Fig. 1. Infrared transmission window of some materials^[25-29]

Fig. 2. Surface topographies of ChG films deposited by different methods. (a) AFM image of GeAsSe films deposited by magnetron sputtering^[69]; (b) optical microscopy image of Ge₂Sb₂Te₅ thin film deposited by fs-PLD^[65]; (c) AFM image of As₃₃S₆₇ film fabricated by sol-gel process^[70]; (d) As₂S₃ films deposited on SiO₂ microdisks by thermal evaporation^[66]; (e) 4 inch Ge₂₅Sb₁₀S₆₅ film fabricated by thermal evaporation^[67]; (f) As₅₅S₄₅ film fabricated by PECVD^[68]

Fig. 3. Morphology images of ChG waveguides obtained by different waveguide fabrication methods. (a) SEM images of Ge₂₂As₂₀Se₅₈ waveguide fabricated by direct laser writing^[71]; (b) schematic of 80GeS₂-15Ga₂S₃-5Sb₂S₃ waveguide fabricated by He⁺ ions implanting^[85]; (c) SEM images of As₂S₃ waveguide fabricated by thermal printing^[76]; (d) SEM image of As₂Se₃ waveguide fabricated by wet-etching^[86]; (e) AFM image of Ge₂₃Sb₇S₇₀ waveguide fabricated by lift-off ^[80]

Fig. 4. Morphology images of ChG waveguides fabricated by dry-etching method. (a) Schematic of As₂S₃ waveguide pre-patterned and redeposited on SiO₂ substrate^[66]; (b) SEM image of Ge₂₃Sb₇S₇₀ waveguide prepared with SF₆ as etching gas^[91]; (c) SEM image of Ge₂₃Sb₇S₇₀ waveguide prepared with chlorine as etching gas^[92]; (d) SEM image of Ge₂₃Sb₇S₇₀ waveguide prepared with CHF₃/CF₄ as etching gas^[93]; (e) SEM image of As₂S₃ waveguide prepared with CF₄/O₂ as etching gas^[94]; (f) SEM images of Ge₂₅Sb₁₀S₆₅ waveguide sidewall prepared with and without optimized parameters of Ar/CHF₃/CF₄/O₂^[53]

Fig. 5. Applications of integrated ChG devices in ultrabroad wavelength regions. (a) Narrow bandwidth Bragg grating filter based on chalcogenide glass in 1.55 μm band^[109]; (b) graphene waveguide integrated detector in 2 μm band^[115]; (c) schematic of photon aerosol spectrometer module in 3 μm band^[116]; (d) schematic of hybrid integrated detector based on spiral chalcogenide waveguide and PbTe film directly integrated under chalcogenide waveguide^[112]; (e) cross-section structure and (f) transmission spectrum of As₂Se₃ waveguides in mid-infrared band^[83]; (g) optical microscope image of spiral single-mode ChG waveguide detector and (h) absorption spectra of methane and nitrous oxide based on device in 7-9 μm wavelength region^[117]

Fig. 6. Applications of ChG phase change material-based photonic integrated devices. (a) Optical switch based on Si₃N₄ microresonator and GST^[118]; (b) 1×2 and 2×2 phase change material-based optical switchs^[122]; (c) GSST based polarization rotary optical switch^[125]; (d) working principle and multi-level signal processing of all-optical on-chip memory device^[127]; (e) GST based photonic memory cell and photonic structure of matrix-vector multiplication^[128]; (f) photonic tensor cores for convolution operations^[8]; (g) optical switches based on symmetric and asymmetric microring resonators^[129]

Fig. 7. Results of ChG optical waveguides fabricated by laser direct writing method^[132]. Optical microscope images of (a) unexposed area, (b) single straight waveguide, and (c) Y-coupler on ChG film; (d) top view of microring resonator obtained by optical microscope^[133]; (e) fabrication schematic of optical Bragg grating through interference caused by dislocation of shadow masks^[134]

Fig. 8. Results of mode splitting caused by photorefractive in ChG microresonators^[136]. (a) Optical microscope image of ChG microresonator; (b) schematic of Bragg grating in ChG microresonator; (c) pattern splitting results caused by different irradiation time; (d) variation curves of mode splitting width Δλ_split and corresponding Bragg grating reflectivity r_B with exposure time; (e) reflection spectra of "writing" and "erasing" selected mode in As₂S₃ microcavity based on photoinduced refractive index variation^[137] (upper image is reflection spectrum including three modes, lower left image is reflection spectrum of three modes after erasing mode 2, lower middle image is local reflection spectrum before erasing mode 2, and lower right image is local reflection spectrum after erasing mode 2)

Fig. 9. Hybrid integrated ChG-based devices. (a) ChG and lithium niobate hybrid integrated microring modulator and MZI modulator^[139]; (b) As₂S₃ vertically integrated optical phased array on lithium niobate substrate^[141]; (c) acoustooptic modulator with lithium niobate/GeSbS hybrid integrated MZI structure^[144]; (d) GeSbSe waveguide integrated black phosphorus photodetector^[147]; (e) hybrid integrated photodetector based on ChG waveguide and tellurene^[148]

Fig. 10. ChG waveguides and systems for all-optical signal processing^[154]. (a) Cross section of As₂S₃ planar waveguide; (b) simulated group velocity dispersion of TE and TM modes and material dispersion of on-chip waveguide; (c) principle diagrams of transmitter optimization of Tbit/s bandwidth signal and demultiplexing time division multiplexing signal receiver system

Fig. 11. Results of ChG integrated microresonators in optical parametric oscillation applications. (a) Optical parametric oscillation based on As₂S₃ microring resonators^[30];(b) measured threshold power of optical parametric oscillation process (about 5.4 mW)^[30]; (c) four-wave mixing effect based on Ge_11.5As₂₄Se_64.5 microring resonators^[158]; (d) measured conversion efficiency of four wave mixing as a function of input pump power^[158]

Fig. 12. Results of ChG integrated microresonators in optical frequency comb applications^[53,67]. (a) SEM of GeSbS microresonator;(b) bright soliton microcomb; (c) dark-pulse microcomb; (d) Raman-Kerr microcomb

Fig. 13. Results of ChG integrated waveguides in SBS applications. (a) SEM of As₂S₃ rib waveguide; (b) 52 dB Brillouin on-off gain spectrum of As₂S₃ rib waveguide^[164]; (c)(d) As₂S₃-silicon hybrid integrated waveguide and its Brillouin gain spectrum^[165]; (e)(f) As₂S₃-Ge∶SiO₂ hybrid integrated waveguides and its Brillouin gain spectrum^[167]; (g)(h) Ge₂₅Sb₁₀S₆₅ waveguides and its Brillouin gain spectrum^[168]; (i)(j) As₂S₃ waveguides with BCB cladding and its Brillouin gain spectrum^[169]

Fig. 14. Applications of SBS in ChG integrated chips. (a) Microwave photonics filters^[171-174]; (b) low-noise microwave source^[175]; (c) microwave measurement^[176]; (d) narrow linewidth on-chip Brillouin laser^[66,165] ; (e) high spatial resolution sensing^[177]; (f) photonic memory^[179]

Fig. 15. Large bandwidth tunable ChG-based integrated Raman laser^[53]. (a) SEM of GeSbS microresonator; (b) Raman laser spectrum; (c) output power of the first-order Raman laser as a function of pump power; (d)(e) discrete tuning of the first Stokes output spectrum and discrete tuning of power; (f) discrete tuning of second-order Stokes output spectrum; (g) continuous tuning of Raman laser at different temperatures

Fig. 16. Integrated Raman soliton laser based on ChG waveguides^[182]. (a) Optical microscope image and SEM image of fabricated device with varying waveguide width between 800 and 1000 nm; (b) diagram of experimental setup; (c) measured optical spectra of Raman solitons; (d) Raman soliton spectrum under 7.73 pJ pulse energy; (e) Raman soliton pulse train; (f) autocorrelation trace of Raman soliton single pulse; (g) frequency domain analysis of Raman soliton