Parametric transformation and spectral shaping of supercontinuum by high-intensity femtosecond laser pulses

The cascaded nonlinear-optical transformation of high-power ultrashort light pulses in an ionizing gas medium involving supercontinuum generation, followed by a frequency conversion of this radiation in the field of femtosecond laser pulses with an intensity of 10¹⁴–10¹⁵ W/cm² has been demonstrated. Parametric four-wave mixing is shown to allow a highly efficient spectral transformation and shaping of supercontinuum radiation. The maximum efficiency of a parametric frequency conversion of femtosecond laser pulses in an ionizing gas medium achieved under the conditions of our experiments is estimated as 1%.     

Energy transfer and infrared-to-visible upconversion luminescence of Er/Yb-codoped halide modified tellurite glasses

We report on the energy transfer and frequency upconversion spectroscopic properties of Er³⁺-doped and Er³⁺/Yb³⁺-codoped TeO₂-ZnO-Na₂O-PbCl₂ halide modified tellurite glasses upon excitation with 808 and 978 nm laser diode. Three intense emissions centered at around 529, 546 and 657 nm, alongwith a very weak blue emission at 410 nm have clearly been observed for the Er³⁺/Yb³⁺-codoped halide modified tellurite glasses upon excitation at 978 nm and the involved mechanisms are explained. The quadratic dependence of fluorescence on excitation laser power confirms the fact that the two-photon contribute to the infrared to green-red upconversion emissions. And the blue upconversion at 410 nm involved a sequential three-photon absorption process.     

Tunable VUV light generation for resonance ionization mass spectrometry of Krypton

Tunable coherent VUV radiation from 115.8 to 116.9 nm has been produced by non-linear four-wave sum frequency mixing in a xenon-argon mixture. 116.5 nm light generated by this means has been used as the first step in a three color, doubly resonant ionization scheme for Kr. In the process of validating the system the xenon refractive index per atom (STP) at 116.5 nm has been determined to be (n_(Xe) − 1)/N_Xe = −6.8(±0.8) × 10⁻²³ cm³.     

3.62 W of continuous-wave orange-yellow light generated by intra-cavity sum-frequency mixing of Nd:YVO4

A design of laser-diode array (LDA) end-pumped Nd:YVO₄ laser that generates simultaneous laser action at wavelengths 1064 and 1342 nm is presented. Using type-I critical phase matching (CPM) BiB₃O₆, 593.5 nm continuous-wave (cw) Orange-yellow laser is obtained by 1064 nm and 1342 nm in an intra-cavity sum-frequency mixing. The maximum laser output power of 3.62 W is obtained when an incident pump laser of 27.5 W is used. The optical-to-optical conversion is up to 13.2%. To the best of our knowledge, this is the highest conversion efficiency at 593.5 nm in an intra-cavity sum-frequency Nd:YVO₄ laser.     

Low threshold, actively Q-switched Nd:YVO4 self-Raman laser and frequency doubled 588 nm yellow laser

We reported an actively Q-switched, intracavity Nd³⁺:YVO₄ self-Raman laser at 1176 nm with low threshold and high efficiency. From the extracavity frequency doubling by use of LBO nonlinear crystal, over 3.5 mW, 588 nm yellow laser is achieved. The maximum Raman laser output at is 182 mW with 1.8 W incident pump power. The threshold is only 370 mW at a pulse repetition frequency of 5 kHz. The optical conversion efficiency from incident to the Raman laser is 10%, and 1.9% from Raman laser to the yellow.     

Generation of tunable infrared radiation in rubidium titanyl phosphate crystal for the optical measurement of number density, absorption cross-section of methane gas

Tunable near-infrared radiation has been generated in a rubidium titany1 phosphate (RTP) crystal by employing non-collinear difference-frequency mixing (DFM) technique. The input radiation sources are Nd:YAG laser radiation and its second harmonic pumped dye laser radiation. For the generation of 2.0 radiation, the maximum value of the conversion efficiency (quantum) obtained in the process is 49% from the dye (0.6945 μm) to the infrared (2.0 μm) radiation in the 7.9-mm-long crystal. The generated tunable mid-infrared radiation has been used to measure the number density, absorption cross-section and minimum detectable concentration of methane gas in its 2ν₃ band in a multi-pass cell at 30.075 Torr pressure. The number density and column density of the methane molecules are found to be 1.068×10¹⁸ cm⁻³ and 3.02×10²¹ cm⁻², respectively, whereas the minimum still-detectable concentration at 1.658 μm wavelength is estimated to be 4.523×10¹⁷/cm³.     

Hg蒸汽中通过双光子共振四波差频产生调谐真空紫外辐射

利用Hg原子6¹S₀—6¹D₂和6¹S₀—6³D₂双光子共振的四波差频(2ω₁—ω₃),获得了真空紫外相干辐射(ω₄)。当入射光之一(ω₃)波长调谐时,产生的真空紫外光也是调谐的,调谐范围为184.9-187.5nm.本文对非线性频率变换过程的机 关键词：     

Generation of coherent radiation at 89.6 nm through two-photon resonant phase-matched tripling of fourth-harmonic Nd: Glass laser radiation in Hg vapors

Coherent radiation at 89.6 nm has been obtained through third-harmonic conversion of the 268.8 nm pulses in mercury vapors. Tunability of the fourth-harmonic frequency of the Nd: glass laser radiation to the 6s ¹ S ₀–8s ¹ S ₀ double quantum resonance of Hg atoms has been used for the resonant enhancement of the twelfth-harmonic output. Possibility to phase match the generated radiation and driving polarisation in pure vapors without employing of bufer gas has been demonstrated.     

VUV generation by frequency tripling the third harmonic of a picosecond kHz Nd:YLF laser in xenon and mercury vapour

uv ^vac=351.165 nm) of a ps 1 kHz Nd:YLF laser system is frequency tripled in xenon and mercury vapour. About 4×10⁴ photons per pulse, i.e. 4×10⁷ photons/s, are generated in xenon yielding a conversion efficiency of η=3×10^-10. The unusual frequency tripling in xenon takes place in a positive dispersive wavelength region. It is shown that Kerr-induced dispersion in the atomic system and a fifth-order process rather than a third-order process can explain the frequency tripling. For comparison a four-wave mixing process is investigated in negative dispersive mercury vapour. Due to absorption of the generated VUV radiation in the autoionization region of mercury the observed effective efficiency is, in our experimental arrangement, even lower than in xenon. An analysis of the VUV generation with respect to absorption is given. Received: 1 September 1997     

High efficiency generation of 355 nm radiation by extra-cavity frequency conversion

High efficiency extra-cavity third harmonic generation (THG) of 355 nm has been developed. A laser diode (LD) end-pumped, acoustic-optical Q-switched Nd:YAG laser was used as the fundamental wave source. With an input pump power of 25 W, average power of 6.75 W at 1064 nm was generated with the repetition rate 12 kHz and pulse duration 10 ns. Using the extra-cavity frequency conversion of three critical phase match (CPM) LiB₃O₅ (LBO) crystals, 3.2 W third harmonic radiation at 355 nm was obtained. The optical-to-optical (1064 nm to 355 nm) conversion efficiency was up to 47.4%.     

VUV sensitisation of Eu emission by Tb in Ba3Tb(PO4)3-Eu

The luminescence properties of Ba₃Tb_0.9Eu_0.1(PO₄)₃ and Ba₃Gd_0.9Eu_0.1(PO₄)₃ phosphors were studied for excitation over the 120-300 nm wavelength range. It is found that Tb³⁺, which exhibits a strong vacuum-ultraviolet (VUV) absorption band, provides sensitisation of Eu³⁺ emission in this host. This effect can be used to develop phosphors with enhanced conversion efficiency of the VUV radiation into visible light.     

Third-order nonlinear-optical parameters of gold nanoparticles in different matrices

Third-order nonlinear-optical properties of gold nanoparticles embedded in Al₂O₃, ZnO and SiO₂ have been investigated by the Z-scan method at the wavelength of 532 nm using nanosecond Nd³⁺:YAG laser radiation. The nonlinear refractive index, nonlinear absorption coefficient, and the real and imaginary parts of the third-order nonlinear susceptibility are deduced. The results of the investigation of nonlinear refraction using the off-axis Z-scan configuration are presented and the mechanisms responsible for the nonlinear response are discussed. The prevailing influence of the electronic Kerr effect over the possible thermo-optical contribution is demonstrated.     

All-solid-state continuous wave intracavity frequency-tripled Nd:YVO4-LiB3O4 blue laser using double-resonant approach

In the paper, a new way of intracavity frequency-tripled all-solid-state laser to generate continuous wave blue coherent radiation is firstly demonstrated. High-efficiency of continuous wave third harmonic generation using the approach of double-resonance at fundamental frequency and second harmonic is developed by insertion of one wedge prism for the phase control. The maximum output power at the wavelength of 447 nm, which was generated with two long LiB₃O₄ crystals by noncritical phase matching, is about 1.15 W with a beam quality factor of M² of 1.05. From the experimental results, the generation of continuous wave blue light using this way with higher conversion efficiency can be achieved.     

New nanosecond Q-switched Nd:VO4 laser fifth harmonic for fast hydrogen-free fiber Bragg gratings fabrication

We demonstrate the fabrication of fiber Bragg gratings using a novel high-repetition rate nanosecond Q-switched Nd:VO₄ laser fifth harmonic (213 nm) source for the first time in boron and hydrogen-free, Ge doped fiber. Strong gratings are rapidly obtained with the phase mask technique in hydrogen-free B/Ge doped photosensitive fiber with relatively low average power (100 mW), as well as in standard Corning SMF28 fiber. The evolution of the refractive index change during UV-exposure is presented. Photosensitivity of fibers to the 213 nm light is compared to the fourth harmonic (266 nm) light, as well as picosecond 213 nm radiation and is shown to be significantly higher than both. We believe that the photosensitivity of SMF28 fiber is due to a single-photon rather than two-photon absorption process.     

High-power tunable terahertz generation from a surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals

A high-powered tunable terahertz wave (THz-wave) has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO) pumped by a multi-longitudinal-mode Q-switched Nd:YAG laser. The effective parametric gain length was enlarged by employing two MgO:LiNbO₃ crystals. The tunable THz-wave radiation from 0.8 to 2.8 THz was realized via varying phase-matching angle between the pump wave and the Stokes wave. The maximum THz-wave radiation was 173.9 nJ/pulse at 1.7 THz as the pump energy was 82 mJ, corresponding to an energy conversion efficiency of about 2.12 × 10⁻⁶ and a photon conversion efficiency of about 0.035%. The first-order, the second-order and the third-order Stokes waves were observed during the experiments.     

Diode-pumped passively Q-switched 912 nm Nd:GdVO4 laser and pulsed deep-blue laser by intracavity frequency-doubling

A diode-end-pumped passively Q-switched 912 nm Nd:GdVO₄/Cr⁴⁺:YAG laser and its efficient intracavity frequency-doubling to 456 nm deep-blue laser were demonstrated in this paper. Using a simple V-type laser cavity, pulsed 912 nm laser characteristics were investigated with two kinds of Cr⁴⁺:YAG crystal as the saturable absorbers, which have the different initial transmissivity (T_U) of 95% and 90% at 912 nm. When the T_U = 95% Cr⁴⁺:YAG was used, as much as an average output power of 2.8 W 912 nm laser was achieved at an absorbed pump power of 34.0 W, and the pulse width and the repetition rate were ∼ 40.5 ns and ∼ 76.6 kHz, respectively. To the best of our knowledge, this is the highest average output power of diode-pumped passively Q-switched Nd³⁺-doped quasi-three-level laser. Employing a BiBO as the frequency-doubling crystal, 456 nm pulsed deep-blue laser was obtained with a maximum average output power of 1.2 W at a repetition rate ∼ 42.7 kHz.     

Enhanced generation of vacuum-ultraviolet radiation by four-wave mixing in mercury using pulsed laser vaporization

The efficiency of a coherent vacuum ultraviolet (VUV) source at 125 nm, based on two-photon resonant four-wave mixing in mercury vapor, has been enhanced by up to two orders of magnitude. This enhancement was obtained by locally heating a liquid mercury surface with a pulsed excimer laser, resulting in a high-density vapor plume in which the nonlinear interaction occurred. Energies up to 5 μJ (1 kW peak power) have been achieved while keeping the overall mercury cell at room temperature, avoiding the use of a complex heat pipe. We have observed a strong saturation of the VUV yield when peak power densities of the fundamental beams exceeded the GW/cm² range, as well as a large intensity-dependent broadening (up to ∼ 30 cm^-1) of the two-photon resonance. The source has potential applications for high-resolution interference lithography and photochemistry. PACS 42.65.Ky; 52.38.Mf     

Development of a Lyman-α laser system for spectroscopy and laser cooling of antihydrogen

Hydrogen Lyman-α radiation (121.56 nm) is important because it allows for the excitation and detection of ground-state hydrogen atoms by a one-photon process. The trapping of antihydrogen, recently reported by the ALPHA collaboration at CERN, has revived interest in Lyman-α lasers. In order to perform high precision tests of matter-antimatter symmetry violations or gravity-antimatter interactions with antihydrogen, laser cooling using the 1s ? 2p single photon transition is essential. Recent theoretical simulations predict that even with a pulsed Lyman-α source, laser cooling of antihydrogen would be possible. Here we describe the implementation of a high power vacuum-ultraviolet (VUV) laser at the Lyman-α transition of hydrogen. The VUV light was generated using a two-photon-resonant four-wave mixing process in a phase-matched mixture of krypton and argon. Two wavelengths (ω _R → 202.31 and ω _T → 602.56 nm) were mixed in a sum-difference scheme (ω _VUV = 2ω _R ? ω _T ) with a two-photon resonance at (4s ²4p ⁵5p[1/2]₀ ← 4s ²4p ⁶(1S ₀)) transition in Kr. With an Ar/Kr mixture of 3.9:1 we obtained 10 ns pulses of 0.1 μJ of energy at a repetition rate of 10 Hz.     

Blue luminescence in Tm-doped KGd(WO4)2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO₄)₂ single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO₄)₂ single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the ³F₂+³F₃ energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the ³H₆ ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.     

Upconverted VUV luminescence of Nd and Er doped into LiYF4 crystals under XeF-laser excitation

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d-4f radiative transitions in Nd³⁺ ions doped into a LiYF₄ crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser. The maximum upconversion efficiency, defined as the ratio of intensity for 5d-4f luminescence to overall intensity for 5d-4f and 4f-4f luminescence from the ⁴D_3/2 Nd³⁺ level, has been estimated to be about 70% under optimal focusing conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d-4f Nd³⁺ luminescence is observed under changing the excitation power density, which favors the most long-wavelength band (260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation emitted. The upconverted VUV and UV luminescence from the high-lying ²F(2)_7/2 4f level of Er³⁺ doped into a LiYF₄ crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed transition to the ²H(2)_11/2 4f level of Er³⁺, but the efficiency of upconversion for Er³⁺ emission is low, less than 5%.