Single Femtosecond Laser Beam Research Articles

Independent multicolored bottle-beam generation using acousto-optic spatial shaping of a femtosecond laser beam.

We report on the development of a tunable spectral and spatial frequency shaping system for ultrashort laser pulses using acousto-optic filters. The system enables the creation of arbitrary axially symmetric multi-wavelength field configurations in the Ti:sapphire laser emission range near 800 nm and controlling them at a multi-kilohertz rate. We experimentally demonstrate independent generation of two-colored annular intensity distributions from a single femtosecond laser beam and a bottle beam having the hollow cylindrical volume with the aspect ratio of 9:1. This laser beam shaping system can be useful in creating advanced setups for an optical control of cold atoms.

Electronic‐Resonance‐Enhanced Coherent Raman Spectroscopy with a Single Femtosecond Laser Beam

AbstractCoherent Raman scattering (CRS) spectroscopy holds versatile applications in many fields ranging from measuring temperature and concentration in reacting flows to imaging biological molecules in living cells. Although the powerful spectroscopic technique has been studied for a few decades, significant attention is still paid to developing state‐of‐the‐art CRS spectroscopy with the simpler design and higher sensitivity for practical applications in harsh or complex environments. Herein, a novel electronic‐resonance‐enhanced coherent Raman scattering (ERE‐CRS) technique is developed with a single femtosecond laser beam. Such single‐beam ERE‐CRS scheme is accomplished with the narrowband air lasing signal naturally generated in the interaction region as a probe source. The developed technique not only avoids fine spatio‐temporal control in the conventional multibeam configuration, but also accomplishes 1–2 orders of magnitude enhancement of Raman signal due to the resonance of air lasing with electronic transition of the target gas. The dramatic enhancement effect is also manifested through the comparative measurements in CO2 isotopes. The single‐beam ERE‐CRS spectroscopy shows an unprecedented opportunity for the high‐sensitivity standoff detections of gas‐phase molecules and plasmas.

Micropattern-assisted absorption enhancement and wettability surface on ZnO via single femtosecond laser beam tailoring

Herein, we report significant micropattern-assisted absorption enhancement on crystalline ZnO fabricated by a single femtosecond laser beam (400 nm, 35 fs, kHz). This two-dimensional micropattern, which is achieved by one-step femtosecond laser tailoring, sharply contrasts the traditional laser-induced periodic surface structure. The large-area preparation of two-dimensional micropatterns can be achieved via an overlapped scanning method, showing potential practical applications. We find that the absorption shows an increasing trend, with maximum values of 90–99% in the ultraviolet spectral regions and 75–80% in the visible spectral regions. Additionally, theoretical results demonstrate that a strong electric field enhancement phenomenon exists on the two-dimensional structure from the ultraviolet to near-infrared wavelengths. We also determine the liquid contact angle of these micropattern structures, which exhibit controlled wettability characteristics that are dependent on laser energy.

High-efficiency-and-quality nanostructuring of molybdenum surfaces by orthogonally polarized blue femtosecond lasers

Femtosecond laser-induced periodic surface structures featured by a mask-free and one-step procedure provide a new fascinating prospect for the high precision manufacture applicable to diverse materials, but still face challenges in both the fabrication efficiency and the structure quality. Here, a homogenous large-area nanolithography is implemented on molybdenum surface, through using cylindrical focusing of double temporally delayed blue femtosecond lasers with orthogonal polarizations. The achieved grating structures present some unique features including the ablated narrow groove width of 100 nm, the small period of 245 nm, the smooth ridge profiles, and especially the uniform distribution in a mm-scaled range without bending, spitting and interruption. The calculated structure orientation angle shows an unprecedentedly minimum dispersion value of 5°, indicting a significant improvement on the structure uniformity compared with the observations of the single-beam femtosecond laser irradiation. The simulation results reveal that the transient spatiotemporal correlations between the localized electromagnetic field distributions from the double orthogonally polarized laser pulses constitutes a positive feedback mechanism to regularize the growth of the surface structures. A macro-sized surface pattern of such uniform structures displays good monochromatic colors and excellent broadband antireflection performances. This efficient laser nanolithography facilitates the applications in the fields of nanooptics.

Two-dimensional suprawavelength periodic surface structuring of a ZnO single crystal with a UV femtosecond laser.

Herein, we report on the one-step formation of a novel microstructure on the surface of crystalline ZnO in ambient air excited by a single femtosecond laser beam (central wavelength 400 nm, pulse duration 35fs), which has photon energy close to the bandgap of ZnO. A two-dimensional surface structure with a controlled period of ∼2-6 μm is observed, with its orientation independent on the status of laser polarization (linear, circular, or elliptical polarization). We find that the orientation of this two-dimensional structure is defined by the direction of the crystal a and c axes. This structural period of ∼2-6 micrometers and the independence of its orientation on the laser polarization are in sharp contrast with the traditional laser induced periodic surface structure (LIPSS). In the meantime, surface cracks with a feature size of ∼30 nm are observed at the bottom of the valley of the two-dimensional structure and theoretical results show there exists strong electric field enhancement on the cracks under 400 nm femtosecond laser irradiation. In view of these unusual features, we attribute the formation of this two-dimensional structure to the mechanical cracking of the ZnO crystal along its (11-20) and (0001) planes induced by the multiple-cyclic heating due to linear absorption of the femtosecond pulses.

Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets.

All-optical, self-focused laser beam array for parallel laser surface processing.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

Formation of Slantwise Surface Ripples by Femtosecond Laser Irradiation.

We report on the formation of slantwise-oriented periodic subwavelength ripple structures on chromium surfaces irradiated by single-beam femtosecond laser pulses at normal incidence. Unexpectedly, the ripples slanted in opposite directions on each side the laser-scanned area, neither perpendicular nor parallel to the laser polarization. The modulation depth was also found to change from one ripple to the next ripple. A theoretical model is provided to explain our observations, and excellent agreement is shown between the simulations and the experimental results. Moreover, the validity of our theory is also confirmed on bulk chromium surfaces. Our study provides insights for better understanding and control of femtosecond laser nanostructuring.

Single femtosecond laser beam pumped transient diffraction and transient lens effects for ultrafast measurement in background-free geometry

We proposed an ultrafast optical gating configuration for ultrafast measuring the characters of a supercontinuum (SC) probe beam. Single femtosecond laser beam pumped transient diffraction (TrD) and transient lens (TrL) effects were realized at the same time, which ensured the measurement in background-free geometry. Experimental results showed that the proposed ultrafast optical gating was robust for very high signal-to-noise ratio (SNR) detecting.

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响

High precision patterning of ITO using femtosecond laser annealing process

• We have reported a process of fabrication of crystalline indium tin oxide (c-ITO) patterns using femtosecond laser-induced crystallization with a Gaussian beam profile followed by chemical etching. • The experimental results have demonstrated that the ablation and crystallization threshold fluences of a-ITO thin film are well-defined, the line width of the c-ITO patterns is controllable. • Fast fabrication of the two parallel sub-micro (∼0.5 μm) c-ITO line patterns using a single femtosecond laser beam and a single scanning path can be achieved. • A long-length sub-micro c-ITO line pattern is fabricated, and the feasibility of fabricating c-ITO patterns is confirmed, which are expected to be used in micro-electronics devices. High precision patterning of crystalline indium tin oxide (c-ITO) patterns on amorphous ITO (a-ITO) thin films by femtosecond laser-induced crystallization with a Gaussian beam profile followed by chemical etching is demonstrated. In the proposed approach, the a-ITO thin film is selectively transformed into a c-ITO structure via a low heat affect zone and the well-defined thresholds (ablation and crystallization) supplied by the femtosecond laser pulse. The experimental results show that by careful control of the laser fluence above the crystallization threshold, c-ITO patterns with controllable line widths and ridge-free characteristics can be accomplished. By careful control of the laser fluence above the ablation threshold, fast fabrication of the two parallel sub-micro c-ITO line patterns using a single femtosecond laser beam and single scanning path can be achieved. Along-length sub-micro c-ITO line pattern is fabricated, and the feasibility of fabricating c-ITO patterns is confirmed, which are expected to be used in micro-electronics devices.

Compact 180-kV Marx generator triggered in atmospheric air by femtosecond laser filaments

We developed a compact Marx generator triggered in atmospheric air by a single femtosecond laser beam undergoing filamentation. Voltage pulses of 180 kV could be generated with a subnanosecond jitter. The same laser beam was also used to initiate simultaneously guided discharges up to 21 cm long at the output of the generator.

Polarization dependence of the self-organized microgratings induced in SrTiO_3 crystal by a single femtosecond laser beam

In this paper, self-organized microgratings are fabricated in SrTiO(3) crystal just by scanning the focus of a tightly-focused linearly-polarized femtosecond laser beam to form a single line. The polarization direction of the laser beam is rotated by a λ/2 waveplate to check the effect of the polarization azimuth on the micrograting morphology. Fourier analyzing of the microscopic images of the microgratings indicates that the polarization plane azimuth of the laser beam does have influence on the microgratings in the aspects of groove orientation and groove spacing. A possible mechanism of polarization dependence is also proposed.

单光束飞秒激光诱导石英玻璃内部纳米光栅的研究进展

单光束飞秒激光诱导石英玻璃内部纳米光栅的研究进展

Formation mechanism of self-organized nanogratings on a titanium surface using femtosecond laser pulses

We demonstrate the formation of self-organized nanogratings on a titanium surface under the irradiation of a single-beam femtosecond laser. Self-formed, periodic nanogratings are printed on a titanium surface by varying the average pulse energy, pulse width, and number of laser pulses in each spot. The direction of the nanogratings is perpendicular to the direction of the laser polarization. The nanograting period shows obvious dependence on the average pulse energy, pulse width, and num- ber of laser pulses. The period of the self-organized nanogratings shows an increasing trend with the increase of laser energy and pulse width, and a decreasing trend with an increase of number of applied laser pulses. We qualitatively explain the formation mechanism of the self-organized nano- gratings and their dependence on various laser parameters. © 2012 Society of

Two-Photon Induced Self-Structuring of Polymeric Films Based on Y-Shape Azobenzene Chromophore

We report on optically induced spontaneous formation of periodic two-dimensional patterns on newly synthesized polymers based on Y-shape azobenzene molecules. Thin films of this polymer are exposed to a single femtosecond laser beam in the near-infrared (wavelength = 800 nm), normally incident on the free surface of the polymer film. The spontaneous formation of periodic structures is observed after few seconds, both perpendicularly and parallel to the incident laser polarization direction, with a periodicity of about 470 and 250 nm, respectively, and amplitude up to 100 nm. The features of height and periodicity are analyzed as a function of the incident pulse energy, evidencing the presence of a threshold of 50 μJ/pulse. The spontaneously formed structures can be partially canceled by illuminating with incoherent UV light. This approach, allowing patterning areas of few mm2 within a few minutes, constitutes a simple and straightforward method for the fabrication of polymer periodic nanostructures.

Formation mechanism of nanostructures in soda–lime glass using femtosecond laser

This paper reports on the formation of nanostructures produced in soda–lime glass by controlling the irradiation conditions of a single-beam femtosecond laser. Periodic nanoholes are fabricated in the sample with the lowest diameter of 200 nm (approximately one fourth of the incident light wavelength). Self-organized nanogratings with a period of 120 nm are fabricated for the first time on soda–lime glass surface by applying many pulses at the same spot. We discover that the nanogratings’ period decreases with the increase of the applied pulses. We investigate that the direction of the nanogratings is perpendicular to the direction of laser polarization. Further, it is discovered that the microholes, due to the illumination by many pulses, are elliptical in shape with the major axis perpendicular to the direction of laser polarization. Finally, long distance horizontal and vertical gratings are fabricated by scanning a femtosecond laser beam only in the horizontal direction.

Interference pattern generation and simulation in the single beam of a white light continuum

We demonstrate the interference phenomenon in the White Light Continuum (WLC) generated by a single femtosecond laser beam. Different kinds of spatial interference patterns of the WLC generation under various conditions were investigated. The spatial patterns were attributed to interference between the filaments in the WLC generated by the fundamental laser beam yielding the diffraction effect from spatial confinement. Simulations of different patterns were performed. By comparing the results of simulation with those of experiments, the distances of several micrometers between the neighboring filaments can be derived, which agree with the literature values from direct measurements.

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

AbstractCoherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm−1) is employed for simultaneous excitation of the CO2 Fermi dyads at ∼1285 and ∼1388 cm−1. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO2 CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO2 concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Two-dimensional periodic structure induced by single-beam femtosecond laser pulses irradiating titanium

A novel two-dimensional (2-D) periodic structure on the ablated hole wall is formed by using the method of single-beam femtosecond laser pulses irradiating a titanium target. This 2-D structure on the ablated surface, after consecutive irradiation of 10000 pulses, presents different spatial periods which characters a 1500 nm period along the hole wall in the vertical direction and a 400 nm period in the azimuthal direction of the hole. The reported experimental results can be well interpreted by a prophetic theoretical model of Bonch-Bruevich (A. M. Bonch-Bruevich et al., Opt. Eng. 31, 718 (1992)).