The ability to confine and store optical energy in small volumes using resonators has far-reaching implications for both practical devices (optical telecommunications) and for fundamental studies such as cavity quantum electrodynamics (QED).
The figure of merit for a resonator- the quality factor Q- can exceed 109 in silica microspheres and 108 in microtoroids, which result in long photon-storage lifetimes. This number is equivalent to an acoustic bell, which after being struck, rings for more than 15 minutes. In a high-Q microsphere light orbits around in paths along the equator, typically >100000 times before leaving the cavity. This leads to a resonant enhancement of the input optical field, allowing power magnification factors upwards of 100000. By coupling this with the tiny physical size of microcavities, gigantic circulating intensities can be reached with modest input power. The potential of high-Q microresonators for accessing nonlinearities at low input power has been known for some time, however the only experimental demonstrations have been in liquid microdroplets, which have obvious technological disadvantages. Our group has demonstrated a micrometer-scale, nonlinear optical source (Raman laser) based on two different types of silica microcavities: the silica microsphere and the silica microtoroid. The microsphere Raman laser, as reported in Nature (see publication list) attains highly efficient pump-signal conversion (higher than 35%) and pump thresholds nearly 1000 times lower than ever shown before.
This device consists of a high-Q silica microsphere coupled to a fiber-optic taper (see inset of the figure above). This system enables a large reduction in the necessary threshold pump power, with the added benefit that fiber-coupling provides a convenient method of optical field transport. Although a single Raman laser is investigated in this work, the ability to fiber-couple should enable easy scaling to multiple resonant systems along a single fiber. By careful tailoring of both the microsphere and taper properties (and additionally control of the coupling between the taper and microsphere), it is possible to change the lasing properties of the device. The below figure illustrates lasing on a single mode with high conversion efficiency.
This method may represent a route to compact, ultra-low threshold sources for numerous wavelength bands that are usually difficult to access. Equally important, this system can provide a compact and simple building block for studying nonlinear optical effects and the quantum aspects of light. There are several potential avenues that can impact fundamental studies. First, opportunities to explore cavity QED effects in small (sub-20 micron) microsphere samples exist. Additionally, the ability to achieve ultra-low-loss coupling between the resonator and the fiber taper suggests that compound resonant systems could be studied by attachment of multiple resonators along a single fiber taper. Furthermore, additional nonlinearities including four-wave-mixing were also observed in these systems. These nonlinearities can be accessed in a compact volume through a nearly ideal field transport channel (optical fiber) and field coupling junction (the taper). As such, this system can be viewed more broadly as a building block for study of a host of nonlinearities within a high-Q silica resonator and potentially for generation of non-classical photon states and their efficient transport. The ability to load or suitably modify spheres using dopants or quantum dots could also be useful in such studies.
The silica microtoroid Raman laser had threshold values less than 100 microwatts, and conversion efficiencies greater than 45%.
More information can be found in the following paper:
Sean Spillane, Tobias Kippenberg, and Kerry Vahala