Enhanced Backward-Directed Multiphoton-Excited Fluorescence from Dielectric Microcavities

Abstract

We demonstrate theoretically and experimentally that one-, two-, and three-photon excited fluorescence from dye molecules in spherical microcavities has an asymmetrical angular distribution and is enhanced in the backward direction. The enhancement ratios (of intensities at 180 degrees and 90 degrees ) are 9, 5, and 1.8 for three-, two-, and one-photon excitation, respectively. Even larger ratios are expected for microspheres with an index of refraction larger than that used in the experiments. Because of the reciprocity principle and concentration of the incident wave inside particles, the backward enhancement is expected to occur even with nonspherical particles.

Individual bioaerosol particle discrimination by multi-photon excited fluorescence

“…A significant improvement of our set-up in terms of sensitivity would be the excitation of the particles through the same objective as for the fluorescence collection, like in epi-microscopy. MPEF from individual aerosol particles is indeed strongly enhanced in the backward direction [22,23] (up to a factor 10 for 3PEF). The important anti-Stokes shift between the 790 nm excitation and MPEF would allow efficient rejection at the dichroic beamsplitter.…”

Identification of biological microparticles using ultrafast depletion spectroscopy

“…33,34 This unexpected behavior is extremely attractive for remote detection schemes, such as Lidar applications. Localization is achieved by the non-linear processes themselves, which typically involve the nth power of the internal intensity I n (r) (r for position inside the particle).…”

“…The depletion factor d reaches 80% for Trp droplets and 60% for FMN droplets (to be compared to 50% and 15% in liquids, respectively). Some tentative explanations could be invoked for this unexpectedly high efficiency, but the definitive reason is not clear yet: (1) The spatial overlap between pump and probe pulses might be enhanced by the shape of the droplet; (2) the spherical shape induces hot spots inside the droplet where intensities are up to 100 times higher than the incident one, 33,34 but the total hot spot volume is rather small; (3) there might be some surface effects (orientation of molecules on the surface) that would enhance the two photon absorption.…”

“…The strongly anisotropic MPEF emission was demonstrated on individual water microdroplets containing tryptophan, riboflavin, or other synthetic fluorophors in ethanol. [33][34][35] The experiment was performed with the aerosol source described in Section 2. MPEF angular distribution for the one-(400 nm), two-(800 nm) and three-photon (1, 2 mm) excitation show that the fluorescence emission is maximum in the direction towards the exciting source.…”

Discriminating Biomolecules with Coherent Control Strategies

“…We indeed demonstrated that light emission from airborne particles is enhanced in the backward direction, especially if the process is nonlinear. [21][22][23] Flexibility on the type of bacteria is provided by seeding the thermodenuder with two piezo-nozzles, one dedicated to the spread of the bacteria A (e.g. Gram+) and the other to bacteria B (e.g.…”