Plasmonic nanolens focusing light in subwavelength scale

We report a metallic nanolens that can focus light into region comparable to wavelength. According to the finite different time domain (FDTD) method numerical calculation, it was found that the relative phase of emitting light scattered by surface plasmon in a single subwavelength metallic groove can be modulated by the groove depth. Consequently, the focal length of the slit-groove-based focusing structures can thus be adjusted in certain value if the groove depths are arranged in traced profile. With the regulation of the groove depth profile, it is possible to modify the focus position in the precision of nanoscale without increasing the size of the nanodevice. The numerical simulation results verify that the method is effective for the design of nano-optical devices such as optical microprobes. Advantages of the proposed nanolens are apparent. (i) The element is miniaturized and the modulating the groove depth trace profile would not increase the corrugation area and hence make the element compact, making it an excellent candidate for integrated optics. (ii) The obtained focal length is comparable to the wavelength and the focal width is less than a wavelength, which are difficult to obtain via conventional refractive element. (iii) The element's dimension is subwavelength in thickness, which may prove useful to act as surface device that integrated into other optical and optoelectronic elements.