Energy scaling of a carbon nanotube saturable absorber mode-locked femtosecond bulk laser

We report successful energy scaling of a room-temperature femtosecond Cr⁴⁺: forsterite laser by using a single-walled carbon nanotube saturable absorber (SWCNT-SA). By incorporating a q-preserving multipass cavity, a repetition rate of 4.51?MHz was realized, and the oscillator produced 121?fs, 10?nJ pulses at 1247?nm, with an average output power of 46?mW. To the best of our knowledge, the peak power of 84?kW is the highest generated to date from a SWCNT-SA mode-locked oscillator. Furthermore, energy scaling of a femtosecond multipass-cavity laser, mode-locked using a SWCNT-SA, is demonstrated for the first time.     

Visible continuum generation using a femtosecond erbium-doped fiber laser and a silica nonlinear fiber

Supercontinuum extending to visible wavelengths is generated in a hybrid silica nonlinear fiber pumped at 1560 nm by a femtosecond, erbium-doped fiber laser. The hybrid nonlinear fiber consists of a short length of highly nonlinear, germano-silicate fiber (HNLF) spliced to a length of photonic crystal fiber (PCF). A 2 cm length of HNLF provides an initial stage of continuum generation due to higher-order soliton compression and dispersive wave generation before launching into the PCF. The visible radiation is generated in the fundamental mode of the PCF.     

Microlens arrays of high-refractive-index glass fabricated by femtosecond laser lithography

Microlens arrays of high-refractive-index glass GeO₂-SiO₂ were fabricated by femtosecond laser lithography assisted micromachining. GeO₂-SiO₂ thin glass films, which were deposited by plasma-enhanced chemical vapor deposition, have a refractive index of 1.4902 and exhibit high transparency at wavelengths longer than 320 nm. Using a femtosecond laser, three-dimensional patterns were written inside resists on GeO₂-SiO₂ films, and then the patterns were transferred to the underlying films by CHF₃ and O₂ plasma treatments. This combined process enabled us to obtain uniform microlens structures with a diameter of 38 μm. The heights of the transferred lenses were approximately one-quarter the height of the resist patterns, due to differences in the plasma etching rates between GeO₂-SiO₂ and the resist. The lens surfaces were smooth. When 632.8-nm-wavelength He-Ne laser light was normally coupled to the lenses, focal spots with a diameter of 3.0 μm were uniformly observed. The combined process was effective in fabricating three-dimensional surfaces of inorganic optical materials.     

Lithographical laser ablation using femtosecond laser

Lithographical laser ablation was demonstrated using a femtosecond laser with a lithographical optical system. In this method, a femtosecond laser beam passes through a mask and the pattern is imaged on a film by a coherent optical system. As a result, the film is lithographically ablated, and a micron-sized pattern can be generated in a single shot. The resolution of generation was 13 m, and the narrowest width of a generated line was about 4 m. Moreover, the system was applied to transmission gratings as masks, and nano-sized periodic structures such as nano-sized hole matrices and nano-meshes were generated in a single shot. PACS 52.38.Mf; 42.25.Hz; 42.82.Cr; 81.16.-c     

Super-hydrophobic transparent surface by femtosecond laser micro-patterned catalyst thin film for carbon nanotube cluster growth

In this work, super-hydrophobic surfaces were fabricated by femtosecond laser micro-machining and chemical vapor deposition to constitute hybrid scale micro/nano-structures formed by carbon nanotube (CNT) clusters. Nickel thin-film microstructures, functioning as CNT growth catalyst, precisely control the distribution of the CNT clusters. To obtain minimal heat-affected zones, femtosecond laser was used to trim the nickel thin-film coating. Plasma treatment was subsequently carried out to enhance the lotus-leaf effect. The wetting property of the CNT surface is improved from hydrophilicity to super-hydrophobicity at an advancing contact angle of 161 degrees. The dynamic water drop impacting test further confirms its enhanced water-repellent property. Meanwhile, this super-hydrophobic surface exhibits excellent transparency with quartz as the substrate. This hybrid fabrication technique can achieve super-hydrophobic surfaces over a large area, which has potential applications as self-cleaning windows for vehicles, solar cells and high-rise buildings.     

Carbon nanotube synthesis in a flame using laser ablation for in situ catalyst generation

Laser ablation of either Ni or Fe is used to create nanoparticles within a reactive flame environment for catalysis of carbon nanotubes (CNTs). Ablation of Fe in a CO-enriched flame produces single-walled nanotubes, whereas, ablation of Ni in an acetylene-enriched flame produces carbon nanofibers. These results illustrate that the materials for catalyst particle formation and CNT, SWNT or nanofiber, inception and growth in the aerosol phase can be supplied from separate sources; a metal-carbon mixture produced by condensation is not necessary. Both particle formation and CNT inception can begin from molecular species in a laser-ablation approach within the complex chemical environment of a flame. Moreover, SWNTs and nanofibers can be synthesized within very short timescales, of the order of tens of milliseconds. Finally, high-intensity pulsed laser light can destroy CNTs through either vaporization or coalescence induced by melting. PACS 42.62 Fi; 81.05.Tp; 82.80.Ch; 81.15 Fg     

Microstructuring of diamond bulk by IR femtosecond laser pulses

We report the fabrication of graphitic microstructures in the bulk of chemical vapor deposited (CVD) diamond using 120-fs laser pulses at 800-nm wavelength. The nature of the laser-modified region and generation of mechanical stresses in the surrounding diamond is studied with Raman spectroscopy. A spontaneous growth of the laser-modified region from the focal plane towards the laser has been visualized in the process of multipulse irradiation with different pulse energies. The formation of discrete or continuous graphitized structures is revealed depending on the varied local laser intensity. The physical processes governing the appearance of separate graphitic globules and continuous extension of the graphitized region are discussed. Controlling the laser irradiation conditions permits us to fabricate graphitic wires with typical length of 150 μm and diameter of 1.5 μm. The longer, 300-ps pulses, as applied to laser microstructuring of the CVD diamond bulk, are found to be inappropriate due to the stronger influence of structural defects on the damage threshold, the noticeable fluctuation of the structure diameter over the length and the pronounced cracking of the surrounding diamond. PACS 42.62.-b; 61.80.Ba     

Multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser

We demonstrate a novel multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser at 1230 nm. By acquiring the whole nonlinear spectrum in the visible and near-NIR region, this novel technique allows a combination of different imaging modalities, including second-harmonic generation, third-harmonic generation, and multiple-photon fluorescence. Combined with the selected excitation wavelength, which is located in the IR transparency window, this microscopic technique can provide high penetration depth with reduced damage and is ideal for studying living cells.     

550 MHz carbon nanotube mode-locked femtosecond Cr:YAG laser

We demonstrate a femtosecond Cr:YAG laser mode-locked by a carbon nanotube saturable absorber mirror(CNT-SAM) at a repetition rate of 550 MHz. By employing the CNT-SAM, which exhibits a modulation depth of 0.51% and a saturation fluence of 28 μJ∕cm~2 at 1.5 μm, we achieved a compact bulk Cr:YAG laser with selfstarting mode-locked operation near 1.5 μm, delivering an average output power of up to 147 m W and a pulse duration of 110 fs. To our knowledge, this system provides the highest repetition rate among reported CNT-SAM mode-locked Cr:YAG lasers and the shortest pulse duration among saturable absorber mode-locked Cr:YAG lasers with repetition rates above 500 MHz.     

Nanoimprint lithography using IR laser irradiation

A new technique called “infrared laser-assisted nanoimprint lithography” was utilised to soften the thermoplastic polymer material mR-I 8020 during nanoimprint lithography. A laser setup and a sample holder with pressure and temperature control were designed for the imprint experiments. The polymer was spin coated onto crystalline Si <1 1 1> substrates. A prepatterned Si <1 1 1> substrate, which is transparent for the CO₂ laser irradiation, was used as an imprint stamp as well. It was shown, that the thermoplastic resist mR-I 8020 could be successfully imprinted using the infrared CW CO₂ laser irradiation (λ = 10.6 μm). The etching rate of the CO₂ laser beam irradiated mR-I 8020 resist film under O₂ RF (13.56 MHz) plasma treatment and during O₂ reactive ion beam etching was investigated as well.     

Dielectric microsphere mediated transfection using a femtosecond laser

We demonstrate the permeabilization of cell membranes by an enhanced optical field generated under polystyrene microspheres of 1000 nm diameter excited by a femtosecond laser pulse. Fluorescent molecules and short interfering RNA (siRNA) have been successfully delivered to many cells in the irradiated area by a single 80 fs laser pulse at 800 nm wavelength in the presence of antibody-conjugated polystyrene spheres. The ratios of the cells showing permeabilization were 38% and 21% for Fluorescein isothiocyanate-dextran and siRNA, respectively, at the laser fluence of 1.06 J/cm(2). The present method has advantages both in high throughput of many cell treatments and precise processing of minute areas on cell membranes.     

Frequency-resolved coherent lidar using a femtosecond fiber laser

We demonstrate a coherent lidar that uses a broadband femtosecond fiber laser as a source and resolves the returning heterodyne signal into N spectral channels by using an arrayed-waveguide grating. The data are processed incoherently to yield an N-times improvement in the Doppler measurement of a surface vibration. For N=6, we achieve a sensitivity of 153 Hz, corresponding to a 0.12 mm/s motion, in 10 ms despite a signal that is speckle broadened to 14 kHz. Alternatively, the data are processed coherently to form a range image. For a flat target, we achieve a 60 microm range resolution, limited mainly by the source bandwidth, despite the dispersion of 1 km of optical fiber in the signal path.     

Plasma-assisted combustion of methane using a femtosecond laser

We first research the effects of femtosecond-laser-induced plasma (FLIP) on a laminar premixed methane/oxygen/nitrogen flame speed with a wide range of equivalence ratios (0.8-2.0) at atmospheric pressure. It is experimentally found that the flame speed increases by 30.8% at equivalence ratio 1.33, and the effects of the FLIP on the flame speed are more remarkable when the methane is rich. The self-emission spectra from the flame and the plasma are studied, and the presence of the oxygen atom is likely to be a key factor in enhancing flame speed.     

Time-resolved nonlinear refraction in femtosecond laser gain media

We present time-resolved measurements of the electronic and the nuclear contributions to the third-order nonlinearities of Ti:sapphire and Cr:LiSGaF. The results suggest that nonlinear refraction in Ti:sapphire is resonantly enhanced relative to that of undoped sapphire. The implications of the nonlinear responses for short-pulse formation are discussed.     

Photoetching of spherical microlenses on glasses using a femtosecond laser

We fabricated spherical microlenses on optical glasses by femtosecond laser direct writing (FLDW) in ambient air. To achieve good appearances of the microlenses, a meridian-arcs scanning method was used after a selective multilayer removal process with spiral scanning paths. A positive spherical microlens with diameter of 48 μm and height of 13.2 μm was fabricated on the surface of the glass substrate. The optical performances of the microlens were also tested. Compared to the conventional laser direct writing (LDW) technique, this work could provide an effective method for precise shape-controlled fabrication of three-dimensional (3D) microstructures with curved surfaces on difficult-to-cut materials for practical applications.     

High-energy femtosecond stretched-pulse fiber laser with a nonlinear optical loop mirror

A stretched-pulse fiber laser with a nonlinear optical loop mirror (NOLM) that produces 100-fs pulses with 1-nJ energy is demonstrated. These results constitute a 30-fold increase in pulse energy over previously reported femtosecond fiber lasers with a NOLM. Compared with previous stretched-pulse lasers, this laser offers a cleaner spectrum and improved stability, with comparable pulse duration and energy. Implications for the construction of truly environmentally stable lasers are discussed.     

Analytical study on metal microstructures using femtosecond laser

We made considerations for microstructures on chromium film observed after femtosecond laser irradiation through an analytical study. Laser diffraction through an optical system with a rectangular aperture was analyzed for the estimation of intensity distribution at the focal point. Molecular dynamics (MD) simulation was also performed for the examination of laser ablation of metal by taking account of the electron–phonon relaxation. From results of diffraction calculation, it was shown that a typical surface pattern is significantly affected by laser intensity distribution. On the other hand, it may be estimated through the MD simulation that the porosity formation evolves by cumulative pulses with energy close to the ablation threshold and their surrounding grows to microcolumns with volume expansion. PACS 68.08.De; 68.35.-p     

T-shaped gate AlGaN/GaN HEMTs fabricated by femtosecond laser lithography without ablation

Femtosecond laser as a maskless lithography technique is able to fabricate structures far smaller than the diffraction limit to a value within sub-micrometer resolution. We present the femtosecond laser lithography without ablation on the positive photoresist is applied in fabricating T-shaped gate AlGaN/GaN HEMT. The feature sizes of femtosecond laser lithography were determined by the incident laser power, the scan speed of the laser focus, the number of scan times, and the substrate materials. T-shaped gate with the smallest gate length 204?nm could be fabricated by dielectric-defined process using femtosecond laser lithography. The fabricated AlGaN/GaN HEMT with 380?nm T-gate exhibits a maximum drain current density of 500?mA/mm and a maximum peak extrinsic transconductance of 173?mS/mm.     

光子晶体光纤飞秒激光非线性放大系统的耦合动力学过程研究

构建了掺镱大模场面积单偏振光子晶体光纤飞秒激光非线性放大系统. 讨论了腔内净色散量和抽运功率对振荡级输出参数的影响和振荡级参数对放大级输出参数的影响. 在本实验条件下, 当腔内净色散量取较大负色散时, 振荡级直接输出的脉冲更宽, 且携带更少的啁啾. 当振荡级抽运4.53 W时, 选择最接近变换极限的脉冲作为种子脉冲, 放大级在60 W抽运时输出压缩后无基底的短脉冲, 宽度为45.7 fs, 平均功率28 W. 振荡级抽运功率增加到5.08 W, 放大级抽运70 W时, 获得最高输出功率34.5 W, 对应脉宽53.5 fs.