Designs of apertureless probe with nano-slits for near-field light localization and concentration

Abstract

This paper presents recent studies on nano-patterned plasmonic probes that can provide highly localized and enhanced light for the near-field scanning optical microscopy. The mechanism to realize such localized light source is introduced and numerically characterized in the near field. In addition, the attainable wideband operation of the plasmonic probe through the proper design is also discussed with particular attention to developing potential applications in the near-field scanning optical microscopy.

Introduction

Near-field scanning microscopy (NSOM) is a versatile tool for investigating the sample surfaces, allowing the imaging of both optical signature and topographic information. By probing near-field interactions within the localized region of sample, high resolution optical images, which cannot be reached through the conventional optics, has been achieved with various probe designs. In particular, aperture-based NSOM probe [1], [2] in the transmission mode has shown promising results in terms of high-resolution imaging while guiding and focusing electromagnetic (EM) energy over the damping mode of waveguide of the small aperture. To obtain quality optical images in near-field, fundamental, physical requirements have to be satiated. First, the waveguide geometry of a probe including the aperture geometry needs to support the efficient transmission of optical energy; and second, a sharp tip dimension allowing access to the small region of interest is also necessary. By observing the near-field interaction which is coupled through the small aperture, one would achieve the high resolution of optical images better than 50 nm [1], [2]. For the past decades, various aperture-based NSOM probe designs have been reported to satisfy such qualifications and break the diffraction limit of conventional optics, taking advantage of the local near field [3], [4], [5]; however, the inherent low optical transmittance of NSOM probes associated with the small dimension of aperture for the high-resolution imaging in general limits the performance of near-field scanning by impeding the signal discrimination over strong background noise signals.

To overcome such low optical transmission of aperture-based probe designs, the recent progress in NSOM probe design has been made utilizing the property of surface plasmon wave [6], [7], which is following the evanescent mode of propagation at the metal and dielectric interface. In plasmonic nano-photonics, effective focusing and enhancing of near-field light within the small region of interest may rely on the configuration of metal geometry used [8], [9], [10], [11], [12], [13]: the improved throughput with effective light confinement has been achieved by using the geometries for diffractive light focusing [8], build-up of aperture resonance [9], [10], nano-antenna at aperture [11], and geometrical plasmon focusing [12], [13]. Among the prominent progresses, one important probe design was devised [14] through the use of periodic corrugations perforated in metal film. One can further utilize the optical energy, coupled through an aperture, in a more efficient way by focusing plasmonic surface waves before being dissipated by the propagation loss. A metallic probe with a tip aperture and concentric corrugations on its face could feature large field enhancement in the near field of the tip apex [14]. Following the demonstration of such diffractive SPP focusing for NSOM applications, similar probe configurations have been studied in order to provide the enhanced signal to noise ratio (S/N) [15], [16]. Although late studies on localized near-field enhancement of aperture-based probe design claim that the use of surface wave is being effective for the near-field focusing and enhancement, overall optical energy available for near-field scanning is sorely dependent on the limited optical energy coupled through the waveguide geometry of probe and small aperture. This allows the use of only fraction of impinging energy; thus, one may envision the probe designs that can utilize the impinging light in a more efficient way without suffering from the cut-off mode of the waveguide and low transmittance of small aperture.

In this paper, we present our recent theoretical study on novel plasmonic probe designs, providing the mechanism of efficient near-field focusing and localization [17]; and further extended the study on plasmonic probe featuring wideband operation is also presented. The proposed probe configuration involves the designed non-local Surface Plasmon Plariton (SPP) coupler (see Fig. 1) launching surface plasmon waves in a unidirectional way, and metal-coated probe body featuring a sharp dimension of apertureless tip. Slit-based non-local coupler designs [19], [20] are employed to efficiently translate the impinging light into the SPPs on prove faces, and metal-coated sharp pyramidal geometry is considered to geometrically guide the surface energy at small tip apex. Unlike the conventional aperture-based probe designs in which the aperture is located at the cut-off region of probe waveguide, the proposed probe configurations may utilize more optical energy before being dissipated by the modal rejection. Herein, the metal-coated pyramidal probes are presented with various SPP generation mechanisms, showing extremely large field enhancement and high optical resolution. The proposed probes are characterized in the near field with the finite-integration-technique (FIT) based software package, CST Microwave Studio. In addition to the numerical study on the near-field focusing and enhancement for NSOM applications, we discuss potential ways to introduce the wide bandwidth of operation to the probe geometry with a non-local SPP coupler. By employing a proper design of the SPP coupler, a plasmonic probe possibly provides the desired spectral response of operation.