Mapping nanoscale light fields

Abstract

N ear-field microscopy is a powerful tool that allows the study of the complex electromagnetic fields that surround nanophotonic structures. As our control over the feature size of these structures becomes ever finer, the ability to image their near fields becomes ever more crucial. This is because, at the nanoscale, light-matter interactions are intimately linked to an object's geometry and not just to the optical properties of its constituent materials. Consequently, near-field mappings are often the only route to understanding the underlying physical processes of exciting phenomena such as extraordinary optical transmission 1,2 , light propagation through photonic crystal waveguides 3,4 , and the optical response of nanoantennas 5,6 . Likewise, when accurate knowledge of the details of nanoscopic light fields is crucial to the performance of a device, near-field imaging becomes essential. Examples of such situations include the creation of hotspots for nonlinear nanophotonics 7 or sensing applications 8 , the way in which nanophotonic structures direct light flow 9 , or the generation of structured fields for nanomanipulation 10,11 . In all, there are a host of fields that stand to gain from the information available from near-field microscopy.In this Review we discuss recent progress towards a complete mapping of light fields at the nanoscale, suggesting new scientific avenues that are opened by these advances. We begin by briefly reviewing the basics of subwavelength field mapping. We then outline recent progress in the mapping of electric near-field vector components, and progress to mapping that goes beyond the vector nature, such as time-or frequency-resolved measurements. We then address recent progress towards the mapping of magnetic near fields, and in particular towards the ability to simultaneously map the entire electromagnetic near field. Throughout the entire Review, we highlight exciting nanophotonic systems whose near fields can be accessed because of progress in this field. Lastly, we end with a brief outlook on the remaining challenges in the pursuit of a complete nanoscale near-field mapping. Basics of subwavelength field mappingNear fields (Box 1) are intrinsically hard to image directly, as they cannot be viewed by, for example, a CCD array or photodiode. This is because near fields are evanescent in nature and hence, for visible or near-infrared light, these fields are typically found within 10s or 100s of nanometres of a surface. Consequently, an intermediate step is required, whereby some of the near field is converted to far field. This conversion may be achieved in several ways. For example, information about the nanoscopic fields may be accessed by incorporating emitters such as dye molecules into a nanophotonic structure and studying the resultant fluorescence 12 , or by using photoresist which polymerizes at high intensities and henceThe control of light fields on subwavelength scales in nanophotonic structures has become ubiquitous, driven by both curiosity and a multitude of appli...

A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

“…A light wave defined by such exponential decay is termed an evanescent wave, and it experiences an exponential decay in amplitude moving away from its originating location. The exponential decay in the propagation direction away from the originating surface has the consequence of localizing the near-field to the originating emitter structure or interface (within a distance less than 100 nm from the surface), which also results in the light field near a surface being dominated by evanescent light [8]. A consequence of this is the real part of the wavevectors k n is larger than k o , resulting in a near-field structure that contains information about the surface beyond the diffraction-limited maximum resolution of the far-field.…”

“…The generated tip near-field is subsequently scattered off the sample surface, into the far-field to be detected. Early versions of the SNOM technique employed a tapered optical fiber with a metallic tip and a subwavelength opening for this purpose [1,8]. The standard aperture tip possesses a circular opening with a typical diameter ranging from 80 to 250 nm, but can be as small as 10-20 nm using advanced nanofabrication procedures.…”

“…Apertureless-type SNOM systems, or scattering SNOM (s-SNOM), utilize light from an external source scattered from an apertureless tip which acts as an antenna to produce a source of evanescent light near the nanostructured sample surface as shown in Figure 1c. Apertureless-type operation is advantageous in that it does not suffer from the fundamental size limitations of c-SNOM, and can achieve resolution below 10 nm [1,8,13]. However, apertureless-type SNOM systems are more challenging to implement than their aperture-type counterparts.…”

Triggering extreme events at the nanoscale in photonic seas

“…-To image the light-field distributions inside the chaotic resonators we use a home-built NSOM, whose operation is described in detail in [29,41], and therefore we here limit to present the main results. Briefly, an aperture probe that consists of an aluminum-coated tapered SiO2 fiber with a 100-300 nm sized aperture, is placed at a distance of 20 nm of the surface, within the evanescent tail of the electromagnetic field inside the cavity.…”

Integrated nanoplasmonic waveguides for magnetic, nonlinear, and strong-field devices

“…With this integrated device architecture, developing nanoplasmonic waveguide devices based on materials and fabrication processes, which are compatible with electronic and photonic technologies, become desirable. The extraordinary properties of nanoplasmonic devices have motivated significant efforts in several fields of research [10], including sensing [11], light-matter interaction enhancement [12][13][14][15][16][17][18][19][20][21][22][23], light amplification [24][25][26][27][28][29][30][31], sub-diffraction imaging [32], metamaterials [33,34], and planar optical circuitry [35,36]. In this Review, we will focus exclusively on the use of nanoplasmonic waveguides in planar optical circuitry and on related developments of integrated devices with advanced functionalities.…”