High-Efficiency Spectral-Beam-Combined 930 nm Diode Laser Source

Spectral beam combining is an effective way to achieve high-brightness direct diode laser output.We present an experimental study on spectral beam combining with external cavity based on transmission grating.Using a series of cylindrical transform lenses with different focal lengths,spectral beaming combining efficiency is greatly improved,and the results of wavelength intervals are consistent with the theoretical calculations.With the injection current of 90 A,a 75.1 W cw 930 nm output power with wavelength span of 18.6 nm and spectral beam combining efficiency of 92.7%is achieved,the beam parameter product is 5.77 mm·mrad.

Disappearance of Superconductivity and a Concomitant Lifshitz Transition in Heavily Overdoped Bi_2Sr_2CuO_6 Superconductor Revealed by Angle-Resolved Photoemission Spectroscopy

By partially doping Pb to effectively suppress the superstructure in single-layered cuprate Bi_2Sr_2CuO_(6+δ)(Pb-Bi2201) and annealing them in vacuum or in high pressure oxygen atmosphere, a series of high quality Pb-Bi2201 single crystals are obtained with T_c covering from 17 K to non-superconducting in the overdoped region. High resolution angle resolved photoemission spectroscopy measurements are carried out on these samples to investigate the evolution of the Fermi surface topology with doping in the normal state. Clear and complete Fermi surfaces are observed and quantitatively analyzed in all of these overdoped Pb-Bi2201 samples. A Lifshitz transition from holelike Fermi surface to electron-like Fermi surface with increasing doping is observed at a doping level of ~0.35. This transition coincides with the change that the sample undergoes superconducting-to-non-superconducting states.Our results reveal the emergence of an electron-like Fermi surface and the existence of a Lifshitz transition in heavily overdoped Bi2201 samples. This provides important information in understanding the connection between the disappearance of superconductivity and the Lifshitz transition in the overdoped region.

Evidence for Multiple Underlying Fermi Surface and Isotropic Energy Gap in the Cuprate Parent Compound Ca2CuO2Cl2

The parent compounds of the high-temperature cuprate superconductors are Mott insulators.It has been generally agreed that understanding the physics of the doped Mott insulators is essential to understanding the mechanism of high temperature superconductivity.A natural starting point is to elucidate the basic electronic structure of the parent compound.Here we report comprehensive high resolution angle-resolved photoemission measurements on Ca_2CuO_2Cl_2,a Mott insulator and a prototypical parent compound of the cuprates.Multiple underl.ying Fermi surface sheets are revealed for the first time.The high energy waterfall-like band dispersions exhibit different behaviors near the nodal and antinodal regions.Two distinct energy scales are identified：a d-wave-like low energy peak dispersion and a nearly isotropic lower Hubbard band gap.These observations provide new information of the electronic structure of the cuprate parent compound,which is important for understanding the anomalous physical properties and superconductivity mechanism of the high temperature cuprate superconductors.

Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi2Sr2CaCu2O8+δ

High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers.A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced.Here,by in situ vacuum annealing and Rb deposition on the Bi2Sr2Ca0.6Dy0.4Cu2O8+δ(Bi2212)sample surface to push its doping level continuously from deeply underdoped(Tc=25K,doping level p^0.066)to the near-zero doping parent Mott insulator,angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time.Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping,which is quite close to the upper Hubbard band.With increasing hole doping,the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior.When the chemical potential approaches the charge transfer band at a doping level of^0.05,the nodal spectral weight near the Fermi level increases,followed by the emergence of the coherent quasiparticle peak and the insulator-superconductor transition.Our observations provide key insights in understanding the insulator-superconductor transition in doping the parent cuprate compound and for establishing related theories.

First light for the sodium laser guide star adaptive optics system on the Lijiang 1.8 m telescope

A first generation sodium Laser Guide Star Adaptive Optics System （LGS-AOS） was developed and integrated into the Lijiang 1.8 m telescope in 2013. The LGS-AOS has three sub-systems： （1） a 20W long pulsed sodium laser, （2） a 300-millimeter-diameter laser launch telescope, and （3） a 37-element com- pact adaptive optics system. On 2014 January 25, we obtained high resolution images of an my 8.18 star, HIP 43963, during the first light of the LGS-AOS. In this paper, the sodium laser, the laser launch telescope, the compact adaptive optics system and the first light results will be presented.

Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi_2Sr_2CaCu_2O_(8+δ) Superconductor

High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi_2Sr_2CaCu_2O_(8+)with a_(c )of 75 K.Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels:the bonding sheet corresponds to a doping level of 0.14,which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped,giving an overall doping level of 0.20 for the sample.Different superconducting gap sizes on the two Fermi surface sheets are revealed.The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet.The maximum gap difference between the two Fermi surface sheets near the antinodal region is～2 meV.These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity,and the layer-dependent superconductivity in high temperature cuprate superconductors.

Evolution of incommensurate superstructure and electronic structure with Pb substitution in(Bi2-xPbx)Sr2CaCu2O8+δ superconductors

High-quality Bi2-xPbxSr2 CaCu2O8+δ(Bi2212) single crystals have been successfully grown by the traveling solvent floating zone technique with a wide range of Pb substitution(x = 0–0.8).The samples are characterized by transmission electron microscope(TEM) and measured by high resolution laser-based angle-resolved photoemission spectroscopy(ARPES) with different photon energies.A systematic evolution of the electronic structure and superstructure with Pb substitution has been revealed for the first time.The superstructure shows a significant change with Pb substitution and the incommensurate modulation vector(Q) decreases with increasing Pb substitution.In the meantime, the superstructure intensity from ARPES measurements also decreases dramatically with increasing Pb concentration.The superstructure in Bi2212 can be effectively suppressed by Pb substitution and it nearly disappears with a Pb substitution of x = 0.8.We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used;they can become even stronger than the main band when using a laser photon energy of 10.897 eV.These results provide important information on the origin of the incommensurate superstructure and its control and suppression in bismuth-based high temperature superconductors.

A High Conversion Efficiency Q-Switched Intracavity Nd:YVO_4/KTA Optical Parametric Oscillator under Direct Diode Pumping at 880nm

We report a high conversion efficiency Q-switched Nd:YVO_4/KTiOAsO_4(KTA) intracavity optical parametric oscillator(IOPO) operating near 3.5 um based on direct 880 nm laser diode(LD) pumping. A maximum average idler output power of 2.6 W with a pulse width of about 7.9 ns is achieved under an absorbed LD power of 45.4 W at a pulse repetition rate(PRR) of 10 kHz. The maximum optical-optical conversion efficiency from LD power to OPO mid-infrared(MIR) output of 6.74% is achieved. To our knowledge, this is the highest conversion efficiency for a KTA-IOPO by exploiting a Q-switched laser as the parent fundamental pump source. The beam quality factors M^2 of the MIR beam at the full output power with a PRR of 10 kHz are within 2.12 in both the horizontal and vertical directions, indicating a near Gaussian mode.

Photoreflectance system based on vacuum ultraviolet laser at 177.3 nm

Photoreflectance(PR)spectroscopy is a powerful and non-destructive experimental technique to explore interband transitions of semiconductors.In most PR systems,the photon energy of the pumping beam is usually chosen to be higher than the bandgap energy of the sample.To the best of our knowledge,the highest energy of pumping laser in reported PR systems is 5.08 eV(244 nm),not yet in the vacuum ultraviolet(VUV)region.In this work,we report the design and construction of a PR system pumped by VUV laser of 7.0 eV(177.3 nm).At the same time,dual-modulated technique is applied and a dual channel lock-in-amplifier is integrated into the system for efficient PR measurement.The system’s performance is verified by the PR spectroscopy measurement of well-studied semiconductors,which testifies its ability to probe critical-point energies of the electronic band in semiconductors from ultraviolet to near-infrared spectral region.

High-power ultraviolet 278-nm laser from fourth-harmonic generation of an Nd：YAG amplifier in CsB3O5 crystal

We report on the experimental investigation and theoretical analysis of a nanosecond pulse high power ultraviolet（UV） 278 nm laser by fourth-harmonic generation（FHG） of a 1112-nm Nd：YAG amplifier in LiB3O5（LBO） and CsB3O5（CBO） crystals. The UV laser delivers a maximum average power of 10.3 W at 278 nm with peak power of 36.8 k W under input pump power of 41 W at 556 nm. This is, to the best of our knowledge, the highest output power at the specific UV wavelength of 278 nm. We also performed the theoretical investigation on the FHG with a model in the Gaussian approximation of both spatial and temporal profiles, especially accounting for the two-photon absorption effect in CBO crystal for the first time. The average output power, pulse width, and beam spatial distribution of the UV laser were simulated. The theoretical calculations are in close agreement with the experimental results.

The 10 kW Level High Brightness Face-Pumped Slab Nd:YAG Amplifier with a Hybrid Cooling System

We demonstrate a high power, high brightness, slab amplifier based on face-pumped Nd:YAG slab gain modules,having a high efficient hybrid cooling system of the conduction cooling and forced convection cooling. Using a single gain module, a laser output power up to 4.5 kW with a remarkable optical-optical conversion efficiency of 51% is realized, indicating an excellent lasing performance of the Nd:YAG slab module. The amplifier operates at a repetition rate of 700 Hz and delivers a maximum average output power exceeding 10.5 kW with pulse duration of 150us. A good beam quality factor is measured to be β = 1.9. To the best of our knowledge, this is the highest brightness for a 10 kW level Nd:YAG slab amplifier.

Detailed electronic structure of three-dimensional Fermi surface and its sensitivity to charge density wave transition in ZrTe3 revealed by high resolution laser-based angle-resolved photoemission spectroscopy

The detailed information of the electronic structure is the key to understanding the nature of charge density wave （CDW） order and its relationship with superconducting order in the microscopic level. In this paper, we present a high resolution laser-based angle-resolved photoemission spectroscopy （ARPES） study on the three-dimensional （3D） hole-like Fermi surface around the Brillouin zone center in a prototypical quasi-one-dimensional CDW and superconducting system ZrTe3. Double Fermi surface sheets are clearly resolved for the 3D hole-like Fermi surface around the zone center. The 3D Fermi surface shows a pronounced shrinking with increasing temperature. In particular, the quasiparticle scattering rate along the 3D Fermi surface experiences an anomaly near the charge density wave transition temperature of ZrTe3 - 63 K）. The signature of electron-phonon coupling is observed with a dispersion kink at -20 meV; the strength of the electron-phonon coupling around the 3D Fermi surface is rather weak. These results indicate that the 3D Fermi surface is also closely connected to the charge-density-wave transition and suggest a more global impact on the entire electronic structure induced by the CDW phase transition in ZrTe3.

A 117-W 1.66-Times Diffraction Limited Continuous-Wave Nd:YVO4 Zigzag Slab Laser with Multilayer Amplified-Spontaneous-Emission Absorbing Coatings

We report a continuous-wave end-pumped Nd:YVη4 zigzag slab laser with multilayer amplified spontaneous emission(ASE)absorbing coatings.The coatings are deposited on the slab faces.A five-layer structure consists of SiO2-Ti-SiO2-Ti-Au,and the thicknesses are 2520 nm,10 nm,160 nm,24 nm and 200 nm,respectively.The designed coatings show good performance for the ASE control in the experimental tests.A stable-unstable hybrid laser oscillator along orthogonal directions in the slab aperture is further configured,achieving the 117 W output at a pump of 328 W.The beam quality factors M2 in the unstable direction and stable direction are 1.57 and1.66,respectively.

Coaxial Multi-Wavelength Generation in YVO4 Crystal with Stimulated Raman Scattering Excited by a Picosecond-Pulsed 1064 Laser

The multiwavelength characteristics of stimulated Raman scattering(SRS)in YVO4 crystal excited by a picosecond laser at 1064nm are investigated theoretically and experimentally.Laser output with seven wavelengths is achieved coaxially and synchronously at 894,972,1175,1312,1486,1713 and 2022 nm in a YVO4 crystal.The maximum total Raman output energy is as high as 2.77mJ under the pump energy of 7.75mJ.A maximum total Raman conversion efficiency of 47.8%is obtained when the pump energy is 6.54 mj.This is the highest order of Stokes components and the highest output energy generated by YVO4 reported up to date.This work expands the Raman spectrum of YVO4 crystal to the near-IR regime,with seven wavelengths covered at the same time,paving the way for new wavelength generation in the near-IR regime and its multiwavelength application.

Evidence of Electron-Hole Imbalance in WTe2 from High-Resolution Angle-Resolved Photoemission Spectroscopy

WTe2 has attracted a great deal of attention because it exhibits extremely large and non-saturating magnetore- sistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concen- tration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range,and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identify a fiat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a fiat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.

Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)superconductor in the superconducting and pseudogap states by super-high resolution laser-based angleresolved photoemission spectroscopy.The precise determination of the superconducting gap for the nearly optimally doped Bi2212(T_(c)=91 K)at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form(cos(2Φ)).It can be alternatively fitted by including a high-order term(cos(6Φ))in which the next nearest-neighbor interaction is considered.We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi_(2)Sr_(2)CuO_(6+δ)superconductors.This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors.These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.

High-energy,hundred-picosecond pulsed 266 nm mid-ultraviolet generation by a barium borate crystal

We present a high-energy,hundred-picosecond(ps)pulsed mid-ultraviolet solid-state laser at 266 nm by a direct second harmonic generation(SHG)in a barium borate(BaB_(2)O_(4),BBO)nonlinear crystal.The green pump source is a 710 mJ,330 ps pulsed laser at a wavelength of 532 nm with a repetition rate of 1 Hz.Under a green pump energy of 710 mJ,a maximum output energy of 253.3 mJ at 266 nm is achieved with 250 ps pulse duration resulting in a peak power of more than 1 GW,corresponding to an SHG conversion efficiency of 35.7%from 532 to 266 nm.The experimental data were well consistent with the theoretical prediction.To the best of our knowledge,this laser exhibits both the highest output energy and highest peak power ever achieved in a hundred-ps/ps regime at 266 nm for BBO-SHG.

Advances in deep ultraviolet laser based high-resolution photoemission spectroscopy

We briefly review recent results on photoemission spectroscopy based on the deep and vacuum ultraviolet diode pumped solid-state lasers which we have developed.Cascaded second harmonic generation with the nonlinear crystal KBe2BO3F2(KBBF)is used to generate deep ultraviolet and vacuum ultraviolet laser radiation,which complements traditional incoherent light sources such as gas discharge lamps and synchrotron radiation,and has greatly improved resolution with respect to energy,momentum,and spin of photoemission spectroscopy.Many new functions have been developed with the advantages of high photon energy,narrow linewidth,high photon flux density,and so on.These have led to the observation of various new phenomena and the amassment of new data in the fields of high temperature superconductivity,topological electronics,Fermi semi-metals,and so forth.These laser systems have revived the field of photoemission spectroscopy and provided a new platform in this frontier research field.