Energy efficient generation of white light has become an important issue in recent years. Technology of white-light emitting diodes (LEDs) is one of the promising directions. The main challenges in the LED production are understanding scattering, absorption and emission from ab-initio, and obtain chromaticity independent emission directions. Physical understanding of multiple light scattering in the LED can provide us with simple tools for extracting optical parameters of this system.
We have studied the transport of light through phosphor diffuser plates that are used in commercial solid-state lighting modules (Fortimo). These polymer plates contain YAG:Ce+3 phosphor particles that both elastically scatter light and Stokes shift light in the visible wavelength range (400-700 nm). We excite the phosphor with a narrowband light source, and measure spectra of the outgoing light. The Stokes shifted light is spectrally separated from the elastically scattered light in the measured spectra. Using this technique we isolate the elastic transmission of the plates. This result allows us to extract the transport mean free path ltr over the full wavelength range by employing diffusion theory. Simultaneously, we determine the absorption mean free path labs in the wavelength range 400 to 530 nm where YAG:Ce+3 absorbs. The diffuse absorption (µ_a=1/l_abs ) spectrum is qualitative similar to the absorption coefficient of YAG:Ce+3 in powder, with the diffuse spectrum being wider than the absorption coefficient. We propose a design rule for the solid-state lighting diffuser plates.
Diffusion equation describes the energy density inside a scattering medium such as biological tissues and paint . The solution of the diffusion equation is a sum over a complete set of eigensolutions that shows a characteristic linear decrease with depth in the medium. It is of particular interest if one could launch energy in the fundamental eigensolution, as this opens the opportunity to achieve a much greater internal energy density. For applications in optics, an enhanced energy density is vital for solid-state lighting, light harvesting in solar cells, low-threshold random lasers, and biomedical optics.
Here we demonstrate the first ever selective coupling of optical energy into a diffusion eigensolution of a scattering medium of zinc oxide (ZnO) paint. To this end, we exploit wavefront shaping to selectively couple energy into the fundamental diffusion mode, employing fluorescence of nanoparticles randomly positioned inside the medium as a probe of the energy density. We observe an enhanced fluorescence in case of optimized incident wavefronts, and the enhancement increases with sample thickness, a typical mesoscopic control parameter. We interpret successfully our result by invoking the fundamental eigensolution of the diffusion equation, and we obtain excellent agreement with our observations, even in absence of adjustable parameters .
 R. Pierrat, P. Ambichl, S. Gigan, A. Haber, R. Carminati, and R. Rotter, Proc. Natl. Acad. Sci. U.S.A. 111, 17765 (2014).
 O. S. Ojambati, H. Yilmaz, A. Lagendijk, A. P. Mosk, and W. L. Vos, arXiv:1505.08103.
Light scattering is known for blurring images to the point of making them appear as a white halo. For this reason imaging through thick clouds or deep into biological tissues is difficult. Here we discuss in details a method we developed recently to retrieve the shape of an object hidden behind a diffusing screen.
We present a method to map the absolute electromagnetic field strength inside photonic crystals. We demonstrate our method by applying it to map the electric field component <i>E<sub>z</sub></i> of a two-dimensional photonic crystal slab at microwave frequencies. The slab is placed between two mirrors to create a resonator and a subwavelength spherical scatterer is scanned inside the resonator. The resonant Bloch frequencies shift depending on the electric field at the scatterer position. By measuring the frequency shift in the reflection and transmission spectrum versus the scatterer position we determine the field strength. Excellent agreement is found between measurements and calculations without any adjustable parameters and a possible realization is suggested for measurements at optical frequencies.
Scattering of light is considered a nuisance in microscopy. It limits the penetration depth and strongly deteriorates
the achievable resolution. However, by gaining active spatial control over the optical wave front it is possible to
manipulate the propagation of scattered light far in the multiple scattering regime. These wave front shaping
techniques have given rise to new high-resolution microscopy methods based on strong light scattering. This is
based on the realization that scattering by stationary particles performs a linear transformation on the incident
light modes. By inverting this linear transformation, one can focus light through an opaque material and even
inside it. An extremely high resolution focus can be obtained using scatterers embedded in a high-index medium,
where the diffraction limit for focusing is reduced by a factor n. We have constructed a scattering lens made
of the high-index material gallium phosphide (GaP) which is transparent over most of the visible spectrum and
has the highest index of all nonabsorbing materials in the visible range. This yields a focal spot resolution of
less than 100 nm, and it seems theoretically possible to create a focus of order 70 nm. The system resolution of
a microscope based on this lens could be substantially higher.n
We present the first demonstration of control of the emission lifetime of a biological emitter by manipulating the local
density of optical states (LDOS). LDOS control is achieved by positioning the emitters at defined distances from a
metallic mirror. This results in a characteristic oscillation in the fluorescence decay rate. Since only the emitting species
contribute to the emission lifetimes, the radiative and nonradiative decay rates derived from the lifetime changes
characterize specifically the on- states of the emitter. We have thus experimentally determined the decay rates, and by
extension the quantum efficiency and emission oscillator strength, of exclusively the emitting states of the widely used
Enhanced Green Fluorescent Protein (EGFP). This approach is in contrast to other methods that average over emitting
and dark states. The quantum efficiency of the on-states determined for EGFP is 72%. This value is higher than
previously reported values determined by methods that average over
on- and off-states, as is expected for this system
with known dark states. The method presented is especially interesting for photophysically complex systems like
fluorescent proteins, where a range of emitting and dark forms has been observed.
We present ultrafast optical switching experiments on 3D photonic band gap crystals. Switching the Si inverse opal is
achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of
second order Bragg diffraction where the photonic band gap is predicted. We observe a large frequency shift of up to
1.5% of all spectral features including the peak that corresponds to the photonic band gap. We also demonstrate large,
ultrafast shifts of stop bands of planar GaAs/AlAs photonic structures. We briefly discuss how our results can be used in
future switching and modulation applications.