Abstract
We report the fabrication and characterization of a new type of porous Si sensor using the Tamm plasmon resonance. The sensor consists of a photonic crystal created by periodic electrochemical anodization of crystalline Si, followed by partial thermal oxidation. The photonic crystal is transferred to a Au-coated glass substrate to allow optical measurements of surface modes at the metal/porous Si interface. This configuration greatly simplifies sensing since an analyte can be introduced in the pores from the opposite side of the metal layer without disrupting the optical path. The fabricated device exhibits a Tamm plasmon resonance within the photonic bandgap at a wavelength of 794 nm with a quality factor of 25. We observe a wavelength shift of the resonance when the nanosized pores are infiltrated with different concentrations of a toluene/ethanol solution. The measured sensitivity reaches 139 nm/RIU, in agreement with scattering matrix simulations and more than twice larger than those previously reported for Tamm plasmons. The quality factor and sensitivity yield a sensor figure of merit of 4. We also show that the electric field within the Tamm device is confined within a mode volume twice smaller than within a Fabry–Pérot resonator of comparable size according to calculations.
Highlights
INTRODUCTIONSurface plasmons (SPs) on metal films have been widely exploited in sensors due to a high sensitivity to a change in the refractive index in their environment. Tamm plasmons (TPs), a novel type of optical surface waves at the interface of a metal and a 1D photonic crystal (PC), have been actively researched in recent times for the same reason. Contrary to a SP resonance, a TP resonance can be excited at any angles of incidence for both polarizations without requiring a coupling prism or grating, which can be advantageous for sensing. An important limitation to this approach is that the electric field is located within the PC, so openings must be made in the device for the analyte to reach the detection volume. One way around this issue is to use porous layers in the PC, such as porous metal oxides deposited by spin-coating. Inspired by this approach, we have previously demonstrated the existence of a TP resonance in a PC made of porous Si (PSi) and coated with gold. PSi is an interesting nanomaterial for PC fabrication because of the simple preparation of a periodic PSi multilayer with high refractive index contrast from a bulk Si wafer using a simple electrochemical process. PSi transmits light with very low absorption from visible to infrared, and the nanosized pores reduce the energy necessary for vapor condensation, facilitating detection of volatile compounds. The very high specific surface of PSi, around 300 m2 cm−3,14 would be especially useful to capture molecules in gas sensing applications
We have demonstrated that porous Si (PSi) is a viable platform to realize a sensor using the Tamm plasmon (TP) resonance as the transduction mechanism
The fact that the porous multilayer is located on top of the metal is a crucial advance since it allows an analyte to diffuse within the body of the sensor where it can affect the TP resonance
Summary
Surface plasmons (SPs) on metal films have been widely exploited in sensors due to a high sensitivity to a change in the refractive index in their environment. Tamm plasmons (TPs), a novel type of optical surface waves at the interface of a metal and a 1D photonic crystal (PC), have been actively researched in recent times for the same reason. Contrary to a SP resonance, a TP resonance can be excited at any angles of incidence for both polarizations without requiring a coupling prism or grating, which can be advantageous for sensing. An important limitation to this approach is that the electric field is located within the PC, so openings must be made in the device for the analyte to reach the detection volume. One way around this issue is to use porous layers in the PC, such as porous metal oxides deposited by spin-coating. Inspired by this approach, we have previously demonstrated the existence of a TP resonance in a PC made of porous Si (PSi) and coated with gold. PSi is an interesting nanomaterial for PC fabrication because of the simple preparation of a periodic PSi multilayer with high refractive index contrast from a bulk Si wafer using a simple electrochemical process. PSi transmits light with very low absorption from visible to infrared, and the nanosized pores reduce the energy necessary for vapor condensation, facilitating detection of volatile compounds. The very high specific surface of PSi, around 300 m2 cm−3,14 would be especially useful to capture molecules in gas sensing applications. Surface plasmons (SPs) on metal films have been widely exploited in sensors due to a high sensitivity to a change in the refractive index in their environment.. An important limitation to this approach is that the electric field is located within the PC, so openings must be made in the device for the analyte to reach the detection volume.. An important limitation to this approach is that the electric field is located within the PC, so openings must be made in the device for the analyte to reach the detection volume.6 One way around this issue is to use porous layers in the PC, such as porous metal oxides deposited by spin-coating.. The refractive index of the toluene/ethanol mixture was determined from an effective model and the sensitivity of the device evaluated by fitting the wavelength of the resonance as a function of refractive index
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