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An acoustic metagrating (MG) constructed with an inverse-design scheme can be used to realize anomalous reflection, but its efficiency is limited. In this work, we find that the evanescent guided modes are an important component of achieving perfect retroreflection in an MG. Considering the evanescent first-order guided mode, we derive an alternative inverse-design condition to produce perfect retroreflection and improved performance in the MG parameter space compared with the traditional method. We further investigate the corresponding contribution ratios of the guided modes to perfect retroreflection and find an orthogonal relationship between the contribution ratios of odd and even guided modes. These findings are confirmed using analytical calculations and numerical simulations.

Various approaches for imaging through scattering layers have been proposed, but very few of them provide depth-resolved images. Here, we propose and experimentally demonstrate an approach for non-invasive depth-resolved imaging of objects hidden behind a scattering medium based on speckle correlations and parallax. The relative intensities of the objects' autocorrelations and the positions of the cross-correlations, both change from different apertures. The reconstruction of objects is achieved by subtraction of the autocorrelations and the conventional Fienup-type iterative phase-retrieval algorithm, and the objects' relative position is retrieved from the relative position of the cross-correlation to the central; moreover, the depth information of objects is decoded by the displacement of the cross-correlations through different apertures. This multi-aperture technique allows sharp imaging of objects with full actual depth information at once.

Nonvolatile holograms have been recorded in nondoped near-stoichiometric LiTaO3 crystals by use of a near infrared (IR) laser for writing and a ultraviolet beam as a gating source. This material exhibits good overall two-color holographic recording characteristics including a high sensitivity of 0.086 cm/J with a gating intensity of 1 W/cm2 at 350 nm, a large refractive index change of as much as 1×10−4, a high resistance to IR erasure, and a long hologram lifetime of five years. The sensitivity can be further enhanced using writing and gating beams of a shorter wavelength without sacrificing readout nonvolatility.

Nonvolatile holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of a two-color holographic recording method. An IR laser with wavelength of 778 nm is used for writing and a UV laser with wavelength of 350 nm is used for gating. Dependences of two-color sensitivity and M/♯ on gating and writing intensities are presented. Significant improvements of sensitivity, M/♯, and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. The obtained high two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is attributed to large light-induced absorption of small polarons.

The self-enhancement and self-depletion phenomena originated from the beam-coupling effect are observed during both the recording and fixing phases in LiNbO3:Fe:Mn nonvolatile holographic storage experiments. A numerical simulation and a steady-state analysis are developed to provide a mathematical evaluation to the phenomena. From the experimental and theoretical studies, it is seen that the highest diffraction efficiency available from the self-enhancement may be twice as large as the lowest one from the self-depletion, both during the recording and fixing phases. Therefore, the use of the self-enhancement is necessary for the practical applications.

One of the authors M.S. thanks the Matsumae International Foundation for the research fellowship. Another author E.G.G. acknowledges support of the Australian Research Council through its Center of Excellence.