Application of new nonlinear-optical crystals for development of novel methods for nonlinear-optical conversion of solid-state laser radiation into mid-infrared range presents an important task of modern infrared photonics. Significance of this challenge is caused not only by limited choice of solid-state sources of coherent radiation in mid-infrared range, but also by potential applications of such sources in science, technology, medicine, and biology. Efficient method of optical frequency down-conversion is the difference-frequency generation (DFG) allowing the single-pass conversion of the pump and signal optical frequencies lying in the near-IR range into the mid-IR idler wave. The narrowband, frequency stable signal wave for the DFG is generated in our setup by the stimulated Raman scattering (SRS) in a cubically nonlinear crystal (CaCO3, BaWO4, or diamond). In order to present a comparative study, the LiGaSe2 and LiGaS2 crystals with the equal length of 8 mm were used. Narrowband idler waves at the discrete wavelengths of 4.6 / 5.4 / 7.5 / 9.2 μm and high pulse energies in the range 10 - 50 μJ were generated. The measured linewidths were close to the monochromator resolution limit of < 2 cm-1 (~10 nm @ 7.5 μm) and they can be even narrower. It can be supposed that the idler wave linewidth should be comparable with the Raman mode linewidth (ΔνR = 1.2 2.7 cm-1). Generation at 10.8 μJ was achieved in LiGaSe2 only and the output energy was at in the order of ~100 μJ (close to the measuring probe resolution limit).
GaSe crystals that are promising as nonlinear optical converters in the mid- and far-infrared ranges are charac- terized by high Fresnel losses leading to transmittance per surface at the level of 77%. In this study, antireflection microstructures (ARMs) were fabricated on the surface of the GaSe single crystal by single-pulse femtosecond laser ablation. This method makes it possible to increase the transmission up to up to 92%.
A high-quality KTiOAsO4 (KTA) single crystal has been successfully synthesized employing the high temperature solution growth technique with rotating and pooling. The XPS valence-band spectra have been measured for pristine and the 1.5 keV Ar+ ion-bombarded thin (001)KTA plate cut from the crystal part that was without any optical inhomogeneities or domain boundaries. The present XPS measurements have revealed the existence of two O 2s subbands on the XPS spectrum of the pristine (001)KTA surface. It has been established that 1.5 keV Ar+ ion bombardment of the (001)KTA surface causes the complete elimination of the O 2s sub-band related to oxygen atoms involved in the formation of Ti–O–As bonds in KTA. In addition, the XPS results reveal that such a treatment leads to a significant decrease of the relative intensity of the XPS As5+ 3d core-level spectrum and causes the formation of the additional As0 3d core-level spectrum in the topmost layer of the (001)KTA surface. The experimental data are compared to the results of the first-principles band-structure calculations of KTA.
LiGaSe2 and LiInSe2 are promising nonlinear crystals for conversion of laser radiation to the mid-IR spectral range
which are transparent down to the visible and UV. We successfully grew a new mixed crystal as a solid solution in the
system LiGaSe2 - LiInSe2, with a composition of LiGa0.5In0.5Se2 which has the same orthorhombic structure (mm2) as
the parent compounds (LiGaSe2 and LiInSe2). The new crystal is more technological with regard to the growth process
in comparison with LiGaSe2 and LiInSe2 since its homogeneity range is broader in the phase diagram. We established
that about 10% of the Li ions are found in octahedral position with coordination number of 3. The band-gap of
LiGa0.5In0.5Se2 is estimated to be 2.94 eV at room temperature. The transparency at the 0-level extends from 0.47 to
13 μm. The dispersion of the principal refractive indices was measured and Sellmeier equations were constructed. The
fundamental wavelength range for the SHG process extends from 1.75 to 11.8 μm. The nonlinear coefficients of
LiGa0.5In0.5Se2 have values between those of LiGaSe2 and LiInSe2.
High chalcogen volatility and Li interaction with the container walls result in variation of crystal composition and
presence of both extended and point defects in as-grown LiGaS2 nonlinear crystals. Annealing in appropriate
conditions is used to correct the composition and improve the optical quality. We annealed LiGaS2 in vacuum, in the
presence of Li2S, Ga2S3, and S, and studied changes in transmission, photoluminescence and photo-induced
absorption. OH groups, S-H and S-S complexes, sulfur vacancies and cation antisite defects (GaLi) are most important.
Photo-induced absorption is reversible: It appears after illumination with UV/blue light and disappears after
illumination with IR light or by heating the sample.
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.7 to 8.7 μm, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition rate of 100 Hz
(~28 mW of average power).
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.65 to 7.5 μm, with a single crystal, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition
rate of 100 Hz (~28 mW of average power).
We report standardized absorption and scattering losses measurements of the nonlinear crystals LiInSe2 and LiInS2 in IR range by high average power 1064 nm radiation and tunable radiation of optical parametric oscillator (OPO) based on a
periodically poled lithium niobate (PPLN) pumped by a diode-pumped, Q-switched TEM00 mode Nd:YVO4 laser
operated at 1064 nm.
We report standardized absorption and scattering losses measurements of the nonlinear crystals LiInSe2 and LiInS2 in IR
range by high average power 1064 nm radiation and tunable radiation of optical parametric oscillator (OPO) based on a
periodically poled lithium niobate (PPLN) pumped by a diode-pumped, Q-switched TEM00 mode Nd:YVO4 laser
operated at 1064 nm.
Energy (frequency) of phonons is the main parameter which determines ratio between probabilities of radiative and radiationless transitions in luminescence. Single crystals of double chlorides KPb2C15 and bromides KPb2Br5 , which are formed by heavy ions and have a low energy phonon spectrum (h? <200 and 150 cm-1, respectively), of optical quality were obtained using the Bridgmen-Stockbarger technique. We studied the optical spectra and luminescence kinetics of RE —doped crystals (RE= Pr3+, Nd3+, Tb3+, Ho3+, Er3+ etc). Intensity parameters were determined by the Judd-Ofelt method, radiative and non-radiative transition probabilities were calculated. It was shown that low multiphonon relaxation rate in these crystals together with high values of radiative probabilities leads to the evidence of high intensity luminescence in spectral domain from 360 to 9000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers with laser diode pumping.
We studied the optical spectra and luminescence kinetics of double chloride Kpb2Cl5:TR3+ crystals as a new luminescent material promising for UV, VIS and mid-IR lasers, pumped with laser diodes. Intensity parameters were determined by the Judd-Ofelt method, radiative and non- radiative transition probabilities were calculated. It is shown that low multiphoton relaxation rate in the se crystal together with high values of radiative probabilities leads to the evidence of high intensive luminescence in spectral domain from 360 to 5000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers.
Bulk single crystals up to 20 mm in diameter and 40 mm long for LiInS2 and up to 10 mm, 20mm, respectively, for LiInSe2 have been grown. Their color changed from colorless to rose for the first one and from yellow to dark red for the other. All crystals have wurtzite-type lattice, lattice parameters were determined. A band gap was found to be 3.72 and 3.57 eV for LiInS2 and 3.02, 2.86 eV for LiInSe2 at 80 and 300K respectively. Color variations are due to point defects, first of all to interstitial sulfur, resulting in additional wide absorption bands in the shortwave part of transparency range. For LiInS2 the SHG phase matching conditions were found to be similar for samples of different color and some difference from Boyd's predictions of 1973 was shown.
The optical properties of LiInS2 suggested it as a promising material for generation of coherent radiation in the mid-IR region. Before investigating such capabilities its optical and mechanical properties have to be characterized precisely, and especially their evolution with temperature. Sufficiently large and suitably oriented crystals of good optical quality were studied. We first deduced the transparency range of these samples, as well as the frequencies of the optical phonons. We observed a phase- matched second-harmonic generation, using a nanosecond-OPO in the range 2.4 - 2.6 microns as the pump source and estimated a first value of the type-II nonlinearity deff(XY) equals 7.4 pm/V. The thermal expansion, thermo-optic, piezoelectric and electro-optic coefficients were determined along the three principal directions of polarization from -20 degrees Celsius up to + 120 degrees Celsius by means of original interferometric methods. A so-called Fabry-Perot Thermal Scanning (FPTS) interferometric method has been developed to measure accurately the electro-optic coefficients. For LiInS2 the values of ri3 were found to be of the same order of magnitude as its piezoelectric coefficients, but around one order of magnitude smaller than the electro-optic coefficients of the well known KTiOPO4.
In this work we studied the optical spectra of dysprosium and ytterbium doped double chloride MePb2Cl5:RE3+ (Me equals K, Rb), and double fluoride LiYF4:RE3+ (RE3+ equals Dy, Yb) crystals. RE3+ doped double chloride and double fluoride crystals have been grown, by using Bridgeman technique. Optical spectra were studied, intensity parameters are determined using Judd-Ofelt method and radiative probabilities and branching ratio were calculated. Yb3+ -Dy3+ energy transfer processes in KPb2Cl5:Dy3+,Yb3+ and LiYF4:Dy3+,Yb3+ were considered.
Nd-doped LiInS2 single crystals were grown using the Bridgeman technique and their optical properties studied. An intense emission with a dominant line at 1077 nm and groups of lines centered at 600, 760, 820 and 890 nm in absorption/luminescence excitation spectra are related to Nd3+. Doping results in a considerable increase of broad band absorption in the 400 - 460 nm range and a blue emission line at 440 nm at 80 K. Both are associated with transitions between InLi level and valence band.
In single crystals of AgCl, BaCl2, PbCl2, SrCl2, KPb2Cl5 with an unextended phonon spectrum (E less than 200 cm-1), doped by Dy, the luminescence spectrum was found to cover a wide range from visible region to IR (4.3 mkm for KPb2Cl5), including well pronounced 1.31 mkm band, which is important for using in telecommunication amplifiers. The Rare Earth impurity was shown to enter the host matrice as RE3+ in chlorides, the only type of impurity centers being formed in the case of rhombic MeCl2 crystals with Me equals Ba, P, S.
Optical waveguide layers Cs:KTiOAsO4 are realized by ion-exchange method. The doping level as 21 mol. percent Cs is achieved. The function of refractive indices increase profiles is identified as erfc-type. The absence of dispersion of indices increase over visible region is detected. The coefficients between indices increase and Cs concentration are obtained.
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