Second harmonic generation of diamond-blade diced KTiOPO4 ridge waveguides

Circular Dichroism Second-Harmonic Generation Imaging of KTiOPO4 Nanocrystal Through Stratified Media

Potassium titanyl phosphate (KTiOPO_4, KTP) particle of nanometric size (nano-KTP) is an attractive material for nonlinear microscopy, and the optimized growth of large-size KTP single crystals has numerous applications for efficient frequency conversion in laser technology. Its three-dimensional orientation and nanoscale morphology are important for growth optimization. In this paper, we introduce an imaging technique based on circular dichroism second-harmonic generation (CD-SHG) to characterize the 3D distribution of KTP nanocrystal. A rigorous theoretical model of CD-SHG imaging for nano-KTP through stratified media is demonstrated. Circular dichroism analysis is used to probe the orientation of 3-axis with respect to the optical observation axis. The research results show that the azimuthal angle of the peak value (SHG) or valley value (CD-SHG) is strongly related to the excitation polarization when the KTP sample is excited by different circular polarizations. Importantly, the refractive index mismatches and the imaging depth also affect the azimuthal angle. Thus, the proposed framework enables a more precise quantitative analysis of the CD-SHG signal of KTP.

Birefringence phase-matched direct third-harmonic generation in a ridge optical waveguide based on a KTiOPO4 single crystal

Birefringence phase-matched third-harmonic generation at 1594 nm is performed for the first time in a KTiOPO₄ single crystal micrometric ridge waveguide. The energy conversion efficiency reaches 3.4% for a pump energy as low as 2 µJ over a pulse duration of 15 ps at a repetition rate of 10 Hz. Strong agreements between theory and experiments for both phase-matching and conversion efficiency is obtained, which let us envision future triple photon generation quantum experiments.

Channel waveguide lasers in bulk Tm:LiYF4 produced by deep diamond-saw dicing

We report on a novel approach to fabricate channel (ridge) waveguides (WGs) in bulk crystals using precision diamond saw dicing. The channels feature a high depth-to-width aspect ratio (deep dicing). The proof-of-the-concept is shown for a Tm³⁺:LiYF₄ fluoride crystal. Channels with a depth of 200 µm and widths of 10-50 µm are diced and characterized by confocal laser microscopy revealing a r.m.s. roughness of the walls well below 100 nm. The channels obtained possess waveguiding properties at ∼815 nm with almost no leakage of the guided mode having a vertical stripe intensity profile into the bulk crystal volume and relatively low propagation losses (0.20-0.43 dB/cm). Laser operation is achieved in quasi-CW regime by pumping at 780 nm. The maximum peak output power reaches 0.68 W at ∼1.91 µm with a slope efficiency of 53.3% (in σ-polarization). The proposed concept is applicable to a variety of laser crystals with different rare-earth dopants.

KLu(WO4)2/SiO2 Tapered Waveguide Platform for Sensing Applications

This paper provides a generic way to fabricate a high-index contrast tapered waveguide platform based on dielectric crystal bonded on glass for sensing applications. As a specific example, KLu(WO₄)₂ crystal on a glass platform is made by means of a three-technique combination. The methodology used is on-chip bonding, taper cutting with an ultra-precise dicing saw machine and inductively coupled plasma-reactive ion etching (ICP-RIE) as a post-processing step. The high quality tapered waveguides obtained show low surface roughness (25 nm at the top of the taper region), exhibiting propagation losses estimated to be about 3 dB/cm at 3.5 μm wavelength. A proof-of-concept with crystal-on-glass tapered waveguides was realized and used for chemical sensing.

Diamond saw dicing of thulium channel waveguide lasers in monoclinic crystalline films

A surface channel waveguide (WG) laser is produced by diamond saw dicing of a 15 μm thick 10 at. % Tm:KY_1-x-yGd_xLu_y(WO₄)₂ monoclinic double tungstate thin film grown by liquid phase epitaxy on an undoped KY(WO₄)₂ substrate. The WG propagation losses are 1.1±0.5  dB/cm. When pumped at 802 nm, laser operation is achieved with a maximum output power of 262 mW at 1833 nm with a record slope efficiency of 82.6% (versus the absorbed pump power) in a TE₁₀ spatial mode (linear laser polarization, E‖N_m). Diamond saw dicing of double tungstate epitaxies is a promising technology for manufacturing WGs for sensing applications.

Quasi-phase matched second harmonic generation in periodically poled Rb-doped KTiOPO4 ridge waveguide

A 10.8 µm wide ridge waveguide was fabricated by diamond-blade dicing in an ion-exchanged periodically poled Rb-doped KTiOPO₄ sample. The waveguide was used to generate blue second harmonic light at 468.8 nm in the TM₀₀ mode through first order Type I quasi-phase matching, exploiting the large d₃₃ coefficient of the crystal. It was evaluated using a cw Ti:Sapphire laser, and 6.7 µW of blue light was generated with 5.8 mW of fundamental radiation at 933.8 nm coupled through the waveguide. The results presented here pave the way for efficient nonlinear processes in a waveguide format.

Calculations of second harmonic generation with radially polarized excitations by elliptical mirror focusing

Second harmonic generation (SHG) polarization intensity distribution illuminated with radially polarized beams by lens focusing appears two peaks, when the nonlinear optical coefficients dominate that is relevant to the transverse electric field components. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. In this paper, an elliptical mirror based system is proposed in the case of radially polarized beams illumination for SHG. The calculated predictions and numerical simulations demonstrate that for radially polarized beams, the proportion of transverse field components at the focal plane under the condition of elliptical mirror focusing is 2.6 times smaller than that with lens focusing when its numerical aperture (NA) is 1. Furthermore, the full width at half maximum (FWHM) of the total field intensity profile is approximately 81% of that in a lens focusing system. Due to the enhancement of longitudinal components of incident field, the distribution of SHG polarization presents a single-peak pattern, in which two peaks can be observed with lens focusing. The SHG polarizations in collagen fiber, KTiOPO₄ , and LiNbO₃ have been numerical simulated and discussed in detail to verify the validity of the proposed method. LAY DESCRIPTION: A novel focusing mode for second harmonic generation (SHG) microscopy is described in this paper. In the case of radially polarized excitations, the SHG polarization distributions of some specimens appear two peaks pattern with traditional lens focusing. Such two peaks pattern may result in ghosting and the decrement of imaging resolution. By introducing elliptical mirror, the modulation of electric field at the focal plane can be realized. Thus, the distribution of SHG polarization is converted into a single-peak pattern which eliminates the ghosting and improves the spatial resolution on SHG images. This proposed method can be used for SHG in various materials and the full width at half maximum (FWHM) of SHG polarization will compress in different degrees. Besides, there is a significant improvement on signal-to-noise ratio in SHG imaging when an annular aperture illumination is applied. The focusing mode with elliptical mirror can also be utilized in other nonlinear optics areas such as polarized third harmonic generation (THG) measurements and two-photon fluorescence (TPF) microscopy.

Phase-matched second-harmonic generation in a flux grown KTP crystal ridge optical waveguide

Type II second-harmonic generation was performed in a 15.8-mm-long KTiOPO₄ (KTP) micrometric ridge waveguide with an average transversal section of 38  μm². Theoretical predictions are compared with experiments. Strong agreements are obtained for both phase-matching wavelengths and second-harmonic intensity. This work opens wide perspectives for integrated parametric optics.

All-dielectric KTiOPO4 metasurfaces based on multipolar resonances in the terahertz region

We employ ferroelectrics to study the multipolar scattering in all-dielectric metasurfaces based on KTiOPO₄ (KTP) micro-disks for efficient manipulation of electromagnetic waves in the THz spectral region (0.6-1.5 THz). By adjusting the aspect ratio of the disks near the multipolar resonances, we show that the KTP disk array can form a multifunctional metasurface that covers the entire range of the electromagnetic response with resonantly enhanced anisotropic properties. The proposed ferroelectric metasurfaces will provide a versatile platform to manipulate THz waves, and open possibilities to monolithically combine it with THz generation.

Refractive index engineering through swift heavy ion irradiation of LiNbO3 crystal towards improved light guidance

Swift heavy ion irradiation has been widely used to modify refractive indices of optical materials for waveguide fabrication. In this work, we propose refractive index engineering by swift heavy ion (Ar) irradiation via electronic energy deposition to construct waveguides of diverse geometries in LiNbO₃ crystal. The feasibility to modulate the refractive index of LiNbO₃ crystal at variable depths through electronic energy depositions of argon ions at different energies has been experimentally explored. The surface and cladding-like optical waveguides with thicknesses of ~13, ~36 and ~23 μm have been produced by using swift Ar ion irradiation at single energy of ~120, ~240, and double energy of (120 + 240) MeV, respectively. The fabricated waveguides are capable of effective waveguiding in single and multiple modes at 1064 nm, which enables efficient guided-wave second harmonic generation at room temperature. This work paves the way to produce waveguides with diverse geometries in dielectric crystals through electronic damage of multiple swift heavy ion irradiation.

Rb/Ba side-diffused ridge waveguides in KTP

We report on characterization of ridge waveguides fabricated in KTP (KTiOPO₄) by use of diamond-blade dicing and Rb/Ba ion exchange. The waveguides were prepared in substrates that have their z-axis in the surface plane, perpendicular to the waveguide direction. This hinders the RbBa ions from diffusion into the depth, as they are only mobile along the z-axis, and improves the waveguide's resistance against elevated temperature. Attenuation coefficients of 0.3 dB/cm (0.4 dB/cm) for TM (TE) polarization were measured at 1060 nm wavelength. Internal conversion efficiency of up to 3.3%/(W cm²) was determined for type-II SHG of 1064 nm.

Low Loss Nanostructured Polymers for Chip-scale Waveguide Amplifiers

On-chip waveguide amplifiers offer higher gain in small device sizes and better integration with photonic devices than the commonly available fiber amplifiers. However, on-chip amplifiers have yet to make its way into the mainstream due to the limited availability of materials with ideal light guiding and amplification properties. A low-loss nanostructured on-chip channel polymeric waveguide amplifier was designed, characterized, fabricated and its gain experimentally measured at telecommunication wavelength. The active polymeric waveguide core comprises of NaYF₄:Yb,Er,Ce core-shell nanocrystals dispersed within a SU8 polymer, where the nanoparticle interfacial characteristics were tailored using hydrolyzed polyhedral oligomeric silsesquioxane-graft-poly(methyl methacrylate) to improve particle dispersion. Both the enhanced IR emission intensity from our nanocrystals using a tri-dopant scheme and the reduced scattering losses from our excellent particle dispersion at a high solid loading of 6.0 vol% contributed to the outstanding optical performance of our polymeric waveguide. We achieved one of the highest reported gain of 6.6 dB/cm using a relatively low coupled pump power of 80 mW. These polymeric waveguide amplifiers offer greater promise for integrated optical circuits due to their processability and integration advantages which will play a key role in the emerging areas of flexible communication and optoelectronic devices.

Tunable single- and dual-wavelength SHG from diode-pumped PPKTP waveguides

A compact, all-room-temperature, widely tunable, continuous wave laser source in the green spectral region (502.1-544.2 nm) with a maximum output power of 14.7 mW is demonstrated. This was made possible by utilizing second-harmonic generation (SHG) in a periodically poled potassium titanyl phosphate (PPKTP) crystal waveguide pumped by a quantum-well external-cavity fiber-coupled diode laser and exploiting the multimode-matching approach in nonlinear crystal waveguides. The dual-wavelength SHG in the wavelength region between 505.4 and 537.7 nm (with a wavelength difference ranging from 1.8 to 32.3 nm) and sum-frequency generation in a PPKTP waveguide is also demonstrated.