Mid-infrared (mid-IR) silicon photonic integrated circuits (PICs) are attracting growing research interest for diverse applications^[1–5]. Benefiting from the atmospheric transparent windows of 3–5 µm^[6], mid-IR optical communication offers a competitive solution to meet the growing demand for free-space data communication^[7]. Furthermore, molecule species, such as water, methane, carbon dioxide, and nitrogen oxides, exhibit fundamental vibrational absorption bands in the mid-IR spectral region^[8], making them suitable for sensing with high sensitivity and selectivity and thus show promising potential for environmental pollution monitoring^[9,10], disease diagnosis^[11,12], and protein analysis in bio-medicine^[13,14].

To meet the application requirement, significant efforts have been made to develop diverse mid-IR silicon photonic devices, such as the mid-IR waveguides^[15,16], splitters^[17,18], and spectrometers^[19] based on various wafer platforms including silicon-on-insulator (SOI)^[20], germanium-on-insulator^[21], germanium-on-silicon^[22], and silicon-on-sapphire (SOS)^[23] wafers. Among them, mid-IR grating couplers are critical components for light coupling at the interface between fibers and chips^[24,25] and have been studied for a long time to improve coupling efficiencies and coupling bandwidths^[26]. Besides, microring resonators (MRRs) are compact on-chip cavities with light greatly enhanced inside, and are thus widely employed in on-chip sensing^[27,28]. However, mid-IR silicon photonic devices are typically designed on silicon wafers with silicon layer thicknesses of hundreds of nanometers. The light is tightly confined inside the waveguide with a limited evanescent field, thus weakening the interaction between the light and the environment. Previous studies have seldom explored mid-IR silicon photonic devices with giant evanescent fields at long wavelengths.

In this Letter, we demonstrated a suspended nanomembrane silicon (SNS) MRR coupled using a subwavelength grating (SWG) coupler at 3.27 µm wavelengths. By tailoring the refractive index (RI) of SWG structures, the highest coupling efficiency of the SWG coupler for the fundamental transverse electric (TE0) mode was measured to be 10.2% at a central wavelength of 3.27 µm. The SNS MRR exhibits a quality (Q) factor of ∼3500 with a giant confinement factor of 0.89 and a reduced temperature sensitivity of 0.07 nm/°C. Our study is expected to open an avenue to developing mid-IR silicon devices for on-chip sensing.

To achieve the giant evanescent field for strong light-environment interaction, we employed an SNS waveguide with a deep-subwavelength thickness (TSi), as illustrated in Fig. 1(a). The SWG cladding of the SNS waveguide was designed by tailoring RI of the equivalent medium^[29] to support the SNS waveguide after removing the buried oxide (BOX) layer. As shown in Fig. 1(b), the longitudinal and lateral periods of the SWG coupler are represented as Λx and Λy, while the corresponding fill factors are fx and fy. The essential parameters of the SNS waveguide, including SNS waveguide width (WWG), SWG cladding period (Λ), fill factor (f), and width (WSWG), are also shown in Fig. 1(b). Figure 1(c) illustrates the zoomed-in view of the coupling region, with two key parameters denoted: the coupling length (L) and gap width (WGap).

The SNS waveguide and SWG coupler were designed on an SOI wafer with a top layer silicon thickness of 70 nm using finite-difference time-domain (FDTD) software. The SWG period Λy was designed to be 600 nm to satisfy the SWG condition, as well as the fabrication requirements. We simulated the directionality and coupling strength of the SWG coupler. Figure 2(a) shows the variation of the directionality and coupling strength with the fill factors, fx. The SWG coupler achieves a directionality of over 50% and a coupling strength of 0.05μm−1. We then simulated the coupling efficiency of the SWG coupler with different fx and fy, as shown in Fig. 2(b). The 2D-FDTD simulation results show that the SWG coupler with Λx=3660nm, fx=0.5, Λy=600nm, and fy=0.4 has the highest coupling efficiency of ∼25%. Figure 2(c) shows the far-field diffraction electric-field distribution of the optimized SWG coupler, revealing that the ideal coupling angle is around 12°. To improve the performance of the SNS MRR, the SNS waveguide and coupling parameters were optimized. The SWG cladding period (Λ), fill factor (f), and width (WSWG) were designed to be 400 nm, 0.5, and 4 µm, respectively. Figure 2(d) shows the optical leakage loss variation as a function of TSi when WWG equals 2.5 and 3.0 µm. Leakage loss increases exponentially as the waveguide thickness decreases, due to substantial optical field leakage into the substrate at low thicknesses. When the thickness exceeds 70 nm and the WWG is 3 µm, the radiation loss is less than 1 dB/cm. Moreover, the optical radiation loss of the waveguide decreases with the increasing waveguide width WWG. The sidewall scattering loss caused by fabrication errors was also reduced. However, waveguides wider than 3 µm can no longer meet the single-mode condition, leading to an excess optical loss due to the excitation of higher-order modes. Figure 2(e) shows the electric-field distribution of the TE0 mode in SNS MRR with a WWG of 3 µm. The SNS waveguide device exhibits a giant confinement factor of 0.89. According to simulations, the confinement factor of the designed SNS waveguide is approximately three times higher than that of silicon photonic devices with a top silicon layer thickness of 340 nm. The bending radius of the SNS MRR was designed to be 120 µm to ensure low radiation loss. To obtain a critical coupling in the MRR, we optimized the coupling region by simulating the self-coupling power ratio (t2) as a function of the gap width (WGap) under fixed coupling lengths (L) of 6 and 7 µm, as shown in Fig. 2(f). The intrinsic optical loss of the ring cavity (α2) is estimated to be 0.917 from the simulation (red dashed line). By varying the WGap, different coupling states of the SNS MRR could be achieved. When the L is set as 6 µm, the critical coupling could be achieved with a WGap of 0.43 µm, while with an L of 7 µm, the critical coupling could be satisfied with a WGap of 0.46 µm.

The SNS waveguide devices were fabricated using standard CMOS-compatible fabrication technology. The SOI wafers were first immersed in acetone, isopropyl alcohol, and deionized water and then treated with O2 plasma. A 200 nm thick layer of positive photoresist (AR-P, 6200.09) was spin-coated, followed by patterning using an electron beam lithography (EBL) system. The chip underwent etching with a reactive-ion etching system (PlasmaPro, 80 RIE), operating at a silicon etching rate of 50 nm/min. After the device fabrication, the BOX was removed using 16% dilute hydrofluoric acid (HF). The scanning electron microscope (SEM) images of the fabricated silicon photonic devices are shown in Fig. 3. The purple-shaded area represents the core of the SNS waveguide. Figure 3(a) shows the SEM image of the SWG coupler. Figure 3(b) shows the zoomed-in image of the coupling region of the SNS MRR. Figure 3(c) shows the overall view of the SWG coupler and SNS MRR.

We measured the performance of the fabricated devices using a mid-IR waveguide device testing system, as shown in Fig. 4(a). A commercial mid-IR interband cascade laser (ICL) (Nanoplus, NP-ICL-3270) was coupled to an optical fiber. A mid-IR half-waveplate (Thorlabs, WPLH05M-3500) was placed between the ICL and fiber to control the light polarization. The alignment of the optical fibers and chip was accomplished using two three-axis translation stages (Thorlabs, Nanomax 300). The transmitted light from the chip was detected by a spectrometer (YOKOGAWA, AQ6377). The inset shows the variation of laser wavelength with the increasing driving current of the ICL. The coupling efficiency of the SWG coupler with different incident angles is shown in Fig. 4(b). The result of the best incident angle is 11°, slightly different from the simulation results in Fig. 2. The maximum coupling efficiency of the SWG coupler is 10.2%, which is roughly consistent with the 3D-FDTD simulation results (pink line). Using an arbitrary function generator (AFG, Tektronix, AFG3021B), we slightly tuned the wavelength of the ICL with a triangular wave of 1 kHz frequency and 10% duty cycle, enabling the measurement of a resonance of the SNS MRR. Figure 4(c) shows a resonance peak of the SNS MRR and its Lorentz fit. The full width at half-maximum (FWHM) is 0.94 nm, corresponding to a Q factor of ∼3500. Note that both the wavelength and power output of the ICL vary with the drive current, and the resonance peak in Fig. 4(c) is normalized to zero. Figure 4(d) shows the Q factor of the MRRs with different coupling lengths L and gap widths WGap. The low Q factor may be ascribed to the strong overlap between the light and waveguide sidewalls and the light leakage to the silicon substrate, which can be mitigated using wafers with a thicker buried oxide layer. Finally, the temperature stability of the SNS MRR was examined using a thermoelectric cooler (TEC), as shown in Fig. 4(e). Since most of the optical energy is distributed in the air cladding, the SNS MRR shows a weaker thermo-optic effect. The temperature sensitivity is only 0.07 nm/°C, exhibiting excellent thermal stability and sensing accuracy, as the existence of temperature fluctuations is the primary reason that affects the sensing accuracy of MRRs^[30,31].

In summary, we demonstrated an SNS MRR coupled using an SWG coupler at 3.27 µm wavelengths. Based on a 70 nm thick SOI wafer, the SWG coupler achieves a peak coupling efficiency of 10.2% at a wavelength of 3.27 µm. The SNS MRR exhibits a Q factor of ∼3500 with a giant confinement factor of 0.89 and a reduced temperature sensitivity of 0.07 nm/°C. The designed SNS devices enhance light–matter interaction while effectively reducing the impact of temperature fluctuations on sensing performance. Our study facilitates the development of mid-IR on-chip gas sensing applications.