Spectroscopic Enhancement Utilizing Nanoscale Metallic Structures: Bow-tie Antenna and Shell Cavity

Surface plasmons on dielectric/metal interface offer significant advantages for spectroscopic applications. The most important one is the ability to enhance electromagnetic energy density in small sample volumes under study. Secondly, with finely controlled geometry, different nano-structures can be used to steer the focused energy into a subwavelength volume. The resonance features of these nano-structures can be characterized by both their geometries and dielectric functions of the metal in these nano-structures. The scheme of metallic nano-structures may be broadly applicable and adaptable to many fields, where advantages come from concentrating the electromagnetic energy density. If these structures are brought into the vicinity of the sample under study, the light scattering effect is enhanced without increasing the excitation power. This offers the benefit of non-invasive chemical analysis at very small sample volume. Prior studies demonstrated that the total optical scattering cross section can increase significantly when the excitation wavelength is resonant with both the chemical and metallic nano-structures. In this study, we focus experimental measurements and numerical modeling on applications which concentrate the electromagnetic energy density,;· An aluminum bow-tie nano-antenna is used to demonstrate the antenna enhancements with the resonance Raman effect to achieve a major increase in signal from the intrinsically weak Raman signal of benzene in the deep ultraviolet (DUV). Both the nano-antenna and the liquid sample resonate at the excitation wavelength of 258.8nm. In addition to the resonance Raman enhancement at the absorption peak of the 1 A1g →1 B2u transition (~ 103), the contribution from the nano-antenna enhancement is about 105. Thus, an overall gain of hundreds of millions is achieved. This antenna is chosen from the scaling relation between the antenna length and the excitation wavelength. The scaling relation described in classical antenna theory and for optical nano-antennas operating in infrared, visible can also apply to (from this work) DUV wavelengths.;· A dielectric core/Au shell cavity is studied by numerical modeling based on the finite element method. The overall size of this cavity is about half a micron. In addition to resonance in a visible range of spectrum from the gold dielectric properties, this cavity has another resonant wavelength near the infrared wavelength of 980nm. This resonance associated with its geometry is similar to the antenna effect research on the Al bow-tie nano-antenna and optical antennas in infrared and visible studied by other researchers. The upconversion of an upconverting nanocrystal enclosed in this gold cavity is modeled. The enhancement from the gold shell on both spectral intensity and peaks ratio are in good agreement with experimental measurements. An modeling protocol based on finite element method is established.;· A Raman spectroscopy system design that includes a continuously tunable excitation source ranging from the DUV to infrared is used for these studies. The improved instrument uses a Type II optical parametric oscillator. Two beta -- BaB2O4 crystals provide a continuous source of wavelengths, with few-wavenumber resolution in the range of 210 -- 2200nm, based on a Type II phase-matching scheme and angular tuning. The extended effective light-crystal interaction length and a larger birefringence, which is resulted from this phase-matching scheme, narrow the excitation linewidth. The narrower linewidths offer higher spectroscopic resolution, which is beneficial for the studies on resonance Raman/nano-structures.