High-Q Polymer Microresonators

Recently, there have been several breakthroughs in high-Q and ultra-high-Q microcavity research, including the silica ultra-high-Q (UHQ) microtoroid. While these devices can be used to study nonlinear optical processes, enable low-threshold lasers and nonlinear oscillators, alternative fabrication materials may expand their application base. Polymeric materials offer a wide range of properties from conductivity to tunable fluorescence. In addition, many of these polymers are nearly as transparent as the traditional materials used but are significantly stronger. Planar all-polymer microresonators could be used where the conventional materials fail.

We have demonstrated a process to fabricate polymer high-Q microresonators using replica molding. (Figure 1) Using the ultra-high-Q (UHQ) silica microtoroid as an ideal master, we form both Polydimethylsiloxane (PDMS) and Vicast microresonators from the PDMS mold. This non-destructive fabrication process allows for both the masters and the molds to be reused.

A typical broad band (BB) spectrum of a polymer resonator is shown in Figure 2. It replicates the BB spectrum of its ideal master. Significantly, both the PDMS and Vicast microresonator Q factors are material-loss limited because the fabrication process is able to accurately mold the smooth toroidal surface. This suggest that this process can be used to rapidly test the optical loss of new polymeric materials. As an example, the use of Vicast has previously been restricted to consumer products, but the current results show that it provides exceptional performance with respect to optical loss characteristics. Indeed, the highest Q factors in this study were 5x106 and obtained using Vicast. This value represents a 40X improvement over all prior polymer-based microresonator work.

Another interesting aspect of the polymer microresonators is long term storage. The Vicast microtoroids are able to be stored for several weeks in the mold without degradation in quality factor. Since high-Q microresonators can be sensitive to long term environmental exposure, this feature is an important means by which the "shelf-life" of disposable microresonators can be increased.

Because the replica-molded resonators are material limited, this technique can also be used to determine material optical loss.

More information can be found in the following papers:

A. M. Armani, A. Srinivasan, and K. J. Vahala
"Soft lithographic fabrication of high Q polymer microcavity arrays"
Nano Letters, Volume 7, issue 6, June 2007.

A. L. Martin, D. K. Armani, L. Yang and K. J. Vahala
"Replica-molded high-Q polymer microresonators"
Optics Letters, Volume 29, No 6, 533-535, March 2004.

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.

 

polymer figure 1 copy

Figure 1: Replica molding process flow: a) ultra-high-Q microtoroid master array is fabricated b) master is coated with PDMS to form PDMS mold; cured and removed  from the master, then filled with PDMS/Vicast to form c) PDMS/Vicast replica polymer microtoroid array, d) optical micrograph of a 45 micron diameter PDMS high-Q microtoroid.

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Figure 2: Transmission spectrum for a ~45-micron diameter Vicast polymer microtoroid. The Free Spectral Range (FSR) of the polymer high Q microtoroid is in agreement  with the theoretical prediction of ~11.5nm.