Microsensors on the Fly

Abstract

Optically driven microresonators freely floating in liquids, embedded into transparent solid media, or even taken up by life cells, can be exploited as optical microsensors, which yield precise information on the immediate local condition of their respective environment. Besides plain physical information, such as on local refractive indices, mechanical stress, and mixture ratios in composite materials, the sensors may be applied as chemical and even bio-sensors for the specific targeting of biochemical binding reactions if accordingly functionalized. Sensitivity can be further improved by driving the sensors above the lasing threshold, thereby increasing signal intensity and optical resolution at the same time. In this article, we introduce the optical principles, present useful implementations, and illustrate feasibility of this still novel approach to optical sensing by a number of diverse examples.With the advent of the laser diode, microresonators capable of confining light to microscopic volumes have entered everyday's life with applications as laser pointers, light sources in CD, DVD and Blu-ray disk drives, and do-it-yourself metrology tools, to name just the most common examples. Intrinsically shaped into their gain medium, these semiconductor devices can achieve extremely small sizes in the micrometer regime, however, since they are electrically powered, they require a fixed connection to the macroscopic world in the form of some electrical driver, which still renders the entire device macroscopic.Here, we report about optically driven microresonators that overcome the need for fixation to a macroscopic power supply and thus turn into microsensors of 3D quantum system characterized by quantum numbers q, n, m for radial, azimuthal, and polar quantization and polarizations TE, TM (transverse electric / transverse magnetic modes, respectively). Fig. 1 (a) Illustration of a whispering gallery mode with quantum numbers q = 3 (three radial maxima), n = 44 (no. of wavelengths along circumference), and m = n (1 polar maximum); (b) Cross section in radial direction showing the field distribution of the WGM of (a); please note the exponential decay outside of the microresonator, which is a peculiarity of WGM resonators; (c) Fluorescence emission spectra of 10 µm coumarin 6G doped polystyrene microbeads as recorded with a set up as sketched in Fig. 2c; the spectrum plotted in blue origi nates from a spherical bead and exhibits several WGMs with q = 1 and several quantum numbers n (distinguished by a different number of wavelengths fitting into the circumference of the bead). For each quantum number n, there is a pair of modes with differing polariza tion (TE, TM) as indicated in the spectrum. The spectrum plotted in red stems from a particle stained with the same fluorophore, but with an odd shape, so that WGMs are not supported. By subtraction of the two spectra, the non resonant fluorescence background can be removed and the pure WGM spectrum is obtained (spectrum in green, Data of (c) taken from [6]).

Fluorescent and lasing whispering gallery mode microresonators for sensing applications

“…Solid fluorescent microsphere resonators are commonly fabricated from polymers due to their low cost, structural flexibility and ease of fabrication and integration of a gain medium . Beyond the straightforward approach of using a single microsphere resonator, either trapped with optical tweezers or simply deposited onto various substrates , there has been interest in combining fluorescent microspherical resonators with capillaries , or microstructured optical fibers (MOF) , as shown in Fig. (a) and (b).…”