Infra-Red Plasmonic Sensors

Abstract

Plasmonic sensors exploiting the localized surface plasmon resonance (LSPR) of noble metal nanoparticles are common in the visual spectrum. However, bio-sensors near the infra-red (NIR) windows (600-900 nm and 1000-1400 nm) are of interest, as in these regions the absorption coefficients of water, melanin deoxyglobin, and hemoglobin are all low. The first part of this paper reviews the work that has been undertaken using gold (Au) and silver (Ag) particles in metal enhanced fluorescence (MEF) in the NIR. Despite this success, there are limitations, as there is only a narrow band in the visual and NIR where losses are low for traditional plasmonic materials. Further, noble metals are not compatible with standard silicon manufacturing processes, making it challenging to produce on-chip integrated plasmonic sensors with Au or Ag. Therefore, it is desirable to use different materials for plasmonic chemical and biological sensing, that are foundry-compatible with silicon (Si) and germanium (Ge). One material that has received significant attention is highly-doped Ge, which starts to exhibit metallic properties at a wavelength as short as 6 µm. This is discussed in the second part of the paper and the results of recent analysis are included.Fluorescent molecules emitting at wavelengths in the infra-red window, in which penetration depth is high and the autofluorescence minimum is of particular interest, are potentially an attractive technology for bio-applications [27]. However, the low quantum yield and poor photostability of near infra-red (NIR) dyes currently limits their applicability. Design and synthesis of NIR dyes with high quantum yield and photostability have proved to be extremely challenging, due to the complex synthetic routes required for these large, complex molecules [27]. The amplification of light from NIR fluorophores by MEF is a promising strategy for dramatically improving both the detection sensitivity and image enhancement, thereby realizing the potential advantages of the NIR fluorophores. Section 2 of this paper discusses the physical process of MEF and reviews some of the published work by the authors.NIR losses arise in Au and Ag from intraband (or Drude) losses. There is, therefore, only a narrow band in the visual and NIR range where losses are low for traditional plasmonic materials. A further challenge associated with noble metals is that they are not compatible with standard silicon manufacturing processes. Further, noble metals diffuse into the semiconductor, forming deep-level traps which have an adverse effect on device performance. While Au and Ag are the obvious choice for visible and NIR applications, there is a desire and need for chemical and biological sensing in the mid-infrared (MIR) range [29][30][31] using materials that are foundry-compatible with silicon (Si) and germanium (Ge), that might lead to on-chip integration of devices governed by plasmonic effects [25,26]. One material that has received significant attention as a potential plasmonic material in the MIR is highly-...

Key Roles of Plasmonics in Wireless THz Nanocommunications—A Survey

“…Several GFET transistors capable of such functioning and operating in the sub-THz or THz band have been demonstrated so far [65]. Other materials, such as Au and Ag have been successfully used for plasmonic sensors in the visual [66,67] and near infrared [68], as well. Ge plasmonic material in the mid-IR, has been also proposed as a promising material for replacing silicon, as a substrate for MOS devices [68].…”