Spectroscopic properties of Tm3+/Ho3+ co-doped NaLa(WO4)2 single crystal

Tm³⁺/Ho³⁺ co-doped NaLa(WO₄)₂ single crystal was successfully grown by the Czochralski method. The crystal was characterized by room temperature absorption spectra, fluorescence spectra around 2 μm, up-conversion fluorescence and luminescence decay measurements. Spectroscopic properties related to the laser operation around 2 μm of Ho³⁺ ions have been evaluated. The energy level scheme and energy transfer processes of Tm³⁺ and Ho³⁺ were analyzed. The obtained spectroscopic results show the crystal is a potentially host for Ho³⁺ 2 μm infrared laser.     

Upconversion luminescence dynamics of Er-doped fluoride crystals for optical converters

Erbium (Er)-doped fluoride crystals (YLF, BYF, CaF₂, etc.) are well-known as active media for solid-state lasers emitting in IR and VIS spectral domains, and as materials for efficient near-IR to VIS upconversion. In this paper, we report on the study of conversion of IR light from an ～1.5 μm spectral region to an ～1 μm spectral domain in low-phonon RE-doped fluoride crystals CaF₂ (RE=Er³⁺ Yb³⁺). Energy transfer processes taking place at selective pulsed and CW laser excitation are investigated experimentally. It is shown that in the CaF₂:RE crystals efficient conversion of IR radiation from the ～1.5 μm region to the ～1 μm region occurs, and these crystals are perspective for using in spectral converters for enhancing solar cell efficiency.     

Development of an efficient coherent optical source at 6.04 μm

Efficiency as high as 26% is obtained for generation of mid-infrared radiation at 6.04 μm by frequency doubling of ammonia laser emission at 12.08 μm in a 15 mm long type-I cut AgGaSe₂ crystal. The NH₃ laser used for this work is optically pumped by a commercial TEA CO₂ laser operating on 9.22 μm and produces pulsed output of ∼210 mJ with a duration of ∼200 ns at 12.08 μm. The generated radiation at 6.04 μm is separated out from the residual radiation at 12.08 μm by exploiting the principle of polarization dependent diffraction of reflection grating.     

Spectroscopic analysis of Cr,Er:Gd3Ga5O12 crystals as candidates of potential xenon lamp pumped ~ 2.7 μm laser

A series of Cr,Er:Gd₃Ga₅O₁₂ crystals with high concentrations of Er^3 + were grown by Czochralski method. The absorption spectra, the up-conversion, near infrared (NIR) and mid-infrared (Mid-IR) luminescence spectra as well as the luminescence decay curves of Er: ⁴I_13/2 and ⁴I_11/2 levels were measured at room temperature. The spectroscopic properties of Cr,Er:Gd₃Ga₅O₁₂ crystals and Cr–Er energy transfer processes were investigated. The spectroscopy of the Er^3 +:⁴I_11/2 → ⁴I_13/2 transition was centralized to discuss, and the important optical parameters including luminescence lifetimes and the Cr–Er energy transfer efficiency are presented. Based on the comprehensive spectral analyses, 0.6 at.%Cr/50 at.%Er:GGG crystal is preferred as candidate of potential xenon lamp pumped ~ 2.7 μm laser in this work.     

YAG laser-induced β-BaB2O4 crystalline dot formation in Sm2O3–BaO–B2O3 glasses

A continuous-wave (CW) YAG laser (power: 0.75–0.9 J/s, irradiation time: 15 s–15 min) with a wavelength of 1064 nm is irradiated to 11.1Sm₂O₃·44.4BaO·44.4B₂O₃ glass, and the formation of β-BaB₂O₄ (β-BBO) crystalline dots with a diameter of 30–150 μm is confirmed from micro-Raman spectra. β-BBO crystals with around 200 μm length grow towards the interior of the glass. The incorporation of Sm³⁺ into β-BBO crystalline dots is suggested from micro-Raman and fluorescence spectra. The second harmonic generation is detected from the array (10×10=100 dots) of β-BBO crystalline dots, indicating that each crystalline dot formed by YAG laser irradiation is a nonlinear optical crystal. CW YAG laser irradiation to glass with Sm³⁺ ions is a nice technique for a spatially controlled crystal growth.     

A tunable and single-longitudinal-mode Ho:YLF laser

A 1.94 μm Tm-doped fiber laser pumped tunable single-longitudinal-mode Ho:YLF laser with double etalons was reported for the first time. The maximum single-longitudinal-mode output power of 345 mW at 2051.6 nm was achieved at the absorbed pump power of 11.9 W, corresponding to a slope efficiency of 5.5% and an optical conversion efficiency of 2.9%. By regulating the angle of the F–P etalons, the output wavelength of the laser can be tuned from 2051.6 nm to 2063.3 nm. The single-longitude-mode Ho:YLF laser operating at 2 μm can be used as the seed laser source of coherent Doppler lidar, differential absorption lidar and so on.     

IR-induced features of AgGaGeS4 crystalline semiconductors

Complex investigations of the photoconductivity and photoinduced absorption together with the piezoelectric features were performed for the AgGaGeS₄ semiconducting single crystals under the influence of 3.5 μs CO₂ (80 mJ) pulsed laser emitting at 10.6 μm. These crystals are transparent in the wide spectral range 0.4–17 μm, which allows operating due to their properties in the spectral range covering the excitation of the phonons and electron subsystem. The piezoelectric properties show substantial increment during illumination by microsecond CO₂ laser and irreversible relaxation after swathing off the laser excitation. The temperature dependent studies of absorption and photoconductivity confirm the main role of intrinsic defects forming the tails of electronic states below the bottom of conduction band gap. Principal role of IR-induced electron–phonon interactions in the observed changes of the piezoelectricity is demonstrated.     

Comparison of eye-safe KTA OPOs pumped by Nd:YVO4 and Nd:YLF lasers

An eye-safe KTA OPO pumped by a Nd:YLF laser is demonstrated and a comparison with that pumped by a Nd:YVO₄ laser is performed. Although the slope efficiency of the continuous-wave free-running Nd:YLF laser is lower than that of the Nd:YVO₄ laser, the performance of KTA OPOs pumped by the Q-switched Nd:YLF laser is better, especially at lower repetition rates. The slope efficiency of KTA OPO pumped by a Nd:YLF laser is 14.6% at 30 kHz and 11.04% at 10 kHz. The better energy storage ability of Nd:YLF makes it an excellent laser medium in IOPOs.     

Ho:LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers

Optical properties of a Ho-doped LaF₃ single crystal have been detailed investigated as a promising material for 2 μm and 2.9 μm lasers for the first time. Judd–Ofelt theory was applied to analyze the absorption spectrum to determine the J–O intensity parameters Ω_t(t=2,4,6), based on which the emission probabilities, branching ratio and radiative lifetime for the as-grown crystal were all calculated. The stimulated emission cross-sections of the ⁵I₇ → ⁵I₈ and ⁵I₆ → ⁵I₇ transitions were obtained by using the Fuchtbauer–Ladenburg method. The gain cross-section for 2 μm emission becomes positive once the population inversion level reaches 30%. The Ho:LaF₃ crystal shows long fluorescence lifetime of ⁵I₇ manifold (25.81 ms) as well as ⁵I₆ manifold (10.37 ms) compared with other Ho³⁺-doped crystals. It can be proposed that the Ho:LaF₃ crystal may be a promising material for 2 μm and 2.9 μm laser applications.     

Passively Q-switched diode-pumped Raman laser with third-order Stokes eye-safe oscillation

We demonstrate a passively Q-switched all-solid-state laser system with intracavity Raman frequency conversion to the eye-safe spectral region. Laser oscillation at the 1.064 μm wavelength with a pulse repetition rate of several kilohertz was provided by a Nd:YAG crystal and a Cr:YAG passive absorber. Third Stokes oscillations at the 1.599 and 1.494 μm wavelengths were obtained in Ba(NO₃)₂ and PbWO₄ crystals with output pulse energies of 5 μJ and 6 μJ, respectively. The results of the numerical simulation of the pulse dynamics are in good agreement with the experimental data.     

Improved short-circuit photocurrent densities in dye-sensitized solar cells based on ordered arrays of titania nanotubule electrodes

We report on the synthesis of highly ordered arrays of titania nanotubules and their applications in enhanced photoelectrochemical cells. Ordered arrays of titania nanotubules of ∼120 nm external diameter, ∼100 nm internal diameter, and ∼5 μm length were fabricated on transparent conductive oxide (TCO) glass substrates by sol–gel processes using in-house prepared anodic alumina templates. After thermal bonding and template removal, the resultant nanotubule structures were applied in dye-sensitized solar cells (DSCs). Overall photoconversion efficiency of nearly 4.8% was achieved with Ru-bipyridine dye, N719, and iodolyte liquid electrolyte. This remarkable performance, for electrodes only ∼5 μm thick, is attributed to an unexpectedly high short-circuit photocurrent density of 16 mA/cm² for masked cells and up to 17 mA/cm² for unmasked cells. The enhanced short-circuit photocurrent (J_sc) is attributed to the high surface area (roughness factor ca. 1207) of the nanotubules and thus improved dye adsorption to the electrodes. The improved J_sc is also attributed to the parallel and vertical orientation of the nanostructures and thus to a well-defined electron diffusion path.     

Infrared fluorescence,energy transfer process and quantitative analysis of thulium-doped niobium silicate-germanate glass

Infrared fluorescence, energy transfer process, thermal stability and quantitative analysis of Tm³⁺ doped novel niobium silicate-germanate glasses have been investigated. The thermal stability changes indicate that the introduction of La₂O₃ to substitute for Nb₂O₅ can improve the anti-crystallization of present glass system. Intense 1.8 μm fluorescence has been achieved and the value of emission cross section can reach as high as 12.2 × 10⁻²¹ cm². Besides, the microparameters of energy transfer were analyzed by the extended overlap integral method. Hence, the results indicate that the excellent spectroscopic characteristics of this kind of silicate-germanate glass together with the good thermal properties may become a promising matrix applied for 1.8 μm band near-infrared laser.     

Micromachined silicon lenses for terahertz applications

Silicon microlenses are a very important tool for coupling terahertz (THz) radiation into antennas and detectors in integrated circuits. They can be used in a large array structures at this frequency range reducing considerably the crosstalk between the pixels. Drops of photoresist have been deposited and their shape transferred into the silicon by means of a Reactive Ion Etching (RIE) process. Large silicon lenses with a few mm diameter (between 1.5 and 4.5 mm) and hundreds of μm height (between 50 and 350 μm) have been fabricated. The surface of such lenses has been characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), resulting in a surface roughness of about ∼3 μm, good enough for any THz application. The beam profile at the focal plane of such lenses has been measured at a wavelength of 10.6 μm using a tomographic knife-edge technique and a CO₂ laser.     

Near-infrared broadband luminescence of Yb-doped PbClF crystals

Yb-doped PbClF crystals were obtained by a modified Bridgman method. Broadband emission (FWHM=58 nm) with long lifetime (5.33 ms) was observed in 2.0% Yb³⁺-doped PbClF crystals. The Stark splitting of Yb^{3+ 2}F_7/2 level was calculated to be 801 cm^?1 in 0.5% Yb³⁺-doped PbClF crystals, and the corresponding population of lower laser level is about 2.13%. All the results indicated that Yb-doped PbClF crystals should be a promising material to ultrashort pulses output at 1.0 μm.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

Realization of compatible stealth material for infrared,laser and radar based on one-dimensional doping-structure photonic crystals

To inhibit the radiant infrared energy between 8 and 14 μm, which is the infrared atmospheric window, and decrease the echo power of detecting laser and radar, to achieve compatible stealth, a doping structural one-dimensional photonic crystal (1-D PC) with Ge, ZnSe and Si was fabricated; and then combine it with radar absorbing material (RAM) to make a compound. After that, the reflection spectra of this compound was tested, and the result shows a high average reflectance (89.5%) in 8–14 μm waveband, and a reflective valley (39.8%) in the wavelength of 10.6 μm, which is the wavelength of CO₂ laser; and the reflectance in radar band shows that at high frequency, especially between 7.8 and 18 GHz, the radar power is strongly absorbed by this material and the reflected energy attenuate over 10 dB within the range from 11.1 GHz to 18.3 GHz, even 24.5 dB to the most in the frequency of 14.6 GHz.     

Nonlinear optics in glasses: How can we analyze?

Maximal optical nonlinearity obtainable in amorphous materials at telecommunication wavelengths of ∼1.5 μm is predicted. Applying a semiconductor concept, we suggest that nonlinear properties become greater in the materials with smaller optical gaps. This trend makes the chalcogenide glass such as As₂Se₃ promising for fiber devices (∼1 m), including optical switches, intensity stabilizers, and stimulated Raman amplifiers. However, for integrated devices with optical path lengths of ∼1 cm, greater nonlinearity is needed.     

Diode-pumped monolithic Er3+:Yb3+:YAl3(BO3)4 micro-laser at 1.6 μm

Efficient eye-safe 1.6 μm monolithic laser was realized in a c-cut, 0.7-mm-thick Er³⁺:Yb³⁺:YAl₃(BO₃)₄ microchip end-pumped by a quasi-continuous-wave 970 nm diode laser. At incident pump peak power of 20.4 W, a maximum output peak power of 2.6 W with a slope efficiency of 19% was obtained when the waist radius of pump laser beam was 220 μm. The spectra and profiles of output beam of the Er³⁺:Yb³⁺:YAl₃(BO₃)₄ monolithic laser were measured. The influences of the waist radius of pump laser beam on the slope efficiency and threshold of the monolithic laser were also investigated.     

Intense 974 nm emission from ErxYb2−xSi2O7 films through efficient energy transfer up-conversion from Er3+ to Yb3+ for Si solar cell

The optical properties of the Er_xYb_2?xSi₂O₇ thin films were investigated by photoluminescence measurements and the intense 974 nm light emission was observed. The 974 nm emission was mainly from the transition ²F_5/2 to ²F_7/2 level of Yb³⁺ upon exploring energy-transfer via up-conversion at Er^{3+ 4}I_13/2 level. Under 972 nm excitation, the lifetime at Er^{3+ 4}I_13/2 level reaches up to 4 ms for film containing 2 at% Er³⁺, while decreases to about 20 μs as the film is pumped by 488 nm. This confirmed that the energy transfer up-conversion process was the dominant transition at Er^{3+ 4}I_13/2 level. This may be of interest to improve the solar cells′ efficiency by placing this film at the rear of cell, converting the near-infrared photons between 1480 nm and 1580 nm to just above the Si bandgap.     

Laser diode end-pumped passively Q-switched Tm,Ho:YLF laser with Cr:ZnS as a saturable absorber

Output performance of a continuous-wave (CW) laser diode end-pumped passively Q-switched Tm,Ho:YLF laser is demonstrated with a Cr:ZnS crystal as the saturable absorber. We particularly investigate the influence of saturable absorber's position in the resonator when the Cr:ZnS crystal is placed close to and far from the laser beam waist. We compare the experimental results at the two different positions, and find that the laser shows unusual output characteristics when the Cr:ZnS saturable absorber is placed close to the beam waist. The pulse width and the pulse energy almost keep constant, measured about 1.25 μs and 4 μJ respectively, when the pump power is changed in the range of 1–1.9 W. Moreover, the pulse repetition frequency can be tuned between 1.3 kHz and 2.6 kHz by changing the pump power. The output wavelength of the passively Q-switched laser shifts to 2053 nm from 2067 nm in CW operation.