A Brief Review on Nonlinear Photonic Crystals Induced by Direct Femtosecond Laser Writing

Abstract

Direct femtosecond laser writing or inscription is a useful technique, and it has been employed to engineer various materials in many applications including nonlinear photonic crystals, which are of periodically patterned second-order nonlinearity to get and control the coherent light at new frequencies. By manipulation of second-order nonlinearity, either erased or poled, quasi-phase matching has been achieved in several crystals, especially three-dimensional nonlinear photonic crystals have been originally proposed and proved to be truly three-dimensional. Here we shortly review on the recent advances in the research field of nonlinear photonic crystals inscribed by femtosecond laser, as well as look into the future in this field.

Femtosecond Laser Writing of Photonic Crystal in LiNbO3 Crystal

“…Several techniques are used for the fabrication of waveguide structures like thermal ion implantation, ion indiffusion, ion/proton exchange, and laser femtosecond (fs) micromachining [5][6][7][8]. Recently, fslaser micromachining has become one of the most efficacious techniques used in 3D volume transparent material microstructure, the fs-laser micromachining technique has been exceedingly used to make optical waveguides from dissimilar materials like glasses, crystals, ceramics, and polymers [9][10][11][12]. When the high-intensity fs laser beam focuses inside the transparent crystal, some of the modification happens in the modified region due to a change in the optical properties to induce the refractive index modulations.…”

“…This micromachining permits to design of integrated optical devices with a great degree of freedom such as buried integrated laser sources, single-photon detectors, amplifiers, nonlinearfrequency converters, etc., [9]. It is possible to create Type I directly written optical waveguides, Type II stress-induced waveguides, and Type III depressed cladding waveguides by utilizing fs-laser micromachining [11][12][13][14].…”

“…Moreover, the stress fields produce an increase of the adjusted volume region and then spread over the nearby regions, causing local alterations in the refractive indices, which are enhanced in either the ordinary or extraordinary index direction where the direction of the stress depends on it, and this led to the type II modification [10]. Yet, the dual line waveguide geometry in crystals is typically the most popular way due to the stress-induced and produces in the irradiated tracks changes of negative refractive index [10][11][12][13].…”

“…Depressed cladding is formed from a waveguide core surrounded by several low index cladding tracks, the position of the core is slightly away from the tracks and the separation distance between the tracks is close to each other (a few μm). This structure is a quasi-continuous low-index potential barrier wall permitting light confinement inside it [12]. More importantly, the possibility of manufacturing different diameters and shapes with all the flexibility of the waveguide cores cross section which makes the depressed cladding enables to guide the light from visible to mid-IR wavelength ranges and from single mode to highly multimode.…”

“…More importantly, the possibility of manufacturing different diameters and shapes with all the flexibility of the waveguide cores cross section which makes the depressed cladding enables to guide the light from visible to mid-IR wavelength ranges and from single mode to highly multimode. Further, in a 3D vision, where the morphology of 2D waveguide cladding appears as the guiding tube, making the depressed cladding allows an excellent interconnection with other commercial fibres for the construction of fiber-waveguide-fiber integrated photonic systems, supporting the guidance to the longer wavelength region [11][12][13]. A very important aspect of 2D waveguide cladding cross-section in most crystals may support the guidance which is very much identical along TE and TM polarizations [11][12][13], and this makes the Type III structures as depressed cladding as a perfect platform for pumping light sources without polarization or phase-matching systems based on frequency conversion [13].…”