Cavity-Enhanced Photoacoustic Detection Using Acoustic and Fiber-Optic Resonators

A wineglass has been used as an acoustic resonator to enhance the photoacoustic signal generated by laser excitation of absorbing dyes in solution. The amplitude of the acoustic signal was recorded using a fiber-optic transducer based on a Fabry-Pérot cavity attached to the rim of the wineglass. The optical and acoustic properties of the setup were characterized, and it was used to quantify the concentration of phosphomolybdenum blue and methyl red solutions. Detection limits of 1.2 ppm and 8 muM were obtained, respectively.

Absolute Absorption Cross-Section Measurement of a Submonolayer film on a silica microresonator

Conventional absorption spectroscopy is not nearly sensitive enough for quantitative overtone measurements on submonolayer coatings. While cavity-enhanced absorption detection methods using microresonators have the potential to provide quantitative absorption cross sections of even weakly absorbing submonolayer films, this potential has not yet been fully realized. To determine the absorption cross section of a submonolayer film of ethylene diamine (EDA) on a silica microsphere resonator, we use phase-shift cavity ringdown spectroscopy simultaneously on near-IR radiation that is Rayleigh backscattered from the microsphere and transmitted through the coupling fiber taper. We then independently determine both the coupling coefficient and the optical loss within the resonator. Together with a coincident measurement of the wavelength frequency shift, an absolute overtone absorption cross section of adsorbed EDA, at submonolayer coverage, was obtained and was compared to the bulk value. The smallest quantifiable absorption cross section is σmin 2.7 × 10−12 cm2. This absorption cross section is comparable to the extinction coefficients of, e.g., single gold nanoparticles or aerosol particles. We therefore propose that the present method is also a viable route to absolute extinction measurements of single particles.