image: a. After passing through the single-mode fiber and grating coupler, the probe light is coupled into the silicon microdisk via evanescent coupling from the Si waveguide. b. When a 420 nm nanosecond laser is focused onto the top surface of the microdisk, local refractive index changes (∆n₁ and ∆n₂ resulted from FCA and thermal effect, respectively) induce the shifting of the resonance frequencies. For a fixed probe wavelength, the shifts of resonances lead to two opposite changes of transmittance ∆T₁(x, y) and ∆T₂(x, y) that can be measured at the output port. view more
Credit: by Shuai Wang, Shuai Liu, Yilin Liu, Shumin Xiao, Zi Wang, Yubin Fan, Jiecai Han, Li Ge and Qinghai Song
The conventional study of mixed-phase space in optical whispering gallery mode (WGM) microcavities has been focusing on its ancillary signatures using their inputs and the outputs. Directly interrogating the dynamics and associated mode patterns in experiments is quite challenging. The acknowledged techniques include, for example, comparing the experimentally observed free spectral range (FSR) and far-field radiation patterns with the corresponding theoretical predictions. However, such comparisons usually lead to educated conjectures instead of conclusive affirmation. This is especially more crucial to study quantum chaos of deformed optical microcavities, whose mode groups may have similar FSRs and far-field patterns. Thence, the association of internal mode patterns with experimentally observed resonances is highly desirable.
In a new paper published in Light Science & Application, a team of scientists, led by Qinghai Song and Shumin Xiao from Harbin Institute of Technology Shenzhen), China, Li Ge from the City University of New York, USA, and co-workers have developed a simple, robust and contactless architecture to rapidly map the field patterns in silicon microdisks. Compared with other attempts regarding visualizing optical modes inside a cavity, this method allows observing resonant modes with drastically different dynamics, covering phase space structures such as unbroken Kolmogorov-Arnold-Moser (KAM) curves, stable periodic motions, and the chaotic sea.
A significant obstacle to studying chaos-assisted tunneling in the optical domain is our inability to see and verify this physical process with absolute certainty. This embarrassing fact was embedded in the conventional measurement schemes of optical microcavities, which is an ideal platform to study chaos-assisted tunneling and other quantum chaotic behaviors. Usually, these existing schemes focus on verifying the ancillary signatures of chaos-assisted tunneling by the optical inputs and the outputs, instead of directly interrogating the dynamics inside the microcavity. The authors addressed the above dilemma using a novel approach based on local index perturbation that these dynamics and the associated mode patterns with unprecedently assurance can be experimentally observed.
The scientists summarize the operational principle of their novel technique:
“The setup utilizes the conventional measurement scheme for on-chip integrated silicon resonators, with a crucial addition of a nanosecond laser at 420 nm normally focused on the microdisk through a 50× objective lens. The probe laser is injected into and out the bus waveguide through single-mode fibers and grating couplers, whose wavelength is slightly blue tuned to the resonance pending test. When the nanosecond laser spot is focused on the silicon cavity, the induced free-carrier absorption (FCA) as well as thermal heating, will lead to local refractive index changes of silicon. That can result in significant resonant wavelength variations if the laser spots have a strong overlap with the optical mode. Otherwise, the resonant wavelength is barely affected, e.g. in the middle of the microdisk or far away from the microdisk. By performing a two-dimensional scan on the microdisk surface with a fixed pump density while measuring the corresponding resonant wavelength variations, the resonant mode distributions inside the cavity can be exactly readout. Moreover, instead of measuring this wavelength shift directly, an easier approach is to measure the change of the transmission intensity at a fixed wavelength within the resonant linewidth, set by the probe laser close to each unperturbed resonance.”
“Using this technique, we have mapped different resonant mode families in both circle and deformed microcavities. We then applied this technique to confirm the CAT process with unprecedented assurance, as we can directly see the mode distributions instead of prediction. Meanwhile, in contrast to the conventional belief of chaotic system, the time-reversal process of CAT has also been experimentally verified for the first time.” They added.
“These findings, as well as the technique itself, can significantly push us to find more beautiful physics of quantum chaos. In addition, we also expect new directions to emerge in the study on light-matter interactions in optical microcavities as a result.” the scientists forecast.
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Journal
Light Science & Applications