Non-invasive three-dimension control of light between turbid layers using a surface quasi-point light source for precorrection

Abstract

Manipulating light non-invasively through inhomogeneous media is an attractive goal in many disciplines. Wavefront shaping and optical phase conjugation can focus light to a point. Transmission matrix method can control light on multiple output modes simultaneously. Here we report a non-invasive approach which enables three-dimension (3D) light control between two turbid layers. A digital optical phase conjugation mirror measured and conjugated the diffused wavefront, which originated from a quasi-point source on the front turbid layer and passed through the back turbid layer. And then, because of memory effect, the phase-conjugated wavefront could be used as a carrier wave to transport a pre-calculated wavefront through the back turbid layer. The pre-calculated wavefront could project a desired 3D light field inside the sample, which, in our experiments, consisted of two 220-grid ground glass plates spaced by a 20 mm distance. The controllable range of light, according to the memory effect, was calculated to be 80 mrad in solid angle and 16 mm on z-axis. Due to the 3D light control ability, our approach may find applications in photodynamic therapy and optogenetics. Besides, our approach can also be combined with ghost imaging or compressed sensing to achieve 3D imaging between turbid layers.

Figure 1

The Schematic of the presented approach for light control between two turbid layers. (a) The schematic of our experimental setup. The scenario we consider here is a light-transparent space confined by two turbid layers. The sample beam (green path) is focused by a lens to form a quasi-point source on the internal surface (right surface) of the front turbid layer (left one), which is used as a reference spot for pre-characterization of the back turbid layer (right one). A spatial light modulator is imaged onto the external surface (right surface) of the back turbid layer (imaging lens not shown here, see Fig. S1 in Supplementary Information). The played-back reference beam (red path) is modulated by the SLM according to the applied phase map, which is a stacked one consisting of three parts shown in (b), (c) and (d), respectively. (b) The conjugated phase map of the sample beam. (c) A quadratic phase map which is used to cancel out the quasi-spherical wave emitted from the quasi-point source. (d) A pre-calculated phase map for the intended intensity pattern (presented by the smiling face). The modulated reference beam can be seen as a loaded carrier wave with the phase-conjugated wave and the quadratic wave together as the carrier, and the pre-calculated wavefront as the load. The carrier wave suppresses the turbidity of the back turbid layer and thus can transport the pre-calculated wavefront through this layer, which would generated/projected the intended intensity pattern.

Figure 2

Simulation of the influences of the quasi-point source size on the performance of our approach. (a–d) Discrepancies between an ideal spherical wavefront and the quasi-spherical wavefronts emitted from quasi-point sources with size of 1 μm, 10 μm, 20 μm and 30 μm, respectively. The field of view is a 2 mm * 2 mm area. (e–h) The foci generated by superposing a lens with a focal length of 10 mm on the under-compensated wavefronts shown in a-d, respectively. The field of view is a 15 μm * 15 μm area on the focal plane. (i) Peak-to-background ratio (PBR, the blue curve) and maximum-to-submaximum ratio (MSR, the green curve) of the generated focus versus the size of the quasi-point source. The smooth curve is obtained by averaging 100 curves for different random phase distributions inside the quasi-point source.

Figure 3

Focusing light between two ground glass plates. (a) Experimental setup for measurement of the focus size. A magnifying system, consisting of a CCD camera and a 20X objective lens, provides 21X magnification for precise measurement of the focus size. (b) The theoretical values (the blue dots) and experimental values (the green dots) of the focus size versus the distances between the focal plane and the back ground glass plate. (c) Observed multi-foci on an x-y plane 10 mm away from the back ground glass plate. (d) Observed multi-foci on z-axis. The z-positions for the three foci are 5 mm, 10 mm and 15 mm, respectively.

Figure 4

Controlling light between two ground glass plates. (a) Experimental setup for observation of the projected intensity patterns. (b) Intended 2D intensity pattern of capital letters ‘SIOM’ (0.55 mm * 0.15 mm) on an x-y plane 10 mm away from the back ground glass plate. (c) Image of the generated/projected 2D intensity pattern on the target x-y plane. The phase map H is calculated from the intensity pattern shown in (b) with an iterative Fourier transformation algorithm. (d) Images of the generated/projected 3D intensity patterns on three x-y planes with z-positions of 5 mm, 10 mm and 15 mm, respectively. The phase map H in this case is a superposition of three phase maps which are calculated from three intensity patterns, capital letter ‘A’ (0.22 mm * 0.26 mm), capital letter ‘B’ (0.12 mm * 0.19 mm) and capital letter ‘C’ (0.13 mm * 0.17 mm), respectively, with the iterative Fourier transformation algorithm.