nm. Retention of memory can be seen up to 2500 s showing long-term potentiation which is useful for memory and learning abilities. Long-duration memory retention using optical spiking is achieved and the decay characteristics of the memory are studied. A transition from short-term plasticity (STP) to long-term plasticity (LTP) can be induced by tuning the optical pulse width. Energy consumption is found to increase exponentially with the applied optical pulse width. The conductance decay follows the Ebbinghaus forgetting curve. We have demonstrated the effect of various measurement parameters such as pulse width, pulse frequency, and pulse amplitude on memory decay. The synaptic device can be operated with energy consumption as low as 1.2 fJ which paves the way for very low energy-consuming synaptic devices for neuromorphic applications.

Biocompatibility and drug release kinetics of TiNbZrSn femtosecond laser-induced superhydrophilic structures

As load-bearing components, metallic implants are frequently used for orthopedic prostheses due to their superiority over conventional ceramic and polymeric biomaterials. Metal-based orthopedic implants have been subjected to various fabrication and treatment methods to enhance their biological activities at the host site. Modifying the structure, improving the hydrophilicity, and developing controlled-release drug delivery systems can improve cellular adhesion, proliferation, osseointegration, and differentiation. Ultrafast lasers have recently attracted interest in surface engineering. Laser surface structuring permits the alteration of sample topography, the chemical makeup of the surface, and the material physical properties. This study presents the surface modification of a TiNbZrSn shape memory alloy using a femtosecond laser to produce laser-induced periodic structures with better drug release than that of a pristine sample. We also present the in vitro cell viability and biocompatibility of the alloy system. All samples structured with femtosecond laser exhibited superhydrophilic nature with 0° contact angle. In addition, the laser structured surfaces showed cell viability above 80 % and minimal cytotoxicity towards human keratinocytes. Moreover, a well-defined hydroxyapatite layer developed on the laser structured surface. In general, the laser structuring process and the induced changes on the surface in terms of roughness and oxide formation result in slower drug release (up to 10 %) compared to pristine specimens.

Ultrafast laser direct writing of in-line polarizers based on nano-gratings.

In-line polarizers play an important role in the emerging quantum computing, integrated photonics, and ultrafast science. However, to our knowledge, no one has actually used in-line polarizers based on nano-gratings (NGs) in those research fields. Here, we present a novel approach to write a fiber polarizer that utilizes nano-gratings directly within the fiber core for the first time to our knowledge. An optical fiber polarizer measuring 1 mm in length, 2.5 dB in polarization-dependent loss (PDL), and 2 dB in insertion loss was developed and initially utilized for the artificially saturable absorber nonlinear polarization evolution (NPE) experiment. In the experiment, several nonlinear transmission curves are measured. In-line polarizers based on nano-gratings are expected to be used in future integrated optical chips because of their small size and easy direct writing.