## 4 Applications of Integrated Photonics
### WG Lasers  
WG lasers are advanced compact light sources ideal for seamless integration into diverse photonic systems [98]. Innovative techniques in fabricating low-loss WGs within laser crystals of varying geometries have established economical platforms for generating solid-state lasers in compact form factors [99]. By harnessing high intracavity optical intensities, WG lasers achieve remarkably low lasing thresholds while maintaining efficiencies comparable to traditional bulk and fiber laser systems. Recent breakthroughs in photonic structure design have enabled customized beam shaping capabilities, including beam splitting and ring-shaped transformations, thereby significantly enhancing the versatility and applicability of WG lasers [100]. These attributes firmly establish WG lasers as highly promising miniature light sources tailored for PICs [101; 102; 103]. Figure 3: (**a**) Graphical illustration of the sensor layout [71]; (**b**) Cross-sectional view of a sensing patch, illustrating the WG, isolation layer, and surface chemistry location [71]; (**c**) photographic image  
showing light propagation along the WG chip [71], (**d**) schematic of a broadband NV hybrid EO plasmonic switch. The inset shows a cross-sectional view of the plasmonic slot WG coated with GST [90]. Mode profiles of the PCM-coated plasmonic slot WG at a telecom wavelength of 1550 nm: (**e**) amorphous phase and (**f**) crystalline phase, calculated for TE polarization. Geometric parameters used for mode profile calculations: metal height \"h\" = 220 nm, slot width \"w\" = 180 nm, PCM layer thickness \"t\" = 20 nm [90], (**g**) all-optical modulator utilizing a Si rib WG structure [97], (**h**) E-field profile of the propagating quasi-TM mode within the WG-integrated modulator, with the dashed white line indicating the graphene sheet plane [97], (**i**) experimental setup schematic for passively Q-switched laser operation in Nd:YVO\({}_{4}\) superficial cladding WG. The inset features a microscope image of the laser-inscribed WG [104], (**j**) room-temperature fluorescence emission spectra (\(\upmu\)PL) depicting transitions from \({}^{4}\)F\({}_{3/2}\) \(\rightarrow\) \({}^{4}\)I\({}_{9/2}\) and \({}^{4}\)F\({}_{3/2}\) \(\rightarrow\) \({}^{4}\)I\({}_{11/2}\) states of Nd\({}^{3+}\) ions. Data obtained from the laser-inscribed WG volume is shown in the red solid line, while bulk Nd:YVO\({}_{4}\) crystal emission is represented by the blue solid line. The inset displays the 2D spatial distribution of Nd\({}^{3+}\) emission intensity captured from the WG's end-face, corresponding to the region marked within the red dashed square in the microscope image insert (**i**) [104]. [PERSON] et al. presented groundbreaking findings on room-temperature subnanosecond WG laser operation at 1064 nm using a Nd:YVO\({}_{4}\) crystal WG, employing Q-switching facilitated by phase-change nanomaterial vanadium dioxide (VO\({}_{2}\)) [104]. VO\({}_{2}\)'s capability to transition between insulating and metallic phases provided crucial low-saturation-intensity nonlinear absorption properties, essential for generating subnanosecond pulses. The low-loss WG was fabricated via femtosecond (fs) laser writing with a depressed cladding geometry. Under optical pumping at 808 nm, efficient pulsed laser emission was achieved in the Nd:YVO\({}_{4}\) WG, with a minimum pulse duration of 690 ps and a maximum output average power of 66.7 mW. Comparative studies involving Q-switched lasers utilizing two-dimensional (2D) materials like graphene and transition metal dichalcogenides (specifically WS\({}_{2}\)) coated mirrors revealed that while these materials can also generate 1064 nm laser pulses in the same WG platform, they typically produce shorter pulses lasting around 22 ns, with higher maximum output average powers reaching approximately 161.9 mW.