Nanophotonics

Nanophotonic crystals have multiple applications that support detection, communications, and display technologies. Their influence, which originates in their orientation and index of refraction, shows tunable selectivity of the distribution of light by emission angle and wavelength. Wavelength- and angle-resolved cathodoluminescence (WARCL) is a very powerful technique that reveals the distribution of light by wavelength and angle (or the energy and momentum bases, respectively) in relation to the electron beam excitation position.

Plasmonics

Plasmons are a collective oscillation of electrons in a material occurring because of the co-mingling of electric and magnetic fields. Plasmonics can refer to shapes, materials, and interfaces that exhibit behaviors like plasmon propagation (polariton) or resonance. Plasmons can occur in a volume or at a surface, known as a surface plasmon (SP), and can have an effective wavelength much shorter than that of their free space counterpart.

The interaction of light causes the resonance of the free electrons. This leads to greatly enhanced electromagnetic fields, particularly in noble metal structures, which can be used in waveguides, to enhance light or particle detection, and catalysis. Plasmonic metal-based nanoparticles attract significant attention due to a plethora of novel optical device designs that utilize a nanostructure’s size, shape, and composition to enhance light emission and absorption. With qualities such as durability, high spatial resolution, and tunable viewing angle, plasmonics offer significant potential for optical elements in high-resolution color printing and high-density data storage. Recent advancements in nanofabrication technology offer fabrication simplicity with control over the shapes of nanoparticles.

The identification of the plasmon resonance modes and hot spot locations is critical. Still, it presents a significant experimental challenge due to weak light emission levels and the particle size being smaller than the diffraction limit.

Cathodoluminescence (CL) is a natural technique for plasmonic investigations for myriad reasons. Plasmons can be stimulated through interaction with photons (as in a photoluminescence (PL) experiment) or energetic electrons of an electron microscope. The electron beam probe size used in CL microscopy is on the order of a few nanometers, thus providing much a higher spatial resolution than PL while also generating other signals to determine sample size, shape, structure, and composition. Furthermore, the energetic electron probe can be considered a broadband excitation source whereby the plasmonic modes excited do not rely on the wavelength (energy), direction, or polarization specificity of the excitation source. CL output data would not contain influence from the excitation source as it may in a PL experiment.

As an example, results from an Au-Pd nanostar deposited on a (non-luminescent) silicon substrate are shown. Secondary electron imaging (a) revealed the nanostar to be approximately 100 nm on edge, and CL spectrum imaging revealed intense emission (wavelength of 600 nm) emitted when the electron beam was positioned at the nanostar tips indicative of plasmonic resonance between the nanostar tips. Polarization-filtered spectrum imaging revealed the resonance to be dipolar modes from opposing tips (red and blue in (b)).

Angle-resolved CL (ARCL) emission patterns were recorded from each tip of the nanostar (e.g., each resonance node). The difference in emission patterns from each resonance mode is displayed in (c) highlighting distinct emission patterns associated with each mode.

Experimental brief

Investigating the optical properties of nanophotonic materials far below the diffraction limit

Metamaterials

Metamaterials are a class of material with widely tunable optical properties and show promise for exotic applications such as negative refractive index materials. They are composed of materials with contrasting permittivity (ε) and permeability (μ) arranged in discrete periods below the diffraction limit. The material period should be much smaller than the wavelength (or wavelength range) of interest such that the impingent radiation experiences the effective material properties, and can be periodic in one-dimension (1D), two-dimensions (2D), or three-dimensions (3D) to offer control over the effective index of refraction anisotropically.

Metamaterials are commonly characterized by their effective index of refraction, n, where n2ε.μ. Many examples of metamaterials employ plasmonic metal shapes taking advantage of their resonance behaviors to fabricate selective absorbers, emitters, and waveguides.

Due to the nanometer-scale probe size, electron-based excitation, and the recent advent of angle-resolved (AR) and wavelength- and angle-resolved (WAR) modes of operation, cathodoluminescence (CL) proves to be an ideal technique for investigating metamaterials. In the case of the Au-based asymmetric split ring (ASR) resonator metamaterial, CL has shown to be useful for observing their emission spectrum and directionality.


The spectral emission of an Au thin film asymmetric split ring resonator metamaterial across the material surface boundary from flat gold to several microns into the ASR region.

The difference between photonic crystals and metamaterials is period: To have the photonic bandgap the atoms and the lattice constant in the former have to be comparable in size with the wavelength, while for the latter the periodicity should be much smaller than the wavelength.

Photonic Structures

Photonic crystals, also known as photonic bandgap materials, are the subject of numerous investigations because of their unique characteristics, with prospective applications ranging from gas sensing to optical filters, inkless printing, and reflective displays. Photonic crystals are highly ordered materials with a periodic dielectric constant, whose period is on the order of the wavelength (or wavelength range) of interest. The effect of confining and controlling the propagation of light stems from the photonic bandgap, a band of frequencies in which light propagation in the photonic crystal is forbidden. Through variations in the refractive index and periodicity, it is possible to design one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) photonic crystals.

In order to combat the issues with using flat emitters, researchers turn their attention to three-dimensional microLEDs. Such emitters have the obvious advantage of increased surface area. Further, the increased dimensionality allows for the formation of quantum wells on the sides of the emitter. The increased dimensionality also provides access to various crystal faces. In the case of GaN, it includes the m-plane, that benefits from a reduced native electric field near the surface reducing the influence of the quantum confined stark effect (QCSE), thereby increasing emitter efficiency.


Schematic of microLED GaN/InGaN core-shell pillar cross-section (left) and a top-down microLED pillar array secondary electron image (right).


WARCL pattern at 580 nm.

To determine the photonic behavior of such an array, we employ the wavelength and angle-resolved CL (WARCL) technique. The WARCL pattern shows distinct anisotropy at several wavelengths relevant to the pillar material, dimensions, and periodicity.

This shows the anisotropy in the light emission as a function of wavelength, which is directly useful in potential display technology. However, the wavelength-angle bases are not ideal for the analysis of photonic modes; for this one, they may transform from wavelength-angle to energy-momentum bases.


Energy-momentum dispersion along 110 degrees from the +x axis.

In this way, CL is useful to probe the local density of optical states (LDOS) in the photonic specimen, which is of paramount importance in the design of nanophotonic structures.