Optical detection of nanoparticle with ultra-high sensitivity plays an important role in bio- / nano- and their relative research fields. In our recently developed method, each single particle exhibits unique 4-lobes pattern both in the amplitude and phase images respectively, based on which we explored the possibility of resolution improvement by a particle pair. In this paper two polystyrene beads at the diameter of 100nm were employed with the gap distance ranging from 100-400nm. The amplitude and phase images of the particle pair were simulated by FDTD solver. The images are sensitive to geometrical parameters of the two particles, such as gap distance and direction. The simulation results lead to a resolution of 100nm.
Noble metallic nanoparticle exhibits unique optical properties in visible to near infrared band when its localized surface plasmon resonance is excited. For example a sharp absorption peak at 509nm for a gold nanoparticle at the diameter of 60nm. The plasmonic properties heavily depend on its geometrical structure. In this paper, we theoretically calculate the optical properties of a single nanoparticle with different structure such as solid and core-shell. The simulation results show that the core-shell structure can reach a much broader tunable band and in which the shell thickness plays a dominant role. Further by employing a core-shell pair, more flexible properties can be reached.
Due to the effect of plasmonic coupling, gold nanoparticle dimers have been paid more attentions in bio-imaging. The coupling effect existing at the gap between a closely linked particle pair can make the local field strongly enhanced and in which the angle between the excitation polarization and the dimer axis plays a dominant role. We calculated the amplitude distribution under a highly focused illumination objective. The simulation results show that for such a model, 45 degrees between the excitation polarization and the dimer axis can produce an optimum signal. The enhancement thus obtained is ~10.78 fold while the variation between peak-peak can reach 6.59 fold compared to a single plasmoic particle during the rotation of the polarization.
It is well known that when total internal reflection occurs at the interface between high to low refractive index, evanescent field will go into the media with low refractive index. This field can be scattered by a small dielectric particle on the surface. In this paper, with the aim to enhance the scattering field we introduced a thin gold film, the filed modified by the metallic film was theoretically calculated by FDTD solver. Further a polystyrene bead at the diameter of 200nm and 800nm was employed to test the model. Theoretical and experimental results agree well with each other that the locally excitated surface plasmon play a dominant role in the field enhancement scattered by the sphere.
Gold nanoparticles exhibit unique plasmonic optical properties in visible to near infrared band. Especially the coupling effect existing at the gap between a closely linked particle pair can make the local field strongly enhanced. These properties make gold particles more attractive to be employed as molecular probes in biomedical related fundamental and clinical researches. However in the bio-system exist many large molecules or groups, whose optical signals can strongly depress the gold particles without detectable. In this paper, we proposed a method to extract the targets which are labelled by gold dimer pairs from large scattering background.
The unique optical properties such as brightness, non-bleaching, good bio-compatibility make gold particles ideal label candidates for molecular probes. Due to the strongly enhanced field, aggregation of gold nanoparticles finds themselves plenty of applications in bio-imaging. But limited by its small cross-section associated with nanometer sized particle, it is a big challenge to employ it in a single molecular detection. The field enhancement results from the effect of plasmonic coupling between two closely attached gold nanoparticle under the right excitation condition. With the aim to apply the gold dimer probe to find the molecules in our recently established optical detection method, we compared of the amplitude enhancement by the dimer relative to a single particle. The amplitude distribution under a highly focused illumination objective was calculated, whose results suggest that at the optimized excitation condition, the local field can be enhanced ~190 fold. In consequence, experimental detection was carried out. Gold dimers were linked together by the hybridization of two single chain DNAs. Dimer and single particle probes were mixed together in one detection. Overwhelming contrast between these two kinds of probes were clearly exhibited in the experimental detection image. This method can provide a way to a high specific detection in early diagnosis.
The interactions between light and sub-wavelength sized dielectric particles have been paid more attentions especially in biotechnology and photonics. Base on Mie scattering method, we calculate the electrical field distribution scattered by a single dielectric particle which is focused by a high numerical aperture (NA) objective. The theoretical result indicates a size dependent amplitude and phase distribution, which agrees well with the experimental measurement. This method provides a way to evaluate the particles size down to the range of less than diffraction limitation.
The unique advantages such as brightness, non-photobleaching, good bio-compatibility make gold nanoparticles desirable labels and play important roles in biotech and related research and applications. Distinguishing gold nanoparticles from other dielectric scattering particles is of more importance, especially in bio-tracing and imaging. The enhancement image results from the localized surface plasmon resonance associated with gold nanopartilces makes themselves distinguishable from other dielectric particles, based on which, we propose a dual-wavelength detection method by employing a high sensitive cross-polarization microscopy.
In this work, we present a model to calculate the electric amplitude and phase field distribution of single nanoparticle by
using finite-difference time-domain (FDTD) method. We model the light-nanoparticle interaction by using a liner
polarization light to illuminate the single nanoparticle through immersion oil and glass substrate. The illumination is set
as a cone of plane waves limited by the aperture of the objective. The scattering field summarized on a single detector is
amplified by heterodyne interference with a reference light. The amplitude and phase distribution of particles with
different diameters ranging from 50 nm to 2 micron are calculated.