Recording, erasing, and rewriting of ripples on metal surfaces by ultrashort laser pulses.

Recording, erasing, and rewriting of ripples are achieved by applying femtosecond laser pulses on tungsten surfaces. Ripples oriented perpendicular to the polarization direction of the writing beam can be recorded on a metal surface by exposing the sample to a series of linearly polarized pulses. When applying the second series of pulses with varied polarization direction on the same place, the original ripples can be erased, and new ripples are rewritten with the orientation perpendicular to the polarization of the second group of pulses. The simulation shows that when original ripples exist, laser intensity is focused above the grooves with polarization parallel to original ripples, which can erase the ripples. However, when the polarization is perpendicular to the existing ripples, laser intensity is almost confined in the grooves, which accelerates the formation of ripples.

Low chromatic Fresnel lens for broadband attosecond XUV pulse applications.

Fresnel zone plates show a great potential in achieving high spatial resolution imaging or focusing for XUV and soft/hard X-ray radiation, however they are usually strictly monochromatic due to strong chromatic dispersion and thus do not support broad radiation spectra, preventing their application to attosecond XUV pulses. Here we report on the design and theoretical simulations based on the design of an achromatic hybrid optics combining both, a refractive and diffractive lens in one optical element. We are able to show by calculation that the chromatic dispersion along the optical axis can be greatly reduced compared to a standard Fresnel zone plate while preserving the temporal structure of the attosecond XUV pulses at focus.

Ultrashort pulsed laser induced heating-nanoscale measurement of the internal temperature of dielectrics using black-body radiation.

The nanoscale measurement of temperature in the bulk of dielectrics initiated by a single ultrashort laser pulse was first investigated by black-body radiation. A structureless broad continuum emission has been recorded at an interval delay of 2 ns with a temporal gate of 2 ns and spectral resolution of about 0.137 nm, which provides the highest temporal and spectral precision ever. The temporally resolved emission spectrum was proved to be black-body radiation in nature, and temperature was obtained by fitting the radiation with the Planckian formula. Pulse energy was varied from 110 to 270 µJ at 600 fs and a pulse duration of 0.83 ns was also used. The temperature exhibited a small variation with an increasing pulse energy at 600 fs. However, due to the energy transfer from heated electrons to lattice, the temperature was sharply increased at pulse duration of 0.83 ns. It was estimated that heat accumulation started at 0.42-0.47 MHz for a laser pulse at 600 fs, while it was 0.25 MHz for a laser pulse at 0.83 ns.

Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics.

We report on the first terahertz (THz) emitter based on femtosecond-laser-ablated gallium arsenide (GaAs), demonstrating a 65% enhancement in THz emission at high optical power compared to the nonablated device. Counter-intuitively, the ablated device shows significantly lower photocurrent and carrier mobility. We understand this behavior in terms of n-doping, shorter carrier lifetime, and enhanced photoabsorption arising from the ablation process. Our results show that laser ablation allows for efficient and cost-effective optoelectronic THz devices via the manipulation of fundamental properties of materials.

Optical absorption and photocurrent enhancement in semi-insulating gallium arsenide by femtosecond laser pulse surface microstructuring.

We observe an enhancement of optical absorption and photocurrent from semi-insulating gallium arsenide (SI-GaAs) irradiated by femtosecond laser pulses. The SI-GaAs wafer is treated by a regeneratively amplified Ti: Sapphire laser of 120 fs laser pulse at 800 nm wavelength. The laser ablation induced 0.74 µm periodic ripples, and its optical absorption-edge is shifted to a longer wavelength. Meanwhile, the steady photocurrent of irradiated SI-GaAs is found to enhance 50%. The electrical properties of samples are calibrated by van der Pauw method. It is found that femtosecond laser ablation causes a microscale anti-reflection coating surface which enhances the absorption and photoconductivity.

Waveguides fabricated by femtosecond laser exploiting both depressed cladding and stress-induced guiding core.

We report on the fabrication of stress-induced optical channel waveguides and waveguide splitters with laser-depressed cladding by femtosecond laser. The laser beam was focused into neodymium doped phosphate glass by an objective producing a destructive filament. By moving the sample along an enclosed routine in the horizontal plane followed by a minor descent less than the filament length in the vertical direction, a cylinder with rarified periphery and densified center region was fabricated. Lining up the segments in partially overlapping sequence enabled waveguiding therein. The refractive-index contrast, near- and far-field mode distribution and confocal microscope fluorescence image of the waveguide were obtained. 1-to-2, 1-to-3 and 1-to-4 splitters were also machined with adjustable splitting ratio. Compared with traditional femtosecond laser writing methods, waveguides prepared by this approach showed controllable mode conduction, strong field confinement, large numerical aperture, low propagation loss and intact core region.

Optical waveguides in TiO2 formed by He ion implantation.

We report on the formation and the optical properties of the planar and ridge optical waveguides in rutile TiO2 crystal by He+ ion implantation combined with micro-fabrication technologies. Planar optical waveguides in TiO2 are fabricated by high-energy (2.8 MeV) He+-ion implantation with a dose of 3 × 10¹6 ions/cm² and triple low energies (450, 500, 550) keV He+-ion implantation with all fluences of 2 × 10¹6 ions/cm² at room temperature. The guided modes were measured by a modal 2010 prism coupler at wavelength of 1539 nm. There are damage profiles in ion-implanted waveguides by Rutherford backscattering (RBS)/channeling measurements. The refractive-index profile of the 2.8 MeV He+-implanted waveguide was analyzed based on RCM (Reflected Calculation Method). Also ridge waveguides were fabricated by femtosecond laser ablation on 2.8 MeV ion implanted planar waveguide and Ar ion beam etching on the basis of triple keV ion implanted planar waveguide, separately. The loss of the ridge waveguide was estimated. The measured near-field intensity distributions of the planar and ridge modes are all shown.

Self-organized tungsten nanospikes grown on subwavelength ripples induced by femtosecond laser pulses.

We report on the formation of nanoscale tungsten spikes generated on subwavelength periodic ripples which built up by single beam 800 nm femtosecond laser pulses. The nanospikes have a diameter ranging from 10 to 100 nm and are up to 250 nm in length. The nanospikes orientate from the ridges toward the valleys of the ripple structures independent of the polarization of the light. The heterogeneous nucleation of the liquid phase at the irradiated surface and the inhomogeneous surface roughness are considered as the mechanism of this nanospike formation.

Optical transfer of periodic microstructures by interfering femtosecond laser beams.

We report on an optical interference method for transferring periodic microstructures of metal film from a supporting substrate to a receiving substrate by means of five-beam interference of femtosecond laser pulses. Scanning electron microscopy and optical microscopy revealed microstructures with micrometer-order were transferred to the receiving substrate. In the meanwhile, a negative copy of the transferred structures was induced in the metal film on the supporting substrate. The diffraction characteristics of the transferred structures were also evaluated. The present technique allows one-step realization of functional optoelectronic devices.

Near infrared broadband emission of bismuth-doped aluminophosphate glass.

Near infrared broadband emission characteristics of bismuth-doped aluminophosphate glass have been investigated. Broad infrared emissions peaking at 1210nm, 1173nm and 1300nm were observed when the glass was pumped by 405nm laser diode (LD), 514nm Ar+ laser and 808nm LD, respectively. The full widths at half maximum (FWHMs) are 235nm, 207nm and 300nm for the emissions at 1210nm, 1173nm and 1300nm, respectively. Based on the energy matching conditions, it is suggested that the infrared emission may be ascribed to 3P1? 3P0 transition of Bi+. The broadband infrared luminescent characteristics of the glasses indicate that they are promising for broadband optical fiber amplifiers and tunable lasers.

Infrared broadband emission of bismuth-doped barium-aluminum-borate glasses.

We report near infrared broadband emission of bismuth-doped barium-aluminum-borate glasses. The broadband emission covers 1.3microm window in optical telecommunication systems. And it possesses wide full width at half maximum (FWHM) of ~200nm and long lifetime as long as 350micros. The luminescent properties are quite sensitive to glass compositions and excitation wavelengths. Based on energy matching conditions, we suggest that the infrared emission may be ascribed to 3P1? 3P0 transition of Bi+. The broad infrared emission characteristics of this material indicate that it might be a promising candidate for broadband optical fiber amplifiers and tunable lasers.

Direct writing computer-generated holograms on metal film by an infrared femtosecond laser.

Writing computer-generated holograms have been achieved by using a near infrared femtosecond laser selective ablation of metal film deposited on glass substrate. The diffraction features with data reconstruction of the fabricated computer-generated holograms were evaluated. Both transmission and reflection holograms can be fabricated in a single process. The process requires no mask, no pre- or post-treatment of the substrate.

Fabrication of internal diffraction gratings in calcium fluoride crystals by a focused femtosecond laser.

We report the fabrication of internal diffraction gratings in calcium fluoride crystals by a focused near-IR 800 nm femtosecond laser. The diffraction efficiency and refractive index change were evaluated after femtosecond laser irradiation and subsequent annealing. The maximum refractive index change was estimated to be 3.57x10(-4). Optical absorption spectra, measured for the crystals before and after the laser irradiation and subsequent annealing, indicate that the absorbance increase after femtosecond laser irradiation and decrease with increasing annealing temperature. The mechanisms of refractive index change are proposed. The results may be useful for fabrication of three-dimensional integrated optics devices in the crystals.

Controllable precipitation and dissolution of silver nanoparticles in ultrafast laser pulses irradiated Ag+-doped phosphate glass.

We report a controllable process of recipitation and dissolution of silver nanoparticles in ultrashort laser pulses irradiated Ag+-doped phosphate glass. Absorption spectra, transmission electron microscopy and refractive index measurement revealed that metallic silver nanoparticles were precipitated in the glass sample after irradiation by an 800-nm femtosecond laser and subsequent annealing at 300 degrees C, and dissolved after further annealing at 450 degrees C. We discuss a mechanism that combines the formation and decoloration of color centers, precipitation and dissolution of silver nanoparticles.