**5. Conclusion**

298 Advanced Photonic Sciences

Fig. 13. (a) The contrast spectrum of experimental data (black line), the simulation result using *nz* =2.0-1.1i (red line), and the simulation result using *nG* = 2.6-1.3i (dash line). (b) The contrast simulated by using both *nG* (blue triangles) and *nz* (red circles), the fitting curve for the simulations (blue and red lines), and our experiment data (black thick

Fig. 14. (a) The contrast image of the sample. (b) and (c) The cross section of contrast image,

The optimized simulation result is shown in Figure 13a reveals a refractive index of single layer graphene *nz* = 2.0-1.1i, whereas the simulation result using the bulk graphite value of *nG* (2.6-1.3i) shows large deviation from our experimental data. Using the optimized refractive index *nz*, we have calculated the contrast of one to ten layers' graphene also as shown in Figure 13b, which agree well with the experimental data with the discrepancy being only 2%. By using this technique, the thickness of unknown graphene sheet can be determined directly by comparing the contrast value with the standard values shown in

Figure 13b. Alternatively, it can be obtained using the following equation:

which corresponds to the dash lines. (Ni et al., 2007).

lines), respectively, for one to ten layers of graphene. (Ni et al., 2007).

In this chapter, we have proposed a far-field CWLR imaging system by combing a small aperture and a small collection fibre core diameter, which is fast, non-destructive and user friendly. It is demonstrated to provide a high spatial resolution about 410 nm, which is capable of resolve two nearest gold nanoparticles with the size and centre-to-centre distance in-between about 300 nm and 200 nm, respectively. Individual single, dimer gold nanospheres, silver nanowires, and graphene sheet were characterized by the imaging system as well. Apart from the dipolar LSP, excitation of multi-polar LSP of individual gold nanospheres was revealed. Compared to the resonance energy of single gold nanosphere, the resonance energy of the dimer is red-shifted due to the EM coupling between the two component nanospheres of the dimer. The near-field EM coupling effect between individual Au nanospheres and the supporting SiO2/Si substrate was also studied by the CWLR imaging method, which reveals a decay length of 0.30 in units of *d R*/ for the coupling strength, qualitatively agreeing well with the 'plasmon ruler' scaling theory. The anisotropic excitation of LSP of single silver nanowire was revealed to get contribution to the polarization dependent images besides their essential reflectivity difference from that of the substrate. It is also demonstrated that the CWLR spectra method provides a standard to identify the thickness of graphene sheet on Si substrate with ~300 nm SiO2 capping layer, from which the refractive index (*nz* = 2.0-1.1i) of graphene below ten layers can also be easily determined. As the CWLR imaging can be preformed at different wavelength, we also expect its other interesting applications such as biomaterial mapping and plasmonic studies in the future.

#### **6. Acknowledgment**

This work was financially supported by the Natural Science Foundation of China (No. 11004103), China Postdoctoral Science Foundation funded project (No. 20100471332), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 0902016C), NUAA Research Funding (No. NS2010186), and NUAA Scientific Research Foundation.

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**12** 

 *Italy* 

Salvatore Tudisco

**Time Resolved Camera:** 

*INFN - Laboratori Nazionali del Sud, Catania* 

**The New Frontier of Imaging Devices** 

Time resolved imaging up to the single photon sensitivity is one of the most ambitious and important goals of photonics. Currently there are no commercial devices able to provide both the information on position (imaging) and arrival time of photons emitted by weak and ultra-weak sources. Only few and very expensive devices (Intensified CCD, electron bombarded CCD etc.) are able to reach the single photon detection threshold together with the possibility to collect photons in small time window (up to few ns). Unfortunately such devices are not able to provide any information on the arrival time of photons, fundamental

for the recent developments on ultra-fast, time correlated optical sensing techniques.

Fluorescence-based imaging (both single and multi-photon) is the research field that has most influenced the development of fast and sensitive optical detectors. Examples of techniques in this class include Förster resonance energy transfer (FRET) (Jares-Erijman et al., 2003), fluorescence lifetime imaging microscopy (FLIM) (Becker et al., 2006), and fluorescence correlation spectroscopy (FCS) (Schwille et al., 1999). The success of these techniques, particularly FLIM, derives from the ability to characterize an environment based on the time domain behaviour of certain fluorophores with high resolution in space domain. This characterization can be done today with high levels of accuracy in 3D with minimal interference from the surroundings and almost no dependence on fluorophore

Many others scientific areas like astronomy, biophysics, biomedicine, nuclear and plasma physics etc. can benefit from a time resolved imaging device; it can improve the actual

In astronomy and astrophysical science, one of the toughest problems affecting groundbased telescopes is the presence of the atmosphere, which distorts the spherical wave-front, creating phase errors in the image-forming ray paths. Even at the best sites, ground-based telescopes observing at visible wavelengths cannot achieve an angular resolution in the visible better than telescopes of 10 to 20 cm diameter, because of atmospheric turbulence alone. The cause is random spatial and temporal wave-front perturbations induced by turbulence in various layers of the atmosphere; one of the principal reasons for flying the Hubble Space Telescope was to avoid this image smearing. In addition, image quality is affected by permanent manufacturing errors and by long timescale wave-front aberrations

detection limits providing physical information otherwise inaccessible.

**1. Introduction** 

concentration.

