Near-infrared Femtosecond Laser Research Articles

Multi-ionization of the Cl2 molecule in the near-infrared femtosecond laser field.

The multi-electron ionization and subsequent dissociation of the Cl2 molecule in a near-infrared femtosecond laser field was investigated via the dc-sliced ion imaging technique. The single charged molecular ions, Cl2+, dissociate from two excited states, 2Πu and 2Σg+, with the electrons ionized from the HOMO−1 and HOMO−2 orbital, respectively. For the multi-charged molecular ions, Cl2n+ (n = 2–8), our results showed that the stretch of the inter-nuclear distance benefitted the ionization of the electrons to produce highly-charged molecular ions. In addition, compared with the traditional charge resonance enhanced ionization (CREI) model, the critical distance (Rc) for the Cl2 molecule in our experiment was a short range that depended on the charge state rather than a single point.

Highly sensitive fiber optic temperature and strain sensor based on an intrinsic Fabry-Perot interferometer fabricated by a femtosecond laser.

In this Letter, we report on a simple, but highly sensitive sensor based on two intrinsic Fabry-Perot interferometers (FPIs) inscribed in a standard single-mode optical fiber. A brief theoretical study on the Vernier effect is presented, in which a simulation of the sensitivity magnification factor dependence on the FPI's length is performed. Based on the simulation results, the FPIs were fabricated using a custom micromachining setup that integrates a near-infrared femtosecond laser and a motorized XYZ platform. Using the Vernier effect, sensitivities of 145pm/με and 927pm/°C were obtained for strain and temperature, respectively. The sensor's performance combined with its versatile and customizable configuration allows real-time and in situ strain and temperature monitoring under harsh environments.

Upconversion luminescence in europium doped Y2O3 powder excited by absorption of three, four, and five infrared photons

Frequency upconversion (UC) luminescence was observed in yttrium oxide (Y2O3) powders doped with trivalent europium ions (Eu3+) when the samples were irradiated with a femtosecond laser system operating either at 1275 nm or 1500 nm. The samples, prepared by low temperature combustion synthesis, were characterized by X-ray diffraction, scanning electronic microscopy, spontaneous Raman scattering and photoluminescence. The UC luminescence, corresponding to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions of Eu3+, characterized by measuring the amplitude of the UC signal versus the laser intensity, is attributed to the simultaneous absorption of three, four and five infrared photons. The results indicate that Eu3+ doped Y2O3 powder is an efficient upconverter for excitation using femtosecond near-infrared lasers.

In2Se3 nanosheets with broadband saturable absorption used for near-infrared femtosecond laser mode locking

Indium selenide (In2Se3) has attracted tremendous attention due to its favorable electronic features, broad tunable bandgap, high stability and other attractive properties. However, its further applications for nonlinear optics have not yet been fully explored. In this work, we demonstrate that few-layer α-In2Se3 nanosheets exhibit strong saturable absorption properties over a wide wavelength range covering 800, 1064 and 1550 nm. The few-layer α-In2Se3 nanosheets used for this experiment are fabricated via a simple ultrasonic exfoliation in liquid. Stable ultrafast mode-locking laser pulses are obtained from both ytterbium-doped and erbium-doped fiber laser systems operating at 1064 and 1550 nm, respectively. A pulse duration as short as 215 fs was achieved in the Er-doped fiber laser system. Stable output pulses over 6 h of operation were obtained in both laser systems. The pulse energy and peak power of the laser output pulses were increased by reducing the In2Se3 thickness. These results indicate that α-In2Se3 nanosheets with low layer numbers are promising candidates for broad ultrafast photonics devices, such as optical switchers, Q-switchers and mode lockers.

Filamentation in Atmospheric Air with Tunable 1100\u20132400\u2009nm Near-Infrared Femtosecond Laser Source

Intense femtosecond pulse filamentation in open-air has been utilized for long distance optical communication and remote sensing, but it results in nonlinear-effect driven eye hazards which are not addressed by current eye safety standards. A systematic study of filamentation in atmospheric air was performed using a tunable 100 fs near-infrared laser (1100 nm–2400 nm). While undergoing filamentation, each source wavelength was spectrally broadened resulting in supercontinuum and third harmonic generation in the visible and near-IR spectrum. We record the spectra at the center and fringes of the supercontinuum as it is imaged onto a planar surface. In a full beam collection regime, we report the energy of the sub-1000 nm light generation for source wavelengths from 1100 nm to 1600 nm and compare the energy density to the maximum permissible exposure values under the ANSI Z136.1 laser safety standard.

Fiber Bragg Grating (FBG) Sensors in a High-Scattering Optical Fiber Doped with MgO Nanoparticles for Polarization-Dependent Temperature Sensing

The characterization of Fiber Bragg Grating (FBG) sensors on a high-scattering fiber, having the core doped with MgO nanoparticles for polarization-dependent temperature sensing is reported. The fiber has a scattering level 37.2 dB higher than a single-mode fiber. FBGs have been inscribed by mean of a near-infrared femtosecond laser and a phase mask, with Bragg wavelength around 1552 nm. The characterization shows a thermal sensitivity of 11.45 pm/°C. A polarization-selective thermal behavior has been obtained, with sensitivity of 11.53 pm/°C for the perpendicular polarization (S) and 11.08 pm/°C for the parallel polarization (P), thus having 4.0% different sensitivity between the two polarizations. The results show the inscription of high-reflectivity FBGs onto a fiber core doped with nanoparticles, with the possibility of having reflectors into a fiber with tailored Rayleigh scattering properties.

Mid-infrared long-pass filter for high-power applications based on grating diffraction.

A gold-coated silicon grating has been developed, enabling efficient spatial separation of broadband mid-infrared (MIR) beams with wavelengths >5 μm from collinearly propagating, broadband, high-power light in the near-infrared (NIR) spectral range (centered at 2μm). The optic provides spectral filtering at high powers in a thermally robust and chromatic-dispersion-free manner such as that necessary for coherent MIR radiation sources based on parametric frequency downconversion of femtosecond NIR lasers. The suppression of a >20 W average-power, 2μm driving pulse train by three orders of magnitude, while retaining high reflectivity of the broadband MIR beam, is presented.

Generation of Multi-Wavelength Light Pulses by Femtosecond Bessel Laser Beam in Silica Glass

When the near-infrared femtosecond Bessel laser beam (fs-BLB) is obliquely incident on the silica glass, annular beams (ABs) with bandwidths of several tens of nanometers and different peak wavelengths discretely distributed in a visible and near-infrared range are generated simultaneously. It is considered that four-wave mixing (FWM) and the corresponding cascaded parametric processes are responsible for the generation of multi-wavelength ABs. It is also found that the broken symmetry of the optical setup due to the obliquely oriented silica glass and the absorption band of the sample significantly enhance the generation efficiency of ABs. Since the diffraction angles of these colorful ABs mainly depend on their wavelengths and are insensitive to the input laser energy, this technique may find important applications where multi-wavelength ultrashort pulses are needed. For example, these multi-wavelength ABs can be used as light sources in nonlinear dark-field microscopy and multiphoton imaging to further suppress the background noise and improve resolution. They can also be used in multidimensional multiplexing optical communications.

Springtail-Inspired Triangular Laser-Induced Surface Textures on Metals Using MHz Ultrashort Pulses

Considering the attractive surface functionalities of springtails (Collembola), an attempt at mimicking their cuticular topography on metals is proposed. An efficient single-step manufacturing process has been considered, involving laser-induced periodic surface structures (LIPSS) generated by near-infrared femtosecond laser pulses. By investigating the influence of number of pulses and pulse fluence, extraordinarily uniform triangular structures were fabricated on stainless steel and titanium alloy surfaces, resembling the primary comb-like surface structure of springtails. The laser-textured metallic surfaces exhibited hydrophobic properties and light scattering effects that were considered in this research as a potential in-line process monitoring solution. The possibilities to increase the processing throughput by employing high repetition rates in the MHz-range are also investigated.

Terahertz hyperspectral imaging with dual chip-scale combs

Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive artifact testing. Contemporary terahertz imaging systems typically rely on nonlinear optical downconversion of a fiber-based near-infrared femtosecond laser, requiring complex optical systems. Here, we demonstrate hyperspectral imaging with chip-scale frequency combs based on terahertz quantum cascade lasers. The dual combs are free-running and emit coherent terahertz radiation that covers a bandwidth of 220 GHz at 3.4 THz with ∼10 μW per line. The combination of the fast acquisition rate of dual-comb spectroscopy with the monolithic design, scalability, and chip-scale size of the combs is highly appealing for future imaging applications in biomedicine and the pharmaceutical industry.

Real-time observation of X-ray-induced intramolecular and interatomic electronic decay in CH2I2

The increasing availability of X-ray free-electron lasers (XFELs) has catalyzed the development of single-object structural determination and of structural dynamics tracking in real-time. Disentangling the molecular-level reactions triggered by the interaction with an XFEL pulse is a fundamental step towards developing such applications. Here we report real-time observations of XFEL-induced electronic decay via short-lived transient electronic states in the diiodomethane molecule, using a femtosecond near-infrared probe laser. We determine the lifetimes of the transient states populated during the XFEL-induced Auger cascades and find that multiply charged iodine ions are issued from short-lived (∼20 fs) transient states, whereas the singly charged ones originate from significantly longer-lived states (∼100 fs). We identify the mechanisms behind these different time scales: contrary to the short-lived transient states which relax by molecular Auger decay, the long-lived ones decay by an interatomic Coulombic decay between two iodine atoms, during the molecular fragmentation.

Single nanowire defined emission properties of ZnO nanowire arrays

We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100 nm diameter wires with an average distance of 200 nm.

(Invited) Biological Multiphoton Imaging Techniques for Tracking Non-Fluorescent Carbon Nanomaterials

Multiphoton Microscopy (MPM) has enabled an unprecedented level of dynamic exploration within living cells and organisms. The physical principle behind this imaging technique involves the use of near-infrared femtosecond lasers to excite optical processes in fluorescent molecules using two or more photons1. The use of long wavelengths (700 nm – 1000 nm) enables deep tissue penetration (up to 1000 μm) without inducing harmful biological effects. Previous studies include the examination of membrane potentials on the single-molecule scale2, the non-invasive observation of embryo development3, and the simultaneous multiplane imaging of calcium transportation in transgenic mice4. An excellent review of this field can be found in the literature5. Herein we demonstrate a novel multiphoton-based technique for imaging negligible-to-non-fluorescent carbon-based nanomaterials such as C60 fullerenes, single-walled carbon nanotubes, and graphene, which usually can only be imaged by conjugation of specific fluorescent tracers thereby altering their inherent dynamics within biological systems. More specifically, we demonstrate optical and photoluminescent activity in water-soluble C60-serinol, highly enriched (95 %) metallic and semiconducting single-walled carbon nanotubes, and graphene. This multiphoton imaging technique was also applied to in vitro and in vivo animal model studies. Various concentrations of the above-mentioned nanomaterials were given to human pancreatic ductal adenocarcinoma (Panc-1) and hepatocellular carcinoma (Heb3B) cancer cell lines, as well as normal, healthy pancreatic ductal epithelial cells (HPDE). In all cases, imaging of these nanomaterials was possible using the MP imaging technique even though the nanomaterials exhibit weak, almost zero inherent fluorescent properties. We also examined the ability to image these nanomaterials, real-time, in vivo, using mice with orthotopic 4T1 breast tumors. Finally, we also include a range of spectroscopy data to examine the specific optical characteristics of the carbon nanomaterials. References Denk, W.; Strickler, J.; Webb, W. Science 1990, 248, (4951), 73-76.Peleg, G.; Lewis, A.; Linial, M.; Loew, L. M. Proceedings of the National Academy of Sciences 1999, 96, (12), 6700-6704.Chu, S.-W.; Chen, S.-Y.; Tsai, T.-H.; Liu, T.-M.; Lin, C.-Y.; Tsai, H.-J.; Sun, C.-K. Opt. Express 2003, 11, (23), 3093-3099.Cheng, A.; Goncalves, J. T.; Golshani, P.; Arisaka, K.; Portera-Cailliau, C. Nat Meth 2011, 8, (2), 139-142.Hoover, E. E.; Squier, J. A. Nat Photon 2013, 7, (2), 93-101.

Characterization of 20-fs VUV pulses by plasma-mirror frequency-resolved optical gating.

We demonstrate that vacuum ultraviolet (VUV) pulses (λ∼160 nm) with 20-fs duration can be fully characterized by plasma-mirror frequency-resolved optical gating. Plasma is formed as an ultrafast optical switch on a fused silica surface by irradiation of an intense near-infrared (NIR) femtosecond laser pulse. We measure the spectra of the VUV pulses reflected off plasma as a function of the delay with respect to the NIR pulse and reconstruct it to simultaneously extract a nearly Fourier-transform limited VUV pulse shape and a time-dependent reflectivity of plasma using the least-squares generalized projections algorithm. We show that there is almost no spatial dependence of the VUV pulse shape, whereas the plasma mirror reflectivity strongly depends on the spatial region.

Coherent modulation of superradiance from nitrogen ions pumped with femtosecond pulses

Singly ionized nitrogen molecules in ambient air pumped by 800 nm femtosecond laser give rise to superradiant emission. Here, we study this superradiance by injecting a pair of resonant seeding pulses at different intensity ratios inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiant emission with a period of 1.3 fs is observed when the delay between the two seeding pulses is finely tuned. The modulation contrast is increased and then decreased with the delay time when the second seed pulse is stronger than the first one, and the maximum modulation contrast occurs at longer delay time when the second seeding pulse is stronger. This reveals the increase of the macroscopic polarization with time after the seeding pulse. Moreover, these observations provide a new level of control on the "air lasing" based on nitrogen ions, which can find potential applications in optical remote sensing.

Spectral broadening effects on metal photoemission by femtosecond laser pulses

Emitted electronic densities from an Al surface have been computed for various temporal shapes of near-infrared femtosecond laser pulses (less than or equal to 20 fs) over a wide range of low and intermediate laser intensities. At low intensities photoemission originates from the high-energy tail of the pulse spectrum leading to linear absorption in all cases, even if several photons are necessary to eject an electron at higher intensities. We propose an analytical method to calculate the transition intensity in which the behavior of emitted densities evolves from the single-photon ${I}_{m}$ slope to the multiphoton ${I}_{m}^{n}$ one (with ${I}_{m}$ the maximum intensity and $n$ the number of absorbed photons). We show that the use of a flat-top profile or at least of a super-Gaussian envelope enables strong electron emission at very low intensities for realistic pulse durations and should allow experimental observation of the transition between these two regimes of photoemission.

Abnormal elemental redistribution in oxyfluoride glasses induced by high repetition rate femtosecond laser

We report on the elemental redistribution behavior in oxyfluoride glasses with a high repetition rate near-infrared femtosecond laser. Elemental analysis by an electro-probe microanalyzer demonstrates that the redistributions of Ca2+ and Yb3+ ions change dramatically with pulse energy, which are quite different compared with previous reported results. Confocal fluorescence spectra of Yb3+ ions demonstrate that the luminescence intensity changes obviously with the elemental redistribution. The mechanism of the observed phenomenon is discussed. This observation may have potential applications in the fabrication of micro-optical devices.

Open-path gas detection using terahertz time-domain spectroscopy

We report on the development of an experimental setup for remote sensing of gas atmosphere in open air conditions using broadband terahertz pulses. Terahertz time-domain spectroscopy technique is used to solve the problems of gas detection and recognition in the presence of water vapor absorption lines. Acetonitrile vapors were detected and gas concentration was successfully estimated. Organic crystal with a 2% conversion efficiency pumped with near-infrared femtosecond laser pulses was used to generate single-cycle terahertz pulses with energies up to 10 µJ. Application of a novel high-power source of broadband terahertz radiation (0.2–3 THz) opens up new perspectives for further increasing the open space distances in remote sensing problem.

Discover Patent Landscape of Two-photon Polymerization Technology for the Production of 3D Nano-structure Using Claim-based Approach.

The TPP technology is a "nano-optics" 3D printing technology that develops in recent years. Compared with the traditional 3D printing technology, the TPP-3DP technique that takes near-infrared light femtosecond pulse laser as the light source can break through the limitation of optical diffraction and manufacture nanoscale 3D structures in arbitrary shapes with high resolution. The research develops a claim-based patent analysis method to identify technical features via the analysis of claim, especially the analysis of plural elements of independent claim. Furthermore, we analyze the subject matters and technical features of claims through claim-base analytical method to observe the patent trend of competitors and understand the current situation of competitors' technical distribution in related industries, in order to provide useful information for enterprises to determine R&D strategies. The result shows 3M Company possesses numerous patent portfolios that base on the diversified innovative applications of TPP technology. Nanoscribe Company has a long-term advantage in monopolizing this technology. The patents in TPP-3DP field can be grouped into five categories relating to application, including micro-optics device, photonic crystal, biomedical, microfluidics and MEMS. The findings of this review confirm the TPP-based 3D printing (TPP-3DP) technology has broad application prospects, of which the hydrogel material applied in tissue engineering and drug delivery is the future trend. Highly-initiated water-soluble initiators are particularly needed to increase its resolution and will be the direction of emerging technology in this domain.

Surface third and fifth harmonic generation at crystalline Si for non-invasive inspection of Si wafer's inter-layer defects.

Detection of inter-layer and internal defects in semiconductor silicon (Si) wafers by non-contact, non-destructive and depth-resolving techniques with a high lateral and depth resolution is one of the challenging tasks in modern semiconductor industry. In this paper, we report that nonlinear optical harmonic generation can be of great virtue therein because it enables non-invasive inspection of inter-layer defects with sub-micrometer depth resolution in extensive penetration depth over several millimeters. Compared to existing inspection methods for inter-layer defects, such as ultrasound, photoacoustic and photothermal imaging, the proposed technique provides higher lateral and depth resolution as well as higher interfacial selectivity. For in-depth understanding of nonlinear harmonic generation at Si wafer surfaces, the spectral power distributions of third and fifth harmonics from Si wafers with various crystal orientations and dopants were carefully analyzed under different incident polarizations and excitation depths using a near-infrared (NIR) femtosecond laser as the excitation light source. We finally demonstrated that inter-layer defects inside stacked Si wafers, such as delamination or stacking faults, can be inspected with a high lateral and depth resolution in a non-contact and non-destructive manner. These findings will pave the way for nonlinear optical harmonic generation to the fields of interfacial studies of crystalline materials, high-resolution detection of sub-diffraction-limit surface defects, and high-resolution imaging of internal structures in stacked semiconductor devices.