Hongjie Liu

Self-guiding of 100 TW femtosecond laser pulses in centimeter-scale underdense plasma

An experiment for studying laser self-guiding has been carried out for the high power ultrashort pulse laser interaction with an underdense plasma slab. Formation of an extremely long plasma channel and its bending are observed when the laser pulse power is much higher than the critical power for relativistic self-focusing. The long self-guiding channel formation is accompanied by electron acceleration with a low transverse emittance and high electric current. Particle-in-cell simulations show that laser bending occurs when the accelerated electrons overtake the laser pulse and modify the refractive index in the region in front of the laser pulse.

Advanced Mitigation Process (AMP) for Improving Laser Damage Threshold of Fused Silica Optics.

The laser damage precursors in subsurface of fused silica (e.g. photosensitive impurities, scratches and redeposited silica compounds) were mitigated by mineral acid leaching and HF etching with multi-frequency ultrasonic agitation, respectively. The comparison of scratches morphology after static etching and high-frequency ultrasonic agitation etching was devoted in our case. And comparison of laser induce damage resistance of scratched and non-scratched fused silica surfaces after HF etching with high-frequency ultrasonic agitation were also investigated in this study. The global laser induce damage resistance was increased significantly after the laser damage precursors were mitigated in this case. The redeposition of reaction produce was avoided by involving multi-frequency ultrasonic and chemical leaching process. These methods made the increase of laser damage threshold more stable. In addition, there is no scratch related damage initiations found on the samples which were treated by Advanced Mitigation Process.

High power laser antireflection subwavelength grating on fused silica by colloidal lithography

In this study we report on an efficient and simple method to fabricate an antireflection subwavelength grating on a fused silica substrate using two-step reactive ion etching with monolayer polystyrene colloidal crystals as masks. We show that the period and spacing of the obtained subwavelength grating were determined by the initial diameter of polystyrene microspheres and the oxygen ion etching duration. The height of pillar arrays can be adjusted by tuning the second-step fluorine ion etching duration. These parameters are proved to be useful in tailoring the antireflection properties of subwavelength grating using a finite-difference time-domain (FDTD) method and effective medium theory. The subwavelength grating exhibits excellent antireflection properties. The near-field distribution of the SWG which is directly patterned into the substrate material is performed by a 3D-FDTD method. It is found that the near-field distribution is strongly dependent on the periodicity of surface structure, which has the potential to promote the ability of anti-laser-induced damage. For 10 ns pulse duration and 1064 nm wavelength, we experimentally determined their laser induced damage threshold to 32 J cm−2, which is nearly as high as bulk fused silica with 31.5 J cm−2.

Reaction ion etching process for improving laser damage resistance of fused silica optical surface

Laser induced damage of fused silica optics occurs primarily on optical surface or subsurface resulting from various defects produced during polishing/grinding process. Many new kinds of surface treatment processes are explored to remove or control the defects on fused silica surface. In this study, we report a new application of reaction ion etching (RIE)-based surface treatment process for manufacture of high quality fused silica optics. The influence of RIE processes on laser damage resistance as a function of etching depth and the evolution of typical defects which are associated with laser damage performance were investigated. The results show that the impurity element defects and subsurface damage on the samples surface were efficiently removed and prevented. Pure silica surface with relatively single-stable stoichiometry and low carbon atomic concentration was created during the etching. The laser damage resistance of the etched samples increased dramatically. The increase of roughness and ODC point defect with deeper etching are believed to be the main factors to limit further increase of the damage resistance of fused silica. The study is expected to contribute to the development of fused silica optics with high resistance to laser induced degradation in the future.

Effect of UV Laser Conditioning on the Structure of KDP Crystal

Multiparametric raster scanning experiments for KDP crystals are carried out to study the laser conditioning efficiency as a function of laser fluence, fluence step, and pulse sequence by using ultraviolet (UV) laser irradiation with pulse duration of approximately 7 ns. It indicates that damage resistance of KDP can be enhanced after conditioning process. And laser conditioning efficiency depends on the maximal fluence which is below the damage threshold. Raman spectra and photothermal absorption have also been studied on KDP crystals before and after multiparametric laser conditioning. Photothermal absorption data reveal that absorbance of conditioned KDP crystal decreases with the increase of laser fluence and the damage threshold of low absorption area is higher. Raman analysis reveals that the effectiveness of laser conditioning relies mainly on the individual mode of PO4 molecule.

The effect of RIE-modified surface contamination on opticalperformance of fused silica

A series of fused silica surface have been created by reaction ion etching to determine the effect of the contamination level on surface state and optical performance of the optics. The results show that both impurity elements contamination and scratches of fused silica surface can be removed dramatically during RIE process. The laser induced damage threshold is raised by 37.6% when the polishing layer is removed for a thickness of 6μm, and the laser weak absorption doesn’t increase obviously. The results can provide technique support for improving laser induced damage performance of fused silica.

Collimation of hot electron beams by external field from magnetic-flux compression

AbstractIn fast ignition of inertial confinement fusion, hot electron beam is considered to be an appropriate energy source forignition. However, hot electrons are divergent as they are transporting in over-dense plasma. So collimating the hotelectrons becomes one of the most important issues in fast ignition. A method to collimate hot electron beam byexternal magnetic field is proposed in this paper. The external field can be generated by compressing a seed magneticfield at the stage of laser-driven implosion. This method is confirmed by particle-in-cell simulations. The results showthat hot electrons are well collimated by external magnetic field from magnetic-flux compression.Keywords: Collimation; External field; Hot electron; Magnetic-flux compression INTRODUCTIONRelativistically, intense laser pulses interacting with over-dense plasmas or solid targets has stimulated considerableinterest in relativistic plasma physics (Cai et al., 2009; Liuet al., 2005; Teng et al., 2010; Williams et al., 2009; Zhouet al., 2008) for its practical applications, such as the gener-ation of high-energy particle beam (Chen et al., 2009; Rothet al., 2001), radiography of dense plasma (Li et al., 2006,2009; Marshall et al., 2009), cancer treatment (Malkaet al., 2004), and fast ignition (FI) in inertial confinementfusion (Tabak et al., 1994). The FI has the possibility ofhigher gain, lower driver energy, and less implosion sym-metry requirements than the conventional inertial fusion. IntheFI,hotelectronbeamisconsideredtobethemostsuitableenergy source for ignition. In order to improve the couplingefficiency of the incident laser to the fuel and reduce theignition energy requirement, the hot electrons should be col-limated to propagate through the over-dense plasma and de-posit their energy to a hot spot of deuterium-tritium fuel.However, numerous simulations and experiments haveshown that the hot electron beams have large divergencewhile transporting in plasma (Green et al., 2008; Kodamaet al., 2001; Stephens et al., 2004).In the pastyears, several methods forcollimating electronshave been introduced. These methods include the cone-wire(King et al., 2009), the two-layer targets (Wu et al., 2009),the funnel-guided target (Zhou et al., 2010), and the conicaltarget (Galloudec et al., 2009), etc. In the cone-wire scheme,wires attached to cones have been used to collimate andguide the energetic electron beams by strong self-generatedmagnetic field around the wires. In the two-layer targets ap-proach, two intense laser pulses interact with plasma consist-ing of two parts with different densities. The resistivemagnetic field induced by the electron current generated bythe first pulse along the density interface collimates the hotelectronsgeneratedbythesecondpulse.Inthefunnel-guidedtargets approach, a gold funnel is attached to a gold slab, andthe funnel is embedded in Deuterium-Ttritium plasma. Themagnetic field generated by fast electrons entering the goldfunnel makes electron beam transport along the funnel. Inthe approach of conical target, a channel is attached to thecone target, and the electron beam can travel into the channelfor a long distance. All the approaches are based on the ideathat the self-generated resistive magnetic field can constrainthe divergence of hot electron beams. However, few re-searchers haveattemptedtocollimatethehotelectronsbyex-ternal magnetic field from magnetic-flux compression.579

Influence of bulk defects on bulk damage performance of fused silica optics at 355 nm nanosecond pulse laser

We demonstrate the effects of typical bulk defects in fused silica on the bulk damage threshold under nanosecond UV pulse in this study. A new test method is proposed to accurately evaluate laser induced bulk damage performance. The bulk bubble, hydroxyl, metal impurity, and weak absorption of the 355 nm laser are respectively characterized. The effects of bulk defects on bulk damage performance are analyzed statistically based on the correlation principle. For synthetic fused silica, metal impurities and hydroxyl have a weak correlation coefficient with the bulk damage threshold, while there is strong correlation between weak UV absorption and the bulk damage threshold. The influence of bulk damage threshold on surface damage performance is also discussed.

Photoluminescence defects on subsurface layer of fused silica and its effects on laser damage performance

Subsurface defects of polished fused silica optics are responsible for igniting laser damage in high power laser system. A non destructive measurement technique is developed to detect subsurface photoluminescence defects of fused silica. The fused silica samples polished by different vendors are applied to characterization of subsurface defects and measurement of damage performance. Subsurface photoluminescence defects of fused silica are evaluated by confocal fluorescence microscopy system. Laser induced damage threshold and damage density are measured by 355 nm nanosecond pulse laser. The results show a great differential subsurface quality of fused silica samples. Laser induced damage performance has a good correlation with subsurface defects. This paper shows a new non destructive measurement technique to detect photoluminescence defects on the subsurface layer of polished fused silica. It is very valuable to increasing laser damage performance and improving production-manufacturing engineering of optics.

Analysis on preferential free running laser wavelength and performance modeling of Tm 3+ -doped YAP and YLF

The preferential free running laser wavelength at room temperature for different axes cuts of Tm³⁺-doped YAP and YLF is comparatively analyzed in this paper. The polarized gain spectrum of Tm:YAP and Tm:YLF with different product values of Tm³⁺-doped concentration and crystal length is theoretically calculated under various cavity output mirror transmissions. From the gain spectrum, it straightforwardly determines the preferential free running laser wavelength for a given light polarization. In addition, a rate equation model is further used to model and compare the laser output performance for both the free running and some common artificially selected oscillating wavelengths, including 1.99 and 1.94 μm of Tm:YAP, and 1.89, 1.91, and 1.94 μm of Tm:YLF, respectively. To achieve an expected laser oscillating wavelength with acceptable output performance, our analysis presented here is very beneficial for one to choose the most suitable axis cut of crystal.