After the first experiment of light focusing through a scattering medium using wavefront shaping (see A pioneer experiment), the same group demonstrated in [I. M. Vellekoop et al., Nat. Photon., 4, 320, (2010)] that a random medium can improve the sharpness of the focus. The scattering in a medium behind a lens randomizes the direction of the light. The speckle pattern shows high spatial frequencies not allowed by the lens alone because of its finite numerical aperture. After optimization of the input wavefront, the focus spot obtained is sharper than the resolution limit of the lens. In these experiments, the intensity profile was always measured in the far field, i.e. at least several wavelengths away from the surface, where only the propagating waves contribute to the optical field. In the present paper, J. H. Park and his colleagues optimize the input wavefront impinging on turbid media to increase the intensity measured in the near field at a given position. Subwavelength focusing is achieved thanks to the contributions of the evanescent waves.
It as been shown in [A. Apostol and A. Dogariu, Phys. Rev. Lett., 91, 093901, (2003)] that the spatial correlation length of the optical field in close proximity of a highly scattering medium can be significantly smaller than the wavelength. Since there is a linear relation between the input far field excitation and the output scattered near field, it is then possible to coherently control the near field by shaping the input wavefront. The principle of the experiment is shown in Figure 1.
Figure 1. Principle of the experiment. Image from J. H. Park et al., Nat. Photon., (2013).
The input wavefront is controlled using a phase only spatial light modulator and is projected onto the sample with a microscope objective. The near field intensity is measured using a near-ﬁeld scanning optical microscopy (NSOM) setup. The phases of several hundreds of segments of the SLM are optimized using a feedback algorithm to increase the intensity at the position of the near-field probe. After optimization, the intensity distribution at the surface of the scattering medium is measured (Figure 2). The authors show a full width at half maximum of the intensity peak of λ/3.9, which is twice as small as what can be obtained in free space with a good microscope objective.
Figure 2. Experimental subwavelength focusing. Image from J. H. Park et al., Nat. Photon., (2013).