High Q and Ultra-high Q (UHQ) silica optical microresonators can perform as highly sensitive detectors; they derive their excellent transduction abilities from long photon lifetimes within the whispering gallery of the microcavity. Unlike their optical waveguide counterparts, where the photon interacts with a functionalized surface only once, recirculation within the microcavity allows photons to interact with the surface many times (e.g., using a Q of 1x108 and a resonator diameter of 80 mm the photon makes 100,000 orbits). For example, silica microsphere resonators, with a properly sensitized surface, were recently used to distinguish between two strands of DNA. However, while detailed studies have been performed to determine the limits of a resonatorís quality factor in air, no such studies have been performed in water. Because most detection experiments are performed in a water-based solution, it is important to thoroughly understand the impact of this environment on the relationship between the diameter of the microtoroid resonator and the operational wavelengths of interest.
The model used to describe the microtoroid resonators immersed in water accounted for two loss mechanisms: radiation-loss and absorption-loss. The model predicts that the quality factor is dominated by radiation-loss when the microtoroid diameter is small due to reduced refractive index contrast. However, when the microtoroid diameter passes a critical size, the quality factor is limited by the absorption of the environment. This is significantly different from the model which accurately described microtoroid quality factors in air where, at large microtoroid diameters, the quality factor is limited by surface scattering, not absorption. The difference between the two models arises from the fact that air is a significantly less absorbing medium than water.
To this end, UHQ silica microtoroid resonators were fabricated over a wide range (50-250micron) of major toroid diameters using a previously outlined process. Experiments were performed in both water (H2O) and deuterium oxide (D2O) (figure 1). The D2O was purchased from Aldrich. D2O was chosen as the second liquid because it has the same refractive index and, in turn, the same radiation-loss as H2O. However, its absorption at all wavelengths tested is significantly less. This allowed for selective probing of the absorption-loss mechanism and verification of the model developed to describe this system.
The intrinsic Q factors measured in the 680 nm, 1300nm and 1550nm band plotted versus toroid major diameter are presented in Fig. 2-4 (triangular and circular points). Q factors trend to larger values with increasing toroid size. This behavior is in good agreement with predictions of the model (also shown in these figures). The maximum quality factor achieved in H2O in the visible was 2.3x108 and in D2O was 1.3x108. These values are notable as they represent a 100x improvement over previous resonator operation in an aqeuous environment. The highest Q reported previously was 106 in a silica microsphere. Measurements beyond Q factors of 500 million were not possible in this experiment owing to laser linewidth stability. In principle, however, larger toroid diameters should exhibit quality factors as high as 1x109, in water, and 1x1010 in D2O.
Both radiation-loss-limited operation and absorption-loss-limited operation were observed in 1300nm and 1550nm and agreed well with the predictions of a numerical model. However, the quality factors were significantly lower due to the increase in absorption.
More information can be found in the following papers:
A. M. Armani and K. J. Vahala,
"Heavy water detection using ultra-high-Q microcavities"
Optics Letters, Volume 31, issue 12, June 2006.
A. M. Armani, D. K. Armani, B. Min and K. J. Vahala, S. M. Spillane
"Ultra-high-Q microcavity operation in H2O and D2O"
Applied Physics Letters, vol. 87, issue 15, October 2005.
D. K. Armani, T. Kippenberg, S. M. Spillane and K. J. Vahala
"Ultra-high-Q toroid microcavity on a chip"
Nature, vol. 421, pp. 925-929, 27 February 2003.