Search

Epsilon‐near‐zero (ENZ) materials have shown strong refractive nonlinearities that can be fast in an absolute sense. While continuing to advance fundamental science, such as time varying interactions, the community is still searching for an application that can effectively make use of the strong index modulation offered. Here, the effect of strong space‐time index modulation in ENZ materials is combined with the beam deflection technique to introduce a new approach to optical pulse characterization that is termed a space‐time knife edge. It is shown that in this approach, temporal and spatial information of a Gaussian beam can be extracted with only two time resolved measurements. The approach achieves this without phase‐matching requirements (<1 µm thick) and can achieve a high signal to noise ratio by combining the system with lock‐in detection, facilitating the measurement of weak refractive index changes (Δn ≈$ \approx $ 10−5) for low intensity beams. Thus, the space‐time knife edge can offer a new avenue for ultrafast light measurement and demonstrates a use case of ENZ materials. In support of this, temporal dynamics for refractive index changes in non‐colinear experiments opening avenues are outlined for better theoretical understanding of both the spatial and temporal dynamics of emerging ENZ films. [ABSTRACT FROM AUTHOR]

Coherent wave control is of key importance across a broad range of fields such as electromagnetics, photonics, and acoustics. It enables us to amplify or suppress the outgoing waves via engineering amplitudes and phases of multiple incidences. However, within a purely spatially (temporally) engineered medium, coherent wave control requires the frequency of the associated incidences to be identical (opposite). In this work, this conventional constraint is broken by generalizing coherent wave control into a spatiotemporally engineered medium is broken, i.e., the system featuring a dynamic interface. Owing to the broken translational symmetry in space and time, both the subluminal and superluminal interfaces allow interference between scattered waves regardless of their different frequencies and wavevectors. Hence, one can flexibly eliminate the backward‐ or forward‐propagating waves scattered from the dynamic interfaces by controlling the incident amplitudes and phases. The work not only presents a generalized way for reshaping arbitrary waveforms but also provides a promising paradigm to generate ultrafast pulses using low‐frequency signals. It has also implemented suppression of forward‐propagating waves in microstrip transmission lines with fast photodiode switches. [ABSTRACT FROM AUTHOR]

Terahertz (THz) radiation sources based on two‐color femtosecond plasmas in air are becoming a mature technology for coherent spectroscopy and strong‐field physics across the extended THz range to several tens of THz. The field‐resolved detection of such THz transients relies on the third‐order nonlinearity of the detection medium. Here, a comparative measurement is demonstrated with air‐biased coherent detection (ABCD) and solid‐state biased detection (SSBCD) as a novel method to measure the dispersion of the magnitude and phase of the relevant third‐order nonlinearity χ(3)(2ω±Ω;ω,ω,±Ω)$\chi ^{(3)}(2\omega \pm \Omega;\omega,\omega,\pm \Omega)$ for fused silica (SiO2${\rm SiO}_2$) and silicon nitride (SiN). Based on the development of the ultrabroadband SSBCD device with a detection bandwidth exceeding 30 THz, χ(3)$\chi ^{(3)}$ measurements are obtained across the 1–28 THz frequency range, hence covering the THz and far‐infrared. It is shown that the vibrational modes in SiO2${\rm SiO}_2$ and SiN in the THz range lead to strong resonant enhancement and dispersion of the nonlinearity. The SSBCD devices operate down to nanojoule (nJ) probe energy, and their is demonstrated by measuring the dielectric function of the Lorentzian line profile of transverse‐optical (TO) phonon mode at 9 THz in single‐crystal gallium arsenide (GaAs) and observing the weak phonon combination bands near the TO phonon. [ABSTRACT FROM AUTHOR]

Copyright of Frontiers of Information Technology & Electronic Engineering is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Ultrashort light pulses have strong research and application values, while nonlinear inverse Thomson scattering has been considered as a unique source of zepto-yoctosecond pulses. Here, the mechanism of nonlinear inverse Thomson scattering of a high-energy electron colliding with a tightly focused intense laser pulse is investigated through numerical simulation. The time-compression effect was proposed to explain the origin of ultrashort pulses and the nonlinear phenomenon of electron radiation in the time–space joint distribution. It is found that the time scale of electron radiation is orders of magnitude shorter than that of electron motion, and the increases in laser intensity and electron initial energy will result in stronger and shorter pulses. Yoctosecond pulses can be generated by a laser pulse with an intensity of 1.384 × 10 20   W / c m 2 and an electron with an initial energy of 51.1   M e V . These results provide theoretical and numerical basis for generating shorter light pulses. [ABSTRACT FROM AUTHOR]

In this article, the impact of the excited electromagnetic surface modes in a comprehensive investigation of the formation of laser‐induced periodic surface structures (LIPSS) is analyzed. It is demonstrated that the electromagnetic origin of low spatial‐frequency LIPSS (LSFL) is the frequency detuning between propagating and localized modes due to their coupling/hybridization. The influence of the pattern profile, inhomogeneity, and material type on the coupling strength, electric‐field spatial distribution, and associated near‐field scattering are highlighted. Exploiting the potential of the approach, evidence of a universal manifestation of LSFLs is provided irrespective of the material and the authors are able to predict and validate the experimentally‐proven lower limit of LSFL periodicity (i.e., λL/2, where λL stands for the laser wavelength). Furthermore, the analysis of the electromagnetic modes predicts that the periodicity of LSFL is practically unaffected by the laser fluence, while a suppression of LSFL at high excitation levels or large number of pulses is also predicted. It is also shown that plasmonic‐active materials are not necessary for LSFL formation perpendicular to polarization. Toward these directions, an important generic metric, namely the resonance quality factor, is additionally inserted. The approach can, thus, serve as a guide for controlling laser‐induced surface topography. [ABSTRACT FROM AUTHOR]

We describe a family of paraxial and quasi-monochromatic optical wave packets with finite energy and smoothly shaped amplitude in space and time that develops a singularity in the intensity when spatio-temporally focused by imparting a converging spherical wavefront and a negative temporal chirp. This singular behavior upon ideal focusing is manifested in actual focusing with finite apertures and in media with high-order dispersion with "exploding" behavior featuring an indefinitely increasing concentration of the energy when opening the aperture radius, thus exercising continuous control on the focal intensity and spatial and temporal resolution. These wave packets offer a new way of focusing that outperforms what can be achieved with standard Gaussian wave packets in terms of focal intensity and resolution, providing new possibilities in applications where energy concentration and its control are crucial. [ABSTRACT FROM AUTHOR]

Pulse self‐compression can offer a more compact and efficient solution for improving the peak power of ultraintense laser pulses. By solving a modified nonlinear Schrödinger equation considering the fifth‐order susceptibility, it is found that self‐compression occurred even in a normally dispersive medium owing to the negative fifth‐order susceptibility inducing a mass of negative frequency chirp. Furthermore, negatively prechirped pulses enabled self‐compression at lower intensities, thus preventing damage to the medium. The optimal choices for prechirp, input intensity, and medium length are numerically analyzed. A proof‐of‐principle experiment confirmed the aforementioned theoretical findings. It is expected that petawatt or even exawatt laser pulses with 25 fs/15 fs transform‐limited pulse duration can be self‐compressed to ≈ 9.9 fs/7.6 fs in normally dispersive media, such as fused silica glass plate. [ABSTRACT FROM AUTHOR]

Stimulated Raman scattering (SRS) microscopy is a high-speed imaging modality based on intrinsic molecular vibrations, producing chemical maps in living systems. Such capability, allowing for direct visualization without the perturbation of biological processes, has enabled a plethora of biological and medical applications. In this review, after introducing the basic theory and competitive effects of SRS, some crucial features for SRS microscopy implementations, such as noise, spectral bandwidth, speed, chemical sensitivity, spatial resolution, and quantum enhancement, are discussed. Finally, some SRS applications in biological and medical imaging are described. Even if certainly not exhaustive, we aimed to offer a broad overview, providing guidance for newcomers and hinting at a more detailed investigation to interested researchers in this rapidly growing field. [ABSTRACT FROM AUTHOR]

The emerging field of lightwave electronics is driving optoelectronic information processing beyond current classical limitations and has the potential to eventually reach petahertz frequencies. One of the major obstacles in reaching not only higher switching frequencies but also higher repetition rates with low‐power light sources is the efficiency of related devices. A device is presented based on a multilayered material that shows a 13‐fold enhancement in terms of converting light to electric current compared to bulk solids. Furthermore, the device exhibits an almost flat intensity response within its working range. The outstanding properties of engineered multilayered devices promise to push technology for lightwave electronics applications. [ABSTRACT FROM AUTHOR]

Femtosecond pulsed lasers are widely used in applied technologies, such as laser processing, nonlinear optical microscopy, and time-of-flight measurement, wherein narrowing the pulse duration is particularly important for improving the processing efficiency and signal-to-noise ratio of measurement systems. This paper proposes a method for shortening temporal pulse waveforms using spectral intensity filtering. Symmetrical spectral filtering was specifically applied at the central wavelength of the femtosecond pulsed laser spectrum. To investigate the pulse-narrowing effect, we numerically simulated the pulse duration as a function of the filtering width and cut-off wavelength position. The results of these simulations showed that the pulse duration decreased as the filter wavelength approached the central wavelength of the light. Furthermore, increasing the filter width reduced the pulse duration. Additionally, we implemented a spatial light modulator-based pulse-shaping system to realize a spectral intensity filter. We experimentally demonstrated that the duration of a telecom-band pulse was reduced by 9.8% when using a spectral intensity filter with a width of 1 nm. [ABSTRACT FROM AUTHOR]

The acousto-optic modulation over a broad near-infrared (NIR) spectrum with high speed, excellent integrability, and relatively simple scheme is crucial for the application of next-generation opto-electronic and photonic devices. This study aims to experimentally demonstrate ultrafast acousto-optic phenomena in the broad NIR spectral range of 0.77–1.1 eV (1130–1610 nm). Hundreds of GHz of light modulation are revealed in an all-optical configuration by combining ultrafast optical spectroscopy and light–sound conversion in 10–20 nm-thick bismuth selenide (Bi2Se3) van der Waals thin films. The modified optical transition energy and the line shape in the NIR band indicate phonon–photon interactions, resulting in a modulation of optical characteristics by the photoexcited interlayer vibrations in Bi2Se3. This all-optical, ultrafast acousto-optic modulation approach may open avenues for next-generation nanophotonic applications, including optical communications and processing, due to the synergistic combination of large-area capability, high photo-responsivity, and frequency tunability in the NIR spectral range.

With the development of high-frequency carriers and high-capacity communications, ultrahigh-speed spectrum analyzer based on all-optical methods has been demonstrated in many fields. Limited by current data processing algorithms, ultrahigh-speed spectrum analyzers based on time-lens, such as frequency-domain light intensity spectrum analyzer (f-LISA) and parametric spectro-temporal analyzer (PASTA), are not yet to automatically process acquired data, which hinders their practicality. In this paper, a frame segmentation algorithm based on time-domain waveform analysis is proposed to achieve the automatic data processing of these analyzers, which can directly generate the spectro-temporal image of the signal under test. In a single-soliton test based on the f-LISA platform, the algorithm achieves an accuracy of 93.28% when the soliton changes and 90.04% when the soliton is stable. And in this test, the single-frame processing time of the algorithm is 419us. We believe the proposed algorithm will facilitate the practical application of spectrum analyzers like f-LISA and PASTA and provide a reference for other real-time measurement schemes.

Understanding the fundamentals of laser-matter interactions is crucial for developing and optimizing ultrafast laser processing strategies. In optically transparent solids, the key event by which energy is deposited in the material is through the generation of an electron–hole plasma via nonlinear excitation mechanisms. As the energy stored in the plasma relaxes, local distortions of the lattice may occur, such as point defects. These defects give rise to new discrete energy states located in the bandgap. In this study, we investigate how the presence of these energy states influences the transmission of ultrashort near-infrared laser pulses in fused silica. Experimental results of laser pulse transmission and photoluminescence from defects are correlated with optical microscopy of the irradiated spots, allowing us to identify different nonlinear interaction regimes. Numerical simulations indicate that photo-induced defects influence the nonlinear losses of ultrashort laser pulses and explain why a non-destructive damage regime with detectable excitation is only observed for a narrow intensity range in multipulse experiments. [ABSTRACT FROM AUTHOR]

Ultrashort electron beams with femtosecond to picosecond bunch durations offer unique opportunities to explore active research areas ranging from ultrafast structural dynamics to ultra‐high dose‐rate radiobiological studies. It presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few‐cycle, mJ‐class femtosecond infrared laser. It demonstrates experimentally that electrons can reach up to 1.4 MeV at a dose‐rate of 0.15 Gy/s, providing enough dose rate for radiation therapy applications. 3D Particle‐In‐Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Relativistic peak intensities up to 1019 Wcm−2 are reached in ambient air due to a very low B‐integral accumulation during focusing, which prevents intensity clamping. Furthermore, it discusses the scalability of this method with the continuing development of mJ‐class high average power lasers, and providing a promising approach for FLASH radiation therapy. [ABSTRACT FROM AUTHOR]

With the development of laser technology, microstructured optical fibers (MOFs) have become an important part of ultrafast optics, providing excellent platforms for ultrafast laser pulse generation, amplification, and compression, promoting the development of fiber laser systems to generate high power, high pulse energy, and few-cycle duration pulses. MOFs extend the ultrafast laser spectrum to the vacuum ultraviolet (VUV) and even extreme ultraviolet (EUV) regions based on dispersive wave emission and high harmonic generation, as well as to the mid-infrared region based on soliton self-frequency shift (SSFS), contributing compact and low-cost light sources for precision microscopy and spectroscopy. In this paper, first several common types of MOFs are introduced, then the various applications of MOFs in ultrafast optics are discussed, mainly focusing on the aspects of ultrafast laser pulse scaling in pulse energy and spectral bandwidth, and finally the possible prospects of MOFs are given. [ABSTRACT FROM AUTHOR]

Ultrashort light pulses have strong research and application values, while nonlinear inverse Thomson scattering has been considered as a unique source of zepto-yoctosecond pulses. Here, the mechanism of nonlinear inverse Thomson scattering of a high-energy electron colliding with a tightly focused intense laser pulse is investigated through numerical simulation. The time-compression effect was proposed to explain the origin of ultrashort pulses and the nonlinear phenomenon of electron radiation in the time–space joint distribution. It is found that the time scale of electron radiation is orders of magnitude shorter than that of electron motion, and the increases in laser intensity and electron initial energy will result in stronger and shorter pulses. Yoctosecond pulses can be generated by a laser pulse with an intensity of 1.384×1020 W/cm2 and an electron with an initial energy of 51.1 MeV. These results provide theoretical and numerical basis for generating shorter light pulses.

We describe a family of paraxial and quasi-monochromatic optical wave packets with finite energy and smoothly shaped amplitude in space and time that develops a singularity in the intensity when spatio-temporally focused by imparting a converging spherical wavefront and a negative temporal chirp. This singular behavior upon ideal focusing is manifested in actual focusing with finite apertures and in media with high-order dispersion with “exploding” behavior featuring an indefinitely increasing concentration of the energy when opening the aperture radius, thus exercising continuous control on the focal intensity and spatial and temporal resolution. These wave packets offer a new way of focusing that outperforms what can be achieved with standard Gaussian wave packets in terms of focal intensity and resolution, providing new possibilities in applications where energy concentration and its control are crucial.

Scattering-type scanning near-field optical microscopy (s-SNOM) has become a powerful tool for subwavelength-resolved imaging and optical spectroscopy. The technique is particularly useful in the terahertz or other long-wavelength regimes, where the diffraction limit prohibits resolving even micron-sized objects. This approach can also drastically improve the spatial resolution of nonlinear measurements such as laser terahertz emission microscopy (LTEM), in which a broadband THz pulse is generated after photoexcitation by a near-infrared (NIR) pulse. However, in all prior near-field LTEM experiments, the pump spot has been localized to the scattering probe, to couple to the generated THz radiation at the center of the illuminated region. Here, we demonstrate the first nonlocal near-field LTEM experiments, in which an ultrafast NIR pulse photoexcites a sample at a location that is laterally shifted from the location of the near-field probe which couples the THz signal to the far field. Increasing lateral shifts of the pump spot produce larger time delays in the arrival of the emitted broadband THz pulse, consistent with drift of the subsurface dipole between the pump location and probe tip. Monte Carlo simulations corroborate the time shift for the dipole formation, which in turn produces the THz emission with a relative delay. The simulation results show excellent agreement with experiments. This nonlocal s-SNOM approach to LTEM offers a new opportunity for studying lateral transport on the nanoscale and may be particularly useful in anisotropic materials. [ABSTRACT FROM AUTHOR]

We present an experimental study of optical time-refraction caused by time-interfaces as short as a single optical cycle. Specifically, we study the propagation of a probe pulse through a sample undergoing a large refractive index change induced by an intense modulator pulse. In these systems, increasing the refractive index abruptly leads to time-refraction where the spectrum of all the waves propagating in the medium is red-shifted, and subsequently blue-shifted when the refractive index relaxes back to its original value. We observe these phenomena in the single-cycle regime. Moreover, by shortening the temporal width of the modulator to ∼5–6 fs, we observe that the rise time of the red-shift associated with time-refraction is proportionally shorter. The experiments are carried out in transparent conducting oxides acting as epsilon-near-zero materials. These observations raise multiple questions on the fundamental physics occurring within such ultrashort time frames, and open the way for experimenting with photonic time-crystals, generated by periodic ultrafast changes to the refractive index, in the near future.

Stimulated Raman scattering (SRS) microscopy is a high-speed imaging modality based on intrinsic molecular vibrations, producing chemical maps in living systems. Such capability, allowing for direct visualization without the perturbation of biological processes, has enabled a plethora of biological and medical applications. In this review, after introducing the basic theory and competitive effects of SRS, some crucial features for SRS microscopy implementations, such as noise, spectral bandwidth, speed, chemical sensitivity, spatial resolution, and quantum enhancement, are discussed. Finally, some SRS applications in biological and medical imaging are described. Even if certainly not exhaustive, we aimed to offer a broad overview, providing guidance for newcomers and hinting at a more detailed investigation to interested researchers in this rapidly growing field.

The rate of the removal of materials coated with nano−crystalline diamonds by femtosecond laser etching was examined by adjusting the repetition rate of the femtosecond laser, the energy−flux density of the concentrated spot, and the scanning speed. The observational results of the white-light interferometer and the numerical fitting approach were used to develop the removal rate function model of the nano-crystalline diamond-covered material etched by the femtosecond laser. The findings demonstrated that the rate of material removal was not greatly affected by the repetition frequency and that the amount of laser energy accumulated over time on the coated surface is steady. The processing outcomes under different laser scanning speeds are different, and the material removal rate tends to increase and then decrease with an increase in scanning speed. The greater the energy−flux density of the focused spot, the greater the etching intensity, and the greater the material removal rate. With an increase in scanning speed, the rate at which the material is removed often rises initially before falling. [ABSTRACT FROM AUTHOR]

We present a detailed numerical analysis of the optical parametric amplification (OPA) process in 5 % MgO doped PPLN photonic crystals, for the efficient generation of signal at 3.4 μ m. The current work calls attention to understand the dynamics of femtosecond pump laser for transferring cascaded energy from the pump to signal under the influence of the self-action effect which is a corollary of Kerr nonlinear response, cascaded χ (2) nonlinearities, and higher-order dispersion effects. Here, the dynamics of frequency shift of generated photon in a PPLN crystal is studied through OPA process by solving coupled nonlinear partial differential equation. It is observed that a small digression in the domain width from the coherence length profoundly increases the efficiency of the signal. The optimization condition for this frequency conversion process is discussed in detail using the pump pulse energy, domain size and input pulse width parameters along with the effects of cubic nonlinearity. [ABSTRACT FROM AUTHOR]

As an exceptional nonlinear material, graphene offers versatile appealing properties, such as electro-optic tunability and high electromagnetic field confinement in the terahertz regime, spurring advance in ultrashort pulse formation, photodetectors and plasmonic emission. However, limited by atomic thickness, weak light–matter interaction still limits the development of integrated optical devices based on graphene. Here, an exquisitely designed meta-cavities combined with patterned graphene is used to overcome this challenge and promote THz-graphene interaction via terahertz location oscillation. By using an 800 nm pump laser, the local field-induced strong interaction allows sensitive responses to the ultrafast energy transfer from the ultrafast optical pump to graphene electron heat, enabling 46.2% enhancement of terahertz transparency. Such optical modulation of terahertz waves shows ultrafast response in delay less than 10 ps. Moreover, thanks to the nature of graphene, the device shows unique potential for electrically dynamic tuning and further bandwidth broadening.

Chalcogenide phase‐change materials (PCMs) have offered an appealing material solution by acting as a switchable dielectric layer to tune the electromagnetic properties of terahertz metamaterials and metasurfaces. Here, this work demonstrates large‐scale and lithography‐free manufacturing of all‐PCM terahertz metasurfaces based on direct laser switching of crystalline micro‐domains in a thin film with high switching ratio of the emerging plasmonic PCM, In3SbTe2 (IST). The fabricated high‐quality IST metasurfaces achieve efficient plasmonic resonances and a large modulation depth with ultrafast response (full width at half maxima of the modulation time ≈1.6 ps) in a deep‐subwavelength switching volume. For the dynamic evolution of terahertz resonance modes, theoretical modeling reveals a delicate interplay between amorphous and crystalline IST due to the bonding‐structure‐induced different carrier lifetimes and spatially localized electric fields. These studies open new avenues for realizing all‐PCM terahertz ultrafast nanophotonics. [ABSTRACT FROM AUTHOR]

Direct measurements of the electric field of light enable new observations of light–matter interactions. In the near‐infrared and visible spectral ranges, this typically relies on techniques that exploit nonlinearities in gases or solids, which limits their sensitivity. Here, a method for the detection of broadband near‐infrared fields spanning more than one octave from 110 to 220 THz based on linear absorption in a semiconductor is demonstrated. This technique, which avoids complex vacuum setups and works under ambient conditions, employs linear photoconductive sampling (LPS) in gallium phosphide. Simulations reveal that the response function of LPS is concerned with the intensity envelope of the gate field, in contrast to electro‐optic sampling, relaxing the stringent temporal requirements on the gate pulse. [ABSTRACT FROM AUTHOR]

This work presents a general framework for quantum interference between processes that can involve different fundamental particles or quasi‐particles. This framework shows that shaping input wavefunctions is a versatile and powerful tool for producing and controlling quantum interference between distinguishable pathways, beyond previously explored quantum interference between indistinguishable pathways. Two examples of quantum interference enabled by shaping in interactions between free electrons, bound electrons, and photons are presented: i) the vanishing of the zero‐loss peak by destructive quantum interference when a shaped electron wavepacket couples to light, under conditions where the electron's zero‐loss peak otherwise dominates; ii) quantum interference between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that a free electron and an atom must be close by to significantly affect each other's processes. Conclusions show that emerging quantum wave‐shaping techniques unlock the door to greater versatility in light‐matter interactions and other quantum processes in general. [ABSTRACT FROM AUTHOR]

With the development of laser technology, microstructured optical fibers (MOFs) have become an important part of ultrafast optics, providing excellent platforms for ultrafast laser pulse generation, amplification, and compression, promoting the development of fiber laser systems to generate high power, high pulse energy, and few-cycle duration pulses. MOFs extend the ultrafast laser spectrum to the vacuum ultraviolet (VUV) and even extreme ultraviolet (EUV) regions based on dispersive wave emission and high harmonic generation, as well as to the mid-infrared region based on soliton self-frequency shift (SSFS), contributing compact and low-cost light sources for precision microscopy and spectroscopy. In this paper, first several common types of MOFs are introduced, then the various applications of MOFs in ultrafast optics are discussed, mainly focusing on the aspects of ultrafast laser pulse scaling in pulse energy and spectral bandwidth, and finally the possible prospects of MOFs are given.

We study the coherence properties of continuum generation in YAG crystals seeded by 180 fs pulses at 1035 nm when the driving beam exhibits small fluctuations of the spatial phase. The relative stability of the continuum spectral phase is first assessed as a function of the driver wavefront aberrations. Furthermore, we evidence and quantify a coupling mechanism between these fluctuations and the spectral phase of the continuum. The coupling coefficients increase with the spectral broadening and are also unexpectedly large (up to tens of rad/rad at $\simeq$ 750 nm). Experimental evidence supports that longitudinal shifts of the position of the filament within the crystal are responsible for such strong effects.

We investigate the generation of ultrabroadband femtosecond ultraviolet (UV) radiation via third-order harmonic generation in highly confined gas media. A dual-stage differential-pumping scheme integrated into a glass microfluidic chip provides an exceptional gas confinement up to several bar and allows the apparatus to be operated under high-vacuum environment. UV pulses are generated both in argon and neon with up to ∼0.8 μ J energy and 0.2% conversion efficiency for spectra that cover the UVB and UVC regions between 200 and 325 nm. Numerical simulations based on the unidirectional pulse propagation equation reveal that ionization plays a critical role for extending the spectral bandwidth of the generated third-harmonic pulse beyond the tripled 800 nm driving laser pulse bandwidth. By delivering UV supercontinua supporting Fourier transform limits below 2 fs, as well as comparable pulse energies with respect to capillary-based techniques that typically provide high spectral tunability but produce narrower bandwidths, our compact device makes a step forward towards the production and application of sub-fs UV pulses for the investigation of electron dynamics in neutral molecules.

Abstract This work presents a general framework for quantum interference between processes that can involve different fundamental particles or quasi‐particles. This framework shows that shaping input wavefunctions is a versatile and powerful tool for producing and controlling quantum interference between distinguishable pathways, beyond previously explored quantum interference between indistinguishable pathways. Two examples of quantum interference enabled by shaping in interactions between free electrons, bound electrons, and photons are presented: i) the vanishing of the zero‐loss peak by destructive quantum interference when a shaped electron wavepacket couples to light, under conditions where the electron's zero‐loss peak otherwise dominates; ii) quantum interference between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that a free electron and an atom must be close by to significantly affect each other's processes. Conclusions show that emerging quantum wave‐shaping techniques unlock the door to greater versatility in light‐matter interactions and other quantum processes in general.

Ultrafast optical switching in plasmonic platforms relies on the third-order Kerr nonlinearity, which is tightly linked to the dynamics of hot carriers in nanostructured metals. Although extensively utilized, a fundamental understanding on the dependence of the switching dynamics upon optical resonances has often been overlooked. Here, all-optical control of resonance bands in a hybrid photonic-plasmonic crystal is employed as an empowering technique for probing the resonance-dependent switching dynamics upon hot carrier formation. Differential optical transmission measurements reveal an enhanced switching performance near the anti-crossing point arising from strong coupling between local and nonlocal resonance modes. Furthermore, entangled with hot-carrier dynamics, the nonlinear correspondence between optical resonances and refractive index change results in tailorable dispersion of recovery speeds which can notably deviate from the characteristic lifetime of hot carriers. The comprehensive understanding provides new protocols for optically characterizing hot-carrier dynamics and optimizing resonance-based all-optical switches for operations across the visible spectrum. [ABSTRACT FROM AUTHOR]

Birefringence, which modulates the polarization of electromagnetic wave, has been commercially developed and widely used in modern photonics. Fostered by high-frequency signal processing and communications, feasible birefringence technologies operating in gigahertz (GHz) range are highly desired. Here, a coherent phonon-induced GHz optical birefringence and its manipulation in SrTiO3 (STO) crystals are demonsrated. With ultrafast laser pumping, the coherent acoustic phonons with low damping are created in the transducer/STO structures. A series of transducer layers are examined and the optimized one with relatively high photon-phonon conversion efficiency, i.e., semiconducting LaRhO3 film, is obtained. The most intriguing finding here is that, by virtue of high sensitivity to strain perturbation of STO, GHz optical birefringence can be induced by the coherent acoustic phonons and the birefringent amplitudes possess crystal orientation dependence. Optical manipulation of both coherent phonons and its induced GHz birefringence by double pump technique are also realized. These findings reveal an alternative mechanism of ultrafast optical birefringence control, and offer prospects for applications in high-frequency acoustic-optics devices. [ABSTRACT FROM AUTHOR]

Birefringence, which modulates the polarization of electromagnetic wave, has been commercially developed and widely used in modern photonics. Fostered by high-frequency signal processing and communications, feasible birefringence technologies operating in gigahertz (GHz) range are highly desired. Here, a coherent phonon-induced GHz optical birefringence and its manipulation in SrTiO3 (STO) crystals are demonsrated. With ultrafast laser pumping, the coherent acoustic phonons with low damping are created in the transducer/STO structures. A series of transducer layers are examined and the optimized one with relatively high photon-phonon conversion efficiency, i.e., semiconducting LaRhO3 film, is obtained. The most intriguing finding here is that, by virtue of high sensitivity to strain perturbation of STO, GHz optical birefringence can be induced by the coherent acoustic phonons and the birefringent amplitudes possess crystal orientation dependence. Optical manipulation of both coherent phonons and its induced GHz birefringence by double pump technique are also realized. These findings reveal an alternative mechanism of ultrafast optical birefringence control, and offer prospects for applications in high-frequency acoustic-optics devices. [ABSTRACT FROM AUTHOR]

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on "three-step scheme"—ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion). [ABSTRACT FROM AUTHOR]

Generation of coherent extreme ultraviolet (XUV) pulses is a nonlinear process of high harmonic generation (HHG). HHG produced in molecular nitrogen (N2) medium has been obtained using ultrashort intense laser pulses. The strong laser field was focused onto a gas cell to produce short wavelengths having photon energy up to 54 eV. The highest photon energy is experimentally observed as the 35th order. The absorption of harmonics restricts the harmonic yield efficiency. The harmonic yield is affected by the interaction length and medium pressure. The harmonic yield changes with the medium parameters. The simulation for the absorption length was performed using the Mathematica program. Variation of the harmonic signal has been attributed to the absorption of harmonics, and it is compared with the absorption length.

Pure Q-switching (self-pulsating) behaviour is shown for the first time in an optically injected two-section quantum-dot laser. Upon CW optical injection, it was possible to switch the operation regime from free-running mode-locking to locked Q-switching. Under this regime, the pulse repetition rate of around 1 GHz was tunable with injection power (by more than 100 MHz) and absorber reverse bias. Moreover, through injection-locking, the Q-switched output was spectrally tunable by around 7 nm, by tuning the master laser. The seamless switch between mode-locking and Q-switching with optical injection as well as the tunability and control afforded by injection-locking could open up a range of applications, such as in multi-modal imaging. On a more fundamental level, this investigation has also generated new insights into the dynamics of quantum-dot lasers and the optical injection process.

Abstract Ultrafast optical switching in plasmonic platforms relies on the third‐order Kerr nonlinearity, which is tightly linked to the dynamics of hot carriers in nanostructured metals. Although extensively utilized, a fundamental understanding on the dependence of the switching dynamics upon optical resonances has often been overlooked. Here, all‐optical control of resonance bands in a hybrid photonic‐plasmonic crystal is employed as an empowering technique for probing the resonance‐dependent switching dynamics upon hot carrier formation. Differential optical transmission measurements reveal an enhanced switching performance near the anti‐crossing point arising from strong coupling between local and nonlocal resonance modes. Furthermore, entangled with hot‐carrier dynamics, the nonlinear correspondence between optical resonances and refractive index change results in tailorable dispersion of recovery speeds which can notably deviate from the characteristic lifetime of hot carriers. The comprehensive understanding provides new protocols for optically characterizing hot‐carrier dynamics and optimizing resonance‐based all‐optical switches for operations across the visible spectrum.

Abstract Birefringence, which modulates the polarization of electromagnetic wave, has been commercially developed and widely used in modern photonics. Fostered by high‐frequency signal processing and communications, feasible birefringence technologies operating in gigahertz (GHz) range are highly desired. Here, a coherent phonon‐induced GHz optical birefringence and its manipulation in SrTiO3 (STO) crystals are demonsrated. With ultrafast laser pumping, the coherent acoustic phonons with low damping are created in the transducer/STO structures. A series of transducer layers are examined and the optimized one with relatively high photon–phonon conversion efficiency, i.e., semiconducting LaRhO3 film, is obtained. The most intriguing finding here is that, by virtue of high sensitivity to strain perturbation of STO, GHz optical birefringence can be induced by the coherent acoustic phonons and the birefringent amplitudes possess crystal orientation dependence. Optical manipulation of both coherent phonons and its induced GHz birefringence by double pump technique are also realized. These findings reveal an alternative mechanism of ultrafast optical birefringence control, and offer prospects for applications in high‐frequency acoustic‐optics devices.

A method to provide bandwidth compressed time-mapped spectrogram analysis based on temporal Talbot effect and temporal magnification is proposed and numerically demonstrated in this paper. The signal under test (SUT) is firstly sampled by a periodic pulse train, then passes through a dispersive medium under the Talbot condition and the spectrogram of the SUT is mapped to the time domain, so-called time-mapped spectrogram (TM-SP). Subsequently, a temporal magnification technique is implemented to stretch the time axis and compress the spectrum bandwidth, so that the effect of detection bandwidth on the frequency resolution is alleviated. Though the TM-SP cannot be acquired in a continuous way, the SUT is analyzed with ultrahigh frame rate in the temporal aperture. Additionally, a method to improve acquisition frame rate is proposed. The proposed bandwidth compressed TM-SP analysis is compared with the dispersion-based Fourier transform, and the related tradeoffs are provided in detail. The bandwidth compressed TM-SP analysis with a frequency resolution of 0.1 GHz, a measurable frequency range of 4.85 GHz, an analysis period of 0.1 ns, and an observation period of 4 ns is achieved by simulation. [ABSTRACT FROM AUTHOR]

We present a numerical approach for analysing simultaneous amplification and compression of a chirped hyperbolic secant soliton using a tapered Yb-doped photonic crystal fiber (PCF) based self-similar compressor. Theoretical studies reveal the conservation of input soliton during the amplification process due to self-similar property, which in turn helps for high quality compression of amplified soliton. In order to satisfy the self-similar condition, we note that the Yb-doped fiber should exhibit a decreasing group velocity dispersion (GVD). Such a constraint is met using a tapered PCF in our work, which is modeled using a fully vectorial effective index method (FVEIM). Influence of spatial and spectral dependent gain, saturation energy, amplified spontaneous emission (ASE), dispersion and nonlinearity on pulse propagation are analyzed using the combined solution of dynamic rate equations (DRE) and the generalized nonlinear Schrödinger equation (GNLSE). Numerical outcomes upon solving the aforementioned equations show that a 1 ps pulse is compressed down to a pulsewidth of 55.4 fs with a peak power of 1.03 kW in a total fiber length of $L=2.5$ m and taper ratio of 2.34. Overall quality of the compression scheme is evidenced by a relatively high compression factor (Fc) = 18.12, and low percentage of pedestal energy ($PE$) = 10.95 % [ABSTRACT FROM AUTHOR]

A simple, practical method based on electro‐optic gating is experimentally shown to improve the temporal resolution of single‐photon detection by more than 16 times. Delay times between ultrafast single photons and a reference clock are stretched by a desired programmable sampling gate factor, allowing reconstruction of delay histograms with ≈0.001 photons per pulse to within 60 ps. By transferring the bandwidth of RF electronics to single‐photon counting, complex single‐photon signals with large time‐bandwidth products > 2000 are temporally stretched up so that they can be resolved by slow detectors, irrespective of the quality of the detector instrument response function. This method is also applied to biphotons, in order to reconstruct the 2D histogram of joint detection delays with 15 times better resolution. The phenomenon of nonlocal dispersion, which is not resolvable directly with the slow detectors, is then observable to within the 98 ps level. The proof‐of‐concept demonstration uses off‐the‐shelf commercial fiber‐integrated LiNbO3 modulators and RF electronics, and the method is readily integrable on‐chip and to speeds >100 GHz, offering a practical solution to ultrafast time‐correlated single‐photon counting beyond the research laboratory. [ABSTRACT FROM AUTHOR]

Three-dimensional integrated circuits (3D-ICs) are becoming more significant in portable devices, autonomous vehicles, and data centers. As the demand for highly integrated and high-performance semiconductor devices grows, recent 3D integration technologies focus on lowering the size of the micro-structures on such devices for high density. In order to inspect the 3D semiconductor devices, it is critical to measure the heights, depths, and overall surface profiles of the micro-structures made with silicon materials. Here, we demonstrate precise surface imaging for silicon devices by using a femtosecond mode-locked laser centered at 785 nm wavelength and an electro-optic sampling-based time-of-flight detection method with sub-10-nanometer axial precision. We could successfully measure the surface profiles as well as the step heights of silicon wafer stacks and micro-scale structures on silicon substrates. [ABSTRACT FROM AUTHOR]

In this letter, we present a photoreceiver front-end circuit that consists of an InP p-i-n photodetector (PIN PD) on an InP transimpedance amplifier (TIA) with double HBTs (DHBTs), in which the key integrated fabrication route of a transfer-printing (TP) process is introduced. Prior to integration, PDs and TIAs are individually fabricated on their respective wafers. Then, a discrete PD with only active layers is transferred onto the destination on a TIA wafer, thereafter, an on-chip coplanar waveguide connection is integrated. The PIN PD and DHBT TIA exhibit excellent performance, characterized by a responsivity of 0.51 A/W at $1.55~\mu \text{m}$ with a bandwidth of 20 GHz and a transimpedance gain of 36 dB $\Omega $ with a bandwidth of 27 GHz, respectively. Finally, the monolithically integrated circuit demonstrates a better frequency response by maximally reducing the parasitic effect, with an open clear eye diagram under a 20 Gb/s nonreturn-to-zero pattern, which evidently manifests its ability for ultrahigh-rate data operation. The present work, by employing a TP process, overcomes the issues in integrating discrete optoelectronic devices and proposes a unique approach for monolithic optoelectronic integrated circuit fabrication. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms and controlling the possibilities of surface nanostructuring is of crucial interest for both fundamental science and application perspectives. Here, we report a direct experimental observation of laser-induced periodic surface structures (LIPSS) formed near a predesigned gold step edge following single-pulse femtosecond laser irradiation. Simulation results based on a hybrid atomistic-continuum model fully support the experimental observations. We experimentally detect nanosized surface features with a periodicity of ∼300 nm and heights of a few tens of nanometers. We identify two key components of single-pulse LIPSS formation: excitation of surface plasmon polaritons and material reorganization. Our results lay a solid foundation toward simple and efficient usage of light for innovative material processing technologies.

Structuring light–matter interaction at a deeply subwavelength scale is fundamental to optical metamaterials and metasurfaces. Conventionally, the operation of a metasurface is determined by the collective electric polarization response of its lithographically defined structures. The inseparability of electric polarization and current density provides the opportunity to construct metasurfaces from current elements instead of nanostructures. Here, we realize metasurfaces using structured light rather than structured materials. Using coherent control, we transfer structure from light to transient currents in a semiconductor, which act as a source for terahertz radiation. A spatial light modulator is used to control the spatial structure of the currents and the resulting terahertz radiation with a resolution of 5.6±0.8 μm $5.6{\pm}0.8\mathrm{\,\mu m}$ , or approximately λ/54 $\lambda /54$ at a frequency of 1 THz. The independence of the currents from any predefined structures and the maturity of spatial light modulator technology enable this metasurface to be reconfigured with unprecedented flexibility.

The rate of the removal of materials coated with nano−crystalline diamonds by femtosecond laser etching was examined by adjusting the repetition rate of the femtosecond laser, the energy−flux density of the concentrated spot, and the scanning speed. The observational results of the white-light interferometer and the numerical fitting approach were used to develop the removal rate function model of the nano-crystalline diamond-covered material etched by the femtosecond laser. The findings demonstrated that the rate of material removal was not greatly affected by the repetition frequency and that the amount of laser energy accumulated over time on the coated surface is steady. The processing outcomes under different laser scanning speeds are different, and the material removal rate tends to increase and then decrease with an increase in scanning speed. The greater the energy−flux density of the focused spot, the greater the etching intensity, and the greater the material removal rate. With an increase in scanning speed, the rate at which the material is removed often rises initially before falling.

Ultrafast optical imaging techniques for light propagation are useful in many fields, such as physics, chemistry, and biomedicine. In this letter, we propose an imaging technique for obtaining dual motion pictures of propagating light pulses with an ultrashort time difference. To realize the proposed technique, we employed a polarization imaging camera to record dual motion pictures of propagating light pulses without superposition. We also constructed and introduced an optical delay setup for generating light pulses with ultrashort time differences. In addition, the capturing of two propagating light pulses with ultrashort time differences given by the optical delay setup was experimentally demonstrated. Specifically, we recorded the first and second light propagations for 302 fs with an 890 fs time difference. [ABSTRACT FROM AUTHOR]

Recently, fully optical-driving neuromorphic devices have attracted growing interest. However, the current device faces great challenges due to the lack of negative photoconductivity materials, which seriously hinders the further development of the visual perceptual applications using photosensitive device. Here, we propose an ionotronic neuromorphic InGaZnO4 phototransistor to establish a fully optical-driving artificial neural network. Optical neuronal paired-pulse facilitation can be switched to optical depression characteristics by tuning the gate bias more negatively through the ion-coupling bioelectrolyte. Moreover, the biological high-pass, low-pass, and band-stop photofilter behaviors can be successfully mimicked in such an all-in-one phototransistor. Finally, a fully optical-driving artificial neural network is constructed to perform artificial visual perception with an accuracy of ~90%. This device may open new avenues for the fascinating applications, such as artificial visual system, intelligent bionic robots, and smart photoelectric devices. [ABSTRACT FROM AUTHOR]