Fig. 1. Route of chip-integrated optical sensing technology. (a) The waveguide type includes: (a1) Fingerprint light absorption waveguide sensor ^[55]; (a2) Refractive index sensor based on waveguide microresonator ^[57]. (b) The free-space type includes: (b1) Low-pass filter integrated fluorometer ^[52]; (b2) Micro/nano-resonant structure vertically integrated detector ^[58]; (b3) Optical sensor with in-situ photodetection ^[54]

Fig. 2. On-chip optical sensing technology based on non-dispersive light absorption of the waveguide mode. (a) Monolithic gas sensor with GeSbS spiral optical sensing waveguide and PbTe waveguide detector ^[65]; (b) Monolithic gas sensor with silicon optical sensing waveguide and graphene waveguide detector on CaF₂ substrate ^[66]; (c) Monolithic integration of epitaxially grown inter-band cascade laser and deposited As₂Se₃ optical sensing waveguide on GaSb substrate ^[67]; (d) Monolithic sensor with quantum cascade laser, quantum cascade detector and surface plasmon sensing waveguide on InP substrate ^[55]; (e) Monolithic integration of broadband light source and detector based on Al-AlO_x-Au tunnel junction on glass substrate, which are connected by surface plasmon waveguide^[56]; (f) Monolithic integration of InGaAs detector and chalcogenide glass optical sensing waveguide on polymer flexible substrate ^[68]

Fig. 3. On-chip refractive index sensing technology based on waveguide-coupled micro-resonators. (a) Monolithic silicon photonic sensor with microring sensing unit and GeSi detector operating with an off-chip tunable laser ^[81]; (b) Monolithic silicon photonic sensor with electrically scanned dual-microring sensing unit and GeSi detector operating with an off-chip amplified spontaneous emission broadband light source^[51]; (c) A broadband tunable mid-infrared laser based on tunable cascaded silicon microrings and bonded III-V active layer for gas sensing by monitoring the absorption fingerprint ^[83]; (d) A biochemical sensor based on heterodyne signal detection with monolithically integrated dual tunable lasers and detector on InP substrate ^[84]

Fig. 4. On-chip optical sensing technology based on vertically integrated optical sensor and optical detector. (a) Chiral sensor based on a nanograting polarizer integrated silicon photodiode on a CMOS platform ^[92]; (b) electric readout optical sensor based on a gold nanodisk array integrated silicon PN junction detector ^[95]; (c) electric readout optical sensor based on a silicon pn-junction detector integrated with silver film covered polymer gratings ^[96]; (d) electric readout optical sensor based on a gold nanodisk array integrated silicon photodiode on a CMOS platform ^[58]; (e) fluorescence sensor based on a copper nanograting filter integrated silicon photodiode on a CMOS platform ^[28]; (f) CMOS image sensor integrated with a gold nanohole array for multi-wavelength detection ^[103]

Fig. 5. On-chip optical sensing technology with in-situ photodetection. (a) Optical sensor based on a prism-coupled metal-semiconductor-metal multilayer ^[109]; (b) Optical sensor based on nanohole array integrated metal-insulator-semiconductor multilayer ^[113]; (c) Optical sensor based on gold film covered silicon nanogratings ^[54]; (d) Optical sensor based on gold nanoparticles covered Au-IGZO-Au photoconductive diode ^[121]; (e) Optical sensor based on a vanadium oxide bolometer integrated with gold film covered silicon nanogratings^[53]; (f) Optical sensor based on a pyroelectric detector integrated with lithium tantalate and metamaterial absorber ^[122]; (g) Optical sensor based on a pyroelectric detector integrated with ZnO and metal nanodisk array ^[123]; (h) Optical sensor based on a tunnel junction consisting of gold nanorod array and liquid gallium indium ^[124]