Abstract
Abstract Simultaneous realization of high quality factor (Q), sensitivity, and figure of merit (FOM) play a pivotal role in building the THz sensor. For such purpose, we propose an all-polymeric Bloch surface wave (BSW) structure that supports a bright BSW mode and a dark surface Fano state that is embedded in the continuum, both of which coupled to the same radiation channels. The existence of the sharp dip with a maximum depth of Fano line could be interpreted with the physics of Friedrich–Wintgen bound states in the continuum (FW-BICs), because of the destructive interference between bright BSW and dark surface Fano modes. A strong angular- and frequency-dependent Q was found. Related influential factors to Q value may also include an asymmetric arrangement of top and grating layers, together with the weak coupling provided by photonic crystals. One numerically optimized design shows a quality factor Q of the Fano mode as 23,670, which is almost two orders higher than that in conventional metallic-metamaterial-based designs. The optimized sensitivity can numerically reach 4.34 THz/RIU in the frequency domain, which is one order higher than that reported in all-dielectric metasurfaces. We infer the high sensitivity is related to the phase-matching condition provided by near-subwavelength gratings. The associated FOM can reach 8857/RIU. Besides, the proposed design also numerically demonstrates high sensitivity in the angular domain ∼125.5°/RIU. Considering it poses no specific requirement for materials that own high contrast of permittivity in the THz regime, large interfacing area, the mechanical and chemical robustness offered by polymers and low cost in fabrication, such all-polymeric BSW structure that supports novel Fano resonance in THz window may give access to rich applications in hazardous gas detection and label-free bio-sensing.
Highlights
The terahertz wave has received intensive attention because of its wide applications in communications [1], safety inspections [2], and sensing [3,4,5]
The proposed design numerically demonstrates high sensitivity in the angular domain ∼125.5°/RIU. Considering it poses no specific requirement for materials that own high contrast of permittivity in the THz regime, large interfacing area, the mechanical and chemical robustness offered by polymers and low cost in fabrication, such all-polymeric Bloch surface wave (BSW) structure that supports novel Fano resonance in THz window may give access to rich applications in hazardous gas detection and label-free bio-sensing
Based on the numerical fact that the thickness of the top layer could be as sufficiently large as that of a substrate, and the associated Fano dip shows with maximum depth, it may make the design experimentally feasible and applicable for building a physical THz sensor
Summary
The terahertz wave has received intensive attention because of its wide applications in communications [1], safety inspections [2], and sensing [3,4,5]. Rakhshani et al [7] proposed a sensing application with a frequency sensitivity of about 6400 nm/ RIU and figure of merits (FOM) of 10,000 based on metalbased Fano resonance (FR) It is noted, most of the MMs as applied in the field of THz sensing today, are mostly limited to noble materials such as gold [8], silver [9, 10], and graphene [6], which inevitably suffer from high intrinsic loss. The proposed design features as numerically high performance, low contrast in permittivity, large interfacing area, mechanical and chemical robustness, and low fabrication cost, we believe the exploration on utilizing BICs for producing Fano surface wave will facilitate the development of THz sensing
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