Field Of High-power Lasers Research Articles

Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

The second-order nonlinear-polarization originated from the interaction between thin-film materials with second-order nonlinear susceptibility (χ (2)) and high-power laser is essential for integrated optics and photonics. In this work, strong second-order nonlinear-polarization was found in a-axis oriented Zn1-x Mg x O (ZnMgO) epitaxial thin-films with Li incorporation, which were deposited by radio-frequency magnetron sputtering. Mg incorporation (x > 0.3) causes a sharp fall in the matrix element χ 33 of χ (2) tensor, although it widens optical bandgap (E opt). In contrast, moderate Li incorporation significantly improves χ 33 and resistance to high-power laser pulses with a little influence on E opt. In particular, a Zn0.67Mg0.33O:Li [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ 33| of 36.1 pm V−1 under a peak power density (E p) of 81.2 GW cm−2, a resistance to laser pulses with E p of up to 124.9 GW cm−2, and an E opt of 3.95 eV. Compared to that of ZnO, these parameters increase by 37.8%, 53.4%, and 18.6%, respectively. Specially, the Zn0.67Mg0.33O:Li shows higher radiation resistance than a Mg-doped LiNbO3 crystal with a comparable E opt. First-principle calculations reveal the Li occupation at octahedral interstitial sites of wurtzite ZnO enhances radiation resistance by improving structural stability. X-ray photoelectron spectroscopy characterizations suggest moderate Li incorporation increases χ 33 via enhancing electronic polarization. These findings uncover the close relationship between the octahedra interstitial defects in wurtzite ZnMgO and its nonlinear-polarization behavior under the optical frequency electric field of high-power laser.

Theoretical and experimental investigation of Al3+ ion-suppressed phase-separation structures in rare-earth-doped high-phosphorus silica glasses.

Rare-earth-doped silica-based composite glasses (Re-SCGs) are widely used as high-quality laser gain media in defense, aerospace, energy, power, and medical applications. The variable regional chemical environments of Re-SCGs can induce new photoluminescence properties of rare-earth ions but can cause the selective aggregation of rare-earth ions, limiting the application of Re-SCGs in the field of high-power lasers. Here, topological engineering is proposed to adjust the degree of cross-linking of phase-separation network chains in Re-SCGs. A combination of experimental and theoretical characterization techniques suggested that the selective aggregation of rare-earth ions originates from the formation of phase-separated structures in glasses. The decomposition of nanoscale phase separation structures to the sub-nanometer scale, enabled by incorporating Al3+ ions, not only maintains the high luminescence efficiency of rare earth ions but also increases light transmittance and reduces light scattering. Furthermore, our investigation encompassed the exploration of the inhibitory mechanism of Al3+ ions on phase-separation structures, as well as their influence on the spectral characteristics of Re-SCGs. This work provides a new design concept for composite glass materials doped with rare-earth ions and could broaden their application in the field of high-power lasers.

The Growth and Spectroscopic Properties of Er, Nd: YSGG Single Crystal Fibers

Single crystal fiber (SCF) is a novel solid gain medium and technique which combines the advantages of glass fiber and single crystal, showing great potential in the field of high-power lasers. In this paper, Er, Nd: YSGG single crystals with diameters of 2 mm and lengths of 80 mm were successfully grown using the micro-pulling-down method for the first time. Then, the measurements of Laue spots and Er3+ distribution indicated that the as-grown crystals were of a high quality. The effect of co-doped Nd3+ on the Er: YSGG was systematically discussed, which demonstrated that Nd3+ can decrease the fluorescence lifetime of Er: 4I13/2 that solve the self-termination bottleneck accordingly. These results demonstrate that Er, Nd: YSGG SCFs are promising materials for the further 3 μm laser generations.

Strontium fluorapatite (S-FAP) nano-grained laser ceramics

The use of large-sized, highly transparent polycrystalline strontium fluorapatite (S-FAP) ceramics could be a breakthrough in the high-power laser field due to its excellent laser gain properties. However, its hexagonal crystal structure makes it difficult to obtain laser quality in polycrystalline ceramics owing to birefringence. In this study, fully densified transparent Nd- and Yb-doped S-FAP ceramics with an average grain size of ∼100 nm can successfully be fabricated via liquid-phase nano-powder synthesis followed by spark plasma sintering, thereby achieving laser oscillation of the Nd:S-FAP ceramics for the first time. Furthermore, we elucidated the discrepancy in the emission properties between single-crystal and ceramics with randomly oriented crystal grains. We believe that these findings can provide important insights into the design and development for efficient non-cubic ceramic laser systems.

Transient Relativistic Plasma Grating to Tailor High-Power Laser Fields, Wakefield Plasma Waves, and Electron Injection.

We show the first experiment of a transverse laser interference for electron injection into the laser plasma accelerators. Simulations show such an injection is different from previous methods, as electrons are trapped into later acceleration buckets other than the leading ones. With optimal plasma tapering, the dephasing limit of such unprecedented electron beams could be potentially increased by an order of magnitude. In simulations, the interference drives a relativistic plasma grating, which triggers the splitting of relativistic-intensity laser pulses and wakefield. Consequently, spatially dual electron beams are accelerated, as also confirmed by the experiment.

Subwavelength Structures on Fused Silica for High-Power Laser Antireflection Applications

In the field of high-power lasers, subwavelength structures with both high transmittance and high laser damage resistance are required. However, in most of the studies, structural design tends to focus on optical properties. In this study, we research transmittance and surface electric field together. The surface light intensity, as an important damage factor, is investigated and quantitatively calculated using finite element method. Our results show that the surface light intensity has asymmetry on the front and rear surfaces, and is greatly regulated by the structural parameters. Based on this study, the subwavelength structures are designed which have both high transmittance and high laser damage resistance. Our research provides a new perspective to design of subwavelength structures, which is important for high-power laser antireflection applications.

Setup of a dipole trap for all-optical trapping* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0307500 and 2017YFA0304401), the National Natural Science Foundation of China (Grant Nos. 11634013 and 11774388), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB21030100), the CAS Youth Innovation Promotion Association (Grant Nos. 2018364

Micromotion induced by the radio-frequency field contributes greatly to the systematic frequency shifts of optical frequency standards. Although different strategies for mitigating this effect have been proposed, trapping ions optically has the potential to provide a generic solution to the elimination of micromotion. This could be achieved by trapping a single ion in the dipole trap composed of a highpower laser field. Here, we present the setup of the dipole trap composed of a 532 nm laser at a power of 10 W aiming to optically trap a single 40Ca+ and we observe an AC-Stark shift of the fluorescence spectrum line of ∼22 MHz caused by the 532 nm dipole beam. The beam waist of the dipole laser is several microns, which would provide a dipole potential strong enough for all-optical trapping of a single 40Ca+ ion.

High laser damage threshold liquid crystal optical switch based on a gallium nitride transparent electrode.

High laser damage threshold optical switches and spatial light modulators are in urgent demand in high power laser fields. In this Letter, liquid crystal optical switches using Si-doped GaN and Mg-doped GaN as transparent electrodes are fabricated, and the influence of the conductive properties of GaN is analyzed. The transmission and absorption characteristics of GaN and its sapphire substrate are tested. The results show that the liquid crystal device based on gallium nitride can be expected to play an important role in the fields of visible and near-infrared laser regions with a high laser damage threshold of more than 1J/cm2.

Bridgman growth and magneto-optical properties of CeF3 crystal as Faraday Rotator

In this work, CeF3 crystal with maximum diameter of 35 mm was successfully grown with the Bridgman-Stockbarger (BS) method. The crystal diffraction peaks match well with the standard pattern of CeF3 (PDF#01-089-0967), which proofs that obtained samples are of high purity. The transmittance of CeF3 crystal is relatively high in the UV-NIR region and reach 92% in comparison with TGG crystal. The UV cut-off absorption edge of CeF3 crystal is located at 270 nm contrary to 400 nm for TGG. The Verdet constant of CeF3 crystal is largest at 450 nm wavelength and reach 247 rad/T m. The results show that CeF3 possess better optical and magneto-optical properties than TGG. CeF3 can be a potential candidate for Faraday Rotators used in the field of high-power lasers.

Characterization of optical properties and laser behavior in [formula omitted]/[formula omitted] co-doped microstructure optical fiber

A microstructure optical fiber with Yb3+/Al3+ co-doped glass core is fabricated using high-temperature laser sintering process combined with stack-capillary-draw technology. No crystal is observed in the core glass and its fluorescence lifetime is measured to be 1.31 ms. The bonding features of glass and fiber, together with the structure of fiber core and cladding, are provided and analyzed. Moreover, the uniform element distributions and the axial stress profiles show a highly symmetrical variation trend which is in good agreement with the intensity distribution of Raman signal in Micro-Raman mapping. Excited by 976-nm LD, the laser slope efficiency of microstructure optical fiber reaches up to 72.5 % at 1032 nm. The near-field beam intensity distribution of fiber laser with spectrum FWHM of 8 nm exhibits an excellent laser beam quality. This work strongly suggests Yb3+/Al3+ co-doped silica preform fabricated by high-temperature sintering method is applicable for the field of high-power laser.

Sets of microlens arrays to finely shape homogenized focal spots

This study aims to demonstrate the effectiveness of sets of microlens arrays to homogenize and finely shape focal spots, for optimizing optical pumps of interest for Yb-doped slabs in the field of high-power lasers, or for use in materials processing and industrial applications. Homogenization basically involves optical integration in the Fourier plane of a large focusing lens, the input light source being consistently sampled. The generic capabilities of three complementary optical designs are demonstrated thanks to combining transfer-matrix computations with highly resolved ray-tracing modeling. This helps to benchmark a variety of experimental results. Fully generic trends are evidenced in terms of the shaping capabilities, for highly efficient focusing, in the form either of a single spot or of multiple sub-spots. Peak power densities as elevated as some hundreds of kW/cm2 or a few MW/cm2, respectively, may be produced by a couple of multi-kilowatt diode stacks.

The characteristics of high-quality Yb:YAG single crystal fibers grown by a LHPG method and the effects of their discoloration.

Single crystal fibers (SCFs), especially ytterbium (Yb) doped crystal fibers, have great potential in the field of high-power lasers. Colorless Yb:YAG single crystal fibers were fabricated using a laser heated pedestal growth (LHPG) method with a diameter fluctuation of less than 2% and a length to diameter ratio greater than 320 : 1. An abnormal color issue exists with respect to Yb:YAG crystals. The origin of coloration was studied via density functional theory, single-crystal X-ray diffraction, XPS and Raman spectroscopy and it was confirmed that the cyan coloration of Yb:YAG crystals is due to oxygen vacancies. Yb:YAG SCFs prepared via the LHPG method could avoid this type of defect due to the large specific surface area and melt convection caused by surface tension. The fundamental properties of the cyan Yb:YAG crystal source rod and colorless Yb:YAG SCFs were systematically investigated. The colorless Yb:YAG SCFs have higher infrared transmittance and thermal conductivity. The distributions of Yb3+ along the radial and axial directions were also measured. Meanwhile we demonstrated the propagation loss and a fiber laser using the colorless Yb:YAG SCFs, obtaining a minimum loss coefficient of 0.008 dB cm−1 and a maximum continuous-wave (CW) output power of 3.62 W. The colorless Yb:YAG SCFs with good thermal conductivity, low propagation loss, wide transparency and uniform ion distribution show promise for acting as the host material in single-mode lasers.

Development of longer Nd:LGGG crystal for high power laser application

In order to further improve the Nd3+:(LuxGd1-x)3Ga5O12 (Nd:LGGG) crystal performance in high power laser field, a long Nd:LGGG crystal with dimensions of Φ 23×112mm3 has been grown successfully by the Czochralski (Cz) method for laser rod fabrication. Compared with the normal size LGGG crystals (like 30–50mm in length), we overcame several difficulties in the growth of longer ones, including crystal cracking by a large longitudinal temperature gradient, spiral growth by a small radial temperature gradient, and growth instability and even constitutional super cooling by Ga2O3 volatilizing continuously. The doping concentrations of Nd3+ and Lu3+ in the as-grown crystal and the crystal optical quality have been measured. The performance of diode-side-pumped Nd:LGGG rod laser has been preliminarily tested for the first time, simply by replacing the Nd:YAG crystal rod inside a commercial laser module. Under an incident pump power of 160W, the maximum continuous wave output power of 38Whas been obtained, corresponding to an optical-optical conversion efficiency of 23.8% and a slope efficiency of 40.8%, respectively.

Third-harmonic generation in optical breakdown of a monatomic gas by an intense laser field

The theory of third-harmonic generation in optical gas breakdown by a high power laser field is developed. The theory treats third-harmonic generation in the presence of a self-generated quasi-static magnetic field. The distribution function of originated electrons and nonlinear third-harmonic current density are evaluated based on the kinetic theory. We have shown that the third-harmonic is generated by beating of electron's longitudinal velocity component vz and laser magnetic field B(r, t). The influences of the laser field polarization and magnetic field strength on the third-harmonic efficiency are discussed, separately. The results show that the third-harmonic exists even in the absence of the self-generated magnetic field; however, the magnetic field smoothly enhances the third-harmonic power with polarization near to the circular polarization. Furthermore, calculations revealed that the third-harmonic is never generated by a circularly polarized laser field, while it is more effectively generated in optical gas breakdown by a linearly polarized laser field. The results are in good agreement with previous theoretical investigations and also experimental observations of third-harmonic generation in optical breakdown of air.

Highly efficient reflective Dammann grating with a triangular structure.

A highly efficient reflective Dammann grating with a triangular structure operating at 1064 nm wavelength under normal incidence for TE polarization is designed and fabricated. Rigorous coupled wave analysis and particle swarm optimization algorithms are adopted to design and analyze the properties. The triangular reflective grating could cancel the 0th order, and the mechanism is clarified by the simplified modal method. The gratings are fabricated by direct laser writing lithography. The diffraction efficiency of fabricated grating is more than 86% at 1064 nm wavelength (97.6% in theory). This reflective grating should be a useful optical element in the field of high-power lasers as well as other reflective applications.

Fabrication and Laser Amplification Behavior of Yb3+/Al3+ Co-Doped Photonic Crystal Fiber

Yb3+/Al3+ co-doped silica glass with the composition of 0.1Yb2O3-1Al2O3-98.9SiO2 was prepared by sol–gel method combined with high-temperature sintering. Using this glass as active core, a photonic crystal fiber (PCF) with 105- $\mu \text{m}$ core was fabricated by the stack-capillary-draw technology. The refractive index fluctuation of this core rod is as low as $2 \times 10^{\mathrm {\mathbf {-4}}}$ , implying that highly homogenous silica preform can be realized in this method. Furthermore, this PCF achieved a maximum amplified power of 255 W, with a slope efficiency of 65.7%. It indicates that Yb3+-doped silica preform prepared by the sol–gel method can be employed in the field of high-power laser.

Subwavelength structures for high power laser antireflection application on fused silica by one-step reactive ion etching

In this paper we report a simple method to fabricate a novel subwavelength structure surface on fused silica substrate using one-step reactive ion etching with two-dimensional polystyrene colloidal crystals as masks. The etching process and the morphologies of the obtained structure are controlled. We show that the period of the obtained fused silica pillar-like arrays were determined by the initial polystyrene nanoparticle size. The height of pillar arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the antireflection properties of subwavelength structures surface. The novel subwavelength structures surface exhibit excellent broadband antireflection properties, but the size of the pillar affects the antireflective properties in short wavelength region. We anticipate this method would offer a convenient and scalable way for inexpensive and high-efficiency high power laser field designs.

Resonant plasmon-stimulated nonlinear absorption in three-level systems.

Mechanisms of resonant nonlinear absorption in the hybrid exciton-plasmon systems based on gold nanoparticles and macroheterocyclical compounds are presented. We used novel substituted subphthalocyanines (SubPc) with 14 π-electronic system as powerful excitonic 3 – level systems, and 50 nm gold nanoparticles (AuNp) as plasmonic structures. We succeeded to define mechanisms of resonant nonlinearities and to show that local field factor (LFF) and Purcell factor (PF) lead to dramatic enhancement in SubPc nonlinearity. We suggest a theoretical model which describes exciton-plasmon interaction of porphyrin/phthalocyanine related compounds with gold nanoparticles in the resonant high power laser field.

Investigation of a novel all-solid large-mode-area photonic quasi-crystal fiber

Large-mode-area single-mode fibers play an important role in the field of high power lasers, high power delivery, and high sensitivity sensor. A novel all-solid large-mode-area single-mode photonic quasi-crystal fiber with extremely low loss is proposed. This kind of fiber contains a hexagonal quasi-crystal array of slightly fluorine-doped silica rods in a silica background. Its structure and properties are simulated numerically in virtue of finite element method. Effects of variation of d/Λ, or Λ on fiber loss and effective mode-area properties are investigated. Numerical results demonstrate that an effective mode-area of 5197 μm2, low confinement loss of 10-5 dB/km for fundamental mode and high confinement loss of 100 dB/km of second-order mode at a wavelength of 1064 nm. Numerical simulations show that this fiber can operate effectively in single-mode and remove the conflict between large-mode-area and low loss. Moreover, the bending loss for a bending radius of 10 cm is as low as 0.01 dB/m. This fiber can increase the thermal damage threshold of the PQF, decrease the coupling loss and simplify the fabrication process. The design of new fibers is highly meaningful for the development of high power delivery, fiber lasers, and fiber amplifiers.

Analysis on FM-to-AM conversion induced by spatial filter in a high-power laser system

In order to avoid stimulated Brillouin scattering and smooth the final focal spot that reaches the D-T capsule, spectral broadening is essential. However, the modulation of spectral structure might result in frequency-to-amplitude modulation (FM-to-AM) conversion. The spatial filter pinhole is used to cut off the high-frequency transmission laser, and to ensure the desired pass. Improper parameter of pinhole would lead a negative impact. Using the spatial filter pinhole, we analyze the characteristic of intensity modulation using several pinholes of improper parameters. We then compare these results with the intensity modulation obtained from an experiment. Experimental diagnosis and design of an appropriate pinhole parameter would be highly beneficial to the field of high-power lasers.