Nanotechnologies / Materials
Optoelectronics, All-optical switch, Super resolution imaging

Giant nonlinear optical scattering by Si nanoresonator
Department of Applied Physics, Graduate School of Engineering
Professor Junichi Takahara
Researchmap https://researchmap.jp/read0185115?lang=en

Abstract
It is found that extremely large nonlinear optical scattering of
100,000 times compared to natural bulk values occurs in an individual nano-optical resonator of single crystalline silicon (c-Si), andthis effect is caused by photothermal effect by Mie resonance. Asa result, it is possible to use nonlinearity of Si with continuous wave
light without using ultrashort light pulse. This is expected to be applied to all-optical switching devices, super-resolution imaging, etc.
in silicon photonics elements.

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Nanotechnologies / Materials▍ Background & Results
In order to control light with light, it is necessary to increase optical nonlinearity of materials. In order to enhance the nonlinearity ofSi, research has been conducted to strongly confine light into various nanostructures of such as micro-ring resonators or photonic
crystals etc. Since above structures are complicated, more simple
methods were required.
In this study, we focused continuous laser beam to a cubic Si
nanoresonator (Fig. 1a) to investigate its scattered spectrum experimentally. We have found that nonlinear optical scattering that saturates the scattered light intensity when a light beam power of 105 W/ cm 2 is applied to the resonator (Fig. 1b). This power density is 105
times smaller than that of bulk Si. In addition, observed saturation
phenomenon was reversed to supersaturation depending on thesize of the resonator (Fig. 1c).
If the size of a nanostructure is the same order of the wavelengthof the incident light, Mie resonance occurs at specific wavelengths.
At the resonance, light is strongly scattered and confined into a
resonator to cause a rapid temperature rise over several hundred
Kelvin. As a result, refractive index changes rapidly due to photothermal effect, and the scattering intensity is deviated largely fromthe linearity. On the other hand, since the volume of the resonatoris smaller than 0.001 μm 3, heat is immediately escaped to thesubstrate when the laser is turned off. As a result, the scattering
intensity of signal light of 543 nm can be modulated in ON / OFF
switching by controlled light with 592 nm (Fig. 2).

Fig.1

Fig.2

▍ Significance of the research and Future perspective
The fact that natural nonlinear optical constant of Si is significantly amplified has an impact to silicon photonics. Conventionally,
the thermal optical effect has been considered to be slow, butsince the relaxation time of this effect is confirmed to be the orderof nano (10-9) seconds, it can operate at high speed at 10 GHz in
the future. Hence, this product is expected to be applied to lightcontrol devices, super-resolution imaging (Fig. 3), etc., including
all-optical switches in silicon photonics.

Fig.3

P a t e n t
Tr e a t i s eTang, Yu-Lung: Yen, Te-Hsin; Nishida, Kentaro et al. Mie-enhanced photothermal/thermo-optical nonlinearity and applications on all-optical switch andsuper-resolution imaging [Invited]. Opt Mater Express. 2021; 11(11): 3608-3626. doi: 10.1364/OME.431533
Duh, Yi-Shiou; Nagasaki, Yusuke et al. Giant photothermal nonlinearity in a single silicon nanostructure. Nature Commun. 2020; 11: 4101. doi: 10.1038/
s41467-020-17846-6

URLK e y w o r d Mie resonator, Kerr effect, silicon photonics, all-optical switch