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Applied Physics Express

Photothermal effect in graphene-coated microsphere resonators

Chenzhi Yuan, Wei Zhang, Yidong Huang

Published Jun 19, 2018• 2 citations
Publisher

Abstract

In this study, graphene-coated silica microsphere resonators with Q values of ∼10 5 are fabricated. After wet transfer of a graphene layer onto the surface of the microsphere, the deterioration of the Q value due to the graphene layer is overcome by a laser treatment. The photothermal effect in the fabricated resonators is investigated experimentally. Using a control-probe scheme, the resonance shift by the photothermal effect is demonstrated, with a high tuning slope. Optical bistability is also observed, with a low threshold and high switching contrast. The experimental results show that the graphene-coated microsphere resonator is a promising platform to investigate the interaction between graphene and light field.

DOI
10.7567/apex.11.072503

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In this study, graphene-coated silica microsphere resonators with Q values of ∼10 5 are fabricated. After wet transfer of a graphene layer onto the surface of the microsphere, the deterioration of the Q value due to the graphene layer is overcome by a laser treatment. The photothermal effect in the fabricated resonators is investigated experimentally. Using a control-probe scheme, the resonance shift by the photothermal effect is demonstrated, with a high tuning slope. Optical bistability is also observed, with a low threshold and high switching contrast. The experimental results show that the graphene-coated microsphere resonator is a promising platform to investigate the interaction between graphene and light field.
Silica microspheres behave as efficient optical microresonators when small-volume whispering-gallery modes (WGMs) are used. We have realized a cryogenic set-up to work with microspheres immersed in a superfluid helium bath. Quality factors up to 109 have been obtained at 2 K. In this environment, we have been able to observe a dispersive bistable behavior of the WGM resonances, due to the weak intrinsic Kerr optical nonlinearity of silica, with a threshold power of 10 (mu) W only. This result opens the way to the realization of thresholdless microlaser, based on a rare-earth doped silica microsphere and to other Cavity-QED projects with microspheres at low temperatures.
A kind of graphene-based perfect absorber which can generate low-threshold and high-extinction-ratio optical bistability in the near-IR band is proposed and simulated with numerical methods. The interaction between input light and monolayer graphene in the absorber can be greatly enhanced due to the perfect absorption. The large nonlinear coefficient of graphene and the strong light-graphene interaction contribute to the nonlinear response of the structure, leading to relatively low switching thresholds of less than 2.5 MW/cm2 for an absorber with a Q factor lower than 1000. Meanwhile, the extinction ratio of bistable states in the absorber reaches an ultrahigh value of 47.3 dB at 1545.3 nm. Moreover, the influence of changing the structural parameters on the bistable behaviors is discussed in detail, showing that the structure can tolerate structural parametric deviation to some extent. The proposed bistable structure with ultra-compact size, low thresholds, high extinction ratio, and ultrafast response time could be of great applications for fabricating high-performance all-optical-communication devices.
In nonlinear plasmonics, the switching threshold of optical bistability is limited by the weak nonlinear responses from the conventional Kerr dielectric media. Considering the giant nonlinear susceptibility of graphene, here we develop a nonlinear scattering model under the mean field approximation and study the bistable scattering in graphene-coated dielectric nanowires based on the semi-analytical solutions. We find that the switching intensities of bistable scattering can be smaller than 1 MW cm−2 at the working frequency. To further decrease the switching intensities, we show that the most important factor that restricts the bistable scattering is the relaxation time of graphene. Our work not only reveals some general characteristics of graphene-based bistable scattering, but also provides a guidance to further applications of optical bistability in the high speed all-optical signal processing.
A proposal on optical bistability at ultralow switching threshold and lower Fermi-level of graphene around 2 THz is implemented analytically through the proposed long-range surface plasmon resonance configuration by employing local field enhancement effects owing to the excitation of a graphene symmetric mode within graphene sheets. Reported threshold intensity for the optical bistability to date is 1.6 kW/cm2 within the terahertz region and 1.83 MW/cm2 at near-IR range. Whereas the proposed scheme explores the possibility of reducing this threshold down to 143.68 W/cm2, this technique proffers potential applications in nanoillumination, optical memory, and all-optical switching at an ultra-low threshold.
In this work, based on a dual-laser pumping scheme, an all-optical controlled coupled-mode induced transparency (CMIT) in a silica microsphere resonator (MSR) has been demonstrated, experimentally and theoretically. This proposed scheme is stable, simple and power efficient. More interestingly, by changing the power of one of the lasers, which functions as the heat source, the wavelength shifting rate of the CMIT can be changed. Our experimental results reveals that when the power of the laser acting as the heat source increases from 1 mW to 5 mW, the wavelength shifting rate experiences a nearly two times increase.
In this paper, we propose a modified Otto configuration to realize tunable and low-threshold optical bistability at terahertz frequencies by attaching multilayer graphene sheets to a nonlinear substrate interface. Our work demonstrates that the threshold of optical bistability can be markedly reduced (three orders of magnitude) by covering the nonlinear substrate with multilayer graphene sheets, due to strong local field enhancement with the excitation of surface plasmons. We present the influences of the Fermi energy of graphene, the incident angle, the thickness of air gap and the relaxation time of graphene on the hysteresis phenomenon and give a way to optimize the surface plasmon resonance, which will enable us to further lower the minimal power requirements for realizing optical bistability due to the strong interaction of light with graphene sheets. These results are promising for realization of terahertz optical switches, optical modulators and logical devices.