Near-infrared Femtosecond Laser Research Articles

High Harmonic Generation near the Low-Frequency Edge of a Plateau under Nonlinear Propagation of 1.24-μm Near-Infrared Femtosecond Laser Radiation in a Dense Argon Jet

High (15–25) harmonic generation in the vacuum ultraviolet spectral range (83–50 nm) has been realized by focused (NA = 0.033) near-infrared femtosecond laser radiation (wavelength λ = 1.24 μm) with a vacuum intensity of ~7.5 × 1014 W/cm2 irradiating a dense gas jet. It has been shown experimentally that the use of such a high-numerical aperture focusing requires high (up to 10 bar) gas jet pressures to optimize phase matching. The use of the dense gas jet results in a noticeable manifestation of nonlinear propagation effects for generating radiation, which affect the generation process through the change in the phase matching conditions. Furthermore, it has been shown that the prechirping of the generating pulse makes it possible to compensate a chirp appearing due to self-phase modulation and to increase the harmonic generation efficiency because of the nonlinear compression of the generating pulse. This approach has allowed 17th (73 nm) harmonic generation with an energy of 2 pJ in a pulse and a generation efficiency of 5.4 × 10–9. The estimates obtained have shown that this radiation can be used for single-pulse maskless photolithography in the extreme ultraviolet range.

Light-Induced Metastable Hidden Skyrmion Phase in the Mott Insulator Cu2 OSeO3.

The discovery of a novel long-lived metastable skyrmion phase in the multiferroic insulator Cu2 OSeO3 visualized with Lorentz transmission electron microscopy for magnetic fields below the equilibrium skyrmion pocket is reported. This phase can be accessed by exciting the sample non-adiabatically with near-infrared femtosecond laser pulses and cannot be reached by any conventional field-cooling protocol, referred as a hidden phase. From the strong wavelength dependence of the photocreation process and via spin-dynamics simulations, the magnetoelastic effect is identified as the most likely photocreation mechanism. This effect results in a transient modification of the magnetic free energy landscape extending the equilibrium skyrmion pocket to lower magnetic fields. The evolution of the photoinduced phase is monitored for over 15 min and no decay is found. Because such a time is much longer than the duration of any transient effect induced by a laser pulse in a material, it is assumed that the newly discovered skyrmion state is stable for practical purposes, thus breaking ground for a novel approach to control magnetic state on demand at ultrafast timescales and drastically reducing heat dissipation relevant for next-generation spintronicdevices.

Strong-field photoionization/dissociation of vinyl bromide in near-infrared and ultraviolet femtosecond laser fields

Ionization of polyatomic molecules in strong femtosecond laser fields is a pivotal process that may induce a variety of highly nonlinear molecular physical phenomena. We performed an experimental study on photoionization and dissociation of vinyl bromide molecules in 800 nm or 400 nm femtosecond strong laser fields using a time-of-flight mass spectrometer. The dependence of the ion yields on laser intensity and polarization was investigated, and the results are compared in different wavelength laser fields. The kinetic energy releases from Coulomb explosion of vinyl bromide, and the angular distributions of different fragment ions were obtained. The mechanism of strong-field ionization/dissociation of vinyl bromide in different laser fields is discussed based on the experimental results.

Design of refractive–diffractive hybrid lenses for space–time focusing ultrashort laser pulses

We propose a simple formulation for the baseline design of a hybrid lens, consisting of refractive and diffractive elements, intended for focusing ultrashort laser pulses. Lens dispersions up to the second order are considered to obtain easy-to-use formulas. Using this formulation, we designed a refractive–diffractive triplet for near-infrared femtosecond laser pulses and observed that once radius-dependent temporal distortions are removed, chromatic distortions are removed as well, and vice versa. Furthermore, we found that the derived equations for removal of pulse distortions can be readily transformed to traditional lens design equations for optical imaging. This finding facilitates the design of hybridized triplets for ultrafast focusing optics.

Two-Photon Stereolithography—Optical Nanolithography

Free-radical photopolymerization has been widely used in additive technologies, in particular, stereolithography using single- and two-photon initiated polymerization. The single-photon stereolithography affords the objects with about 100 μm resolution. The two-photon stereolithography initiated with a femtosecond near-infrared laser can afford arbitrary 3D microstructures with ultrahigh resolution at micro- and nanoscale level (~100 nm). Herein each of the mentioned method and the mechanisms of single- and two-photon excitation are reviewed. The recent results on the components of the photopolymerizable resin as well as the approaches to decrease the size of the elements of objects and accelerate their formation have been generalizated and systematized.

Mesoporous Silica-Encapsulated Gold Nanorods for Drug Delivery/Release and Two-Photon Excitation Fluorescence Imaging to Guide Synergistic Phototherapy and Chemotherapy.

Photothermal therapy is a promising light-based medical treatment that relies on light absorption agents converting light irradiation into localized heat to destroy cancer cells or other diseased tissues. It is critical to enhance the therapeutic effects of cancer cell ablation for their practical applications. This study reports a high-performance combinational therapy for ablating cancer cells, including both photothermal therapy and chemotherapy to improve therapeutic efficiency. The prepared AuNR@mSiO2 loading molecular Doxorubicin (Dox) assemblies were highlighted by merits of facile acquisition, great stability, easy endocytosis, and rapid drug release in addition to improved anticancer capability upon irradiation with a femtosecond pulsed near-infrared (NIR) laser, where AuNR@mSiO2 nanoparticles afforded a high photothermal conversion efficiency of 31.7%. Two-photon excitation fluorescence imaging was introduced into confocal laser scanning microscope multichannel imaging to track the drug location and cell position in real time for monitoring the process of drug delivery in killing human cervical cancer HeLa cells and then to realize imaging-guiding cancer treatment. These nanoparticles exhibit widespread potential in photoresponsive utilizations including photothermal therapy, chemotherapy, one- and two-photon excited fluorescence imaging, and 3D fluorescence imaging and cancer treatment.

Femtosecond Inscription of a Fiber Bragg Grating Spectral Array in the Same Spatial Location.

A five fiber Bragg grating (FBG) array is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of a near-infrared femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The tunability of the center Bragg wavelength is achieved by a defocusing lens, and by translating the PM, which results in a different magnification of the PM. A first FBG is inscribed, followed by four cascading FBGs, which are inscribed exactly at the same spot only after the translation of the PM. The transmission and reflection spectra of this array are measured, showing a second-order Bragg wavelength at ~1.56 µm with a transmission dip of ~-8 dB. The spectral wavelength shift between each consecutive FBG is ~2.9 nm, and the total wavelength shift is ~11.7 nm. The reflection spectrum of the third-order Bragg wavelength is measured at ~1.04 µm, showing a wavelength separation of ~1.97 nm between neighboring FBGs, and the total spectral span between the first FBG and the last one is ~8 nm. Finally, the wavelength sensitivity to strain and temperature is measured.

Nanoscale local modification of PMMA refractive index by tip-enhanced femtosecond pulsed laser irradiation

Tip-enhanced optical effects have enabled a wide palette of nanoscale investigation techniques and recently they were successfully used to modify and manipulate matter with nanoscale spatial resolution. We contribute to this latter area by reporting a viable method to modify the refractive index of polymethyl methacrylate (PMMA). Local surface changes of the refractive index are accomplished by focusing a femtosecond pulsed near-infrared laser beam on the apex of a metalized nanosized tip, traditionally used in scanning probe microscopy (SPM). The refractive index is changed over a circular area with diameter <200 nm under 790 nm illumination, representing a four-fold increase in resolution compared to the current state-of-the-art. These results are obtained at low laser pulse energies (<0.5 nJ), which represents a two orders of magnitude advantage over previously reported methods. Besides enabling modifications of the refractive index with high lateral resolution, this method can pave the way towards other important applications such the fabrication of photonic crystal lattices and surface waveguides.

Room-temperature synthesis of CsPbBr3@CsPb2Br5 microcrystals and their upconversion luminescence

In this work, a convenient and environmental-friendly aqueous-phase method is used to synthesize CsPbBr3@CsPb2Br5 microcrystals (MCs) at room temperature, of which structural and optical properties are characterized by powder X-ray diffraction (PXRD), absorption, and photoluminescence (PL) spectroscopic techniques. Two obvious absorption peaks at 307 and 515 nm are attributed to CsPb2Br5 matrix and CsPbBr3 impurity, respectively. Full-width at half-maximum (FWHM) of PL for MCs is 83.5 meV at room temperature, and its emission peak is observed at 518 nm. With the temperature changed from 80 to 340 K, the fitted value of the exciton binding energy is 84.1 meV from temperature-dependent steady-state PL. Upconversion luminescence (UCL) can be observed obviously under near-infrared femtosecond laser pulse excitation, which is compared with normal luminescence from steady-state and time-resolved PL measurements. The experimental investigations show that CsPbBr3@CsPb2Br5 MCs have excellent stability against thermal, water and light irradiation, which will provide a new choice for the commercial applications to lasering and displaying.

Angle-Resolved Time-of-Flight Electron Spectrometer Designed for Femtosecond Laser-Assisted Electron Scattering and Diffraction

We developed an apparatus for measuring kinetic energy and two-dimensional angular distributions of femtosecond laser-assisted electron scattering (LAES) signals with a high detection efficiency, consisting of a photocathode-type ultrashort pulsed electron gun, a gas injection nozzle, an angle-resolved time-of-flight analyzer, and a time-and-position sensitive electron detector. We also established an analysis method for obtaining the kinetic energy and two-dimensional angular distributions of scattered electrons from raw data of their flight times and the detected positions at the detector recorded using the newly developed apparatus. From the measurement of the LAES processes of Ar atoms in a femtosecond near-infrared intense laser field, we obtained a two-dimensional angular distribution image of the LAES signals and showed that the detection efficiency of the LAES signals was raised by a factor of 40 compared with that achieved before in 2010.

Submicron structures created on Ni thin film by submicron focusing of femtosecond EUV light pulses

Using a newly developed submicron focusing system equipped with a large-aperture Wolter mirror with a precisely figured surface, we irradiate a Ni thin film coated on a silicon substrate with femtosecond extreme ultraviolet (EUV) laser pulses, generated as high-order harmonics of near-infrared femtosecond laser pulses. At the fluence of up to 100 mJ cm−2 at the sample surface, we show that the surface morphology can be modified with a submicron spatial resolution and that a protruding structure is formed when the fluence is 60–70 mJ cm−2, exhibiting a characteristic feature of the femtosecond EUV light processing of a thin metal surface.

Near-infrared multiphoton absorption and third harmonic generation with CsPbBr3 quantum dots embedded in micro-particles of metal–organic frameworks

A hybrid compound of CsPbBr3 quantum dots embedded in ZIF-8 microparticles shows both multiphoton excited (MPE) photoluminescence and third harmonic generation (THG) when excited by near-infrared femtosecond laser pulses.

Influence of film thickness and substrate roughness on the formation of laser induced periodic surface structures in poly(ethylene terephthalate) films deposited over gold substrates

A study of the formation of Laser Induced Periodic Surface Structures (LIPSS) using near-infrared femtosecond pulsed laser irradiation on poly(ethylene terephthalate) (PET) films deposited over gold substrates has been carried out. We report the influence of the gold substrate roughness and the PET film thickness on LIPSS formation and analyze it in terms of the features of the electric field distribution obtained by computer simulations using COMSOLTM. We obtain LIPSS with periods close to the irradiation wavelength as long as the aforementioned substrate and film parameters remain below certain threshold values, in particular for polymer thicknesses below 200 nm and substrate roughness of few nm. However, experiments show the impossibility of LIPSS formation for rough substrates as well as thick films above these threshold values. In our numerical simulations, we notice the generation of Surface Plasmon Polariton (SPP) in the film-substrate interface that gives rise to a periodical field pattern on the surface of the thin film. This periodicity is broken for a certain level of substrate roughness or film thickness. Moreover, the evolution of the period of the SPP as the substrate roughness and film thickness change for given laser parameters is qualitatively in good agreement with the experimental LIPSS period (below but close to the irradiation laser wavelength). In conclusion, the experimental findings are explained by the formation and behavior of SPP in the thin film-substrate interface. On these grounds, we propose that, for our case of study, this SPP formation and the subsequent inhomogeneous rise in temperature induced by the periodic field on the surface of the sample is the leading mechanism contributing to LIPSS formation.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study.

The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

Coherent XUV Multispectral Diffraction Imaging in the Microscale

The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free electron laser (XFEL) facilities currently provide such coherent sources of directional and high-brilliance X-ray radiation. These facilities require large financial investments for development, maintenance, and manpower, and thus, only a few exist worldwide. In this article, we present an automated table-top system for XUV coherent diffraction imaging supporting the capabilities for multispectral microscopy at high repetition rates, based on laser high harmonic generation from gases. This prototype system aims towards the development of an industrial table-top system of ultrafast soft X-ray multi-spectral microscopy imaging for nanostructured materials with enormous potential and a broad range of applications in current nanotechnologies. The coherent XUV radiation is generated in a semi-infinite gas cell via the high harmonic generation of the near-infrared femtosecond laser pulses. The XUV spectral selection is performed by specially designed multilayer XUV mirrors that do not affect the XUV phase front and pulse duration.

Hot Carrier-Induced Nonlinear Photoluminescence in a Thin Indium Tin Oxide Layer Patterned by Ga Ion Beam Milling

This paper explores the nonlinear photoluminescence emitted by indium tin oxide (ITO) thin layers patterned by focused gallium ion beam milling. Using tightly focused near-infrared femtosecond pulsed laser excitation, a broad up-converted luminescence spectrum spanning the visible is detected. The intensity of the luminescence follows a nonmonotonous relationship with milling doses and can be related to the modification of the ITO electronic band structure by the implantation of Ga ions. The shape and the power dependence of the spectrum share strong similarities with those of nonlinear photoluminescence arising from metals. The results are consistent with a nonlinear process originating from the radiative decay of photogenerated hot carriers. The thermal coefficient relating the carrier temperature to the laser intensity is determined as a function of milling dose.

Two-photon collagen crosslinking in ex vivo human corneal lenticules induced by near-infrared femtosecond laser.

Myopia and keratoconus have become common corneal diseases that threaten the quality of human vision, and keratoconus is one of the most common indications for corneal transplantation worldwide. Collagen crosslinking (CXL) using riboflavin and ultraviolet A (UVA) light is an effective approach for treating ophthalmic disorders and has been shown clinically not only to arrest further progression of keratoconus but also to improve refractive power for cornea. However, CXL surgery irradiated by UVA has various potential risks such as surface damage and endothelial cell damage. Here, near-infrared femtosecond laser-based two-photon CXL was first applied to ex vivo human corneal stroma, operating at low photon energy with high precision and stability. After two-photon CXL, the corneal stiffness can be enhanced by 300% without significantly reducing corneal transparency. These findings illustrate the optimized direction that depositing high pulses energy in corneal focal volume (not exceeding damage threshold), and pave the way to 3D CXL of in vivo human cornea with higher safety, precision, and efficacy.

Second harmonic generation nanoparticles enables Near-Infrared Photodynamic Therapy from visible light reactive photosensitizer conjugates.

Combination of photosensitizers (PS) with nanotechnology can improve the therapeutic efficiency of clinical Photodynamic Therapy (PDT) by converting visible light reactive PSs into Near-Infrared (NIR) light responsive molecules using Harmonic Nanoparticles (HNP). To test the PDT efficiency of HNP-PS conjugates, pathogenic S. aureus cell cultures were treated with perovskite (Barium Titanate) Second Harmonic Generation (SHG) nanoparticles conjugated to photosensitizers (PS) (we compared both FDA approved Protoporphyrin IX and Curcumin) and subjected to a femtosecond pulsed Near-Infrared (NIR) laser (800 nm, 232–228 mW, 12–15 fs pulse width at repetition rate of 76.9 MHz) for 10 minutes each. NIR PDT using Barium Titanate (BT) conjugated with Protoporphyrin IX as HNP-PS conjugate reduced the viability of S. aureus cells by 77.3 ± 9.7% while BT conjugated with Curcumin did not elicit any significant effect. Conventional PSs reactive only to visible spectrum light coupled with SHG nanoparticles enables the use of higher tissue penetrating NIR light to generate an efficient photodynamic effect, thereby overcoming low light penetration and tissue specificity of conventional visible light PDT treatments.

Laser-induced galfenol embedded multi-layer graphene-oxide in solution

The proposed work demonstrates the direct synthesis of nanomaterial-embedded laser-induced few-layer graphene-oxide by directly ablating galfenol in a water-based solution for the first time. Laser-induced multilayer graphene-oxide (GO) embedded with galfenol (gallium–iron alloy) nanoparticles (NPs) is created through a method of direct laser inscription of bulk galfenol in deionized (DI) water with femtosecond laser ablation. The NP-embedded GO is achieved by irradiating a near-infrared (near-IR) femtosecond laser at 1040 nm on a bulk galfenol material submerged in a solution comprising DI water and a small concentration (5%/wt.) of polyvinylpyrrolidone followed by a second ablation in pure DI water. Results show nanoparticles with a mean diameter of ∼30 nm embedded in GO sheets with visible folds spaced at ∼0.63 nm. The composition of iron and gallium shifts by less than 2% during the laser ablation process, and the few-layer GO sheets exhibit similar Raman peaks to bulk graphite.

Two-color laser PEEM imaging of horizontal and vertical components of femtosecond surface plasmon polaritons

Explicit visualization of different components of surface plasmon polaritons (SPPs) propagating at dielectric/metal interfaces is crucial in offering chances for the detailed design and control of the functionalities of plasmonic nanodevices in the future. Here, we reported independent imaging of the vertical and horizontal components of SPPs launched from a rectangular trench in the gold film by a 400-nm laser-assisted near-infrared (NIR) femtosecond laser time-resolved photoemission electron microscopy (TR-PEEM). The experiments demonstrate that distinct imaging of different components of SPPs field can be easily achieved by introducing the 400-nm laser. It can circumvent the risk of sample damage and information loss of excited SPPs field that is generally confronted in the usual NIR laser TR-PEEM scheme. The underlying mechanism for realizing distinct imaging of different components of the SPPs field with two-color PEEM is revealed via measuring the double logarithmic dependence of photoemission yield with the 800-nm and 400-nm pulse powers of different polarizations. Moreover, it is found that the PEEM image quality of the vertical and horizontal components of the SPPs field is nearly independent of the 400-nm pulse polarization. These results pave a way for SPPs-based applications and offer a possible solution for drawing a space–time field of SPPs in three dimensions.