Study On Novel Optical Micro/Nano Structures In Mid- And Far- Infrared Enhanced Sensing Application

In recent years,optical sensing has a wide range of application needs in medical diagnosis,health monitoring,security detection,biomedicine and other fields because of its advantages of high accuracy,low latency and imageability.With the rapid development of information technology such as big data and Internet of Things,the demand for miniaturization and portability of sensors is increasingly urgent,especially for automotive and electronic products.The sensors based on micro/nano structures with the advantages of high tunability,small size and high repeatability have received wide attention.Mid-and far-infrared spectroscopy can identify molecular species,but to ensure the identification of molecules with enhanced sensing sensitivity,which requires the use of artificial resonant structures and the resonance matches the absorption intrinsic frequency of the substance.There are two problems:1.how to increase the Q factor and thus enhance the local electric field enhancement and characteristic absorption at the resonant frequency;2.how to design the artificial structure so that the resonance matches the absorption intrinsic frequency of the substance to be detected.This thesis addresses these two problems and innovatively constructs a series of optical sensors based on micro/nano structures based on the concepts of bound states in the continuum,toroidal dipoles,and quasi-guided modes,and systematically investigates their sensing enhancement mechanisms,and verifies the feasibility and efficiency of the sensors with numerical simulations and micro/nano processing experiments.The main contents of the paper are as follows:1.Most artificial structures are affected by radiation and material losses,and their Q-factors at resonance are only a few tens or even lower,which limits them from having ultra-narrow bandwidths and thus difficult to be applied to mid-and far-infrared sensing.An effective way to achieve the goal of ultra-high Q-factor in the mid-and far-infrared band is to explore the concept of bound states in the continuum.Bound states in the continuum have theoretically infinite Q-factors,but usually bound states in the continuum cannot be directly excited by incident plane waves.By breaking the structural symmetry,the coupling with the incident plane wave can be realized to form quasi-bound states in the continuum resonance while the Q-factor can be kept at a high order of magnitude,which is important for the sensor development.In this thesis we theoretically design and study an asymmetric rectangular air slot structure in the terahertz band based on the continuous domain bound state concept.The fingerprint detection of low concentration of hydrogen cyanide gas at 2 ppm has been achieved by finite element algorithm simulation.Most importantly,the structural size of this structured hydrogen cyanide gas fingerprint detector is less than 100μm,which facilitates device integration as well as multi-channel gas detection.In addition,we have confirmed the feasibility and high sensitivity of the c bound states in the continuum concept-based sensing application in the mid-infrared band through theoretical simulations and experimental validation.2.Using photonic structures that resonate at the characteristic absorption frequency of the target molecule to detect the target molecule can enhance absorption and improve sensing sensitivity in many spectral regions.However,the requirement for accurate spectral matching poses a significant challenge to the fabrication of structures,and the use of external means（e.g.,electric gating）to actively modulate the resonance of a given structure can make the system very complex.In the present work,we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q-factors and wavevector-dependent resonances over a large operating bandwidth.These modes are supported in a distorted photonic lattice,where the period doubles due to the band folding effect.The detection of nanoscaleα-lactose films is achieved by using a composite grating structure on a silicon slab waveguide.Our results show that the transmittance at resonance is highly dependent on theα-lactose thickness and that the structure can detect thicknesses as small as 0.5 nm.3.The toroidal dipole resonance is a special electromagnetic excitation phenomenon that is independent of the two fundamental multipole families.The weak coupling of the toroidal dipole to free space provides an effective design method for designing optical resonances with high Q-factors.In addition,the far-field radiation energy distribution of the toroidal dipole is the same as that of the conventional electric dipole,but in opposite phase.Simultaneous excitation of a toroidal dipole and an electric dipole will enable anapole resonance without far-field radiation through phase-dissipative interference.In this thesis,we investigate the optical properties of both toroidal dipole and anapole based resonances in the mid-infrared band,and the related work is as follows:（1）We design a mid-infrared sensing with ring dipole resonance supported by an"E"shaped metasurface.Both numerical and experimental results show that the position of the toroidal dipole spectrum is highly dependent on the thickness of polymethyl methacrylate,and a minimum measurable thickness of less than 10 nm can be stably achieved experimentally and the sensitivity of the structure can reach 420 nm/RIU.（2）The"lucky knot"shaped metasurface based on the toroidal dipole response is further designed based on the anapole mode.This mode can minimize the energy loss due to the nanostructure to achieve greater field enhancement and higher sensitivity.Theoretically,we find that the transmission spectrum exhibits polarization-independent properties as the polarization of the incident wave of this structure varies withfor 0°,45°and-45°.In addition,the sensitivity of the hypersurface can reach 685 nm/RIU,which is improved compared with the previous"E"shaped metasurface that supports the toroidal dipole response.4.The mid-infrared spectral range（wavelength 2-20μm）is known as the"molecular fingerprint"region,where various gas molecules exhibit highly rotational or vibrational characteristics,and notably,in the mid-infrared region,the absorption cross section of molecular leaps is typically 10-1000 times larger than that of the visible or near-infrared region.Therefore,mid-infrared spectroscopic gas sensors can be used to identify and quantify the presence of substances with high sensitivity and selectivity.To address the problem of very low energy utilization of commonly used non-dispersive infrared sensors,this thesis designs a gas sensing and detection device based on a narrow-band thermal radiation source to realize the construction of an ultra-sensitive mid-infrared gas detection system with an experimentally detectable CO₂concentration of 40 ppm and a Q-factor of 81 at resonance.This work provides a new way for mid-infrared spectral gas sensors.