Ultrafast Processing Research Articles

Pulsed polarized vortex beam enabled by metafiber lasers

Pulsed polarized vortex beams, a special form of structured light, are generated by tailoring the light beam spatiotemporally and witness the growing application demands in nonlinear optics such as ultrafast laser processing and surface plasma excitation. However, existing techniques for generating polarized vortex beams suffer from either low compactness due to the use of bulky components or limited controlment of pulse performance. Here, an all-fiber technique combining plasmonic metafibers with mode conversion method is harnessed to generate high-performance pulsed polarized vortex beams. Plasmonic metafibers are utilized as saturable absorbers to produce Q-switched pulses with micro-second duration, while the offset splicing method is employed to partially convert the fundamental transverse mode (LP01\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{01}$$\\end{document}) to higher-order mode (LP11\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{11}$$\\end{document}). Eventually, a polarized vortex beams laser is achieved at the telecom band with a repetition frequency of 116.0 kHz. The impact of geometrical parameters including period of metafibers and offset of splicing on the spatiotemporal properties of pulsed polarized vortex beams is systematically investigated. Our findings could pave the way for design, control and generation of all-fiber pulsed polarized vortex beams, and also offer insights into the development of other types of structured laser sources.

Localized surface plasmon resonance induced nonlinear absorption and optical limiting activity of gold decorated graphene/MoS2 hybrid

The synergistic supremacy of a hybrid nanocomposite made of reduced graphene oxide (rGO), molybdenum disulfide (MoS2) and gold nanoparticles (Au) for Q-switching and optical limiting applications is investigated. Hydrothermally synthesized Au-rGO-MoS2 hybrid with varying concentrations of Au (2.5, 5, 7.5 and 10 wt%) was subjected to interact with Q-switched Nd:YAG nanosecond green laser pulses. The intensity dependent nonlinear absorption studies revealed a switch over in the nonlinear behaviour from saturable absorption (SA) to reverse saturable absorption (RSA) behaviour. SA occurs due to ground state bleaching at comparatively lower laser irradiances serving the need for Q-switching and optical communications. The RSA trait at higher laser irradiance is attributed to sequential two-photon absorption which serves as the platform for optical limiting behaviour prompting laser safety and effective photothermal therapy applications. The strong mid-visible and UV absorption promotes the availability of multiple energy bands for energy transitions of excited state absorption (ESA). Enhanced local fields due to localized surface plasmon resonance effect, structural defects, hierarchical morphology, increased absorption cross-section and plasmon induced energy transfer promotes robust ESA process. These tunable nonlinear optical properties make Au-rGO-MoS2 hybrid promising futuristic candidates for applications in nonlinear photonic devices, ultrafast optical switches and all-optical signal processing systems.

Mixed precision quantization of silicon optical neural network chip

In recent years, the field of neural network research has witnessed remarkable advancements in various domains. One of the emerging approaches is the integration of photonic computing, which leverages the unique properties of light for ultra-fast information processing. In this article, we establish a mixed precision quantization model to silicon-based optical neural networks and evaluates their performance on the MNIST and Fashion-MNIST datasets. Through a genetic algorithm-based optimization process, we achieve significant parameter compression while maintaining competitive accuracy. Our findings demonstrate that with an average quantization bitwidth of 4.5 bits on the MNIST dataset, we achieve an impressive 85.94% reduction in parameter size compared to traditional 32-bit networks, with only a marginal accuracy drop of 0.65%. Similarly, on the Fashion-MNIST dataset, we achieve an average quantization bitwidth of 5.67 bits, resulting in an 82.28% reduction in parameter size with a slight accuracy drop of 0.8%.

Characterization of a four-channel surface plasmon polaritons emitter based on a combined grooves coupling structure using photoemission electron microscopy

In various functional devices relying on femtosecond surface plasmon polaritons (SPPs), actively controlling the propagation direction of the SPPs field is crucial for designing plasmon-based functional devices. This capability is particularly valuable in optical logic circuits and optical communication systems. In this study, we employed photoemission electron microscopy (PEEM) to conduct near-field imaging of the SPPs field generated by a novel combination of wide and narrow grooves etched on a flat silver film. Experimental results revealed that femtosecond SPPs fields excited by laser pulses with different linear polarizations (±45°) can be emitted in four distinct directions, showcasing the combined groove structure's potential as a four-channel SPPs emitter. Additionally, Finite-Difference Time-Domain (FDTD) simulations were utilized to model the time evolution envelope of SPPs modes excited by groove coupling structures irradiated with laser pulses of varying polarization directions. The physical mechanism behind the combined groove structures as a four-channel SPPs emitter was thoroughly analyzed, and the experimental findings closely aligned with the FDTD simulation results. This research is of significant importance for advancing novel ultra-fast micro-nano optoelectronic devices with exceptional properties and for constructing ultra-fast information processing systems in plasmonics nanocircuits.

The influence of ultrafast laser processing on morphology and optical properties of Au-GaAs composite structure

The results of direct femtosecond laser structuring of GaAs wafer coated with continuous semitransparent gold (Au) film are presented. The obtained structures demonstrate a combination of different features, namely laser-induced periodic surface structures (LIPSS) on semiconductor and metal film, nanoparticles, Au islands, and fragments of exfoliated Au film. The properties of Au-GaAs samples are studied with scanning electron microscopy (SEM), Raman scattering, and photoluminescence (PL) spectroscopy. The behaviour of phonon modes and enhancement of band-edge PL of Au-GaAs composite sample are discussed. The Raman spectra of Au-GaAs sample processed at different levels of irradiation pulse energy reveal forbidden TO and allowed LO phonon modes for selected geometry of experiment, as well as the manifestation of GaAs surface oxidation and amorphization. A 12-fold increase of PL intensity for Au-GaAs sample with LIPSS compared to initial GaAs surface is observed. The detected PL enhancement is caused by an increase of absorption in GaAs due to the light field enhancement near the Au nanoislands and a decrease of nonradiative surface recombination. The blue shift of PL band is caused by the quantum size effect in GaAs nano-sized features at laser processed surface. The combination of GaAs substrate with surface micro- and nanostructures with Au nanoparticles can be useful for photovoltaic and sensorics applications.

Ultrafast Laser Processing for High-Aspect-Ratio Structures.

Over the past few decades, remarkable breakthroughs and progress have been achieved in ultrafast laser processing technology. Notably, the remarkable high-aspect-ratio processing capabilities of ultrafast lasers have garnered significant attention to meet the stringent performance and structural requirements of materials in specific applications. Consequently, high-aspect-ratio microstructure processing relying on nonlinear effects constitutes an indispensable aspect of this field. In the paper, we review the new features and physical mechanisms underlying ultrafast laser processing technology. It delves into the principles and research achievements of ultrafast laser-based high-aspect-ratio microstructure processing, with a particular emphasis on two pivotal technologies: filamentation processing and Bessel-like beam processing. Furthermore, the current challenges and future prospects for achieving both high precision and high aspect ratios simultaneously are discussed, aiming to provide insights and directions for the further advancement of high-aspect-ratio processing.

Random laser ablated tags for anticounterfeiting purposes and towards physically unclonable functions

Anticounterfeiting tags affixed to products offer a practical solution to combat counterfeiting. To be effective, these tags must be economical, capable of ultrafast production, mass-producible, easy to authenticate, and automatable. We present a universal laser ablation technique that rapidly generates intrinsic, randomly distributed craters (in under a second) on laser-sensitive materials using a nanosecond pulsed infrared laser. The laser and scanning line parameters are balanced to produce randomly distributed craters. The tag patterns demonstrate high randomness, which is analyzed using pattern recognition algorithms and root mean square error deviation. The optical image information of the tag is digitized with a fixed bit uniformity of 0.5 without employing any debiasing algorithm. The efficacy of tags for anticounterfeiting is presented by securing the challenge associated with each tag. Statistical NIST tests are successfully performed on responses generated from both single and multiple tags, demonstrating the true randomness of the sequence of binary digits. The single(multiple) tag(s) achieved an actual encoding capacity of approximately 10391 (10518) and a low false rate (both positive and negative) on the order of 10−58 (10−50). Our findings introduce a laser-based method for anticounterfeiting tag generation, allowing for ultrafast and straightforward product processing with minimal fabrication and tag cost.

Substrate‐Free Terahertz Metamaterial Sensors With Customizable Configuration and High Performance

AbstractMetamaterials based on quasi‐bound states in the continuum (qBICs) with manipulable resonance quality (Q) factors have provided a standout platform for cutting‐edge terahertz (THz) sensing applications. However, most so far have been implemented as conventional metal patch structures with adjacent substrate layers, incurring the limitation of insufficient light‐matter interaction due to substrate effects. Here, qBIC‐driven metamaterials with substrate‐free metallic aperture structures for tailoring light‐matter interactions and exhibiting near‐ideal sensing performance is introduced. Specifically, it is incorporated ultrafast femtosecond laser processing technology to fabricate H‐type metallic aperture metamaterials with accessible high‐contrast Q factor resonances allowed by in‐plane symmetry breaking. Correspondingly, stronger light field energies are applied to the interactions due to completely eliminating the confinement of the substrate effect, enabling experimental sensitivity of up to 0.86 THz RIU−1 for the qBIC resonance, 1.9 times that of the conventional dipole resonance. Moreover, a high Q qBIC resonance achieved by optimized asymmetry parameter is exploited for detecting ultrathin layers of L‐proline molecules as low as 0.87 nmol. It is envisioned that this approach will deliver insights for real‐time, precise, and high‐performance detection of trace biomolecules, and open new perspectives for realizing ideal performance metadevices.

Generation of multi-photon Fock states at telecommunication wavelength using picosecond pulsed light

Multi-photon Fock states have diverse applications such as optical quantum information processing. For the implementation of quantum information processing, Fock states should be generated within the telecommunication wavelength band, particularly in the C-band (1530-1565 nm). This is because mature optical communication technologies can be leveraged for transmission, manipulation, and detection. Additionally, to achieve high-speed quantum information processing, Fock states should be generated in optical pulses with as short a duration as possible, as this allows embedding lots of information in the time domain. In this paper, we successfully generate picosecond pulsed multi-photon Fock states (single-photon and two-photon states) in the C-band with Wigner negativities for the first time, which are verified by pulsed homodyne tomography. In our experimental setup, we utilize a single-pixel superconducting nanostrip photon-number-resolving detector (SNSPD), which is expected to facilitate the high-rate generation of various quantum states. This capability stems from the high temporal resolution of SNSPDs (several tens of picoseconds in our case and also in general) allowing us to increase the repetition frequency of pulsed light from the conventional MHz range to the GHz range, although in this experiment the repetition frequency is limited to 10 MHz due to the bandwidth of the homodyne detector. Consequently, our experimental setup is anticipated to serve as a prototype of a high-speed optical quantum state generator for ultrafast quantum information processing at telecommunication wavelength.

Fabrication of nitrogen-hyperdoped silicon by high-pressure gas immersion excimer laser doping

In recent years, research on hyperdoped semiconductors has accelerated, displaying dopant concentrations far exceeding solubility limits to surpass the limitations of conventionally doped materials. Nitrogen defects in silicon have been extensively investigated for their unique characteristics compared to other pnictogen dopants. However, previous practical investigations have encountered challenges in achieving high nitrogen defect concentrations due to the low solubility and diffusivity of nitrogen in silicon, and the necessary non-equilibrium techniques, such as ion implantation, resulting in crystal damage and amorphisation. In this study, we present a single-step technique called high-pressure gas immersion excimer laser doping (HP-GIELD) to manufacture nitrogen-hyperdoped silicon. Our approach offers ultrafast processing, scalability, high control, and reproducibility. Employing HP-GIELD, we achieved nitrogen concentrations exceeding 6 at% (3.01 × 1021 at/cm3) in intrinsic silicon. Notably, nitrogen concentration remained above the liquid solubility limit to ~1 µm in depth. HP-GIELD’s high-pressure environment effectively suppressed physical surface damage and the generation of silicon dangling bonds, while the well-known effects of pulsed laser annealing (PLA) preserved crystallinity. Additionally, we conducted a theoretical analysis of light-matter interactions and thermal effects governing nitrogen diffusion during HP-GIELD, which provided insights into the doping mechanism. Leveraging excimer lasers, our method is well-suited for integration into high-volume semiconductor manufacturing, particularly front-end-of-line processes.

Ultrafast reverse saturable absorption and all-optical computing with boron dipyrromethene (BODIPY) chromophore derivatives

Ultrafast reverse saturable absorption (RSA) in chromophore-based borondipyrro-methenes (BODIPYs) derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenyl-vinyl)phenyl)-boron-diuoride azadipyrromethene (ZL-22), has been theoretically analyzed with femtosecond (fs) laser pulses at 800 nm and 850 nm. The results are in good agreement with the reported experimental results. The effect of input pulse intensity, pulse width, nonlinear absorption (NLA) coefficients, sample thickness and laser pulse shaping on transmittance has been studied and optimized for high contrast and low-power operation. All-optical fs NOT and the universal NOR and NAND Boolean logic gates have been designed with ZL-61 and ZL-22. The logic gates with top-hat input pulse profile have a sharp switch ON/OFF time compared to the Gaussian input laser profile. Ultrafast operation at relatively low pump intensities opens up exciting prospects for the applicability of BODIPY derivatives for ultrafast low-power all-optical computing and information processing.

All-Optical Control of Ultrafast Switching between the Hybridized Plasmonic Fields of Au Nanorod Dimer in fs-nm Scale with Dispersed Femtosecond Laser.

With the increasing demand for ultrafast communication and information processing in future optical chips, arbitrary manipulation of electromagnetic fields in the femtosecond-nanometer spatiotemporal scale has attracted great attention in integrated optics. Manipulation of the nanoscale light field in the real femtosecond temporal domain is challenging work. Here, we have demonstrated all-optical control of ultrafast switching between the hybridized plasmonic fields of a Au nanorod dimer in the fs-nm scale using a dispersed femtosecond laser and revealed the transformation process with ultrahigh spatiotemporal resolved technology via the combination of a pump-probe technique and photoemission electron microscopy (PEEM). The results show that we can actively and coherently control the transformation sequence and time (with the shortest temporal interval of around 15 fs) of the two hybridized modes in the Au nanorod dimer by tuning the dispersion of the laser pulse. The nanoscale light manipulation achieved by all-optical control may contribute to the design of high-speed miniaturized signal-processing systems.

Ultrafast spatiotemporal control of orthogonally directional surface plasmon polariton via a compact nanoantenna

Ultrafast spatiotemporal control over the launch direction of surface plasmon polariton (SPP), which is highly desirable for the realization of integrated nanophotonic devices and ultrafast data processing systems within plasmonic nano-circuitry, remains a significant challenge. In particular, ultrafast spatial-temporal switching of SPP directional emissions in orthogonal directions has not yet been reported. Here, we propose a novel compact plasmonic nanoantenna structure design for spatiotemporal control of SPP launch at the nano−femto scale. This device possesses the capacity to launch the SPP directionally into different orthogonal output channels by changing the polarization direction of incident light. More importantly, ultrafast switching of the SPP directional launch at a femtosecond time scale has been proposed via adjusting the instantaneous polarization state of the excitation light. This study can lay the foundation for the development of miniaturized signal processing systems for high-speed data processing, and highly integrated nanophotonic devices, as well as for enhancing the functionality of next-generation optical nanocircuits.

A Review of an Investigation of the Ultrafast Laser Processing of Brittle and Hard Materials.

Ultrafast laser technology has moved from ultrafast to ultra-strong due to the development of chirped pulse amplification technology. Ultrafast laser technology, such as femtosecond lasers and picosecond lasers, has quickly become a flexible tool for processing brittle and hard materials and complex micro-components, which are widely used in and developed for medical, aerospace, semiconductor applications and so on. However, the mechanisms of the interaction between an ultrafast laser and brittle and hard materials are still unclear. Meanwhile, the ultrafast laser processing of these materials is still a challenge. Additionally, highly efficient and high-precision manufacturing using ultrafast lasers needs to be developed. This review is focused on the common challenges and current status of the ultrafast laser processing of brittle and hard materials, such as nickel-based superalloys, thermal barrier ceramics, diamond, silicon dioxide, and silicon carbide composites. Firstly, different materials are distinguished according to their bandgap width, thermal conductivity and other characteristics in order to reveal the absorption mechanism of the laser energy during the ultrafast laser processing of brittle and hard materials. Secondly, the mechanism of laser energy transfer and transformation is investigated by analyzing the interaction between the photons and the electrons and ions in laser-induced plasma, as well as the interaction with the continuum of the materials. Thirdly, the relationship between key parameters and ultrafast laser processing quality is discussed. Finally, the methods for achieving highly efficient and high-precision manufacturing of complex three-dimensional micro-components are explored in detail.

Selective removal of variable thickness multilayer materials based on photoelectric detection

An ultrafast laser processing system with near-axis monochromatic photoelectric detection for selective removal of variable thickness multilayer materials is proposed. The correlation between signal evolution and ablation progress of silver layer with the variable thickness is established, and a signal attenuation threshold is determined to ensure non-removal of the substrate, providing criteria for the processing strategy combining high and low energies. The proposed strategy is experimentally validated, which balances the quality of low-energy removal and the efficiency of high-energy removal.

TRENDS OF THE MICROWAVE PHOTONIC RADARS

Today, the world's leading countries are intensively working on the development of new generation radars - microwave photonic radars. Microwave photonic radars make it possible to significantly reduce the mass and size characteristics of radar stations, to increase the information capability and range of target detection due to the reduction of losses in long communication lines when using optical fiber, to ensure high immunity due to the significantly lower sensitivity of optical-electronic equipment and fiber-optic lines of communication connection to external electromagnetic influences. Microwave photonics provides wide bandwidth, flat response, low loss transmission, multi-dimensional multiplexing, ultra-fast analog signal processing and immunity to electromagnetic interference. Radar implementation in the optical domain can provide better resolution, coverage, and speed performance, which would be difficult to implement with traditional electronics. The review article examines the state of development and system architectures of such photonic radars as optoelectronic hybrid radars, all-optical radars, multifunctional microwave photonic radar systems, distributed microwave photonic radars, software-defined radars, and cognitive radars. New technologies in this field and possible future directions of research are discussed. As an example, a broadband microwave photon radar reproduced on the basis of a microcircuit is considered. The broadband signal generator and receiver are built into the silicon crystal on the insulator. A high-precision distance measurement with a resolution of 2.7 cm and an error of less than 2.75 mm was obtained. Visualization of multiple targets with complex profiles has been implemented. But the performance of most integrated microwave photonic chips is not yet satisfactory for practical radar applications. Monolithic integration of key microwave photonic subsystems is also not mature enough for practical applications, so hybrid integration of devices fabricated on their optimal integration platforms is of practical interest. At present, indium phosphide, silicon nitride and silicon on insulator are the three leading platforms for photonic integration

Ultrafast Laser Hyperdoped Black Silicon and Its Application in Photodetectors: A Review

Based on the ultrafast and extremely strong interaction between laser pulses and materials, ultrafast laser irradiation can break the solid solubility constraints and enable hyperdoping of impurities. This process overcomes the bandgap constraints of crystalline silicon, resulting in heightened absorption across a broad spectral range spanning from ultraviolet to infrared wavelengths, therefore commonly referred to as black silicon (b‐Si). The b‐Si demonstrates significant changes in optoelectronic properties, making it highly promising for applications in silicon photonics. Specifically, b‐Si photodetectors exhibit distinct advantages in terms of high photoelectric gain at low voltage, ultrabroadband spectral responsivity, large dynamic range, and suitability for operation over a wide temperature range. These properties address the limitations of traditional silicon photodetectors, showcasing great potential for applications in optoelectronic integration, artificial intelligence, information technology, energy devices, and beyond. This review focuses on b‐Si achieved through ultrafast laser processing, with a special emphasis on its applications in photodetectors. The mechanism of ultrafast laser irradiation and the properties of hyperdoped silicon are discussed. Then, the research progresses and state‐of‐the‐art b‐Si photodetectors are introduced, as well as working mechanism and potential application expansion. Finally, the development prospects of b‐Si photodetectors based on ultrafast laser hyperdoping are predicted.

Role of spatial coherence in diffractive optical neural networks

Diffractive optical neural networks (DONNs) have emerged as a promising optical hardware platform for ultra-fast and energy-efficient signal processing for machine learning tasks, particularly in computer vision. Previous experimental demonstrations of DONNs have only been performed using coherent light. However, many real-world DONN applications require consideration of the spatial coherence properties of the optical signals. Here, we study the role of spatial coherence in DONN operation and performance. We propose a numerical approach to efficiently simulate DONNs under incoherent and partially coherent input illumination and discuss the corresponding computational complexity. As a demonstration, we train and evaluate simulated DONNs on the MNIST dataset of handwritten digits to process light with varying spatial coherence.

Construction of microstructures on the Cu substrate using ultrafast laser processing to enhance the bonding strength of sintered Ag nanoparticles

Silver nanoparticle (Ag NP) pastes become a potential die-attachment material with the increased electronic power density. However, the weakness of bonding interface between sintered Ag NPs and bare Cu substrate limits the applications of the Ag NPs paste, thereby reducing the shear strength of the sintered joint. In this work, ultrafast laser processing is utilized to enhance the bonding strength of the sintered Ag joint by fabricating a microstructure interface. The microstructure dimensions are tunable by controlling laser parameters, and then high-strength joints could be obtained. Different substrate microstructures were constructed, and the enhanced bonding mechanism was analyzed by characterizing the cross section and fracture surface morphologies of joints. The ultrafast laser processing could increase the surface energy of Cu substrates to form a more reliable connection with Ag NPs and more energy required for crack extension with the increasing connection area, thereby resulting in a significant improvement in the shear strength of the Ag NP joints. The patterned microstructures on the Cu substrate using this technique showed improved surface energy and increased number of connection areas on the substrate, showing potential for the use in third-generation semiconductors for highly reliable packaging.

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations.

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.