Sensors based on microscopic optical resonators and arrays of such resonators have been proposed for detecting trace amounts of specific molecules in small gaseous, liquid, and solid samples. In addition to compactness, these sensors would offer ultrahigh sensitivity - in some cases, enough sensitivity to detect single molecules. These sensors could be especially useful in biochemical and biomedical applications.

The proposed sensors would be enhanced versions of the sensors reported in "Microsphere and Microcavity Optical-Absorption Sensors" (NPO-21061), NASA Tech Briefs, Vol. 25, No. 4 (April 2001), page 12a. To recapitulate:

The transducer in a sensor of this type is a fiber-optic-coupled optical resonator in the form of a transparent microspheroid or a microcavity optically equivalent to a microspheroid. Resonance is achieved through grazing-incidence total internal reflection in one or more "whispering-gallery" modes, in which light propagates in equatorial planes near the surface, with integer numbers of wavelengths along closed circumferential trajectories. In the absence of external influences, and assuming that the microspheroid or microcavity is made of a low-loss material, the high degree of confinement of light in whispering-gallery modes results in a high resonance quality factor (highQ).

Suppose that the resonator is illuminated by laser light at its resonance wavelength and is immersed in a sample liquid or gas that has an index of refraction less than that of the resonator material and contains a highly diluted chemical species of interest that absorb light at the resonance wavelength. In that case, theQ of the resonator is diminished through absorption by molecules of that species in the evanescent field of the whispering-gallery modes. Because of the smallness of microresonators (typical diameters from tens to hundreds of optical wavelengths), the smallness of the effective volumes of the evanescent fields (as small as 10-10 cm3), and the low level of optical losses intrinsic to resonators themselves, it is possible to detect very small amounts of optically absorbing chemical species through decreases in Q; calculations have shown that in some cases, it should be possible to detect amounts as small as single atoms or molecules. This completes the recapitulation of information from the cited prior article.

The basic principle of operation as described thus far does not, by itself, afford the sensitivity and selectivity needed to detect a specific chemical species in a quantity as small as a single molecule. What is needed to realize the desired capability is a means of obtaining optical absorption and/or emission at one or more specific wavelengths characteristic of transitions between energy levels of the molecule of interest. Toward this end, the present proposal calls for one or more of the following enhancements:

  • The molecules of interest could be marked by use of fluorescent dye molecules that site-specifically bind to them. The microresonator for use in this case would be one that was designed to resonate at a wavelength at which the dye fluoresces. Provided that the microresonator had a Q of at least 109 in the absence of the fluorescent dye and the species of interest, it should be possible to detect the absorption of resonator light by a single tagged molecule. The emission of light from the fluorophore could serve, in addition to the absorption-induced decrease in Q, as an amplified indication of the molecular specifics of interest.
  • A microresonator would be coated with fluorophores that bind to the molecules of interest. The thickness of the fluorophore coat (typically less than 100 nm) would be small enough that the coat would not significantly alter the Q of the resonator in the absence of the molecules of interest and would be less than the characteristic decay length of the evanescent field. Once a molecule of interest became bound to the coat, the Q of the resonator would change, indicating the presence of the molecule.
  • A fluorophore coat like the one described in the preceding paragraph could be applied at an interface between an optical fiber and a microresonator, instead of over the surface of the resonator. In this case, the fluorophore coat by itself would exert little effect on the coupling between the optical fiber and the resonator, but in the presence of a molecule of interest, the optical-transmission characteristic of the sensor would change measurably.

Going a step further, a sensor according to the proposal could be constructed as an array of multiple fiber-optic-coupled microresonators microfabricated on a silicon or other substrate. The microfabrication of the sensor array on the substrate could include etching of channels to support flows of sampled fluids. Each microresonator could be coated with a fluorophore that binds to a different molecule of interest, so that the sensor could detect many molecular species of interest simultaneously.

This work was done by Lutfollah Maleki and Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21239.

This Brief includes a Technical Support Package (TSP).
Enhanced Optical Microresonators for Detecting Molecules

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This article first appeared in the November, 2001 issue of Photonics Tech Briefs Magazine.

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