Ultraviolet (UV) plasmonic nanostructures hold promises in enabling label-free sensing of biomolecules using their native fluorescence. Several UV plasmonic structures have been explored to enhance native fluorescence of biomolecules, including metallic thin film, particle array, hole array using aluminum, magnesium, indium, etc. However, the enhancement factor of them is quite small, with less than 80 times for nucleic acids and less than 15 times for amino acids. In order to achieve higher enhancement factor, we study a bowtie nano-antenna (BNA) made of aluminum (Al) in the ultraviolet region. The effect of the native oxide layer on Al is also investigated. The numerical simulation has shown 1026x net enhancement with the optimal geometry.
Despite of increasing understandings of UV plasmonic materials, materials that can enable active tuning of UV plasmonic resonance has not been reported. Here, we demonstrate a modification of UV SPR on an aluminum (Al) hole-array by coupling Graphene π plasmon resonance with Al SPR. Graphene monolayer exhibits an abnormal absorption peak in the UV region (270-290nm) due to π plasmon resonance. The location and intensity of the absorption peak depend on the position of Fermi-level, which can be adjusted by electric or chemical doping. Al SPR is shown here to be modified by coupling Graphene π plasmon resonance with Al SPR.
FDTD simulation shows the modification of Al hole-array transmission by adding a single layer of Graphene on top. The shifts of transmission dips after adding a Graphene layer shows a distinct transition at around the Graphene π plasmon position. For transmission dips that are located at shorter wavelength compared to Graphene π plasmon, up to 8nm blue shifts occur after adding Graphene. On the other hand, up to 20nm redshifts occur for transmission dips that are at a longer wavelength relative to Graphene π plasmon. This change in the sign of shifts of transmission dips corresponds to the change in the sign of the real permittivity of Graphene. The amount of shifts diminishes as the transmission dip moves further away from Graphene π plasmon resonance into the visible spectrum. Experimentally we have observed redshifts of SPR dips but not blue shifts possibly due to the poor light collection below 250nm.
The intrinsic fluorescence of biomolecules such as proteins and nucleic acids lies in the ultraviolet (UV) range of the spectrum. UV plasmonic nano-structures have been shown to enhance the fluorescence quantum yield and reduce the lifetimes of various biomolecules. Fluorescence enhancement is contributed to by both excitation rate and emission rate enhancement. Since biomolecules are prone to photon-degradation in the UV range, excitation rate enhancement should be minimized, while radiative rate enhancement should be maximized. Although numerous nano-structures have been proposed both numerically and experimentally to enhance the fluorescence of native biomolecules, very few studies have achieved more than 10x radiative rate enhancement. Here we report systematic studies of fluorescence enhancement by equilateral bowtie nano-antennas (BNA) made of aluminum (Al) or magnesium (Mg) in the ultraviolet region. We modeled the emission rate enhancement using the excitation and emission peak wavelength of tryptophan. The quantum yield of tryptophan is also taken into account. Our results show that with the optimal geometry, Al BNA with oxide yields an excitation enhancement of 21× at the excitation wavelength of tryptophan (270nm), a radiative enhancement of 37×, a quantum yield enhancement of 5×, and a net fluorescence count rate enhancement of 64× at the emission wavelength of tryptophan (340nm). Mg BNA with oxide sustains the highest Purcell factor enhancement, 14×. The effect of the native oxide layer on both metals is investigated. The studies reported here are meaningful in the design of better UV plasmonic nano-structures for label-free sensing of biomolecules.
We report a new type of structural defect in β-Ga2O3 homoepitaxial thin films grown by metalorganic vapor phase epitaxy, which we have dubbed as “sympetalous defects.” These consist of a line defect (for example, a nanotube defect) in the underlying substrate combined with a multi-faceted inverted polycrystalline pyramid in the epitaxial film, which may also be decorated with twinned polycrystalline grains. In plan-view atomic force, scanning electron, or optical microscopies, the sympetalous defects appear similar in shape to polygonal etch pits observed for single crystals. Photoluminescence microscopy exposed spots of polarization-dependent luminescence at these defects, different from the single crystal films' luminescence. Furthermore, some of the defects exhibited circular dichroism in their luminescence that we correlated with partial helices formed within the pits by the arrangement of linearly dichroic polycrystalline grains. Finally, the density of sympetalous defects agrees with the etch pit densities of the substrates. Understanding and controlling these defects will be of importance as they modify the local properties of films, affect fabricated device yields, and influence characterization experiments.
Abstract Optoelectronic devices in the UV range have many applications including deep-UV communications, UV photodetectors, UV spectroscopy, etc. Graphene has unique exciton resonances, that have demonstrated large photosensitivity across the UV spectrum. Enhancing UV absorption in graphene has the potential to boost the performance of the various opto-electronic devices. Here we report numerical study of UV absorption in graphene on aluminum and magnesium hole-arrays. The absorption in a single-layer graphene on aluminum and magnesium hole-arrays reached a maximum value of 28% and 30% respectively, and the absorption peak is tunable from the UV to the visible range. The proposed graphene hybrid structure does not require graphene to be sandwiched between different material layers and thus is easy to fabricate and allows graphene to interact with its surroundings.
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Herein we utilize polarized photoluminescence (PL) microscopy and spectral analysis to locate and characterize many different types of µm-scale extended defects present in melt-grown bulk crystals and metal-organic vapor-phase epitaxy (MOVPE)-grown epitaxial thin films of β-Ga 2 O 3 and β-(Al,Ga) 2 O 3 . These include pits, divots, mounds, scratches, rotation domain boundaries, stacking faults, cracks, and other defect categories. Some types of µm-scale defects simply decrease overall PL yield, while others emit different spectra than single crystal regions. We combine PL microscopy with atomic force microscopy (AFM) and scanning electron microscopy (SEM) to provide detailed characteristics of these different types of features which can arise from both bulk crystal growth, surface preparation, and epitaxial growth processes. We show that sample quality (in terms of extended defects) can be determined by using PL and that attributing spectral features to isolated point defects is invalid unless the sample is proven to not contain extended defects.
Abstract The native fluorescence of biomolecules has been used in analytical chemistry to determine the concentration of an analyte. However, detecting biomolecules based on their intrinsic fluorescence at low concentration is challenging due to their small quantum yield and poor photon stability. Ultraviolet plasmonics have been reported to increase the photon yield and the photon stability of the native fluorescence of biomolecules such as DNA, peptides, and proteins. However, the experimentally reported count rate, or net enhancement factor, is small-with < 80× for DNA and < 14× for amino acids. Here we report native fluorescence enhancement of tryptophan on aluminum hole-arrays. By optimizing excitation geometry and the hole spacings, we are able to achieve a 47× net enhancement factor, the highest reported in the literature for tryptophan molecules. We conducted photobleaching experiments and observed a 2.3× reduction in the fast photon bleaching rate and 1.9× reduction in the slow photon bleaching rate on an aluminum hole-array with 300 nm periodicity compared to an aluminum thin film. The enhancement of the total photon yield reaches 58×, which is a result of the enhanced radiative rate. This study shows that periodic aluminum hole-arrays allow detection of tryptophan at concentration levels lower than previously reported, underpinning further research into label-free biosensing.
Abstract We investigated several geometric parameters such as the height, width and length, and the contribution of different plasmonic modes on the enhancement factors of aluminum (Al) bowtie nano-antennas (BNAs) on tryptophan’s native fluorescence in the ultraviolet (UV) to visible range. The highest fluorescence enhancement was produced by the tallest BNAs. Analysis revealed that, in tall BNAs illuminated at normal incidence, phase retardation amplified quadrupole resonances which were exploited to obtain high excitation enhancement. The optimized oxide-free Al BNA predicted 331 × excitation enhancement, 74 x radiative enhancement, 993 × fluorescence net enhancement and the optimized oxidized Al BNA predicted 128 × excitation enhancement, 142 × radiative enhancement and 461 × fluorescence net enhancement. These enhancement factors are the largest reported for simulated UV plasmonic structures in literature using tryptophan as the model molecule. The effect of length and width on the different plasmonic modes were also studied and explained in depth. An oxide layer dampened the excitation enhancement but has negligible effect on emission enhancements. The numerical study conducted in this manuscript sheds light to light–matter interaction in the UV frequency range.
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