Making an opaque medium transparent has been of long-term interest for many researchers. For example, a single atom can be made transparent for an on-resonance light probe in a process known as electromagnetically induced transparency (EIT), a phenomenon resulting from the destructive interference between excitation pathways to the upper level. A further interesting effect of induced transparency is not only the transparency itself, but also the large dispersion at the point of minimal absorption.

Left, a free-space beam coupled to a chaotic optical microcavity where the high-Q cavity mode distribution is displayed in the bottom; Right, two typical transmission spectra showing induced transparency.

Recently, a team led by Professor Yun-Feng Xiao and Professor Qihuang Gong at Peking University reported a way to make a chaotic microcavity transparent for a laser beam. Their experimental results were published in Laser & Photonics Reviews 7, L51-L54 (2013). The narrow transparency peaks have been observed in the transmission spectra, revealing the transparency of chaotic scattering. The brand-new induced transparency is attributed to the destructive interference of two optical pathways: one is to directly excite the continuous chaos from the incident beam, and the other excites the high-Q mode coupling back to the chaos.

Chaos-assisted tunneling plays the key role to make the microcavity transparent. In the world of classical physics, the Fresnel’s law can predict the motion of rays in a microcavity. However, this chaos-assisted tunneling violates the classical law of ray reflection and represents a formal analogue to dynamical tunneling, which is known as a pure quantum mechanical phenomenon. Therefore a free-space beam can indirectly excite the high-Q modes of the microcavity even without phase matching. In particular, this kind of dynamical tunneling produces a Pi phase shift when chaotic light couples to the high-Q mode and returns, because the high-Q mode can be regarded as potential barrier. In tunneling-induced transparency, a steep normal dispersion appears around resonance. It may open up new possibilities in optical information processing, such as a dramatic slow light behavior and a significant enhancement of nonlinear interactions.