Theoretical Study of the Optical Manipulation of Semiconductor Nanoparticles under an Excitonic Resonance Condition

We theoretically study the mechanical interaction between radiation and a semiconductor nanoparticle, based on a microscopic model of confined excitons. The exerted force is calculated by using the Maxwell stress tensor expressed in terms of the microscopic response field. The numerical demonstrations clarify the following: (1) The enhancement of the force by using electronic resonance is remarkable for a particle with a radius of less than 100 nm for semiconductor materials. (2) The spectral peak position of the exerted force is considerably sensitive to nanoscale-size changes which would be useful for highly accurate size-selective manipulation.

Figure 1

(a),(b) Frequency dependence of the radiation force. (c) Size dependence of the maximum value of the acceleration induced by radiation force in 0–4 eV. In (a)–(c), “Non-res.” means the results in the absence of the resonance term in Eq. (1). In (c), the gravitational acceleration is also shown.

Figure 2

(a) Frequency dependence of the radiation force. (b) The scattering and absorption cross sections (normalized by $\pi R^2$). (c),(d) Frequency dependence of the radiation force near the peak of the second-lowest level of the TM mode exciton (the most prominent one in the spectrum). The scattering and absorption cross sections are shown together.

Figure 3

The spatial distribution of the absolute value of the electric field for a 50 nm radius. The direction of polarization is normal to the $xz$ plane. A particle occupies the inside of the white circle in each figure. (a) The considered geometry. (b) $\hslash \omega =3.199(\mathrm{e}\mathrm{V})$. (c) $\hslash \omega =3.2023(\mathrm{e}\mathrm{V})$. (d) $\hslash \omega =3.2057(\mathrm{e}\mathrm{V})$.

Figure 4

The force spectra in the case where a particle is irradiated by a standing wave field. The considered geometry is explained in (a). The parameter values for ZnO are ${ϵ}_b=4.0$, $\hslash {\omega }_T=3.37525(\mathrm{e}\mathrm{V})$, ${\Delta }_{LT}=2.16(\mathrm{m}\mathrm{e}\mathrm{V})$, $M_{\mathrm{e}\mathrm{x}}=0.87m_0$, $\Gamma =20(\mu \mathrm{e}\mathrm{V})$.