Hasan Yilmaz

Customizing speckle intensity statistics

We develop a general method for customizing the intensity statistics of speckle patterns. By judiciously modulating the phase-front of a monochromatic laser beam, we experimentally generate speckle patterns with arbitrarily-tailored intensity probability-density functions (PDF). Based on applying a local intensity transformation to a Rayleigh speckle pattern, our method allows for topological changes in the customized speckles while preserving their granularity. In addition to tailoring the functional form of the intensity distribution, we can separately control the intensity range of the PDF and thereby tune the speckle contrast.

Suppressing spatiotemporal lasing instabilities with wave-chaotic microcavities

Taming laser instabilities Broad-area and high-power lasers often suffer from instabilities owing to the chaotic interference of multiple modes within the cavity. Such instabilities can ultimately limit the operation of the laser or damage the cavity. The usual approach to minimizing such instabilities is to limit the number of modes in the cavity. Bittner et al. designed a chaotic cavity that disrupts the formation of self-organized structures that lead to instabilities (see the Perspective by Yang). This approach of fighting chaos with chaos by using the boundary condition of the cavity shape may provide a robust route to stabilizing lasers at high operating powers. Science, this issue p. 1225; see also p. 1201 A chaotic cavity design is used to suppress the spatiotemporal instabilities in lasers. Spatiotemporal instabilities are widespread phenomena resulting from complexity and nonlinearity. In broad-area edge-emitting semiconductor lasers, the nonlinear interactions of multiple spatial modes with the active medium can result in filamentation and spatiotemporal chaos. These instabilities degrade the laser performance and are extremely challenging to control. We demonstrate a powerful approach to suppress spatiotemporal instabilities using wave-chaotic or disordered cavities. The interference of many propagating waves with random phases in such cavities disrupts the formation of self-organized structures such as filaments, resulting in stable lasing dynamics. Our method provides a general and robust scheme to prevent the formation and growth of nonlinear instabilities for a large variety of high-power lasers.

Customizing speckle intensity statistics and correlations (Conference Presentation)

In this work, we develop a general method for customizing the intensity statistics of speckle patterns, on a target plane. By judiciously modulating the phase-front of a monochromatic laser beam -with a spatial light modulator- we experimentally generate speckle patterns possessing either arbitrarily-tailored intensity probability density functions (PDFs) or non-local spatial intensity correlations. Relative to Rayleigh speckles, our speckles with customized intensity PDFs exhibit radically different topologies yet maintain the same spatial correlation length. Furthermore, they are fully developed, ergodic, and stationary: with circular non-Gaussian statistics for the complex field. Furthermore, we can tailor the spatial intensity-correlations of speckle patterns: in particular, the non-local intensity correlations so that the speckle field and intensity spatially fluctuate on different length scales. We can even synthesize speckles with non-isotropic long-range intensity correlations, while the spatial field-correlation remains short-ranged and isotropic. Propagating away from the target plane, however, all of the customized speckles revert back to Rayleigh speckles. This work provides a versatile framework for tailoring speckle patterns with varied applications in microscopy, imaging and optical manipulation.

Transverse localization of transmission Eigenchannels (Conference Presentation)

Coherent light propagation in random scattering media such as biological tissue, fog, and turbulent atmosphere is dictated by the eigenchannels of transmission matrices. The spatial profiles of these channels can be exploited for tailoring light-matter interactions inside a turbid medium. While the spatial structures of transmission eigenchannels in diffusive waveguides are extensively studied, most scattering systems in practical applications have an open slab geometry. Here, we present experimental and numerical studies on the spatial profiles of transmission eigenchannels in disordered slabs of thickness much less than the width. We discover that all transmission eigenchannels are localized in the transverse direction (parallel to the slab). The lateral dimension of each channel increases linearly with the slab thickness and the transport mean free path. Such localization, which are absent for the transmission eigenchannels in quasi-one-dimensional samples, originate from spatial disorder, partial mixing of spatial channels, and non-local correlations of waves in the slab. Experimentally not all input channels can be controlled, and usually only the phase of incident beam is modulated. In this case, light injected to a high-transmission channel remains laterally localized, but the beam in a low-transmission eigenchannel expands laterally as it propagates through the slab. Our results provide physical insight to the transmission eigenchannels in open disordered systems, therefore paving the way for their applications in optical imaging and communication.

Speckle correlation resolution enhancement of wide-field fluorescence imaging(Conference Presentation)

Structured illumination enables high-resolution fluorescence imaging of nanostructures [1]. We demonstrate a new high-resolution fluorescence imaging method that uses a scattering layer with a high-index substrate as a solid immersion lens [2]. Random scattering of coherent light enables a speckle pattern with a very fine structure that illuminates the fluorescent nanospheres on the back surface of the high-index substrate. The speckle pattern is raster-scanned over the fluorescent nanospheres using a speckle correlation effect known as the optical memory effect. A series of standard-resolution fluorescence images per each speckle pattern displacement are recorded by an electron-multiplying CCD camera using a commercial microscope objective. We have developed a new phase-retrieval algorithm to reconstruct a high-resolution, wide-field image from several standard-resolution wide-field images. We have introduced phase information of Fourier components of standard-resolution images as a new constraint in our algorithm which discards ambiguities therefore ensures convergence to a unique solution. We demonstrate two-dimensional fluorescence images of a collection of nanospheres with a deconvolved Abbe resolution of 116 nm and a field of view of 10 µm × 10 µm. Our method is robust against optical aberrations and stage drifts, therefore excellent for imaging nanostructures under ambient conditions. [1] M. G. L. Gustafsson, J. Microsc. 198, 82–87 (2000). [2] H. Yilmaz, E. G. van Putten, J. Bertolotti, A. Lagendijk, W. L. Vos, and A. P. Mosk, Optica 2, 424-429 (2015).