Titania—a 1020 W cm−2 ultraviolet laser

Abstract

The Titania laser system, based around a 42 cm e-beam pumped KrF amplifier, is currently being installed at the Rutherford Appleton Laboratory and will come on line as a user facility in 1996. Like Sprite, its predecessor, it will operate in both CPA (249 nm) and Raman (268 nm) short-pulse modes, delivering up to 10 TW to target in high-quality beams. With brightness expected to reach 10²¹ W cm^?2 sterad^?1, it will be the world's brightest ultraviolet laser. The design of the Titania system includes a number of novel features. The multi-pass Ti:sapphire front-end amplifier uses an unusual beam-folding scheme. The Raman system will involve the first application of Raman multiplexing, combining high KrF efficiency with low transport cost. Reflective coatings with very high damage thresholds are being developed for the CPA compressor gratings and the UV transport optics. A windowless configuration for the final Raman amplifier is presently under analysis, to allow the performance of this maximally stressed component to be extended substantially. Finally the design of the Titania e-beam machine, featuring novel split-cathode diodes, has resulted in unusually high efficiency of electron transport into the laser gas. The laser's infrastructure has involved sophisticated mechanical and electrical design, and a computerized diagnostic, control and safety package is being developed to allow one-man operation of the whole 1000 m² installation.     

Nanomechanical properties of silicon surfaces nanostructured by excimer laser

Abstract

Excimer laser irradiation at ambient temperature has been employed to produce nanostructured silicon surfaces. Nanoindentation was used to investigate the nanomechanical properties of the deformed surfaces as a function of laser parameters, such as the angle of incidence and number of laser pulses at a fixed laser fluence of 5 J cm^?2. A single-crystal silicon [311] surface was severely damaged by laser irradiation and became nanocrystalline with an enhanced porosity. The resulting laser-treated surface consisted of nanometer-sized particles. The pore size was controlled by adjusting the angle of incidence and the number of laser pulses, and varied from nanometers to microns. The extent of nanocrystallinity was large for the surfaces irradiated at a small angle of incidence and by a high number of pulses, as confirmed by x-ray diffraction and Raman spectroscopy. The angle of incidence had a stronger effect on the structure and nanomechanical properties than the number of laser pulses.     

Angularly-resolved High-order Harmonic Generation in Helium

Abstract

Harmonics up to 119th have been generated in helium using a 1·053 μm, 1 ps chirped pulse amplification laser at intensities up to 3 × 10¹⁴W cm^?2, and their far-field spatial distributions have been measured. In the presence of considerable ionization, complex angular distributions are found for harmonics in the plateau region, while near the cut-off, their distribution narrows to approximately that predicted by lowest-order perturbation theory. The experimental results are found to be in good agreement with the predictions of a numerical propagation model which incorporates photoelectron dispersion and strong-field atomic data for the ionization rates and dipole moments.     

An Experimental Study of a Synchronously Pumped Fibre Raman Oscillator

Abstract

A synchronously pumped fibre Raman oscillator has been constructed employing a mode-locked c.w. Nd:YAG laser as a pump source and 150 m of single-mode optical fibre as the Raman-active medium. A detailed spectral and temporal study of the laser has been undertaken. Time-dispersion tuning offered an operating spectral range of 1·0725–1·1220 µm for the first Stokes oscillation and 1·149–1·179 µm for the associated second Stokes component. Pulse durations ～ 100 ps were generated with average output powers of about 40 and 9 mW for the first and second Stokes Raman pulses respectively.     

An analysis of high efficiency stimulated Raman scattering in water with a frequency-doubled pulsed Nd:YAG source

Abstract

Unexpectedly high stimulated Raman scattering (SRS) has been seen with a frequency-doubled Nd:YAG pump laser (532 nm) in water. The pump beam was propagated, both focused and unfocused, through a 4m long tank. Threshold pump irradiance for SRS was measured to be 2 × 10¹³ Wm^?2 and Stokes transitions were identified at approximately 3230 and 3380 cm^?1. The maximum conversion efficiency to the Stokes frequencies seen was 13% of pump input, for a focused beam, 4% for unfocused. SRS from an unfocused beam was seen to arise from high irradiance filaments caused by self-focusing in the water. The term Raman Interaction Function (RIF) has been defined here as the irradiance of the pump beam integrated over the interaction length (distance over which the pump beam irradiance is above threshold). The RIF model was been designed to describe the relationship between pump laser parameters and the output at the dominant Stokes frequencies.     

Generation and propagation of a resonant CARS signal from biomolecules: Application to dipicolinic acid

Abstract

We develop a theoretical approach and perform simulations of coherent anti-Stokes Raman scattering (CARS) with ultrashort laser pulses. The signal is generated by biomolecules having subpicosecond dephasing times, from femtosecond pulses on exact resonance with the molecular transitions. All propagation effects are explicitly accounted for, including pump depletion, Raman amplification, parametric generation and pulse reshaping. Our model predicts that a measurable CARS signal can be generated by the dipicolinic acid biomolecule under realistic conditions.     

Travelling-wave Brewster-angled Stripe InGaAsP Laser Amplifier at 1·3 µm

Abstract

Optical amplification of a Brewster-angled stripe travelling-wave laser amplifier operating at 1·3 µm is described. An actively mode-locked external cavity semiconductor laser generating pulses of duration from 11 to 30 ps was used as a signal source. Maximum single-pass gain of 32 dB was attained for an input of ?24 dB m and a maximum output peak power of 25 dB m was measured for an input power of 8 dB m. Narrowing in the amplified pulse was observed on a synchroscan streak camera when the amplifier gain is too weak to sustain constant temporal amplification across the whole pulse     

Superradiance and lasing properties of new two-photon absorption dye DEASPS

Pumped by picosecond pulses from a Nd:YAG laser, a new lasing dye, trans-4-[4′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated to DEASPS), shows both intense superradiance and strong lasing properties in benzyl alcohol solution. By using streak camera systems, the superradiance and lasing can be distinguished both spectrally and temporally. It has been found that the peak wavelength of lasing is at 620 nm with a red-shift of about 12 nm to the superradiance wavelength. The lasing pulse shows an oscillatory effect that it is not found in the superradiance pulse. The fluorescence lifetime is 529 ± 40 ps and the effective molecular two-photon absorption is (1.25 ± 0.1) × 10^?48 cm⁴ ·s·photon ^?1, measured using a nonlinear transmittance method. This dye shows effective optical limiting of the pumping wavelength.     

Growth and characterization of Ni:DLC composite films using pulsed laser deposition technique

The structure and surface morphology of Ni-incorporated diamond like carbon (Ni:DLC) films have been investigated. These films were deposited on Si substrates using pulsed laser deposition (PLD) technique. A KrF Excimer laser (λ = 248 nm) was used for co-ablation from multi component Ni–graphite target. The concentration of Ni was varied by ablating the Ni part of the target with various numbers of laser pulses. The SEM and AFM analysis reveals that the surface is composed of segregates of Ni which increases with the increase in Ni content during the growth process. The structural investigations by XRD and Raman spectroscopy provided information about the orientation of the incorporated constituent and the ordering of the carbon species. Maximum height of the nano structures which were observed on the surface was ∼50 nm. The G-peak of the graphite was shifted towards higher wave number due to enhancement in SP² sites which have been increased due to the increase in the Ni concentration. A small change in the surface roughness ranging from 7.78 nm to 13.1 nm due to increased Ni concentration was also observed.     

KrF excimer laser crystallization of silicon thin films

A KrF excimer laser operating at 249 nm has been employed to crystallize silicon thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and by RF magnetron sputtering on Corning glass and SiO₂. All films display a substantial improvement in crystallinity after ELC with the optimum laser fluence for as-deposited ECR films being higher than for sputtered films. This is probably related to the presence of Si-H_x bonds in the former. A pronounced bimodality in the Raman spectra of some amorphous, as-deposited ECR samples has been observed after laser crystallization where, in addition to the peak at 520cm^-1, a strong peak at 509cm^-1 is also present. Such behavior has not been reported previously to our knowledge in ELC silicon films. Interestingly, the XRD spectra of these samples do not exhibit any peaks suggesting the films are composed of nano-grain material. The dehydrogenation of ECR films by ELC has been demonstrated to be substantial, the hydrogen content typically decreasing from ~30 at % in an as-deposited film to ~10 at % after a single low fluence laser shot. Raman spectroscopy has shown that the film bonding changes from predominantly Si-H₂ to Si-H after ELC. Electrical resistivity measurements of phosphorus-doped films show a controllable and repeatable change with laser fluence. The results in this paper show that it is possible to crystallize and controllably change the electrical characteristics of ECR PECVD produced silicon thin films by ELC.     

Spectral Filtering Effect of Fused Fibre Couplers in Femtosecond Fibre Soliton Lasers

Abstract

A fused fibre coupler has been used as a spectral filter in a femtosecond fibre soliton laser to eliminate the spectral sideband structure associated with periodic amplification of solitons in the regime where the characteristic soliton period of the generated pulses approaches that of the overall fibre laser loop length.     

On the Action of γ Radiation on Solid C60 and C70 Fullerenes: A Comparison with Graphite Irradiation

Abstract

C₆₀ and C₇₀ fullerene have been treated in sealed flasks under Ar with γ radiation using radiation dosages ranging from 10 to 1000 kGy. The treated samples studied by electronic and FT-IR spectroscopy have not shown any evidence about fullerenes decomposition or radiopolymerization. However, through Raman spectroscopy it was possible to observe that γ radiation induces C₆₀ dimerization and trimerization. It has additionally been discovered that γ-treated C₆₀ (oligomerized) can be easily photopolymerized in the solid state by post-irradiation with laser light at 514 nm while this phenomenon has not been observed by using laser light at 782 nm and considerably higher laser power. Previously to this study, C₆₀ photopolymerization was known to occur only by using ultraviolet light.

For comparison also graphite was irradiated with 1000 kGy of γ radiation. Irradiated graphite shows considerably changes in its Raman spectrum, showing the formation of glassy carbon domains, perhaps carbon onions. The l_d bandshift to 1310 cm^?1 could be interpreted in terms of formation of hexagonal diamond.     

Effects of Laser Repetition Rate and Fluence on Micromachining of TiC Ceramic

Micromachining of titanium carbide (TiC) ceramic is very difficult because of its high hardness and brittleness. Femtosecond pulsed laser was employed to process circular rings on the surface of TiC ceramic. The interaction area between femtosecond laser pulses and TiC at different laser repetition rates and fluences was studied. Morphology and composition of irradiated area were analyzed by scanning electron microscope, energy dispersive spectroscopy, and Raman spectrum. The results indicated that the radius of outer circle was close to the intended radius. Laser fluence had obvious effects on the radius and width of circular rings, compared to laser repetition rate. The width of circular rings increased rapidly with increasing laser fluence from 2.55 × 10^?2 to 1.27 × 10^?1 J/mm², and then stabilized at around 40 µm when laser fluence was above 7.64 × 10^?1 J/mm². The surface of circular rings was characterized by ripples at the lower laser fluence. With increasing laser fluence, four kinds of typical morphology were observed, including ripples, cauliflower-like particles, ball-like particles, and deposited oxide layer. Ball-like particles contained high concentration of titanium, which came from melt ball splashing from ablation area. The others came from the different oxidation stages occurred on the surface of TiC sample.     

Ultrafast laser processing: New options for three-dimensional photonic structures

Abstract

Using tightly focused ultrashort laser pulses allows the direct writing of three-dimensional photonic structures in different glasses and also crystalline media. One of the main drawbacks of this technology is, however, the limited writing speed achieved so far. In this paper we shall review our recent advances in the direct writing of three-dimensional integrated-optical devices and discuss a new approach using a fibre-based femtosecond laser system producing 300 fs pulses with pulse energies of 0.6 μJ at 2 MHz repetition rate. Using this laser system we fabricated low-loss waveguides (less than 0.5 dBcm^?1) at writing speeds of 100 mms^?1 for the first time. The influence of the writing speed on the produced structures as well as their optical properties will be discussed in detail.     

Picosecond optical parametric chirped pulse amplifier as a preamplifier to generate high-energy seed pulses for contrast enhancement

We present the design, implementation, and testing of a novel picosecond optical parametric preamplifier system to generate high-energy seed pulses for the Vulcan laser facility. The preamplifier amplifies 100?fs pulses stretched to 3?ps pulses from 10?pJ to 70 μJ in a single stage of amplification before the pulses are further amplified in the Vulcan high-power Nd:glass laser facility to the petawatt power level. This increased seed energy has led to an improvement of the nanosecond amplified spontaneous emission contrast intensity to 10(-10) of the main pulse, without degrading the output of the laser system.     

The action of ultrashort high-power electron beam pulses on metal targets

The action of high-power subnano-and picosecond electron beam pulses on metal targets has been numerically studied within the framework of a one-dimensional two-temperature model of a metal. The dynamics of strain and stress fields generated in the target by pulses of various durations have been simulated. The rates of tensile straining are significantly higher than the rates of compressive straining. The straining rate is determined by the rate of energy supply and can reach 10⁷?10⁸ s^?1 for tensile strains. A decrease in the time of energy deposition from tens of nanoseconds to ～1 ns and below gives rise to the level of mechanical stresses. It is established that subnanosecond pulses provide a more effective conversion of the electron beam energy into the kinetic energy of a target material and the potential energy of a mechanical stress field.     

Picosecond nonlinear optical studies of gold nanoparticles synthesised using coriander leaves (Coriandrum sativum)

The results are presented from the experimental picosecond nonlinear optical (NLO) studies of gold nanoparticles synthesised using coriander leaf (Coriandrum sativum) extract. Nanoparticles with an average size of ～30?nm (distribution of 5–70?nm) were synthesised according to the procedure reported by Narayanan et al. [Mater. Lett. 2008, 62, 4588–4591]. NLO studies were carried out using the Z-scan technique using 2?ps pulses near 800?nm. Open-aperture data suggested saturation absorption as the nonlinear absorption mechanism, whereas closed-aperture data suggested a positive nonlinearity. The magnitude of third-order nonlinearity was estimated to be (3.3?±?0.6)?×?10^?13?esu. A solvent contribution to the nonlinearity was also identified and estimated. A comparison is attempted with some recently reported NLO studies of similar gold nanostructures.     

Stable Modelocking of a Coupled Cavity Moving Mirror Ring Dye Laser

Abstract

We report the stable operation of a new modelocked standing-wave ring dye laser operating with a repetition rate of 250 MHz. Two external cavities are coupled to a bidirectionally operating continuously pumped ring dye laser. When a moving mirror is placed in one of the external cavities, the laser produces pulses 6 ps in duration. The operation is stable with the modelocked pulses arriving in a constant intensity envelope 5 ms in duration at a repetition rate of 90 Hz.     

Generation of isolated attosecond pulses using unipolar and laser fields

A new scheme to generate isolated attosecond pulses is presented that involves the use of a laser field and of a unipolar field. The laser field has a pulse of intensity I = 1.5×10¹⁴ W cm^?2 and wavelength λ = 820 nm. The unipolar pulse is an asymmetric pulse consisting of a sharp peak, lasting approximately half a laser period, i.e. nearly 1.4 fs, followed by a long and shallow tail. We show that on combining these two fields, it is possible to generate isolated attosecond pulses as short as 1/10 of a laser period, i.e. approximately 270 as. Moreover, it is argued that this scheme is robust either against small variations of the laser envelope, or against small changes in the delay between the laser pump and the unipolar pulse.     

Ultraviolet HeCd laser pumped by a high-frequency electron beam

An electron beam with a large cross section and with a repetition rate of 3×10⁵ pulses per second in a packet has been obtained for the first time. With this beam a HeCd high-pressure laser has been developed and high-frequency lasing on the Cd ion at a wavelength of 325 nm has also been achieved for the first time. Pis’ma Zh. Tekh. Fiz. 23, 44–47 (March 26, 1997)