Laser-written 3D crystalline photonic devices

Compact waveguide devices represent the basis of integrated photonics and are useful for a variety of applications in areas including optical telecommunications, quantum computing, biophotonic sensing, and information processing.^1–3 There is much interest in the integration of these structures into dielectric crystals due to their impressive optical properties, with the aim of fabricating complex 3D waveguide devices. Femtosecond laser writing (FLW), which produces localized material modifications that give rise to refractive index changes, has emerged as one of the most efficient techniques for the direct 3D microfabrication of transparent optical materials.^4–6 Although various 3D waveguide devices in glass have been successfully fabricated using FLW,⁷ the complexity of crystalline structures and anisotropies make it much more difficult to implement in crystals. Novel fabrication approaches are therefore required.⁸

Although several research groups have been working toward the fabrication of photonic devices in crystals using FLW, most research has been restricted to 2D structures based on simple 1D waveguides. This is due to the complexity of the problem. The behavior of laser-induced changes to the refractive index is strongly dependent on bulk properties and irradiation parameters.

For example, FLW waveguides in lithium niobate (LiNbO₃) birefringent crystals are polarization-sensitive due to the increase and decrease of the extraordinary and ordinary refractive indices, respectively, in the weak-damage regime (characterized by slight lattice distortion caused by low-power irradiation). In the severe-damage regime—characterized by a highly damaged lattice caused by high-power irradiation—the stress-field modification (index increase) only occurs along one orientation in LiNbO₃.⁸ As a result, the light is guided in only this direction. Another example is the neodymium-doped yttrium-aluminum-garnet laser crystal, in which polarization-sensitive or insensitive guidance can be implemented by varying the parameters and structural geometries of the laser-writing process.⁸ The wide variety of behaviors across this material class makes using standard FLW in a general way impossible.

Laser-induced refractive index modifications can result in either positive or negative variations, depending on the magnitude and nature of the damage produced on the material. For laser powers under a given threshold, weak damage created in the focal volume is typically accompanied by a positive index change inside the laser tracks. Increasing laser power may irreversibly damage the lattice in irradiated regions while inducing positive index changes near the tracks (instead of inside them) due to lattice compression. The above two cases refer to so-called type-I and type-II laser-induced modification, respectively.⁹

Our approach to realizing 3D photonic structures is based on both types of laser-induced index modification. Because a refractive index is induced directly in the focal region (guiding core) by type-I modification, fabrication consists of simply scanning the sample with the laser focus following the desired waveguide trajectory. However, finding fabrication parameters that are able to produce uniform waveguide profiles presents a challenge. Figure 1(a) shows the FLW process for fabricating 3D waveguiding beamsplitters (1×4) based on type-I modification of crystals. The polarization-dependent properties are very different for the two crystalline waveguides: see Figure 1(b–e). LiNbO₃ supports light propagation along particular orientations, whereas BGO wave guidance is uniform for any polarization.^{10, 11}

Figure 1. (a) Schematic plots of the fabrication of a 3D beam splitter with a 1×4configuration by femtosecond laser writing (FLW). Optical microscope images of the beam-splitter cross section in (b) lithium niobate (LiNbO₃) crystal and (c) bismuth germanate (BGO) crystal. Output beam profiles at (d) 1.06μm in z-cut LiNbO₃ crystal and (e) 4μm in BGO crystal, along the transverse-magnetic (TM) polarization.

In the case of type-II modification, special designs are required for the production of 3D waveguide structures. The typical dual-line approach, or normal depressed cladding geometry, works well in the 2D case. In the 3D case, however, disadvantages emerge due to the possibility of unbalanced guidance along different polarizations, in addition to the complicated architecture required to connect separate channels.

To overcome these shortcomings, we have designed a new family of photonic-lattice-like cladding photonic structures for 3D microfabrication. A typical structure contains a number of periodically written laser tracks with geometry defects. The few-layer laser tracks, with a hexagonal layout, serve as low-index claddings that confine light in the enclosed track-free region.¹² Figure 2 shows the waveguide configuration and modal distribution of a four-layer photonic-lattice-like cladding structure. The light field is confined within the center of the structure, i.e., the region without laser-written tracks.

Figure 2. (a) Schematic of configuration and cross-sectional microscopic image of a four-layer photonic-lattice-like cladding waveguide structure. (b) Measured and (c) simulated modal profiles of the guiding structure along both transverse-electric (TE) and TM polarizations at 1064nm.

Due to the ease with which the periodically arrayed tracks can be engineered, we have been able to realize beam steering by introducing additional defect-line tracks at certain positions. These defect tracks serve as beam blocks or confiners that shape the guided fields during their propagation. The monolithic integration of a few photonic-lattice-like structures in a single chip enables beam manipulation with very low losses. Tailored functionalities, such as beam splitting or ring-shaped transformation, can be introduced using these laser-written structures. Schematic plots of the monolithic crystalline structures for 3D beam splitting and ring-shaped transformation from a single Gaussian beam are shown in Figure 3.

Figure 3. Schematic diagram of the three-element 3D photonic-lattice-like cladding structures for the 1×4beam splitter and ring-shaped transformer. The cross-sectional images of each element are indicated as insets.

Beam propagation through monolithic photonic structures can be simulated numerically, offering an ideal platform for the development of 3D devices based on type-II modification. In addition, from an applications point of view, passive and active devices could both be manufactured in suitable crystalline platforms. We have successfully implemented 3D beam splitters (1×N) and miniature laser sources (with a maximum output power of up to 0.4W at ∼1μm) in the FLW crystals.

In summary, we have demonstrated new 3D-waveguide-device designs and FLW-fabricated dielectric crystals using different laser-induced modifications. The approaches that we have proposed could enable the development of highly compact 3D devices for light guiding and beam steering. In addition, by carefully considering the bulk features of the materials used, more niche applications could be catered for (e.g., nonlinear frequency conversion and quantum computing) in a number of novel 3D spatial geometries. Such geometries could significantly improve the integration of complex photonic devices with compact designs. Due to huge differences in the microscopic properties of crystals, however, each type requires a different 3D fabrication and optimization strategy.

In the next stage of our research, we intend to explore the feasibility of producing new 3D optical devices in other interesting dielectric crystals and to discover features that may prove useful for novel photonic applications.