Fig. 1. Passive devices on an SOI platform for SWIR. (a) and (b) Top view and cross section of scanning electron microscope (SEM) images of the fabricated SWIR grating couplers [57]. (c) SOI single-mode waveguide propagation loss in the SWIR [57]. (d) Microscope image of an AWG [40]. (e) Transmission spectrum of a ring resonator (inset) in wavelength range between 2.28 and 2.32 μm [58]. Figures are reproduced from: (a)–(c) Ref. [57]; (d) Ref. [40]; (e) Ref. [58].

Fig. 2. Passive devices on SOI with a thicker silicon layer. SEM images of strip-waveguide-based (a) bending [61]; (b) MMI device [61]; (c) racetrack resonator [61]. (d) SEM cross section image of a waveguide implemented in the imecAP process where the thickness of the p-Si layer is 160 nm and the thickness of c-Si is 220 nm [39]. (e) SEM image of the mode converter between a strip waveguide and a slot waveguide [62]. Figures are reproduced from: (a)–(c) Ref. [61]; (d) Ref. [39]; (e) Ref. [62].

Fig. 3. Other passive devices on SOI operating beyond SWIR. Optical microscopy images of (a) FTIR spectrometer with conventional asymmetric MZIs [41]; (b) the Vernier architecture [67]; (c) fabricated MZI and the AMMIs’ inputs [68]; (d) output of IAMMI. The inset shows the cross section of the waveguides [68]. Figures are reproduced from: (a) Ref. [41]; (b) Ref. [67]; (c) and (d) Ref. [68].

Fig. 4. Active devices on an SOI platform. (a) Optical microscope image of the asymmetric MZI modulator. Inset: magnified detail of the 50/50 Y-junction, optical path imbalance, thermo-optic heaters, and RF signal inputs [37]. (b) Optical microscope images of a spiral-arm asymmetric MZI [36]. (c) Aluminum heater sits on top of one arm of the spiral MZI [36]. (d) SEM image of the longitudinal cross section of the gain region shown in Fig. 4(e) [69]. (e) Schematic of the III-V-on-silicon DFB laser [69]. (f) Cross section view of the photodetector shown in false color to distinguish the materials. Inset shows schematic of the silicon waveguide with dimensions labeled in nanometers, and with TE mode at 2 μm overlaid [38]. Figures are reproduced from: (a) Ref. [37]; (b) and (c) Ref. [36]; (d) and (e) Ref. [69]; (f) Ref. [38].

Fig. 5. Nonlinear response on an SOI platform. Output transmission spectrum with pump operating at 1946 nm when the input signal is (a) off, (b) on. Parametric amplification of the signal occurs, with simultaneous spectral translation across 62 THz, to an idler at 1620 nm [44]. (c) SEM image of cross section of the etchless silicon microresonator with integrated PIN diode, shown in false colors [43]. (d) MIR broadband frequency comb generation from 2.1 to 3.5 μm in the etchless silicon microresonator [43]. Figures are reproduced from: (a) and (b) Ref. [44]; (c) and (d) Ref. [43].

Fig. 6. Suspended-membrane-based devices on an SOI platform. (a)–(d) SEM images of suspended devices in SOI: (a) focusing SWG coupler; (b) SM waveguide cross section with etch depth ∼240  nm, residual slab thickness ∼100  nm, and width ∼1  μm; (c) straight and bending SM waveguide with bending radius ∼40  μm; (d) SM-based ring resonator with radius ∼10  μm [78]. (e) SM-based ring resonator for MIR photothermal spectroscopy. Heat generated through absorption of the MIR pump beam increases the temperature of the suspended ring resonator and then shifts the resonance wavelength induced by the thermo-optic effect. It leads to a change in the output power of the fixed-wavelength NIR probe light. The absorption spectrum of the analyte can be reconstructed through pumping wavelength scanning [81]. Figures are reproduced from: (a)–(d) Ref. [78]; (e) Ref. [81].

Fig. 7. Suspended-membrane-based PCs on an SOI platform. (a) Mode profile of Ez field component of L3 PC cavity with lattice constant a=1.34  μm, radius r=0.263a, silicon layer thickness t=0.5  μm, and shift of the two edge holes of the cavity s=0.15a, with resonance wavelength of 4.604 μm and Q factor of 24,000 [88]. (b) and (c) SEM images of fabricated device: (b) L3 PC cavity; (c) 45° tilt view of etched side wall of PC cavity hole [88]. (d) SEM image of an MIR W1 PCW [89]. (e) Measured transmission of a PCW with lattice period a=1060  nm. Three colored regions, both in the spectrum and the simulated band diagram (inset), correspond to guided region, above the light line, and bandgap, respectively [89]. Figures are reproduced from: (a)–(c) Ref. [88]; (d) and (e) Ref. [89].

Fig. 8. Suspended-subwavelength-grating-waveguide-based devices on an SOI platform. SEM images of (a) a waveguide with SWG cladding, focusing coupling grating, and taper [35]; (b) the 90° bend [97]; (c) the MMI [97]; (d) a cleaved facet of a 90° bend [97]. Figures are reproduced from: (a) Ref. [35]; (b)–(d) Ref. [97].

Fig. 9. Waveguides on an SOS platform. (a) False-colored SEM image of the cleaved facet of a waveguide. Here silicon is in green while sapphire is in blue [100]. (b) Cut-back loss measurements at λ=5.18  μm for TE polarization along with imaged mode profiles for each length [101]. (c) Ridge waveguide propagation loss for wavelengths ranging from 2.9 to 4.1 μm. Inset shows the cross section of 2400 nm by 480 nm [102]. (d) SEM image of a fabricated slot waveguide working at 3.43 μm wavelength [103]. (e) Close-up of strip waveguide to slot waveguide mode converter [103]. (f) 1×2 MMI-based power splitter [104]. Figures are reproduced from: (a) Ref. [100]; (b) Ref. [101]; (c) Ref. [102]; (d) and (e) Ref. [103]; (f) Ref. [104].

Fig. 10. Grating couplers on an SOS platform. (a) SEM image of shallow-etched uniform grating on the 10-μm-wide waveguide [107]. (b) Zoom-in image of (a) with 405-nm etch depth, 0.4 fill factor, and 1120-nm period [107]. (c) SEM image of full-etched nanoholes subwavelength grating on the 10-μm-wide waveguide [107]. (d) Zoom-in image of (c) with 600 nm etch depth, 253 nm nanoholes radius, and 1250 nm period [107]. (e) SEM image of full-etched subwavelength grating coupler [103]. (f) Magnified view of air holes in (e) with 152 nm width, 825 nm length while the periods in vertical and horizontal directions are 800 nm and 1500 nm, respectively [103]. (g) Optical image of a fully etched 1D grating coupler [106]. Figures are reproduced from: (a)–(d) Ref. [107]; (e) and (f) Ref. [103]; (g) Ref. [106].

Fig. 11. Ring resonators on an SOS platform. (a) Optical micrographs of the primary MIR ring resonator with Q∼3,000 around λ=5.45  μm wavelength (top) and a group of ring resonators with various dimensions (bottom) [105]. (b) Transmission (λ=4.35–4.6  μm) of a ring resonator after resist-reflow and post-fabrication treatment, showing loaded Qt∼151,000, and intrinsic Q0∼278,000 (inset) [106]. (c) Normalized temperature-dependent transmission of a quasi-TE ring resonator working at λ=2.75  μm at 25°C–65°C scanning (left) and 25°C–40°C scanning (right) [110]. Figures are reproduced from: (a) Ref. [105]; (b) Ref. [106]; (c) Ref. [110].

Fig. 12. PCs on an SOS platform. (a) Side view SEM image of the W1 PCW at the PCW-strip waveguide interface [112]. (b) 70° tilt view of the slot mode converter at the input (or output) of the slotted PCW [115]. (c) Top view SEM image of the HPCW [115]. (d) Top view of an L21 PC microcavity side coupled to W1.05 PCW [104]. Figures are reproduced from: (a) Ref. [112]; (b) and (c) Ref. [115]; (d) Ref. [104].

Fig. 13. TEM images of SOS wafer (a) before and (b) after annealing.

Fig. 14. Sensing application of devices on an SOS platform. Change in transmitted light intensity at λ=3.43  μm through silicon devices in SOS for TEP sensing: an 800-μm-long HPCW with lattice constant a=845  nm with introduction of (a) 10 ppm TEP and (b) 50 ppm TEP; an 800-μm-long silicon slot waveguide when introducing of (c) 25 ppm TEP and (d) 28 pph TEP; (e) a silicon strip waveguide in the presence and absence of 28 pph TEP [115]. (f) Comparison of theoretical and measured Q of a MIR ring resonator in zero and 5000 ppmv N2O [118]. (g) Zoom-in spectral of resonance variation in pure N2 and in 5000 ppmv N2O concentration. The upper/lower panel corresponding to resonator line overlaps/away from N2O absorption line [118]. (h) Normalized transmission of a 3-μm-wide multi-mode strip waveguide with 1.2-μm-thick SiO2 up cladding at different D2O–H2O mixtures [119]. Figures are reproduced from: (a)–(e) Ref. [115]; (f) and (g) Ref. [118]; (h) Ref. [119].

Fig. 15. MIR PC TO switch. (a) SEM image of the whole MZI structure; the gold heater is adjacent to one PCW on one arm. Inset shows the zoom-in image of the heater and PCW [121]. (b) Normalized optical intensity from the TO switch against applied heating power at λ=3.43  μm [121]. Figures are reproduced from Ref. [121].

Fig. 16. MIR supercontinuum generation in an SOS waveguide. (a) Experimentally observed output spectra for different coupled input peak powers [102]. (b)–(e) Measured and calculated transmission as a function of coupled intensity at the input of a 5 μm by 0.5 μm SOS waveguide at (b) λ=3.5  μm, (c) λ=3.7  μm, (d) λ=3.9  μm, and (e) λ=4.1  μm [102]. Figures are reproduced from Ref. [102].

Fig. 17. Standard silicon wafer. (a) Fabrication scheme for producing suspended silicon rib waveguide in a standard silicon wafer [34]. (b) SEM image of a fabricated suspended silicon waveguide with dimensions of 2.4 μm wide and 1.07 μm above the membrane, which is 1 μm thick and 17 μm wide [34]. (c) SEM cross section of an MIR silicon T-guide, which only supports a single mode and a single polarization in the range from 1.2 to 8.1 μm [129]. (d) Measured transmission at different lengths for two different waveguide structures [129]. Figures are reproduced from: (a) and (b) Ref. [34]; (c) and (d) Ref. [129].

Fig. 18. Pedestal-type waveguides on a standard silicon wafer. (a) Fabrication procedure to make a pedestal-type waveguide [131]. (b) SEM images of fabricated pedestal MIR devices including waveguide, waveguide bending, and Y-splitter [131]. (c) Real-time trace of toluene using a pedestal-waveguide-based MIR sensor, showing output intensity drops when adding analytes and recovers during evaporation of analytes [132]. (d) Output intensity decreases as toluene ratios increase since the aromatic C-H stretch in toluene strongly absorbs the transmitting light at 3.3 μm wavelength [132]. (e) Absorbance of six different chemicals at 3.55 μm [132]. Figures are reproduced from: (a) and (b) Ref. [131]; (c) and (d) Ref. [132].

Fig. 19. Silicon-on-porous-silicon. (a) Cross section of a silicon waveguide on porous-silicon bottom cladding [136]. (b) Measured propagation loss for oxidized SiPSi waveguides in both NIR and MIR through the cut-back method [60]. Figures are reproduced from: (a) Ref. [136]; (b) Ref. [60].

Fig. 20. Silicon-on-nitride (SON). (a) Schematic of the SON fabrication process [135]. (b) SEM image of the facet of a fabricated silicon waveguide on silicon nitride [135]. (c) Microscope image of an integrated QCL on SONOI platform [137]. (d) Facet of an integrated QCL on SONOI platform [137]. (e) Schematic of an integrated DFB QCL on SONOI. An SONOI waveguide with surface DFB grating (left panel). A DFB QCL is heterogeneously integrated with an SONOI waveguide (middle panel). One taper of the fabricated DFB QCL is removed (right panel) [138]. Figures are reproduced from: (a) and (b) Ref. [135]; (c) and (d) Ref. [137]; (e) Ref. [138].

Fig. 21. Silicon-on-calcium-fluoride. (a)–(f) Schematic of the device fabrication process, showing a silicon membrane is transferred to a CaF2 substrate. (g) Image of a fabricated silicon device (light purple) on CaF2 (transparent). SEM images of (h) a silicon microring resonator on CaF2 and (i) a taper at the edge of the chip for light coupling. (j) Measured transmittance of a fabricated microring immersed in different concentration ratios of ethanol and toluene in a cyclohexane mixture. (k) Derived absorption coefficient (α) and refractive index change (Δn) of the mixture from the measured extinction ratio and resonance peak shift in (j). Calibration samples of ethanol, toluene, and blank solvent are plotted. (l) Calculated concentrations of ethanol and toluene from (k) using linear transformation showing good agreement with FTIR data. Figures are reproduced from Ref. [140].

Fig. 22. Silicon-on-lithium-niobate. (a) Fabrication process for a silicon-on-lithium-niobate chip and the electro-optic modulators on it. (b) SEM image of a fabricated modulator in a silicon-on-lithium-niobate substrate. The applied field will follow the direction shown in white lines. (c) Modulator response (blue) in the time domain. The red line represents the drive voltage divided by 20. The inset shows the modulator response in the frequency domain. Figures are reproduced from Ref. [141].

Table 1. Devices and Applications on an SOI Platform

Table 2. Devices and Applications on SOI with Free-Standing Structure

Table 3. Summary for Devices on SOS

Table 4. Summary of Devices on a Standard Silicon Wafer

Table 5. Summary for SON, SOCF, and SOLN

Table 6. Comparison of MIR Silicon Platforms