Development of Electrically Controllable Mid-Infrared Bandpass Filters for Mechanical-Chopper-Free NDIR Sensing

Molecules exhibit their unique light absorption spectra due to molecular vibration in the mid-infrared region (3–15 μm), and thus, a method to control mid-infrared light is important for various sensing applications including nondispersive infrared absorption (NDIR) sensing. In this study, we demonstrate an electrically controllable mid-infrared bandpass filter that can serve as both a chopper for synchronous detection to improve detection sensitivity and a wavelength-selective filter in NDIR systems. The proposed filter consists of a photonic crystal (PC) slab with multiple quantum wells (MQWs), where the frequency of the intersubband transition in MQWs coincides with those of a transverse-magnetic-like (TM-like) resonance and a transverse-electric-like (TE-like) resonance simultaneously. As a result, this design shows a narrowband bandpass transmission spectrum in the mid-infrared region for vertically incident light owing to the interference of the two resonances. In addition, the magnitude of the peak transmittance can be electrically modulated by changing the electron density in the MQW and modulating the absorption coefficient of intersubband absorption (ISB-T). By fabricating the designed filter, we demonstrate a narrow-bandwidth (Q ∼ 70) transmission spectrum with a transmittance change of 20% and a modulation speed of over 100 kHz at a wavenumber of 1030 cm–1. Moreover, we develop a nonmechanical NDIR system using the proposed filter and successfully demonstrate a detection of fast gas-flow change of C3F8.