**3. Hybrid nanoantennas based on silicon nanoparticles and nanostripes**

#### **3.1 Electrically driven scattering in a hybrid dielectric-plasmonic nanoantenna**

In order to build an electrically controlled silicon nanoantenna, the biggest issue is how to apply voltage on a single nanoparticle and collect the electrically

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

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.

**2. Electrically controlled optical responses of silicon-based**

into electro-optical modulation. This chapter can be divided into two parts: First, we demonstrated the electrically tunable scattering of a single silicon nanoparticle at visible wavelengths. To build the nanoantennas, gold interdigital electrodes with separation distances between 100 and 200 nm were fabricated using photolithography and focused ion beam (FIB) milling. After trapping silicon nanoparticles with different sizes between adjacent two electrodes, the scattering spectra under different voltages can be measured. Interestingly, the scattering experiences blueshift and obvious intensity attenuation when increasing the applied voltages from 0 to 1.5 V. In theory, MIS (metal-insulator-semiconductor) junctions can be formed at Au-SiO2-Si interfaces [28]. Once the bias voltage increases, the inversion and accumulation effect would produce much more free carriers at interfaces [29–31] and then change the permittivity based on Drude model [32, 33]. The proposed hybrid nanoantennas represent a new method to build optoelectronic

In this chapter, we will discuss the applications of silicon-based Mie resonators

Second, we combined silicon nanostripes, a typical Mie resonator, with WS2 to

**3. Hybrid nanoantennas based on silicon nanoparticles and nanostripes**

**3.1 Electrically driven scattering in a hybrid dielectric-plasmonic nanoantenna**

In order to build an electrically controlled silicon nanoantenna, the biggest issue is how to apply voltage on a single nanoparticle and collect the electrically

realize active PL manipulation. In the proposed electro-optical modulator, suspended monolayer and bilayer WS2 are covered on a Si nanostripe. The Si nanostripe not only acts as a nanoscale gate electrode but also a Mie resonator. For both monolayer and bilayer WS2, the PL intensities on the nanostripes are much stronger than those of the suspended one. After applying gate voltages, both the electrostatic doping and strain come into effect. This new tuning mechanism leads to abnormal control of exciton emission from WS2, which is clearly different from that in previous works [34–36]. Considerable PL tuning can also be observed in bilayer WS2 gated by Si nanostripes. Based on the modulation capability, we believe the proposed electro-optical modulator will bring new possibilities for future

**nanoantennas**

*Applications of Nanobiotechnology*

devices based on Mie resonators.

nanophotonic devices.

**78**

modulated signals with low noise. The design of electrically tunable silicon 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 certain probability to be trapped in the gaps.

Before studying the optical properties of Si-Au hybrid nanoantennas, we should 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 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 Si nanoparticle can be obtained.

For isolated Si nanoparticles, the resonant modes depend on particle sizes and particle numbers according to Mie theory. While for Au electrode-loaded Si

#### **Figure 1.**

*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 localized plasmon and magnetic dipole.*

nanoparticles, the mode coupling between nanoparticles and Au electrodes also needs to be considered. The hybrid nanoantennas may exhibit different scattering spectra at visible wavelengths. Therefore, it is necessary to study the scattering spectra without applied voltage first. Although self-assembled process is random, desirable and representative nanoparticles can be found through matching and positioning. **Figure 1e** shows 180 nm Si nanoparticles between two Au electrodes with a spacing slightly less than 180 nm. The measured scattering spectra exhibits a single broad peak around *λ* ¼ 650*nm*. As shown in **Figure 1f**, the simulated spectrum is very similar to experimental spectrum. Corresponding electric and magnetic field distributions in **Figure 1g** demonstrate the existence of circular magnetic field distributions and strong electric field enhancement at interfaces, which means the scattering peak is generated from the interaction between the Mie-type magnetic dipole mode in Si nanoparticles and the localized surface plasmon resonances (LSPR) at Au-Si interfaces.

*Qacc* j j <sup>¼</sup> *Cox*ð Þ *VG* � *VFB* <sup>∝</sup> exp �*β*Ψ*<sup>S</sup>*

where *Qacc* and *Qinv* are the carrier densities at accumulation and inversion regions. *Cox* is the capacitance of thin insulator layer. *VG* is the applied bias. *VFB* and *VT* are accumulation and inversion effects related to voltages. *β* ¼ *q=kBT* is a constant. Ψ*<sup>S</sup>* is the surface potential at interfaces. Based on the electric field we applied and above calculation, the electron or hole concentrations can be increased by more than three orders of magnitude in accumulation or inversion regions, respectively [29]. The Raman signals under different voltages are presented in **Figure 2c**. With 1.5 V applied bias, the resonant peak of silicon just red shifts slightly, and this weak shift means the temperature variation is less than 100 K [37]. Therefore, we can exclude the influence of thermal effect on the refractive index since the refractive

To examine whether the mechanism discussed above could affect the optical properties significantly, the scattering spectra of the typical silicon nanoantenna with applied bias were presented in **Figure 3a**. To ensure stability, all scattering data were collected in 1 min during the increase of voltage from 0 to 1.5 V. Because the interfaces of the fabricated hybrid nanoantenna are symmetric, we only need to collect the scattering spectra under forward bias which is enough to embody the properties of hybrid nanoantennas. For a typical hybrid nanoantenna as shown in **Figure 3a** with a 180 nm Si nanoparticle, we can observe the suppression of hybrid plasmon-Mie resonant peaks when increasing the voltages. The magnetic dipole peak was dominated when no voltage applies. However, when applied voltage reaches 1.5 V, the electric dipole peak at shorter wavelength becomes the more

As discussed above, different applied voltages result in different free carrier concentrations of Si nanoparticles. Further, we should clarify how carrier injection influences the dielectric function of silicon. The modulation mechanism is based on free carrier-induced refractive index change. Although electric field cannot change the refractive index of bulk silicon or whole silicon nanostructures significantly as previous works reported [39], obvious refractive index modification can be realized at accumulation and inversion interfaces. From the field profiles, one can understand the refractive index change on surface is enough to change optical responses because field enhancements and radiative decays mainly come from interfaces. How free carriers contribute to the refractive index change at interfaces can be

*Electrically controlled scattering. (a) Scattering spectra of the 180 nm Si nanoparticles when applied voltages equal to 0, 0.3, 0.6, 0.9, 1.2, and 1.5 V. (b) The variation trend of the real part of permittivity at Au-SiO2-Si interfaces when increasing the carrier concentrations. (c) The calculated scattering spectra of the 180 nm Si*

index of silicon only increases 3*:*<sup>85</sup> � <sup>10</sup>�<sup>4</sup> per degree [38].

*Electro-Optical Manipulation Based on Dielectric Nanoparticles*

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

prominent one.

**Figure 3.**

**81**

described by the Drude model

*nanoparticle under different carrier concentrations at interfaces.*

2

(2)

The electrical properties of the Si-Au hybrid devices were measured using a semiconductor parameter analyzer. The measured I-V curve is shown in **Figure 2a**, and we can conclude that the Si-Au interfaces can be regarded as Schottky junctions. From 0 to 1.5 V, the current increases nonlinearly with the voltage. For the fabricated Si nanoparticles, thin oxide (1–2 nm) shells will be formed inevitably in the air. Therefore, the interfaces are MIS junctions whose current is generated through tunnel effect and plasmon hot electron injection. For MIS junctions, the band bends upward at interfaces when no voltage applies as the schematic diagram shown in **Figure 2b**. Depletion region forms at the interfaces and free carriers move away from interfaces based on the band bending [29]. With applied bias, surface potential at two interfaces increases. The carrier concentration at the MIS junction under lower potential was greatly increased because the downward energy band realizes the accumulation of electrons. The other MIS junction under higher potential could form an inversion layer if the applied voltage is high enough. When the intrinsic energy level crosses the Fermi level [29–31], the hole density would greatly increase under the inversion state. The charge densities at surface at different applied voltages can be estimated by the following equations [29]

$$|Q\_{inv}| = -C\_{ox}(V\_G - V\_T) \propto \exp\left(\frac{\beta \Psi\_S}{2}\right) \tag{1}$$

#### **Figure 2.**

*Analysis on the voltage-induced carrier injection. (a) The I-V curve of a fabricated Si-Au hybrid structure. (b) Schemes for the band bending and carrier distribution with and without applied voltage. (c) The Raman spectrum of a loaded nanoparticle before and after applied voltage (1.5 V).*

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

nanoparticles, the mode coupling between nanoparticles and Au electrodes also needs to be considered. The hybrid nanoantennas may exhibit different scattering spectra at visible wavelengths. Therefore, it is necessary to study the scattering spectra without applied voltage first. Although self-assembled process is random, desirable and representative nanoparticles can be found through matching and positioning. **Figure 1e** shows 180 nm Si nanoparticles between two Au electrodes with a spacing slightly less than 180 nm. The measured scattering spectra exhibits a single broad peak around *λ* ¼ 650*nm*. As shown in **Figure 1f**, the simulated spectrum is very similar to experimental spectrum. Corresponding electric and magnetic field distributions in **Figure 1g** demonstrate the existence of circular magnetic field distributions and strong electric field enhancement at interfaces, which means the scattering peak is generated from the interaction between the Mie-type magnetic dipole mode in Si nanoparticles and the localized surface plasmon resonances

The electrical properties of the Si-Au hybrid devices were measured using a semiconductor parameter analyzer. The measured I-V curve is shown in **Figure 2a**, and we can conclude that the Si-Au interfaces can be regarded as Schottky junctions. From 0 to 1.5 V, the current increases nonlinearly with the voltage. For the fabricated Si nanoparticles, thin oxide (1–2 nm) shells will be formed inevitably in the air. Therefore, the interfaces are MIS junctions whose current is generated through tunnel effect and plasmon hot electron injection. For MIS junctions, the band bends upward at interfaces when no voltage applies as the schematic diagram shown in **Figure 2b**. Depletion region forms at the interfaces and free carriers move away from interfaces based on the band bending [29]. With applied bias, surface potential at two interfaces increases. The carrier concentration at the MIS junction under lower potential was greatly increased because the downward energy band realizes the accumulation of electrons. The other MIS junction under higher potential could form an inversion layer if the applied voltage is high enough. When the intrinsic energy level crosses the Fermi level [29–31], the hole density would greatly increase under the inversion state. The charge densities at surface at different

applied voltages can be estimated by the following equations [29]

*Qinv* j j ¼ �*Cox*ð Þ *VG* � *VT* ∝ exp

*Analysis on the voltage-induced carrier injection. (a) The I-V curve of a fabricated Si-Au hybrid structure. (b) Schemes for the band bending and carrier distribution with and without applied voltage. (c) The Raman*

*spectrum of a loaded nanoparticle before and after applied voltage (1.5 V).*

*β*Ψ*<sup>S</sup>* 2 

(1)

(LSPR) at Au-Si interfaces.

*Applications of Nanobiotechnology*

**Figure 2.**

**80**

$$|\mathcal{Q}\_{\rm acc}| = \mathcal{C}\_{\rm ox}(V\_G - V\_{FB}) \propto \exp\left(\frac{-\beta \Psi\_S}{2}\right) \tag{2}$$

where *Qacc* and *Qinv* are the carrier densities at accumulation and inversion regions. *Cox* is the capacitance of thin insulator layer. *VG* is the applied bias. *VFB* and *VT* are accumulation and inversion effects related to voltages. *β* ¼ *q=kBT* is a constant. Ψ*<sup>S</sup>* is the surface potential at interfaces. Based on the electric field we applied and above calculation, the electron or hole concentrations can be increased by more than three orders of magnitude in accumulation or inversion regions, respectively [29]. The Raman signals under different voltages are presented in **Figure 2c**. With 1.5 V applied bias, the resonant peak of silicon just red shifts slightly, and this weak shift means the temperature variation is less than 100 K [37]. Therefore, we can exclude the influence of thermal effect on the refractive index since the refractive index of silicon only increases 3*:*<sup>85</sup> � <sup>10</sup>�<sup>4</sup> per degree [38].

To examine whether the mechanism discussed above could affect the optical properties significantly, the scattering spectra of the typical silicon nanoantenna with applied bias were presented in **Figure 3a**. To ensure stability, all scattering data were collected in 1 min during the increase of voltage from 0 to 1.5 V. Because the interfaces of the fabricated hybrid nanoantenna are symmetric, we only need to collect the scattering spectra under forward bias which is enough to embody the properties of hybrid nanoantennas. For a typical hybrid nanoantenna as shown in **Figure 3a** with a 180 nm Si nanoparticle, we can observe the suppression of hybrid plasmon-Mie resonant peaks when increasing the voltages. The magnetic dipole peak was dominated when no voltage applies. However, when applied voltage reaches 1.5 V, the electric dipole peak at shorter wavelength becomes the more prominent one.

As discussed above, different applied voltages result in different free carrier concentrations of Si nanoparticles. Further, we should clarify how carrier injection influences the dielectric function of silicon. The modulation mechanism is based on free carrier-induced refractive index change. Although electric field cannot change the refractive index of bulk silicon or whole silicon nanostructures significantly as previous works reported [39], obvious refractive index modification can be realized at accumulation and inversion interfaces. From the field profiles, one can understand the refractive index change on surface is enough to change optical responses because field enhancements and radiative decays mainly come from interfaces. How free carriers contribute to the refractive index change at interfaces can be described by the Drude model

#### **Figure 3.**

*Electrically controlled scattering. (a) Scattering spectra of the 180 nm Si nanoparticles when applied voltages equal to 0, 0.3, 0.6, 0.9, 1.2, and 1.5 V. (b) The variation trend of the real part of permittivity at Au-SiO2-Si interfaces when increasing the carrier concentrations. (c) The calculated scattering spectra of the 180 nm Si nanoparticle under different carrier concentrations at interfaces.*

$$
\Delta \varepsilon' = -\frac{\varepsilon\_{\infty} a\_p^2 \tau^2}{\mathbf{1} + a^2 \tau^2} \tag{3}
$$

$$
\Delta \varepsilon'' = \frac{\varepsilon\_{\infty} \alpha\_p^2 \tau}{\alpha (1 + \alpha^2 \tau^2)} \tag{4}
$$

where *ω<sup>p</sup>* is the plasma frequency which is defined as *ω<sup>p</sup>* ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi *Ne*<sup>2</sup> *=mCε*∞*ε*<sup>0</sup> q . *τ* is 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 controlled scattering signals on single-particle level.

#### **3.2 PL enhancements enabled by silicon nanostripes**

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.

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

*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*

*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.*

*Electro-Optical Manipulation Based on Dielectric Nanoparticles*

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

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

is much more significant as shown in **Figure 5c**. The bilayer WS2 on the Si

**Figure 5.**

**Figure 4.**

*stripe).*

**83**

**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
