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High-harmonic generation (HHG) of extreme ultraviolet (EUV) radiation enables ultrafast spectroscopy and nanoscale coherent imaging with timing resolutions down to the attosecond regime. However, beam manipulations such as steering and focusing remain a major challenge for handy implementation of such applications towards the achievement of a wavelength-scale spatial resolution. Here, we present a solid-based non-collinear HHG scheme mediating the propagation control and helical wavefront generation commanded via a spatial light modulator. The phase-matching-insensitive nature of the solid-HHG leads to the coherent multi-fold conversion of wavefronts enabling active control of the EUV harmonic beam propagation. Further, EUV harmonics generated by double-annular beams were converted to the Bessel vortex beam, for the first time, narrowing the beam diameter to 3.4 wavelengths with a long millimetre-level depth-of-focus without extra EUV-dedicated optical components. Our results will suggest a solid-HHG as a powerful tool for high photon-energy applications with a nanoscale spatial resolution.

Real-time monitoring of temperatures over extensive free-form surfaces of precision machines, smart products, and human bodies with a high resolution can provide invaluable information for smart manufacturing, Internet-of-Things, and advanced healthcare. However, traditional rigid thermistors could not be conformally attached on arbitrarily curved surfaces. In this study, a high-resolution flexible graphene thermistor is demonstrated by transforming wood into laser-induced-graphene via ultrafast laser pulses and subsequent transfer to flexible substrates. This thermistor provides a 16-times higher resolution than the state-of-the-art counterparts which was applied to precise temperature monitoring of an electric motor, glass cup, and human hand.

Hybrid optical elements, which combine refractive and diffractive optical components to enhance optical performance by taking advantage of the optical characteristics of the individual components, have enormous potential for next-generation optical devices. However, there have not been many reports on the simulation methodology to characterize such hybrid optical systems. Here, we present a method for simulating a hybrid optical element realized by attaching an ultra-thin, flexible diffractive optics array onto a refractive optical element. The ultra-thin diffractive optical element is fabricated by direct-laser-writing using a femtosecond pulsed laser as the light source. A systematic investigation of the proposed simulation method, which does not require extensive hardware resources or computational time, but retains resolution and accuracy, is presented. The proposed scheme is validated by comparing simulation and experimental results. The simulation and experimental results on the spot size and focal length for the diffractive Fresnel zone plate (FZP) match well, with typical errors of less than 6%. The aspect ratio of the focal spot sizes at the compound and FZP focal planes of the hybrid optical system from the simulation and experiment also match quite well, with typical errors below 7%. This simulation scheme will expedite the designs for novel hybrid optical systems with optimal optical performances for specific applications, such as microfluidics and aberration-controlled optics. Nanyang Technological University This work was financially supported by a research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA15/027); National Research Foundation of the Republic of Korea (NRF-2012R1A3A1050386, 2020R1A2C2102338, 2021R1A4A1031660); Korea Forest Service (Korea Forestry Promotion Institute) through the R & D Program for Forest Science Technology (2020229C10-2022-AC01); and Basic Research Program funded by the Korea Institute of Machinery and Materials (NK224C).

Laser ablation of metal thin films draws attention as a fast means of clean micropatterning. In this study, we attempt to remove only the metal thin film layer selectively without leaving thermal damage on the underneath substrate. Specifically, our single-pulse ablation experiment followed by two-temperature analysis explains that selective ablation can be achieved for gold (Au) films of 50–100 nm thickness by the lift-off process induced as a result of vaporization of the titanium (Ti) interlayer with a strong electron–phonon coupling. With increasing the film thickness comparable to the mean free path of electrons (100 nm), the pulse duration has to be taken shorter than 10 ps, as high-temperature electrons generated by the ultrashort pulses transfer heat to the Ti interlayer. We verify the lift-off ablation by implementing millimeters-scale micropatterning of optoelectronic devices without degradation of optical properties.

In this investigation, we describe a simple cyclic radial shearing interferometer for single-shot wavefront sensing. Instead of using the telescope lens system used in typical radial shearing interferometry, a single lens is used to generate two diverging radial shearing beams. This simple modification leads to the advantages of conveniently adjusting the radial shearing ratio, compactness of the system, and practical ease of alignment. With the aid of a polarization pixelated CMOS camera, the spatial phase-shifting technique is used to extract the phase with a single image. The most important feature is the fringe contrast enhancement by reducing the aberrations caused by the complicated optical system even though an incoherent light is used. The experimental results show the fringe contrast enhancement is at least 0.1 better than that of the conventional method, and the wavefronts are properly reconstructed with less than 0.071 λ root-mean-squared wavefront error regardless of the coherence of the light.

High harmonic generation (HHG) from solids offers a convenient means of producing coherent extreme ultraviolet (EUV) radiation. However, the achievable harmonic yield is greatly affected by propagation effects that the driving laser pulse experiences with spatiotemporal distortion when passing through solids. Here, we intend to optimize EUV harmonics generated from crystals by pre-shaping the driving laser pulse via spectral phase modulation through a genetic algorithm. Our experimental results obtained from MgO, sapphire and quartz crystals demonstrate EUV yield improvements, without increasing the incident laser fluence, up to an order of magnitude along with cut-off extension and selective harmonic enhancement. As a practical tool of compensating laser propagation effects, our method is expected to facilitate solid-based HHG applications such as high-energy photon spectroscopy and multi-petahertz signal processing.

We investigated the non-periodic nano-structural modulation of crystalline silicon surface enabled by a tightly focused ultrashort pulse laser. The focused annular beam reduces the cross-sectional area of electron multiplication to be smaller than the wavelength of the surface electromagnetic wave which is represented by the surface plasmon polaritons. The silicon wafer surface was amorphized with a consistent narrow line profile beyond the diffraction limit by suppressing the formation of the LIPSS-induced side ripples. We also revealed that the important excitation of free carriers by an ultrashort pulse shorter than 1.7 ps leads to the amorphization below the ablation threshold, thereby bulged uniformly in the range of 6–28 nm with a sub-micron linewidth. This structural change shows its potential applications in nanoscale surface structuring with advanced height-controllability in the nanometer-scale.

Selective thin-film removal is needed in many microfabrication processes such as 3-D patterning of optoelectronic devices and localized repairing of integrated circuits. Various wet or dry etching methods are available, but laser machining is a tool of green manufacturing as it can remove thin films by ablation without use of toxic chemicals. However, laser ablation causes thermal damage on neighboring patterns and underneath substrates, hindering its extensive use with high precision and integrity. Here, using ultrashort laser pulses of sub-picosecond duration, we demonstrate an ultrafast mechanism of laser ablation that leads to selective removal of a thin metal film with minimal damage on the substrate. The ultrafast laser ablation is accomplished with the insertion of a transition metal interlayer that offers high electron-phonon coupling to trigger vaporization in a picosecond timescale. This contained form of heat transfer permits lifting off the metal thin-film layer while blocking heat conduction to the substrate. Our ultrafast scheme of selective thin film removal is analytically validated using a two-temperature model of heat transfer between electrons and phonons in material. Further, experimental verification is made using 0.2 ps laser pulses by micropatterning metal films for various applications., Comment: 19 pages, 4 figures. This manuscript has been submitted to International Journal of Precision Engineering and Manufacturing-Green Technology

We discuss how to realize rigorous single pulse imaging using a fiber mode-locked laser for the purpose of ultrafast interferometric observation of fast varying dynamic objects. Sub-picosecond pulses are readily picked up in synchronization with the camera operation, allocating one pulse per frame, but rigorous ultrashort single pulse imaging is disturbed by the accumulation of amplified spontaneous emission (ASE) over the exposure time of the camera. Here, we propose four distinct methods to eliminate the ASE-accumulated disruption in the ultrashort optical gating by pulse interferometry and then evaluate their merits and limitations individually by experiments. The proposed four methods are referred to respectively as the time averaged phase modulation, unbalanced pulse overlapping, tandem pulse picking, and second harmonic generation.

Substrate side irradiation is widely used for a thin film removal process because high absorption at the film/substrate or film/film interface leads to complete isolation of thin film by single shot irradiation of laser pulse with low energy. However, in the transparent thin film removal process, large thermal expansion or local phase change at the interface cannot be created by substrate side irradiation because of its large optical penetration depth compared to its small thickness. Nevertheless, substrate side irradiation works obviously for single shot film isolation process compared to film side irradiation, and the mechanism of the process was not clear in terms of difference in the irradiation direction. In order to investigate the effect of the irradiation direction, this study focused on the transient interaction between the material and nanosecond laser pulse. Experimental results showed that film was thermally ablated. Variation of temporal profile of nanosecond laser pulse during the process was experimentally investigated to detect plasma shielding. Pulse width and energy transmittance of transmitted pulse decreased by plasma shielding as pulse energy increases regardless of irradiation direction. In addition, temperature distribution in the film during the process was investigated using a 2-dimensional thermal model, which accounts for melting, vaporization, and laser induced plasma shielding. Calculated temperature distribution was used to support the scenario of the process mechanism which was investigated in the experiments. Our findings demonstrate that laser induced backward ablation is a single shot TCO film removal mechanism, and plasma shielding is dominant factor to interrupt absorption of beam thorough the film in the film side irradiation process.

In this study, a high repetition rate laser ultrasonic system is developed for noncontact detection of silicon wafer crack. A high repetition rate pulse laser is used to generate two narrowband ultrasonic waves on a target silicon wafer, whereas a high-speed laser Doppler vibrometer is used to measure the corresponding ultrasonic responses. Nonlinear interaction occurs between the two generated ultrasonic waves due to crack formation, producing nonlinear ultrasonic modulation at the sum and difference of the two excitation frequencies. Outlier analysis is performed on the amplitudes of the nonlinear modulation components, and a fatigue crack is identified when the amplitudes of the nonlinear modulation components exceed a user specified threshold. The uniqueness of this study is as follows. (1) A noncontact, high-speed, and scanning laser ultrasonic system is developed using a high repetition rate laser for ultrasonic generation and a laser Doppler vibrometer for sensing as well as scanning the two laser beams simultaneously in a pitch-catch mode. (2) Two narrowband ultrasonic waves up to 1 MHz are simultaneously generated from a single laser pulse. (3) Micro cracks with widths of less than 10 µm are successfully detected by extracting the crack induced nonlinear ultrasonic modulation components. (4) The probability of detection of the proposed crack detection technique is evaluated. The performance of the proposed technique is experimentally validated using eight silicon wafer specimens with varying crack lengths. Results show that the proposed technique yields a 90% probability of detection in silicon wafers when the crack length is over 3.77 mm.

The fluidic optical device by means of the thermal lens effect has attracted increasing attention as a flexible beam shaper. However, onset of natural convection inhibits operation of the fluidic optical device as it causes asymmetric distortion on shaped beam profile. For implementation of the fluidic beam shaper, this adverse effect should be controlled in terms of the beam profile change. From this perspective, we investigated the influence of natural convection on the propagating beam profile through the fluidic optical device by means of CW laser induced the thermal lens effect. Numerical method taking advantage of beam propagation method was used to calculate modulation of the probe beam profile. The calculated results demonstrated good agreements with several experimental results. Using this numerical model, the parametric study to control asymmetry and quality of the annular beam was performed quantitatively in the dual beam thermal lens system. The core of this study is that the solution of the simple pentadiagonal equation can provide the crucial investigation on the modulation of the beam profile in the fluidic optical device with natural convection.

A solar thermochemical reactor is a device utilizing concentrated solar energy to conduct hydrogen gas production by two-step water-splitting by dissociation of a reactive material, such as zinc oxide (ZnO). Reactor design, heat transfer, and reaction kinetics contribute a significant portion to the achievement of high solar-to-fuel conversion efficiency. In this work, an investigation of an indirect-cavity type reactor design performance has been conducted by numerical simulation method by coupling the computational model of fluid flow, energy equation, discrete ordinate radiation, and species transport. The reactor consisted of a windowed cavity reactor with an array of five tubes containing the flow of the reactive material. Dissociation of ZnO in a steady state condition of the reactor has been assessed under 1,500 sun heat flux from quartz window. A parametric study has been performed for a variation of the particle’s mass flow rate, solar flux peak in, and reactor configuration. The cavity of the reactor was insulated by a ceramic and reflective material to reduce the conduction and radiation losses. Inert gas of Ar was injected into the tube as product carrier. Energy balance analysis and reactor efficiency calculation have been performed to analyze the reactor performance. The results showed that the thermal re-radiation through the window and thermal conduction through the cavity wall dominated the heat losses around 84 % in total. The best operating condition in this study was at a mass flow rate 0.05 g/s, peak-heat-in 2,000 kW/m2, and tubes configuration of the staggered-front dominant. Some recommendations to improve the research include changing the chemical reactant with other metal-oxide which has a lower reactivity, increasing the tubes number to absorb the solar irradiation, combining the metal-oxide decomposition with other processes which require less heat, and applying the special material in window side which can filter the high wavelength from going outside the reactor to decrease the re-radiation losses.