Stefan Haacke

Photonic jet driven non-linear optics: example of two-photon fluorescence enhancement by dielectric microspheres.

The two-photon excited fluorescence from a dye solution is enhanced when a small amount of micro-meter sized silica beads are added. This observation is made in the simple scattering regime (inter-sphere distance four times larger than their radius) and is shown to depend on the concentration of the silica spheres. For a solution of rhodamine B, the enhancement can reach more than 30 %. As complementary experiments show that the fluorescence efficiency is unchanged, we argue that the non-linear absorption is enhanced due to focussing of the incident beam in the near-field of the spheres, a situation previously referred to as photonic (nano-)jets [3]. Our calculations indeed show that for the parameters of the spheres studied near-field focussing leads to an intensity concentration close to the sphere surface. We suggest that these photonic jets could be used to enhance other non-linear optical effects.

Broadband ultrafast spectroscopy using a photonic crystal fiber: application to the photophysics of malachite green

Femtosecond single-pulsed supercontinua (SC) are produced in a short sub-cm piece of photonic crystal fiber. The SC span from 450 nm to more than 1.1 mum with 1-nJ energy injection. UV light down to 340 nm is observed with increased injection power. Using such a single-pulsed SC we implemented a compact transient absorption spectrometer with broadband detection and 150-fs FWHM time resolution to monitor the ultrafast dynamics of the electronic states of malachite green in ethanol excited to the S(2) state. The full spectral evolution is observed from 450 nm to 1050 nm, with high sensitivity and a signal-to-noise ratio as high as 1000.

Impact of Donor Chemical Design on Charge Recombination in Donor-Acceptor Oligomers

Transient absorption spectroscopy is applied to study CT state recombination in a family of self-assembling conjugated D-A oligomers. It reveals that chemical design prolongs the CT state lifetime with benefits for photovoltaic cells.

Towards A Compact Femtosecond Spectrometer Based on Photonic Crystal Fibers with Probe Light in the Near-UV

This work has built a compact broad-band femtosecond spectrometer based on a probe light supercontinuum generated in a birefringent photonic crystal fiber. The near-ultraviolet spectrometer system is based on a Ti:sapphire oscillator with extended cavity operating at 27 MHz and delivering 40 fs pulses of 15-18 nJ energy.

Two-photon absorption enhancement using photonic jets

We point out theoretically and experimentally the possibility to enhance a non-linear effect by using photonic jets. The new possibility to highly concentrate visible light with micrometric dielectric spheres is illustrated simulating the situation in water. The photonic jet is a highly energetic beam with special properties that deviate from geometrical optics. Its ability to enhance non-linear effects is experimentally demonstrated by studying the two-photon excited fluorescence of rhodamine B.