Electro-Optical Manipulation Based on Dielectric Nanoparticles

*Jiahao Yan and Yuchao Li*

## **Abstract**

The ability to dynamically modulate plasmon resonances or Mie resonances is crucial for practical application. Electrical tuning as one of the most efficiently active tuning methods has high switching speed and large modulation depth. Silicon as a typical high refractive index dielectric material can generate strong Mie resonances, which have shown comparable performances with plasmonic nanostructures in spectral tailoring and phase modulation. However, it is still unclear whether the optical response of single silicon nanoantenna can be electrically controlled effectively. In this chapter, we introduce two types of optoelectronic devices based on Mie resonances in silicon nanoantennas. First, we observe obvious blueshift and intensity attenuation of the plasmon-dielectric hybrid resonant peaks when applying bias voltages. Second, photoluminescence (PL) enhancement and modulation are achieved together in the WS2-Mie resonator hybrid system.

**Keywords:** silicon nanoparticles, silicon nanostripes, WS2, active control, photoluminescence manipulation

### **1. Introduction**

Dynamically controlling the optical responses from plasmonic or Mie resonators is significant for future optical signal processing [1, 2]. Among different active tuning methods, electrical tuning is one of the most effective one owing to high switching speed and large tuning ranges [3–5]. Recently, electrical tuning on metamaterials based on plasmonic nanostructures has been reported, and the control mechanisms rely on semiconductor layers [6–8], graphene [9–13], or electromechanical deformation [14, 15]. Nevertheless, there are few works about the optoelectronic modulation on nanoscale devices up to now. Furthermore, how to realize the electrical tuning on single nanoparticles is still a challenge.

Combining optical nanoantennas with atomically thin WS2 may be another method to realize dynamic optical responses. Atomically thin WS2 (monolayer or bilayer) exhibits intriguing electrical and optical properties [16–18]. Monolayer WS2 shows strong excitonic emission peak at visible wavelengths; however, ultrathin thickness hinders further enhancement of excitonic emission. Near-field enhancements at excitation wavelengths can enhance the light absorbance, while that at emission wavelengths would boost the emission rate, so those two factors both enhance the excitonic emission from WS2. Based on it, many efforts have been made to realize field enhancements via plasmonic nanostructures and photonic crystals at both excitation and emission wavelengths.

Silicon nanoantennas as a typical dielectric Mie resonator have wide application prospect in building metasurfaces [19–21], nonlinear optics [22], and biosensing [23]. They may be better choice than plasmonic structures and photonic crystals in building electrically controlled devices. The Mie resonances in silicon nanoantennas can be modulated through changing the sizes [24, 25] or crystallographic phases [20] passively. However, how to realize active control based on the Mie resonances in silicon nanocavities is still a challenge. Besides changing the optical properties of Mie resonators intrinsically, active tuning may also be realized via coupling with 2D materials. Neshev et al. have theoretically demonstrated the PL modulation of 2D materials based on the directional emission caused by Mie resonators [26]. Recently, the first experimental work has been done, in which a forward-to-backward emission ratio of 20 was realized because of the interaction between MoS2 monolayer and Mie resonators [27]. However, both of them were analyzed on passive control.

modulated signals with low noise. The design of electrically tunable silicon

*Electro-Optical Manipulation Based on Dielectric Nanoparticles*

*DOI: http://dx.doi.org/10.5772/intechopen.88616*

certain probability to be trapped in the gaps.

Si nanoparticle can be obtained.

**Figure 1.**

**79**

*localized plasmon and magnetic dipole.*

nanoantenna is shown in **Figure 1a**. First, maskless laser lithography and electronbeam deposition were used to fabricate Au electrodes with the thickness of 100 nm on the Si/SiO2 substrate, and the thickness of SiO2 layer is 300 nm. In our design, several large Au electrodes (200 400 μm) are deposited with a row of holes in the center. Second, the connected area in the center was nano-patterned using FIB milling to form nanoscale interdigital electrode structure. The separation distance between adjacent nano-electrodes is adjusted from 100 to 200 nm to match the size distributions of silicon nanoparticles, since the silicon nanoparticles fabricated through femtosecond laser ablation in liquid (fs-LAL) have a wide size distribution. Finally, during the evaporation process, the silicon nanoparticles in colloid have a

Before studying the optical properties of Si-Au hybrid nanoantennas, we should

nanoparticles are shown in **Figure 1c** and **d**, where a bright dot can be seen in darkfield image which means the scattering from the Si nanoantenna. In spectral measurement, through moving the scattering spot into the center of slit and only extracting the data from the location of nanoparticle, the exact scattering from the

For isolated Si nanoparticles, the resonant modes depend on particle sizes and

particle numbers according to Mie theory. While for Au electrode-loaded Si

*Optical properties of the silicon nanoantenna. (a) A schematic diagram explains the fabrication of Au electrode-loaded Si nanoparticles. (b) The schematic shows different plasmon resonant modes of two types Au electrodes. (c) The scanning electron microscope (SEM) image of Au interdigital electrodes with a silicon NP trapped among them. Inset is the high magnification SEM image with a scale bar of 100 nm. (d) The dark-field scattering image of the sample in (c). The white circle reveals the location of Si nanoparticles. (e, f) Measured scattering spectrum of a 180 nm Si nanoparticle (e) and the corresponding simulated scattering spectrum (f). (g) The electric and magnetic field distributions at 675 nm, which represent the hybrid modes coupling between*

study the Au electrode platform first. For the fabricated Au grating, due to the incident light that comes from a dark-field circle in the objective, wave vectors with different directions at x-y plane cannot launch surface plasmon polariton efficiently. In addition, the plasmon energy mainly decays nonradiatively through near-field coupling between adjacent Au electrodes, so Au gratings cannot show bright scattering as shown in **Figure 1b**. However, if only two electrodes left (see **Figure 1b**), localized surface plasmon can be formed between two Au electrodes. Strong scattering light can be generated from the plasmonic field enhancement in the gap. Therefore, we use Au grating in experiment whose scattering can be ignored compared with Si nanoparticles. Typical Au electrode-loaded Si
