Fig. 1. Schematic diagram illustrating the laser nanofabrication for PIC applications. Insets depict the examples of different PIC components fabricated via laser nanofabrication: optical modulator^[32]; 3D optical interconnect^[33]; 2D material hybrid device^[34]; on-chip light source^[35]; and photonic wire bond^[36].

Fig. 2. Schematic of a typical experimental setup for ultrafast laser nanofabrication. The inset manifests the writing process by translating the focal spot inside the sample. The translation can be achieved using a high-precision scanning stage.

Fig. 3. Mode profiles (TE) of waveguides with (a) circular and (b) elliptical cross-sections. (c) Loss variation of an ellipse waveguide with different feature sizes. The mode profiles and propagation loss data were calculated using COMSOL Multiphysics.

Fig. 4. (a) Schematic illustration of laser-written type-I waveguide. (b) Optical microscope images and mode profiles of waveguides fabricated in x-cut lithium niobate crystals with different laser repetitions at 1030 nm (from top to bottom: 200 kHz, 720 kHz, and 1 MHz)^[76]. (c) Schematic illustration of laser-written type-II waveguide. (d) Optical microscope images and mode profiles of waveguides in yttrium aluminum garnet (YAG) crystals fabricated with different writing schemes (A and B)^[84].

Fig. 5. (a) Schematic for fabricating hybrid beam splitters in LiTaO3 crystals with fs laser direct writing. Microscope images of the input (a) and output (b) end faces. Optical microscope images of the single-line waveguide in the cross-section before (c) and after (d) annealing. The scale bars are 20 µm. (e), (f) The corresponding guided mode distribution of a single-line waveguide excited by a TM polarized laser^[87]. (g) Schematic of fs laser writing of the LiNbO3 waveguide beam splitter. The 40μm×40μm cross-sectioned input arm splits into two 20μm×20μm cross-sectioned channels. (h) Microscope images of the input faces (left) and output faces (middle), and top views (right) of the waveguides 1-3^[95]. (i) Theoretical illustration of O-FIB technique. (j) The relation between scanning speed and line width of nanogrooves fabricated via the O-FIB. Under a fixed pulse energy of 16.3 nJ, the line width of 18 nm could be obtained with a scanning speed of 8 µm/s^[96].

Fig. 6. (a) Colored scanning electron microscope (SEM) image of a triplet microlens attached to an optical fiber. (b) SEM image of a single triplet microlens. (c) SEM image of an array of triplet microlenses^[101]. (d) 3D schematic of micro-optical elements enabled by laser nanofabrication. (e) A free-form lens. (f) A total internal reflection mirror. (g) A free-form lens with a high NA. (h) A total internal reflection mirror. (i) A beam expander. (j) A multi-lens optical device^[103]. (k), (l) Laser nanofabrication process of MLAs in which the first fabrication process fabricates a large-curved surface (k) and the second fabrication process fabricates the MLA on the surface (l). SEM image of the fabricated MLA (m) and the zoomed-in image (n). Scale bar in (m): 20 µm. Scale bar in (n): 10 µm^[115].

Fig. 7. Fabrication and characterization of WBGs in LiNbO3 crystals. (a) Schematic of the experimental setup for fabricating WBGs. (b) Mode profile in a depressed-cladding waveguide at the wavelength of 1550 nm along extraordinary polarization. Microscope images from upper surfaces and cross-sections of WBGs are demonstrated in (c) and (d), respectively^[135].

Fig. 8. (a)–(c) SEM images of 3D/2D PhC heterostructures with a waveguide structure. (d) Schematic of a 3D/2D/3D photonic crystal heterostructure^[149]. (e), (f) Top view SEM images of the silicon woodpile structure fabricated using laser nanofabrication combined with the silicon double-inversion method. (g) Cross-section SEM image of the silicon woodpile structure. (h) SEM image of the original SU-8 woodpile structure^[153].

Fig. 9. Schematics of NPCs. The 1D, 2D, and 3D NPCs are displayed in (a), (b), and (c), respectively^[154]. (d) Nonlinear microscope images of 3D NPCs fabricated in a LiNbO3 crystal by fs laser erasing of the second-order nonlinearity^[160]. (e)–(g) 2D and 3D NPCs fabricated by domain inversion in LiNbO3^[41], BaCaTiO3^[161], and CaBaNbO6 crystals^[162].

Fig. 10. Laser-induced modification of 2D materials: (a) illustration of self-limiting laser thinning of MoS2; (b) atomic force microscopy (AFM) image of thinned MoS2 flake; (c) AFM height profile of the red line in (b) from top right to bottom left^[202]. (d) Illustration of laser reduction of GO; (e) bandgap and (f) refractive index change of GO as a function of irradiance laser fluence;^[215] (g) laser-induced phase change in MoTe2 with the right figure showing the optical microscope image of pristine (top) and laser modified (bottom) sample;^[204] (h) laser-induced spin defects in h-BN; (i) confocal image of a laser-irradiated spot with the spin emitters located at the edge of the hole; (j) optically detected magnetic resonance measurement of the generated spin defects^[214].

Fig. 11. Laser fabricated 2D material patterns: (a) graphene 1D grating pattern;^[216] (b) GO 1D grating pattern;^[217] (c) GO 2D grating pattern;^[218] (d) graphene nanodisk and (e) MoS2 nanohole arrays.^[219] (f) rGO lens fabricated on a fiber facet;^[220] (g) microcircuit on a GO film^[221]. (h) MXene interdigit electrode^[222]. (i) Chinese knot pattern on CsPbI3^[223].

Fig. 12. (a) Schematic of the fabrication process of the graphene nanogratings. (b) SEM image of the fabricated nanograting. Scale bar: 2 µm. (c) Absorption spectrum of the nanograting in the solar spectrum range^[217]. (d) Laser nanofabricated GO gratings working as an ultrathin polarizer in the NIR to MIR region. (e) SEM image of the fabricated GO grating. Scale bar: 10 µm. (f) Photo of a mounted free-standing GO polarizer^[226].

Fig. 13. (a) Schematic of laser nanofabrication of graphene metalenses^[228]. (b) Optical microscope image of a laser-fabricated graphene lens^[220]. (c) Theoretical and experimental focal intensity distributions in the lateral and axial directions of graphene metalenses^[228]. (d) Graphene orbital angular momentum (OAM) metalenses. (e) Measured focal intensity distributions of the two graphene OAM metalenses working at different wavelengths^[234]. (f) Laser-fabricated graphene metalens with broadband focusing capability and varifocal. Bright-field optical images of an unstretched lens (g) and a uniformly stretched graphene metalens (h)^[236].

Fig. 14. (a) Lateral heterojunction photodetector based on laser-modified In2Se3^[238]. (b) rGO grating photodetector: (i) illustration of the rGO grating fabrication process; (ii) fabricated rGO grating photodetector^[239]; (c) perovskite triangular grating photodetector fabricated via direct laser writing^[240].

Fig. 15. (a) Schematic of laser nanofabrication with slit beam shaping. (b) Microscope image of the fabricated waveguides in the y–z plane^[253]. (c) Experimental laser writing setup using an astigmatic beam-shaping technique. CL: cylindrical lens; M: mirror; PBS: polarizing beam splitter; HWP: half-wave plate; P: polarizer^[256]. (d) Schematic of simultaneous spatial and temporal beam shaping based on single-pass parallel gratings. The inset illustrates how the pulse duration evolves around the focal region as a function of the spatial chirp beam aspect ratio βBA^[257]. (e) Calculated laser intensity distributions at the focus without (top) and with (bottom) the simultaneous spatial and temporal beam shaping.

Fig. 16. (a) Schematic setup of adaptive slit beam shaping using an SLM^[261]. The inset (right) shows the example SLM phase pattern. (b) Different spot geometries are realized via SLM beam shaping^[262]. The first column is the hybrid hologram loaded on the SLM. The second column is the laser spot profile on the CCD. The third column shows the laser-modified surface on the sample.

Fig. 17. (a) Principle of aberration compensation in laser nanofabrication^[272]. (b) Laser nanofabricated structures in diamond with and without aberration compensation^[273]. (c) A feedback system is used to optimize the aberration correction process: (1) no correction, (2) with aberration correction, and (3) with adaptive correction^[274]. Laser-fabricated 3D gyroid photonic crystals inside high-refractive-index chalcogenide materials: side (d) and top (f) views of the structures without aberration compensation; side (e) and top (g) views of the structures with aberration correction. (h) Measured lateral and axial feature sizes at the marked depths in the SEM images^[275].

Fig. 18. (a) Schematic of an optical setup for multi-spot laser generation using an MLA^[289]. (b) SEM images of the fabricated micro-letter array of “N” using MLA technique^[283]. (c) Experimental setup of the SLM-based fs multifocal vortex beam writing system. (d) SEM image of a fabricated SR pattern in polymer^[286]. (e) Schematic of the SLM-based fs multifocal direct writing system of circular cross-sectional waveguides. The inset shows a typical energy distribution of multi-foci at the focal plane. (f) The cross-sections and (g) mode field diameters of waveguides fabricated by different multi-foci arrays. The end-face microscope images and mode profiles of waveguides produced with different multi-foci arrays are shown in the insets of (f) and (g), respectively. The blue and red spots in (g) represent mode sizes at 1550 nm for horizontal and vertical polarizations, respectively. The scale bars shown in insets are 5 µm^[249].

Fig. 19. (a) Illustration of a multi-chip system fabricated with PWB. (b) Inverse-taper transition between an SOI nanowire waveguide and a polymer PWB interconnect. (c) SEM image of a fabricated chip-to-chip interconnect^[83]. (d) SEM image of a fabricated hybrid multi-chip module combining passive SiP waveguides with an InP distributed feedback (DFB) laser array. (e) Measured laser emission direction before PWB. (f) Side view of PWB1 in (d)^[292].

Fig. 20. Optical communication engines fabricated with PWB. (a) Illustration of an eight-channel transmitter that combines efficient InP lasers with electro-optic modulators on a silicon photonic chip. The modulator array is electrically driven via an RF fan-in and connected to an array of single-mode fibers. (b) An array of densely spaced on-chip PWB test structures. (c) Histogram of measured insertion losses of 100 on-chip PWB bridges directly after fabrication (blue) as well as after temperature cycling tests, comprising 120 (orange) and 225 (green) cycles^[36].

Fig. 21. Illustration of the hybrid PICs with laser-fabricated 3D optical connectors^[103].

Fig. 22. (a) Coupling between a single-mode fiber array and an array of edge-emitting SiP waveguides using 3D-printed facet-attached microlenses^[296]. (b) 3D vertical coupler structure: part 1 is a planar silicon-SU-8 waveguide mode converter; part 2 is an IPD Euler-bend waveguide; part 3 is an IPD taper waveguide-to-fiber mode converter; part 4 is the IPD supporting pillars^[297]. (c) Photo of the fabricated fused silica fan-out device^[300]. (d) Illustration of a 3D bridge waveguide for polarization conversion. (e) SEM image of the fabricated bridge waveguide^[301].

Fig. 23. Techniques for 2D material integration.

Fig. 24. Integrated devices with 2D materials. (a) False-color SEM image of a silicon waveguide device coated with graphene^[304]. (b) False-color SEM image of a graphene-coated silicon microring resonator^[305]. (c) Optical micrograph image of a fabricated waveguide detector with black phosphorus^[306]. (d) Optical microscope image of a silicon chip with trenches. (e) SEM image of the cross-section of a trench in (d). X-sectional SEM images of the top (f) and side (g) of a trench after deposition of Al2O3 and PtSe2^[309]. (h) Optical microscope image of a silicon waveguide^[29] and (i) SEM image of silicon nanopillars with conformal coated GO^[34].

Fig. 25. Laser-written PICs for optical communication. (a) All-organic MZI modulator, which is composed of the SU-8 waveguide core and nonlinear polymer cladding. (b) Optical image of the fabricated device. (c) Experimental setup for EO characterization of the device. M: mirror; OSC: oscilloscope; FG: function generator; PI: phase inverter; AMP: amplifier^[98]. LiNbO3 type-II waveguide grains for EO modulation: (d) schematic experimental setup of the device fabrication and characterization (left) and the structure illustration of the waveguide gratings (right)^[323]. (e) Illustration of a rotated polarization directional coupler for polarization multiplexing. Each waveguide is composed of two adjacent tracks with different relative positions. (f) Polarization analysis of the 45° rotated parallel coupling region with different linear input states. (g) Normalized transmission power of different polarized lights through a 45° RPDC^[324].

Fig. 26. Laser-written waveguides for quantum applications. (a) Experimental layout for single-photon generation^[99]. The chip is a 4 cm long waveguide fabricated by fs laser writing in an undoped silica chip. SMF: single-mode fiber; DM: dichroic mirror. (b) CNOT gate realization with a laser-written glass chip^[350]. DL: delay line; LPF: long pass filter; HWP: half-wave plate; IF: interference filter; PC: polarization control; TDC: digital converter; SPCM: single-photon counting module. (c) Experimentally constructed CNOT logical truth table. (d) Photograph of a laser-written waveguide-coupled NbN nanowire SPD chip^[351]. (e) Optical image of the marked region on the chip. (f) Partial enlargement of the waveguide and NbN nanowires. The inset shows the SEM image of a single NbN nanowire.

Fig. 27. Laser fabricated PICs for optofluidic devices. (a) An optofluidic chip composed of a curved waveguide and microchannels to perform classification of microspheres and algal cells^[374]. (b) Illustration of an MZI-based optofluidic chip^[375]. (c) Microscope image showing the two arms of the MZI crossing the microfluidic channel, with the sensing arm crossing the microfluidic channel while the reference arm passes over it. (d) 3D-cascade-microlenses optofluidic chip^[380]. (e) Photograph of the fabricated chip, with a five-cent Swiss franc being placed to indicate the general chip size.

Fig. 28. (a) Schematic illustration of a 64×64 nano-photonic phased array (NPA) system^[389]. The inset shows a close-up view of one antenna unit cell. SEM images of (b) the fabricated NPA system on a silicon chip and (c) a single antenna unit cell^[389]. (d) Photograph of an Omnivision 5647CMOS image sensor with laser-fabricated doublet lenses. The CMOS chip has a pixel size of 1.4μm×1.4μm. (e) Image of a part of the USAF1951 resolution test chart at a distance of 30 mm taken through a hexagonal lens arrangement. Scale bar, 70µm=50pixels^[101].

Fig. 29. (a) Schematic illustration of the photonic lanterns composed of color-coded trajectories of the optical waveguides. Transmission microscope image of (b) a 6×6 array multimode input facet where the telescope point spread function is injected and (c) the pseudo-slit where the reformatted diffraction-limited output is formed. The scale bar is 50 µm^[399]. (d) Photograph of an integrated photonic nulling interferometric system employed in the Subaru Telescope. (e) The top-down view of the waveguide arrangement in the chip. To avoid interference of uncoupled light at the input with guided light at the output, a “side-step” design is adopted. The inset shows a zoomed-in detail of the coupling region^[400].

Fig. 30. (a) Schematic illustration of on-chip photonic synapse^[403]. (b) Optical micrograph of the experimentally demonstrated optical neutral network, which realizes both matrix multiplication and attenuation fully optically^[404]. (c) Schematic of 3D network fabrication via two-photon nanolithography. (d) SEM image of a fabricated 3D Steiner tree network with a unit size of 2 µm and a rod diameter of 200 nm^[280].