D. Akbulut

Scattering Lens Resolves Sub-100 nm Structures with Visible Light

The smallest structures that conventional lenses are able to optically resolve are of the order of 200 nm. We introduce a new type of lens that exploits multiple scattering of light to generate a scanning nanosized optical focus. With an experimental realization of this lens in gallium phosphide we imaged gold nanoparticles at 97 nm optical resolution. Our work is the first lens that provides a resolution better than 100 nm at visible wavelengths.

Focusing light through random photonic media by binary amplitude modulation

We study the focusing of light through random photonic materials using wavefront shaping. We explore a novel approach namely binary amplitude modulation. To this end, the light incident to a random photonic medium is spatially divided into a number of segments. We identify the segments that give rise to fields that are out of phase with the total field at the intended focus and assign these a zero amplitude, whereas the remaining segments maintain their original amplitude. Using 812 independently controlled segments of light, we find the intensity at the target to be 75±6 times enhanced over the average intensity behind the sample. We experimentally demonstrate focusing of light through random photonic media using both an amplitude only mode liquid crystal spatial light modulator and a MEMS-based spatial light modulator. Our use of Micro Electro-Mechanical System (MEMS)-based digital micromirror devices for the control of the incident light field opens an avenue to high speed implementations of wavefront shaping.

Optical transmission matrix as a probe of the photonic strength

We demonstrate that optical transmission matrices (TMs) provide a powerful tool to extract the photonic strength of disordered complex media, independent of surface effects. We measure the TM of a strongly scattering GaP nanowire medium and compare the singular value density of the measured TM to a random-matrix-based wave transport model. By varying the transport mean free path and effective refractive index in the model, we retrieve the photonic strength. From separate numerical simulations we conclude that the photonic strength derived from TM statistics is insensitive to the surface reflection at rear surface of the sample.

Observation of Intensity Statistics of Light Transmitted Through 3D Random Media

We experimentally observe the spatial intensity statistics of light transmitted through three-dimensional (3D) isotropic scattering media. The intensity distributions measured through layers consisting of zinc oxide nanoparticles differ significantly from the usual Rayleigh statistics associated with speckle and instead are in agreement with the predictions of mesoscopic transport theory, taking into account the known material parameters of the samples. Consistent with the measured spatial intensity fluctuations, the total transmission fluctuates. The magnitude of the fluctuations in the total transmission is smaller than expected on the basis of quasi-one-dimensional (1D) transport theory, which indicates that quasi-1D theories cannot fully describe these open 3D media.

Measurements on the optical transmission matrices of strongly scattering nanowire layers

Summary form only given. Light incident on a scattering medium is redistributed over transport channels that either transmit through or reflect from the medium. We perform experiments aiming at finding individual transport channels of extremely strongly scattering materials. A small number of transport channels in a scattering sample are open with transmission coefficient close to 1; field transmission mainly takes place through these channels [1-3]. This means that, even if two very different incident fields are sent to the sample, the corresponding transmitted fields are correlated. As the scattering becomes stronger, these correlations become more pronounced.One way to investigate these correlations is to construct a transmission matrix by measuring the fields transmitted through the medium in response to pre-determined incident fields. Recently, microwave and optical experiments have been performed on measurements on the transmission matrices of scattering materials. In these studies, knowledge of the transmission matrix have been used for focusing [4, 5] and enhancing the transmission [6] through disordered media, or to study predictions of random matrix theory [7]. An observation of correlations in the optical transmission matrices of strongly scattering materials have not been reported so far. We measure transmission matrices of strongly scattering layers of disordered GaP nanowires, which are among the strongest scattering materials for visible light. The samples under study have thicknesses varying between -1.5 μm and -6 μm and transport mean free path of -0.2 μm [8]. We investigate the correlations in the measured transmission matrices and compare our experimental findings to a numerical model in order to retrieve physical parameters such as the scattering strength of the samples.

High-resolution phase and amplitude modulation using a digital micromirror device

The ability to spatially control the phase and amplitude of light allows for many exciting applications. In adaptive optics, light fields are modulated to correct for aberrations in the atmosphere. It has recently been shown that by spatially modulating light it is possible to focus and image through and inside opaque materials.

Coherent optical imaging through opaque layers

Scattering of light is usually seen as a nuisance in microscopy. Scattering 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 even far in the multiple scattering regime.[1–3] It was recently shown that in this way scattered light can even be exploited for perfect optical focussing.[4] These wave front shaping techniques pave the way for new microscopy methods based on strong light scattering.[5–8]

Focusing light through turbid media by binary amplitude modulation

Materials such as white paint, paper, milk, tissue appear opaque because they strongly scatter the incident light. It is impossible to focus light through such turbid media using conventional optics. However, it has been shown that it is possible to focus light through or inside such materials using a feedback based algorithm that spatially modifies the phase of the wavefront. When the incident wavefront is optimized in order to match the configuration of the sample a bright spot is created in the desired position [1,2]. Optical phase conjugation based methods have also been successfully utilized for the purpose of focusing light through turbid media [3–5]. Spatial phase modulation of the incident wave is central for all of these experiments. The slow response time of spatial phase modulators is a major obstacle to the in vivo use of these methods.