Critically coupled Fabry–Perot cavity with high signal contrast for refractive index sensing

Abstract

Perfect absorption at a resonance wavelength and extremely low absorption at the wavelength range of off-resonance in a one-port optical cavity is required for refractive index (RI) sensing with high signal contrast. Here, we propose and analyze an absorption-enhanced Fabry–Perot (MAFP) cavity based on a critical coupling condition in a near-infrared wavelength range. For a one-port cavity, a thick bottom Au is used as a mirror and an absorber. To achieve the critical coupling condition, a top dielectric metasurface is employed and tailored to balance the radiation coupling and the absorption coupling rates, and the one-port cavity is theoretically analyzed using temporal coupled-mode theory. We investigate two types of MAFP structures for gas and liquid. The gas MAFP cavity shows a sensitivity of ~ 1388 nm/RIU and a full-width at half-maximum of less than 0.7 nm. This MAFP cavity resolves the RI change of 5 × 10−4 with a reflectance signal margin of 50% and achieves a signal contrast of ~ 100%. The liquid MAFP cavity shows a sensitivity of ~ 996 nm/RIU when RI of liquid changes from 1.30 to 1.38. With tailoring the period of the metasurface maintaining its thickness, a signal contrast of ~ 100% is achieved for each specific RI range.

Broadband-Tunable Vanadium Dioxide (VO2)-Based Linear Optical Cavity Sensor

“…Moreover, a closer observation reveals that with the help of the presented model, it is possible to achieve the transmission modulation between approximately 0.03 (insulated VO 2 ) and 0.8 (metallic VO 2 ) in the frequency range of 125 to 230 THz. Hence, making it a broadband optical modulator in addition to its potential application as an RI sensor [ 72 ].Transmission modulation was obtained because of the presence of the FP-phenomenon in the insulating phase of VO 2 and its absence in its conductive phase. In the next step, for the purpose of optimizing the RI sensor model, resonating modes of the cavity were studied as the function of VO 2 layer thickness on each side of the Si waveguide along with the length of the optical cavity resonator.…”

Temporal Coupled Mode Analysis of Chromaticity in Trilayer Subtractive Structural Colors

“…The materials include silicon (Si), germanium (Ge), titanium (Ti), platinum (Pt), aluminum (Al), silver (Ag), copper (Cu), and gold (Au). As shown in Figure S2, these absorbers have different degrees of light reflection in order to vary the radiative decay rates: silver reflects almost all light across the visible range, silicon reflects only 30% of the spectrum, and copper/gold reflects different wavelength bands due to their absorption from interband transition . In order to effectively compare the color performance of various absorbing materials in the 25 nm TiO 2 /SiO 2 /absorber, the thickness of the middle dielectric layer (SiO 2 ) is carefully tuned due to the nontrivial phase upon light reflection from the interface of SiO 2 /absorber to ensure that all structures give the same hue.…”

“…Considering the law of energy conservation as well as time-reversal symmetry, we can find that with N to be an arbitrary integer. We can then further solve eq for the reflection coefficient as and absorption can be calculated as Therefore, a perfect absorption (i.e., unity absorption) can be achieved at resonance (ω – ω 0 ) only when , which is typically referred to as the critical coupling condition. , Meanwhile, we can also obtain the absorptive and radiative quality factors, respectively, as with U as the energy stored inside the cavity and P diss and P abs represent the dissipation power due to absorption and radiation, respectively. Note that the features of the reflection spectrum are strongly related to the overall decay rate , which determines the broadening/width of the resonance in the spectrum and the ratio δ = γ abs /γ rad determines the minimum reflectance at resonance wavelength.…”