We present an optical encryption approach based on the combination of fractal Fresnel lens (FFL) and fractional Fourier transform (FrFT). Our encryption approach is in fact a four-fold encryption scheme, including the random phase encoding produced by the Gerchberg–Saxton algorithm, a FFL, and two FrFTs. A FFL is composed of a Sierpinski carpet fractal plate and a Fresnel zone plate. In our encryption approach, the security is enhanced due to the more expandable key spaces and the use of FFL overcomes the alignment problem of the optical axis in optical system. Only using the perfectly matched parameters of the FFL and the FrFT, the plaintext can be recovered well. We present an image encryption algorithm that from the ciphertext we can get two original images by the FrFT with two different phase distribution keys, obtained by performing 100 iterations between the two plaintext and ciphertext, respectively. We test the sensitivity of our approach to various parameters such as the wavelength of light, the focal length of FFL, and the fractional orders of FrFT. Our approach can resist various attacks.
Besides a linear momentum, optical fields also carry angular momentum (AM), which has two intrinsic components: one is spin angular momentum related to the polarization state and the other is orbital angular momentum (OAM) caused by the helical phase due to the existence of a topological azimuthal charge. The two AM components of the optical field may not be independent of each other, especially if spin-to-orbit conversion (STOC) under high focusing creates a spin-dependent optical vortex in the longitudinal field. However, it would be very exciting to experimentally manifest and control the local OAM density. Here, we present a strategy for achieving STOC via a radial intensity gradient. The linearly varying radial phase provides an effective way to control the local AM density, which induces a counterintuitive orbital motion of the isotropic microparticles in optical tweezers without intrinsic OAM. Our work not only provides fundamental insights into the STOC of light, but could also have applications in optical micromanipulation.
We propose a novel scheme for designing and generating kaleidoscope-structured vector optical fields (KS-VOFs) by analogy with the principle of multiple mirror reflection in a kaleidoscope. For KS-VOFs with symmetric polarization states, we show the symmetry properties of the focal fields with various shapes for different applications. The redistributing symmetric local spin angular momentum (SAM) density indicates that the design method of the KS-VOFs plays a role as a catalyst to the redistribution process of polarization states and local SAM conversion in the tight focusing process. Meanwhile, the controllable transverse energy flow in the focal plane can be used to transport multiple absorptive particles and then to be fixed at certain locations. Our results may find applications in optical machining, trapping, and manipulation.
Bessel-like beams with controllable rotation of local linear polarization upon propagation are generated, which in fact achieve the evolution of polarization states along the equator of the Poincaré sphere during propagation. Based on the amplitude–phase joint modulation method, the rotation direction and rate of polarizations of the Bessel-like beam can be controlled easily by adjusting the radial indices and intensity ratio of two superposed beams. A rotation angle of
∼
800 deg has been achieved after a propagation distance of 120 mm, corresponding to a rotation rate of
∼
6.7 deg/mm, which is about three times higher than in previous works.
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