Localization of light in an optical microcapillary induced by a droplet

Abstract

Sensing with optical whispering gallery modes (WGMs) is a rapidly developing detection method in modern microfluidics research. This method explores the perturbations of spectra of WGMs propagating along the wall of an optical microcapillary to characterize the liquid medium inside it. Here we show that WGMs in a silica microcapillary can be fully localized (rather than perturbed) by evanescent coupling to a water droplet and, thus, form a high quality-factor microresonator. The spectra of this resonator, measured with a microfiber translated along the capillary, present a hierarchy of resonances which allow us to determine the size of the droplet and variation of its length due to the evaporation. The resolution of our measurements of this variation equal to 4.5 nm is only limited by the resolution of the optical spectrum analyzer used. The discovered phenomenon of complete localization of light in liquid-filled optical microcapillaries suggests a new type of microfluidic photonic devices as well as an ultraprecise method for microfluidic characterization.

Optical bottle microresonators

“…(f) -Femtosecond laser inscription [49]. (g) -A BMR induced by a droplet in the microcapillary [28]. (h) -Depositing glass with lower melting temperature [47].…”

Tunable Near‐ to Far‐Infrared Optical Breakdown in Nonlinear Interactions of Ultrashort Laser Pulses with Water Microdroplets in Ambient Air

“…We note that the number of modes excited by the ultrashort pulse is limited by the moderate quality factor of the resonances Q . This excludes the excitation of whispering‐gallery modes [ 8,9 ] and other high‐quality resonances [ 48 ] with factors as high as . The latter require huge numbers of optical cycles and laser pulses of >100 ps duration.…”

“…Water droplets are omnipresent in biophysical environments and ecosystems, playing an important role in communication through the atmosphere (i.e., aerosols and contaminants, fog, mist, haze, drizzle, clouds) and being involved in a variety of physicochemical reactions and biological processes (respiratory droplets due to breathing, talking, coughing, etc., biological cells, bioaerosols, water clusters, surface adhesion, and wettability). Broad opportunities in analysis, detection, and control over droplet distributions are enabled by the application of powerful lasers used for laser breakdown, [ 1–3 ] electron photoemission [ 4–7 ] and cavity‐enhanced droplet spectroscopy, [ 8,9 ] long‐distance optical communications, [ 10–12 ] and high harmonic generation from water microdroplets. [ 13–15 ] The processes of ultrashort laser excitation and ionization of water droplets and aerosols remain poorly explored for a wide range of photon energies and wavelengths from visible to long‐wave infrared (IR) range.…”

Fundamental limit of microresonator field uniformity and slow light enabled ultraprecise displacement metrology

“…, is zero at this segment (see Supplementary Material in [9] where 𝛽𝛽 𝑚𝑚𝑚𝑚𝑚𝑚 for radially nonuniform 𝑛𝑛 𝑟𝑟 (𝜌𝜌) was determined). Analogous to the design shown in Fig.…”