Second Harmonic Generation In Waveguides Research Articles

Buried annealed proton-exchanged waveguide in periodically-poled MgO:LiTaO3 fabricated by surface-activated bonding for high-power wavelength conversion

Watt-class UV laser lights with symmetric spatial modes are suitable for high-resolution industrial applications. Channel waveguides with a guided mode of several ten microns diameter in MgO:LiTaO3 enhance integration capability while avoiding photorefractive damages during high-power wavelength conversion. We focused on buried waveguides fabricated by proton exchange, surface-activated bonding (SAB), and proton diffusion processes in periodically-poled (PP) MgO:LiTaO3. In this work, the mode profiles were simulated using the proton diffusion coefficients estimated by secondary ion mass spectrometry. Using the simulation results, the buried waveguides with a mode diameter larger than 30 μm were fabricated. By adopting designed PP structures, second harmonic generation (SHG) devices at a wavelength of 532 nm were fabricated. The nonlinear coupling coefficient was estimated to be 0.15 W−1/2 cm−1. Compared to the conventional annealed proton-exchanged waveguide SHG device without SAB, symmetric guided modes were obtained while maintaining the nonlinear coupling coefficient.

Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers.

Waveguides play a key role in the implementation of on-chip optical elements and, therefore, lie at the heart of integrated photonics. To add the functionalities of layered materials to existing technologies, dedicated fabrication protocols are required. Here, we build on laser writing to pattern grating structures into bulk noncentrosymmetric transition metal dichalcogenides with grooves as sharp as 250 nm. Using thin flakes of 3R-MoS2 that act as waveguides for near-infrared light, we demonstrate the functionality of the grating couplers with two complementary experiments: first, nano-optical imaging is used to visualize transverse electric and magnetic modes, whose directional outcoupling is captured by finite element simulations. Second, waveguide second-harmonic generation is demonstrated by grating-coupling femtosecond pulses into the slabs in which the radiation partially undergoes frequency doubling throughout the propagation. Our work provides a straightforward strategy for laser patterning of van der Waals crystals, demonstrates the feasibility of compact frequency converters, and examines the tuning knobs that enable optimized coupling into layered waveguides.

Periodic domain structures formed under electron-beam irradiation in LiNbO3 plates and Ti:LiNbO3 planar waveguides of the Y cut

Regular domain structures on the Y cuts of the LiNbO3 substrates and Ti:LiNbO3 waveguide structures based on these substrates have been fabricated under electron-beam irradiation. It has been revealed that the domains in undoped and titanium-doped LiNbO3 crystals are formed as a result of different processes. It has been demonstrated using chemical etching and waveguide second-harmonic generation that regular domain structures in the Ti:LiNbO3 waveguides are formed at a depth of approximately 8 μm from the surface, where the titanium concentration does not exceed 2 mol %. The quasi-synchronous waveguide optical second harmonic generation with an efficiency of 8.8% has been obtained using the fabricated structures.

Design of semiconductor laser resonator mirrors for optimal second-harmonic generation

The waveguide second-harmonic generation is a reliable mechanism for frequency doubling of laser emission since light can be highly confined to a nonlinear waveguiding medium, besides being compatible with current present semiconductor and solid state lasers. An optimization is achieved based on designing the resonator mirror reflectivity at the fundamental and the second-harmonic wavelengths to obtain optimal trapping and optical losses of the fundamental and the second-harmonic waves inside the resonator.

Theory of design parameters for quasi-phase-matched waveguides and application to frequency doubling in polymer waveguides

A theory of dispersion in single-mode symmetric waveguides is presented for phase-matching second harmonic generation (SHG) of the fundamental modes, based on the approximate analytical waveguide theory of Botez. The theory is used to derive new equations for the maximum phase-matching distance allowed when there are random fluctuations in waveguide thicknesses, under critical and noncritical phase-matching conditions. The theory is also used to calculate the overlap integral as a function of the waveguide parameters V/sup /spl omega// and V/sup 2/spl omega//. A new expression is derived for the efficiency of SHG in waveguides in terms of waveguide parameters that can be used to optimize SHG. Theoretical results are presented for typical LiNbO/sub 3/ and polymer waveguides. Quasi-phase-matched (QPM) waveguides are fabricated from nonlinear optical (NLO) polymers using the techniques of periodic poling and bleaching, and channel waveguides are printed by the bleaching of the NLO polymers. The NLO polymers are characterized for their refractive indexes, optical loss, NLO coefficients, and bleaching characteristics. Phase-matched SHG results are presented for the different fabrication methods over a distance of 0.5 cm, and an assessment is given of the relative strengths and weaknesses of the different fabrication approaches.

Tuning of second-harmonic generation in waveguides induced by photorefractive spatial solitons

We demonstrate experimentally wavelength tuning of second-harmonic generation in waveguides induced by photorefractive solitons in a KNbO3 crystal. By rotating the crystal, we can attain as large wavelength tunability as in bulk but with considerable improvement in conversion efficiency and elimination of walk-off.

Nonlinear optical studies of 3-methyl-4-methoxy-4′-nitrostilbene single-crystal films

We report the second-order optical measurement of single-crystal thin-films of 3-methyl-4-methoxy-4′-nitrostilbene by transverse second-harmonic generation (SHG) technique. Phase mismatch was negligible in the measurements since the thickness of the films was less than the coherence length. Detailed polarization selective measurements were used to determine the magnitudes of the tensor elements of second-order susceptibility. The values of d coefficients at the fundamental wavelength of 1064 nm were measured to be d33=195±10 pm/V and d24=75±5 pm/V. Both type I and type II phase-matched propagation directions were identified in the crystal and the type II phase matching direction lies parallel to the film. The results show that these films are promising for applications such as phase-matched SHG in waveguides.

Phase-matched second-harmonic generation in a ferroelectric liquid crystal waveguide

True phase-matched second-harmonic generation in a waveguide of crosslinkable ferroelectric liquid crystals is demonstrated. These materials allow the formation of macroscopically polar structures whose order can be frozen by photopolymerization. Homeotropic alignment was chosen which offers decisive advantages compared to other geometries. All parameters contributing to the conversion efficiency are maximized by deliberately controlling the supramolecular arrangement. The system has the potential to achieve practical level of performances as a frequency doubler for low power laser-diodes.

Nonlinear optical properties of poly[(4′-acetylphenylaminoformyloxy- 4-styrene)-co-(4-hydroxystyrene)] and guided mode-mode second-harmonic generation in waveguide

Linear and nonlinear optical properties of electrically poled films of newly synthesized poly [(4′-acetylphenylaminoformyloxy-4-styrene)-co-(4-hydroxystyrene)] [poly(AS-co-HS)] are investigated. Linear optical properties of a poly(AS-co-HS) film are measured by the prism-coupling method. Ultraviolet (UV) transparency down to 350 nm identifies that this polymer is a promising candidate for second-harmonic generation (SHG) in the UV-visible light region. Corona-poling technique is used to produce a noncentrosymmetric structure. The induced d33 at λ=1064 nm is 2.8 pm/V. High glass-transition temperature (Tg=160 °C) results in very long decay time of the nonlinearity, d33. The decay time at operating temperature of 80 °C is estimated to be more than ten years. Guided-mode blue and green SHGs are demonstrated using the film as a slab waveguide.

Second-harmonic generation in waveguides induced by photorefractive spatial solitons

We demonstrate experimentally second-harmonic generation in waveguides induced by photorefractive solitons and show that the conversion efficiency is improved considerably. These induced waveguides are flexible and can be generated in any crystalline direction that allows soliton formation, and thus offer broad tunability (by rotation of the crystal), which cannot exist in fabricated waveguides.

Waveguide second-harmonic generation device with broadened flat quasi-phase-matching response by use of a grating structure with located phase shifts

We report on a theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG).We used phase-shifted segments of a periodic grating to obtain a spectrally broadened, nearly flat response simultaneously with high conversion efficiency. We used an x-cut MgO:LiNbO(3) QPM waveguide in our analysis and experiments. The spectral range of the 850-nm fundamental for which SHG conversion exceeded 0.95 of the maximum value broadened from 0.02 to 0.12 nm when a 1-cm-long grating was divided into three segments with optimum phase shift. SHG conversion efficiency was 300%/W for this waveguide. The SHG efficiency and phase-matching characteristics showed good agreement with theoretical results.

Beam-propagation method for second-harmonic generation in waveguides with birefringent materials

Finite difference equations are given for second-harmonic generation in two-dimensional planar structures for coupling between TE polarizations at two frequencies as well as for TM polarizations at two frequencies, including the effect of birefringence in the latter case. Attention is given to obtaining accurate expressions for the finite difference operators for points close to interfaces, and it is shown how the equations for nonlinear coupling can be incorporated into the beam-propagation method and the coupled-mode theory. Computational results of second-harmonic generation incorporating both the coupled mode theory and the beam-propagation method are given and discussed.

Organic crystal isopropyl-4-acetylphenylurea and waveguide second-harmonic generation

A new organic crystal isopropyl-4-acetylphenylurea (IAPU) was identified as a promising candidate for blue second-harmonic generation (SHG). The nonlinear optical d coefficient obtained is d22=30.5 pm/V at the fundamental wavelength 1064 nm, and the crystal's transmitting range is 380–1400 nm. Thin, single crystalline films fabricated on the glass substrate are useful for waveguide-device applications. For the improvement of SHG efficiency and the relaxation of phase-matching conditions, the concept of thickness-noncritical phase matching (TNCPM) in a four-layer waveguide is proposed. Modal dispersion calculations for the various four-layer structures suggested the possibility of TNCPM from the TE0ω mode to TE12ω mode and from the TE0ω mode to the TE22ω mode by adjustment of the refractive indices of the two guiding layers.

High-efficiency LiNbO/sub 3/ waveguide second-harmonic generation devices with ferroelectric-domain-inverted gratings fabricated by applying voltage

The characteristics of the ferroelectric-domain-inverted gratings fabricated by applying voltage at room temperature in Z-cut LiNbO/sub 3/ crystal were examined for waveguide quasiphase-matched second-harmonic generation (QPM-SHG) devices. Conditions suitable for reproducible fabrication of uniform grating structure were clarified. Devices with uniform and fan-out grating for green and blue light generation were fabricated, and the performances were examined. A normalized SHG efficiency as high as 150%/W was obtained in blue light generation device with uniform grating of 3 mm interaction length.

Blue second-harmonic generation in waveguides fabricated in undoped and scandium-doped KTiOPO4

The efficiency of second-harmonic generation at 425 nm has been measured in periodically segmented waveguides in undoped and 120 ppm Sc3+-doped c-axis-oriented KTiOPO4 (KTP) plates. Conversion efficiencies up to 200% and 325%/W cm2 are obtained for the two types of crystals. These efficiencies are found to have decreased by up to a factor of 2 after the generation of 2–4 mW of cw blue output power for more than 5 min, while the transmission of the pump beam is decreased by about 20%. These effects are semipermanent in undoped KTP, and are therefore ascribed to optical damage. A full recovery within a few minutes is, however, found in the Sc3+-doped KTP waveguides. The ionic conductivity along the polar axis of 120 ppm Sc3+-doped KTP has been measured to be about equal to that of undoped KTP, so that the difference in the optical damage characteristics of the waveguides is not caused by a difference in the conductivity of the substrate KTP. The conductivity of 230 ppm doped crystals is found to be about a factor of 30 lower than that of undoped KTP, and has a different dependence on current frequency. In these crystals domain inversion in the exchanged waveguide segments was not obtained.

Second-harmonic generation in single-crystal films of an organic material

Single-crystal films of 2-cyclooctylamino-5-nitropyridine have been prepared by the shear method, and the second-order susceptibility has been measured by second-harmonic generation. Polarization-dependent measurements in an a plate show that the maximum value of deff (~56 pm/V) is approximately 10 times larger than that of LiNbO3 and that the ratio d32/d33 is approximately −4 at a 1064-nm wavelength. This material has a phase-matching direction (type I only) for propagation parallel to the plane of the a plate. The results of these measurements are important for phase-matched second-harmonic generation in waveguides of this material.

Compact blue-green lasers get down to business

The development of solid-state, blue-green lasers, which are emerging as viable alternatives to air-cooled argon and helium-cadmium lasers for such applications as flourescent bioanalysis, optical inspection, and high-end reprographics, is summarized. Methods of efficient doubling in diode-pumped lasers, direct doubling, second harmonic generation in waveguides, and intracavity frequency mixing are discussed. The limited marketing impact of these technologies is also discussed. >

Stable and Large Second Harmonic Generation in Sol-Gel-Processed Poled Silica Waveguides Doped with Organic Azo Dye

Sol-gel-processed silica films doped with organic azo dye, Disperse Red 1 (DR1) in a higher concentration of 21.7 wt%, were prepared at a higher baking temperature with corona poling of higher voltage than in other reports to obtain stable and large second harmonic generation (SHG) and film strength. A large effective SHG coefficient of 75 pm/V was measured for the film of 0.2-µm thickness at a fundamental wavelength of 1064 nm, and the value was unvaried for 700 h with no relaxation. Moreover, a waveguide SHG device of four-layer structure with DR1-doped sol-gel film as a top layer was fabricated for the first time, and Cherenkov-type phase-matched SHG was observed successfully.

First Order Quasi-Phase-Matched Second-Harmonic Generation in LiTaO3 Utilizing a Periodic Domain Inversion Created by an External Stress

AbstractFirst order quasi-phase-matching in a proton exchanged LiTaO3 channel waveguide structure is reported, where the periodic domain inversion is created by the application of a periodic compressive stress near the Curie temperature. The processing conditions, driving force, and effects on resulting domain structure is reported, including the observation of domain structures with periods as small as 0.36 μm. A waveguide second-harmonic generation structure with a periodic 3.45 μm domain inversion structure is fabricated and evaluated to quasi-phase-match 420 nm blue light with 840 nm output available from standard laser diode sources.

New method of domain inversion in LiNbO3 using thermal oxidation of Ti film

A new technique for fabricating domain inversion regions in –c face LiNbO3 by thermal oxidation of Ti is reported. This technique can minimise refractive index changes in such regions, thus it is suitable for quasiphase-matching (QPM) second harmonic generation (SHG). A prototype QPM waveguide SHG device is also demonstrated.