J.-J. Zondy

Frequency measurement of the 5S12(F = 3)−5D52(F = 5) two-photon transition in rubidium

Abstract We have measured the frequencies of three diode lasers stabilized on the 5 S 1 2 (F = 3)−5 D 5 2 (F = 5) two-photon transition in rubidium at λ = 778.1 nm, with an uncertainty of 1 kHz, using BNM-LPTF frequency synthesis chain starting from a CO 2 OsO 4 reference laser at 10.3 μm. We show that this frequency chain is able to reach the 10−13 resolution level. After a discussion of the systematic effects that may shift the resonance, the transition frequency is found to be ν = 385 285 142 378.280 ± 2 kHz.

LiInSe2: A biaxial ternary chalcogenide crystal for nonlinear optical applications in the midinfrared

Single crystals of lithium selenoindate (LiInSe2) are grown by the Bridgman–Stockbarger technique up to sizes of 10 mm in diameter and 20 mm in length. The different phases have color from yellow (as grown) to dark red (after annealing) but all have the same wurtzite type structure mm2 with slightly differing cell parameters. The band gap for the yellow phase at 300 K is at 2.86–2.87 eV. The red color is attributed to point defects and can be removed by proper illumination. Sellmeier equations are constructed for the 0.5–11 μm range and their validity is checked with second harmonic generation which provides first estimations of the nonlinear coefficients of LiInSe2. The potential of LiInSe2 is compared to that of the widely spread and technologically mature AgGaS2.

Optical, vibrational, thermal, electrical, damage, and phase-matching properties of lithium thioindate

Lithium thioindate (LiInS2) is a new nonlinear chalcogenide biaxial material transparent from 0.4 to 12 μm that has been successfully grown in large sizes and with good optical quality. We report on new physical properties that are relevant for laser and nonlinear optics applications. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInS2 displays a nearly isotropic thermal expansion behavior, a 5-times-larger thermal conductivity associated with high optical damage thresholds, and an extremely low-intensity-dependent absorption, allowing direct high-power downconversion from the near-IR to the deep mid-IR. Continuous-wave difference-frequency generation (5–11 μm) of Ti:sapphire laser sources is reported for the first time to our knowledge.

Optical, thermal, electrical, damage, and phase-matching properties of lithium selenoindate

Lithium selenoindate (LiInSe2) is a new nonlinear chalcogenide biaxial crystal, related to LiInS2 and transparent from 0.54 to 10 μm at the 50% level (10 mm thickness), which has been successfully grown in large sizes and with good optical quality. We report on what we believe to be new physical properties that are relevant for laser and nonlinear optical applications and summarize all relevant characteristics, both from the literature and as measured in the present work. With respect to AgGaS(e)2 ternary chalcopyrite materials, LiInSe2 displays a nearly isotropic thermal expansion behavior with three- to five-times-larger thermal conductivities associated with high optical damage thresholds, and low intensity-dependent absorption, allowing direct high-power downconversion from the near-infrared, especially 1064 nm, to the deep mid-infrared. Continuous-wave difference-frequency generation (5.9–8.1 μm) of Ti:sapphire laser sources is reported for the first time as well as nanosecond optical parametric oscillation with a Nd:YAG laser as the pump source at 100 Hz and idler tuning between 4.7 and 8.7 μm.

Growth and characterization of LiInS2 single crystals

Bulk LiInS 2 single crystals were grown using the Bridgman}Stockbarger technique. The crystals were characterized in composition, structure and defects. The composition was determined both in average and local versions, the latter was carried out using the unique di!erential dissolution technique. For all crystals a departure from ideal LiInS 2 stoichiometry, especially for cations, was detected. In the optical absorption spectra a strong band at 360 nm which disappears after annealing in S 2 vapor was, with a high probability, related to sulfur vacancies V S : their oscillator strength is f*1.3 * 10~4. The blue photoluminescence in all crystals is likely to be due to In L* antisite defect with an energy level 0.62 eV below the conduction band. The universal values of forbidden band gap, E ’ are 3.72 and 3.57 eV at 80 and 300 K, respectively, for all crystals studied indicating the similar character of dominant bonds. ( 2000 Elsevier Science B.V. All rights reserved.

Comparative theory of walkoff-limited type-II versus type-I second harmonic generation with gaussian beams

Abstract Starting from a unified treatment for type-I and type-II second harmonic generation (SHG) under the undepleted pump assumption based on the heuristic theory, we compare the effect of walkoff on both types of phase-matching (PM). We show that the beam walkoff between the pump polarization waves in type-II interaction may be regarded as an additional absorption in a type-I interaction experiencing the same walkoff angle. Power saturation is reached at about 2.3 times the aperture length which corresponds to the coherence length. Unlike for type-I SHG, the criterion of optimum focusing becomes meaningless in type-II coupling as the walkoff angle or the crystal length increase. The optimum SHG power or beam shape are then well described by the parallel beam limiting case. Furthermore, it appears that type-II angular far-field intensity distribution remains quasi gaussian whatever the focusing: diffractional effects such as sharp-edged or fringed transverse pattern observed in type-I SH profile under strong focusing do not appear.

Twin-crystal walk-off-compensated type-II second-harmonic generation: single-pass and cavity-enhanced experiments in KTiOPO 4

A twin-crystal device consisting of a pair of two identically cut KTP crystals of length l = 6 mm mounted with their optical axes symmetrically crossed is used to circumvent conversion-efficiency limitations that are due to the aperture effect in type-II (oeo) critically phase-matched second-harmonic generation (SHG) at λ = 1.3 and λ = 2.532 μm. A single-pass increase of as great as 3.2–3.5 times the conversion efficiency of a single crystal is obtained with this angle-tuned device at these wavelengths. This enhancement results from an increased effective coherence length of the interaction compared with that of a single bulk crystal of length 2l. We show that cavity-enhanced critically phase-matched type-II SHG can be easily performed with the twin device, allowing for walk-off as well as phase compensation for the fundamental-resonating ordinary and extraordinary waves at λ = 1.30 μm. Potential implementations of this device in other single-cavity-enhanced parametric interactions, such as doubly resonant optical parametric oscillation and degenerate sum-or difference-frequency mixing, are discussed.

Frequency-comb-referenced singly-resonant OPO for sub-Doppler spectroscopy

We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W between 2.7 and 4.2 μm, which is phase locked to a self-referenced frequency comb. Both pump and signal frequencies are directly phase-locked to the frequency comb of a NIR-emitting fs mode-locked fibre laser, linked, in turn, to the caesium primary standard. We estimate for the idler frequency a fractional Allan deviation of ∼ 3 × 10⁻¹²τ⁻½ between 1 and 200 s. To test the spectroscopic performance of the OPO, we carried out saturation spectroscopy of several transitions belonging to the ν1 rovibrational band of CH₃I, resolving their electronic quadrupole hyperfine structure, estimating a linewidth better than 200 kHz FWHM for the idler, and determining the absolute frequency of the hyperfine components with a 50-kHz-uncertainty.

The effects of focusing in type-I and type-II difference-frequency generations

Abstract Beam aperture functions are derived for type-I and type-II difference-frequency generations using focused TEM 00 Gaussian beams. The effects of walk-off, absorption, focusing and beam waist location are included in the analysis performed under the undepleted input wave approximation, using a Greens function approach. General expressions for the focusing h functions are given for arbitrary values of the input seed-wave confocal parameters, together with their optimized h m ( B , μ ) values as a function of the walk-off and degeneracy parameters ( B , μ ). The criterion of equal confocal parameters (ECP) is found to be optimal only for non-critical phase-matching. For critical phase-matching, unequal confocal parameters (UCP) generally optimize the efficiency. The theory is illustrated with relevant performance comparisons between the various type-I and type-II DFG couplings using KTP and AgGaS 2 .

LiInSe 2 nanosecond optical parametric oscillator

Optical parametric oscillation using the new lithium selenoindate nonlinear crystal is reported for what is to our knowledge the first time. A 17 mm long, type II phase-matched sample is pumped by a 10 ns Nd:YAG laser. The minimum pump energy threshold is 3 mJ for a signal-resonant configuration. The signal and idler waves are tunable between 1.47 and 1.57 microm, and 3.3 and 3.78 microm, with a total output energy of 170 microJ corresponding to a 2.4% energy conversion at 8 mJ pump, only limited by the AR coatings damage. With optimized crystal quality and coatings, lithium selenoindate should show superior performance as compared with AgGaS(e)2 crystals, owing to its 4x larger thermal conductivity.