Incident Pulse Energy Research Articles

X-ray streak camera cathode development and timing accuracy of the 4ω ultraviolet fiducial system at the National Ignition Facility

The convergent ablator experiments at the National Ignition Facility (NIF) are designed to measure the peak velocity and remaining ablator mass of an indirectly driven imploding capsule. Such a measurement can be performed using an x-ray source to backlight the capsule and an x-ray streak camera to record the capsule as it implodes. The ultimate goal of this experiment is to achieve an accuracy of 2% in the velocity measurement, which translates to a ±2 ps temporal accuracy over any 300 ps interval for the streak camera. In order to achieve this, a 4ω (263 nm) temporal fiducial system has been implemented for the x-ray streak camera at NIF. Aluminum, titanium, gold, and silver photocathode materials have been tested. Aluminum showed the highest relative quantum efficiency, with five times more peak signal counts per fiducial pulse when compared to Gold. The fiducial pulse data were analyzed to determine the centroiding statistical accuracy for incident laser pulse energies of 1 and 10 nJ, showing an accuracy of ±1.6 ps and ±0.7 ps, respectively.

Off-focus generation of strong super-continuum emission in fused silica using high power femtosecond laser pulses

We report on the generation of millijoules supercontinuum, which covers waveband from ～ 400 nm to ～ 900 nm wavelength, by intense femtosecond laser pulse propagating in 3 mm thick fused silica. The fused silica is placed in front of the geometric focus of incident laser pulse to prevent the breakdown. The dependences of supercontinuum spectrum on incident pulse energy and off-focus distance of medium are investigated in detail. Our work demonstrates that strong supercontinuum can be generated using off-focus pump of solid transparent medium with energetic incident pulses.

Laser micro- and nanostructuring using femtosecond Bessel beams

We report the generation of femtosecond Bessel beams of conical half-angle 26 degrees using an axicon lens and a beam reduction imaging setup. The generated Bessel beams were applied to the micromachining of nanostructures in glass of length up to 100 μm. The effect of the incident pulse energy on the characteristics of the nano- structures was studied using optical microscopy.

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.

Capturing light pulses into a pair of coupled photonic crystal cavities

We describe finite-difference time-domain simulations of a two-dimensional photonic crystal implementation of a two-resonator system capable of capturing light pulses from a waveguide. As much as 99.61% of incident pulse energy is captured in simulations. The release of near-perfect Gaussian pulses is also demonstrated.

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters ((CD4)n) in a gas jet subjected to intense femtosecond laser pulses (170 mJ, 70 fs) have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5 nm in radius and the laser intensity used was 3 × 1017 W/cm2.The measured maximum and average energies of deuterons produced in the laser–cluster interaction were 60 and 13.5 keV, respectively. From DD collisions of energetic deuterons, a yield of 2.5(±0.4) × 104 fusion neutrons of 2.45 MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5 × 105 per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.

Diode-Pumped Rational Harmonic Mode-Locked Yb:GSO Laser

We experimentally demonstrated a sub-picosecond laser based on Yb:Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> crystal and obtained stable rational harmonic mode-locked laser output. Along with the fundamental repetition rate of 100 MHz, we observed the generation of the 200 MHz harmonics and pulse bunches, depending on the balance between the laser gain and incident pulse energy fluence on the semiconductor saturable absorber mirror. The pulse duration in harmonic mode-locking operation was measured as 600 fs, while pulses in bunch were measured with a duration of 1 ps separated by 20-ps time interval.

Intense femtosecond laser pulse interaction with ultra-low expansion glass: nonlinear responses and optical limiting effect

In this study, the nonlinear responses and optical limiting behaviour of titanium doped silicate glass (7.5TiO2–92.5SiO2), known as ultra-low expansion (ULE) glass, are investigated. The measurements were performed by use of the Z-scan technique and monitoring of the transmitted energy of 200 fs Ti:sapphire laser pulses at a central wavelength of 800 nm. The optical power limiting threshold was measured to be about 37 µJ (peak intensity of 0.55 × 1012 W cm−2) in this sample. The decrease of transmitted intensity was about 60% compared to the theoretical linear transmission for incident pulse energies of 320 µJ (peak intensity of 21 × 1012 W cm−2). For the prediction of the experimental nonlinear behaviour results, we used the three-photon absorption and three-photon-generated free-carrier absorption model within the sample.

Optothermoacoustic Phenomena in Highly Diluted Suspensions of Gold Nanoparticles

An optoacoustic (OA) sensor was designed, fabricated, and used to detect spherical gold nanoparticles (NPs) in diluted suspensions. The sensor, operating in the backward mode, was designed to measure signals from microscopic volumes of nanoparticulate suspensions in water. Thermal nonlinearity was observed in the course of OA signal generation. The irradiation of a microvolume of gold nanoparticles at the wavelength matching the peak of their plasmon resonance absorption gives rise to a multitude of thermomechanical processes, including heating of NPs below the critical temperature of water (374 K). The thermal diffusion from nanoparticles to water takes place; however, formation of vapor nanobubbles is avoided. As a result, a specific acoustic signal is produced exhibiting nonlinear behavior with respect to the incident laser pulse energy. The optoacoustic profile of the laser-induced signal generated in a thin layer of highly diluted suspensions of gold nanospheres was examined, thereby providing a basis for a method for detection of metal nanoparticles with high sensitivity.

Shock wave generation in laser ablation studied using pulsed digital holographic interferometry

Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd–YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.

Femtosecond pulse propagation in temperature controlled gas-filled hollow fiber

We propose in this paper a novel technique of controlling spectrum expansion of intense femtosecond pulses in argon gas by temperature. The simulation demonstrated that the temperature gradient gas-filled hollow fiber allows more incident pulse energy and therefore more spectra broadening. This scheme has the similar effect as differential pumping, while avoiding the disadvantages of gas flowing through the fiber.

Laser-induced breakdown spectroscopy at a water/gas interface: A study of bath gas-dependent molecular species

Single-pulse laser-induced breakdown spectroscopy has been performed on the surface of a bulk water sample in an air, argon, and nitrogen gas environment to investigate emissions from hydrogen-containing molecules. A microplasma was formed at the gas/liquid interface by focusing a Nd:YAG laser beam operating at 1064 nm onto the surface of an ultra-pure water sample. A broadband Echelle spectrometer with a time-gated intensified charge-coupled device was used to analyze the plasma at various delay times (1.0–40.0 μs) and for incident laser pulse energies ranging from 20–200 mJ. In this configuration, the dominant atomic spectral features at short delay times are the hydrogen H-alpha and H-beta emission lines at 656 and 486 nm, respectively, as well as emissions from atomic oxygen liberated from the water and air and nitrogen emission lines from the air bath gas. For delay times exceeding approximately 8 μs the emission from molecular species (particularly OH and NH) created after the ablation process dominates the spectrum. Molecular emissions are found to be much less sensitive to variations in pulse energy and exhibit a temporal decay an order of magnitude slower than the atomic emission. The dependence of both atomic hydrogen and OH emission on the bath gas above the surface of the water was studied by performing the experiment at standard pressure in an atmospheric purge box. Electron densities calculated from the Stark broadening of the H-beta and H-gamma lines and plasma excitation temperatures calculated from the ratio of H-beta to H-gamma emission were measured for ablation in the three bath gases.

Growth of nanostructured ZnO thin films on sapphire

Growth of ZnO nanostructures on c-plane sapphire has been investigated using three different methods. Pulsed laser deposition (PLD) at low incident pulse energies can yield nanorods, the majority of which are aligned at an angle of ∼50° to the substrate plane. Selected area electron diffraction reveals that the nanorods display two distinct epitaxial relationships with the sapphire substrate. Those inclined to the surface normal exhibit the relationships (1124)ZnO//(0001)sap; [1010]ZnO//[1120]sap and (0001)ZnO//(1014)sap; [1010]ZnO//[1120]sap. Members of the second family are aligned along the surface normal, with (0001)ZnO//(0001)sap and [1010]ZnO//[1120]sap; the relative yield of this latter class increases at higher incident pulse energies. Hydrothermal synthesis and chemical vapour deposition on sapphire substrates that have been pre-coated (by PLD) with a thin ZnO layer result, respectively, in well-aligned ZnO microrod and nanorod arrays, both of which satisfy the relationships (0001)ZnO//(0001)sap; [1010]ZnO//[1120]sap. In contrast, employing these latter methods with a bare sapphire substrate results in, respectively, poorly aligned structures and localized islands of growth.

Structural Modification in Borosilicate Glass by Use of Femtosecond Fiber Laser at 1.56 µm

By focusing intense femtosecond laser pulses inside a transparent material, one can induce localized structural modifications including a refractive-index change. This technique can be applied to the fabrication of three-dimensional photonic devices. In this paper, we study on the structural modifications in borosilicate glass by using an amplified femtosecond Er-fiber laser system producing 947-fs optical pulses at a repetition rate of 155.1 kHz and a wavelength of 1.56 μm. In order to optimize the irradiation condition, we investigate the dependence of structural modification on the incident pulse energy, exposure time, and focusing condition. In a static exposure experiment, it is found that refractive-index change can be induced with a wider range of pulse energies compared to silica glass. The magnitude of refractive-index change is estimated to be 1.2 × 10 by measuring the diffraction efficiencies of a Raman-Nath grating fabricated in borosilicate glass.

Nonlinear optothermoacoustic phenomena in highly diluted suspensions of gold nanoparticles

Optoacoustic (OA) sensor was designed, fabricated and used to detect spherical gold nanoparticles (NPs) in suspensions at the level of up to a single particle. The sensor operating in the backward mode was designed to measure signals from microscopic volumes of nanoparticulate suspensions in water. Thermal nonlinearity was observed in the course of OA signal generation. The irradiation of the microvolume of gold nanoparticles gives rise to a multitude of thermomechanical processes, including heating of NPs above the vaporization temperature, thermal diffusion from nanoparticles to water and formation of vapor nanobubbles. As a result, a specific acoustic signal is produced exhibiting nonlinear behaviour with respect to the incident laser pulse energy. The optoacoustic profile of the laser-induced signal generated in a thin layer of highly diluted suspensions of gold nanospheres was examined thereby providing a basis for a method for detection of metal nanoparticles with high sensitivity.

Effect of laser pulse duration in picosecond ultrasonics

An optical grating has been introduced in a picosecond ultrasonics experiment, in order to vary continuously the duration of the laser beam pulse from 0.1to150ps. The evolution of the measured signal has been observed and analyzed through the comparison with a theoretical approach based on a two-temperature model. The latter allows matching the acoustic echoes together with the thermal background and the coincidence peak, for each pulse duration and at any time scale. The broadening of the acoustic echoes and the disappearing of its Brillouin component, along with the diminishing of the thermal coincidence peak, have been demonstrated when increasing the pulse duration. For a constant incident pulse energy, the efficiency of acoustic generation is optimum for the shortest pulses. Nevertheless, for longer pulses designed to obtain thermal conditions below the ablation threshold, acoustic generation could be enhanced.

Residual thermal effects in Al following single ns- and fs-laser pulse ablation

A comparative study of residual thermal effects in aluminum following ns- and fs-laser ablation shows a surprisingly similar trend in their behavior, despite many differences between ns and fs laser-matter interactions. At laser fluences above the ablation threshold where plasmas are produced and at a sufficiently high ambient gas pressure, an enhanced coupling of pulsed laser energy to the sample occurs. This effect appears to be a universal phenomenon for both ns- and fs-laser ablation in gas media. Furthermore, in contrast to the common belief that residual thermal energy is negligible in fs-laser ablation, our study shows that up to 70% of the incident pulse energy can be retained in the sample following single-pulse fs-laserablation in 1-atm air. In both ns- and fs-laser ablation, the major factors governing thermal energy coupling to the sample are the laser fluence and ambient gas pressure. Residual thermal energy deposition decreases with reducing ambient gas pressure.

Subpicosecond saturation of intersubband absorption in (CdS∕ZnSe)∕BeTe quantum-well waveguides at telecommunication wavelength

Ultrafast all-optical switching at an optical communication wavelength has been investigated by utilizing an intersubband transition (ISBT) of II–VI-based multiple quantum wells (MQWs) fabricated in high-mesa waveguide devices. The waveguide structure consists of a CdS∕ZnSe∕BeTe MQW core layer and two top and bottom ZnMgBeSe quaternary cladding layers grown by molecular beam epitaxy on a (001) GaAs substrate. A marked increase in waveguide transmittance was observed only for transverse-magnetic-polarized subpicosecond pulse with increasing incident pulse energy at λ=1.57μm, indicative of the ISBT absorption saturation. The pulse energy necessary for a 10dB transmittance increase is as low as 13.3pJ for a waveguide device with 2.7μm mesa, and the saturation pulse energy can be even further reduced by employing a narrower mesa structure. Ultrafast gate switching within a time window of 0.56ps was also demonstrated with pump pulse at λ=1.57μm and probe pulse at λ=1.63μm in this waveguide device.

Linear and non-linear optical transmission from multi-walled carbon nanotubes

The multi walled carbon nanotubes grown by the CVD technique were suspended in methanol and their linear and non-linear transmission properties have been studied. It is observed that the linear transmission spectrum, measured in the range 450–1100 nm, depicts features similar to those of single walled nanotubes. The observed features may be well characterized as the van-Hove singularities of the one-dimensional electronic density of states. Further, excellent non-linear optical properties have been observed in these suspensions. Experiments are conducted to study the optical limiting behavior in the visible (532 nm) as well as in the near infra-red (1064 nm) region by using a dual beam pulsed pump-CW probe, technique. The grown multi walled carbon nanotubes show a strong limiting behavior at both of these wavelengths. The recovery time as observed from the probe beam transmission is measured at different values of the incident pulse energy. The results are discussed in the light of the existing results.

Simultaneous multi-microhole drilling of soda-lime glass by water-assisted ablation with femtosecond laser pulses

Longitudinal and transverse microholes are drilled in soda-lime glass by water-assisted ablation with femtosecond laser pulses. True three-dimensional microchannels consisting of longitudinal and transverse microholes are presented. At low incident pulse energy, only one transverse microhole is observed. At high incident pulse energy, multiple transverse microholes can be simultaneously drilled. Using a focusing lens with numerical aperture of 0.5, two, three and four transverse microholes are fabricated at 3.2, 4.9 and 9.3microJ/pulse, which is qualitatively explained by the multiple foci process.