*Electro-Optical Manipulation Based on Dielectric Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.88616*

#### **Figure 4.**

Δ*ε* 0

*Applications of Nanobiotechnology*

Δ*ε* 00

where *ω<sup>p</sup>* is the plasma frequency which is defined as *ω<sup>p</sup>* ¼

controlled scattering signals on single-particle level.

**3.2 PL enhancements enabled by silicon nanostripes**

¼ � *<sup>ε</sup>*∞*ω*<sup>2</sup>

<sup>¼</sup> *<sup>ε</sup>*∞*ω*<sup>2</sup>

the damping time equals to *μmC=e* where *e* is the charge of an electron. *m*<sup>C</sup> is the effective mass, *ε*<sup>0</sup> is the vacuum permittivity, and *ε*<sup>∞</sup> is the permittivity of silicon at visible band. *N* is the concentration of free carrier which determines the changes of permittivity. Using the Drude model discussed above, we can calculate how free carriers influence the dielectric function of silicon as shown in **Figure 3b**. Putting different carrier concentrations (1017to 2*:*<sup>0</sup> � <sup>10</sup>20cm�3) into Drude model, one can see the real part of permittivity decreases gradually especially at longer wavelengths from 600 to 900 nm. Because the accumulation and inversion layers are less than 5 nm at interfaces, we only used the free carrier-induced dielectric functions at interfaces for the numerical simulation. As shown in **Figure 3c**, the simulated scattering spectra under different carrier concentrations are very similar to the corresponding measured spectra under different applied bias. For the 180 nm Si nanoparticle (see **Figure 3c**), the hybrid resonant peak experiences blueshift and intensity attenuation when increasing the carrier concentrations in sequence. The attenuation trend of resonant peaks is very similar to the experimental spectra in **Figure 3a**. Our proposed structures provide an opportunity to collect the electrically

Owing to the unique properties of dielectric Mie resonators, researchers are trying to use Mie resonators as an important building block to form new-generation electro-optical modulators. One strategy is to combine Mie resonators with 2D materials as the schematic shown in **Figure 4a**. WS2 monolayers and bilayers were obtained by mechanical exfoliation and all-dry transfer technique. WS2 layers and

Si nanostripes were aligned and contacted under an optical microscope. Si nanostripes were fabricated by FIB milling onto SiO2 coated silicon-on-insulator (SOI) wafers. Gold electrodes were patterned and deposited on WS2 and bare silicon to build source, drain, and gate. A simple cross-section schematic of the substrate in **Figure 4a** shows that there is an insulator layer between the Si substrate and the top Si film. Therefore, the scattering from Si nanostripes is not only pure Mie effect but the Mie resonance combined with the Fabry-Perot effect. In our case, the thickness of the insulator layer (h) is 375 nm. The dark-field scattering spectrum and the corresponding optical image in **Figure 1b** indicate that Si nanostripes have a broadband resonant peak. Dominant peaks are located around 700 nm, and two small peaks can also be distinguished below *λ* ¼ 600 nm. **Figure 4c** is a typical WS2- Si nanostripe hybrid nanostructure. Corresponding SEM images are shown in **Figure 4d**. From SEM images, we can see the width of Si nanostripe is around 650 nm surrounded by two 10 � 30*μm* etched regions. Wrinkles and missing regions are inevitably formed during the transfer and lift-off processes. Fortunately,

these regions can be avoided in the following measurements.

**82**

**Figure 5a** indicates different locations we measured on bilayer and monolayer WS2. From the PL emission spectra of monolayer WS2 as shown in **Figure 5b**, one can conclude Si nanostripes can increase the PL intensities but less than threefold

compared with that in the suspended region. It should be noticed that the

*pτ*2

*pτ*

<sup>1</sup> <sup>þ</sup> *<sup>ω</sup>*2*τ*<sup>2</sup> (3)

*<sup>ω</sup>* <sup>1</sup> <sup>þ</sup> *<sup>ω</sup>*2*τ*<sup>2</sup> ð Þ (4)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

*=mCε*∞*ε*<sup>0</sup>

. *τ* is

*Ne*<sup>2</sup>

q

*Experimental design for the WS2-Si nanostripe hybrid structure. (a) Schematic illustration of the electrically controlled device and the cross section of the SiO2-coated SOI substrate. (b) Dark-field backward scattering of the fabricated Si nanostripe. Inset: The dark-field optical image. (c) The bright-field optical image of a typical device. Monolayer and bilayer regions are labeled as 1L and 2L. (d) The corresponding SEM image.*

#### **Figure 5.**

*PL enhancements in monolayer and bilayer WS2. (a) The optical image showing the detection points on monolayer (M) and bilayer (B) WS2. (b, c) PL spectra of different positions marked in (a). The locations of exciton (A) and trion (A*�*) states are labeled, along with the range of Mie resonant modes (marked by red stripe).*

unpatterned region can also enhance the PL intensity and the enhancement performance is better than that on the Si nanostripe. For bilayer WS2, the PL enhancement is much more significant as shown in **Figure 5c**. The bilayer WS2 on the Si nanostripe possesses nearly 10 times larger PL intensities than the suspended area. Interestingly, this PL enhancement was only observed at λ ¼ 625 nm where direct bandgap transition happens. However, for unpatterned area, PL intensities at both direct and indirect transition wavelengths (λ ¼ 635&750 nm) are enhanced. The performances of PL enhancement for direct transition are comparable for unpatterned area and the Si nanostripe. Moreover, unpatterned area can strengthen the indirect transition more than 10 times. Besides the differences of PL intensities, the line shapes also change in **Figure 5b** and **c** which reveals the conversion between exciton (A) and trion (A�). The locations of exciton and trion emission are labeled in **Figure 5b** and **c**. Further peak fitting demonstrates the decreased trend of trion emission from suspended area and nanostripes to the unpatterned region. Substrate effect and the optical resonant modes may be two reasons that lead to the change of line shapes. The Fabry-Perot mode in the unpatterned substrate and Fabry-Perot mode assisted by Mie resonances in fabricated nanostripes can both influence the PL line shapes.
