We report on the generation of a spinning twin-mode with two bright spots in a biomaterial, bacteriorhodopsin (bR), suspension pumped by an optical vortex. The spinning direction of the twin-mode is fully assigned by the handedness of the incident optical vortex. This phenomenon occurs owing to the spatial soliton effects in the bR suspension.
The receptive field of a visual neuron is the particular region in which a stimulus will modify the firing of that neuron. Retinal ganglion cells have a coaxial-shaped structure formed by the central excitatory and outer inhibitory regions. These regions respond oppositely to light. This shape can be mathematically modeled by a balanced Difference of Gaussian (DOG) function. In digital image processing, the DOG filter is used for detecting objects' edges.
Bacteriorhodopsin (bR) is a photosensitive protein which resembles the visual pigment rhodopsin. When bR is illuminated, it generates a positive transient photocurrent, and, when light is switched off, a negative transient photocurrent is produced. This peculiar behavior is similar to the response of ganglion receptive fields.
In this study, we fabricate a two-dimensional binarized DOG (b-DOG) filter, in which the central part mimics the excitatory part of the ganglion cell receptive fields, whereas the outer ring reproduces the effect observed in their outer inhibitory regions. This filter consists of photosensitive protein bR films and electrolyte solution that are sandwiched between ITO electrodes.
To analyze the spatial-temporal frequency sensitivity, we use moving sine wave gratings with different pitches with a controlled scanning speed. When the temporal frequency is kept constant, the spatial frequency sensitivity matched with the Fourier transform of the b-DOG function. On the other hand, when the spatial frequency does not vary, the temporal frequency sensitivity corresponded well with the Fourier transform of Difference of Gamma function. Difference of Gamma function is known as the impulse response of the visual nerve of animals. We separately analyze the independent spatial and temporal frequencies collected from the spatial-temporal characteristics of the filter. The spatial-temporal frequency characteristics of b-DOG filter are similar to those of the X-type retinal ganglion cells. However, unlike the retinal ganglion cell, the b-DOG filter is a perfect linear filter.
The analog image processing using this b-DOG filter is performed by scanning a standard test image. It is found that an edge can be detected just by scanning the image and plotting the zero-crossing point. With increasing input image size, the spatial frequency peak detected by the b-DOG filter shifts towards higher frequencies. Since there are only sharp edges where high spatial frequency components are present, the detected results of the analog filter are similar to that of the digital filter. Consequently, the phase difference between the input and output image is approximately the same for all pixels, and it agrees with the digital edge detection results.
Some illusion images are scanned on the b-DOG filter to verify the occurrence of the illusion similar to vision. The Hermann grid illusion generated with lower-order vision was observed. It is found that no luminance information is necessary for the Herman grid illusion, because this b-DOG filter does not extract image luminance.
The visual function elements similar to X-type retinal ganglion cells prepared in this study are useful to constructively understand the visual information processing mechanism of the organism.
Optical vortex possesses an annular intensity profile and an optical orbital angular momentum arising from its helical wavefront. In particular, it is noteworthy that optical vortex can twist the irradiated materials, such as silicon, metal, and polymer, to form chiral structures.
In this paper, we report on a spatial symmetry breaking of optical vortex propagating through bacteriorhodopsin (bR) suspensions. A 1 µm picosecond optical vortex mode propagated through bR suspensions (concentration: ~10 µM diluted in a 16 % NaCl solution) was broken into a twin mode with two bright spots. Also, the twin mode rotated towards a clockwise or counter-clockwise direction assigned by the handedness of the incident optical vortex mode. The rotation speed of the twin mode was measured to be 0.05 cycle/second. It was worth mentioning that such symmetry breaking of the optical vortex mode manifests an interaction between a helical wavefront and a helical bacteriorhodopsin. In fact, this phenomenon was never observed by using a NaCl solution without bacteriorhodopsin.
We propose ganglion cell receptive-field-type filters with the use of the photoreceptor protein bacteriorhodopsin. Visual image processing is possible with the use of only one sensing element. We also demonstrate that our difference of Gaussians (DOG) filter, which mimics on-center off-suround ganglion cell receptive fields, has the function of a Laplacian filter and can act as an edge detecor. The X-type receptive field responses obtained by the filter, for a variety of stimuli, are compared with available electrophysiological recodings.
Bacteriorhodopsin (bR) is a promising biomaterial for several applications. Optical excitation of bR at an electrode-electrolyte interface generates differential photocurrents while an incident light is turned on and off. This unique functional response is similar to that seen in retinal neurons. The bR-based bipolar photosensor consists of the bR dip-coated thin films patterned on two ITO plates and the electrolyte solution. This bipolar photocell will function as a biomimetic photoreceptor cell. The bipolar structure, due to the photocurrent being generated in alignment with the cathodic direction, makes the excitatory and inhibitory regions possible. This scheme shows our bipolar cell can act as a basic unit of edge detection and forms the artificial visual receptive field.
Transmission spectra and photoinduced transmission change are observed in periodic waveguide which consist of a quartz grating substrate and a thin protein film of bacteriorhodopsin. We propose a scheme to achieve all optical switching using the photoinduced refractive index change of bacteriorhodopsin.
Polarization holographic gratings convert or transform a polarization state of incident wave front. We have studied the diffraction properties of polarization gratings, which formed with two orthogonal linearly or circularly polarized waves. At a polarization modulation recorded on organic material bacteriorhodopsin, the diffraction efficiency and the diffracted wave polarization strongly depend on the read-out wave. We have practiced several simple theoretical treatments, namely Fraunhofer diffraction integral. The experimental results have been interpreted in terms of these simplified models.
Second order nonlinear optical properties of C60 based multilayered charge transfer structures with TPP, TPD and TPN molecules, acting as electron donors, are studied by the transverse optical second harmonic generation. The studied structures are of ABABAB...and ABCABC...type, where layer A is made from electron accepting molecules, layer B from electron donating and layer C from neutral molecules. The typical thickness of individual layers is about 2 nm. A significant increase of SHG is observed in ABCABC...type structures, where an effective charge transfer is expected with creation of a noncentrosymmetry in the perpendicular direction tot eh layers. The largest (chi) (2) susceptibility is obtained with multilayers containing rubrene. The SHG generation experiments performed on thin films with different thicknesses show that the nonlinear optical response comes from the bulk material and not form the interfaces. A quadratic dependence of SHG intensity on input power is also observed, showing that the charge transfer takes place at the ground state.
Second and third order nonlinear optical properties of C60 based composites and multilayered structures with TPP and TPN are studied by transverse optical second and third harmonic generation, respectively. The results are compared with those obtained from pristine, photolyzed in vacuum and in oxygen atmosphere C60 thin films. Second harmonic generation is observed from structures containing C60 molecules. An enhancement of the quadratic susceptibility is observed from the multilayered structures. The third harmonic generation experiments show an enhancement of the cubic susceptibility from photolyzed C60 thin films. No increase of (chi) (3) in the multilayered structures or composites is observed. A slightly larger response is obtained with the multilayered thin films as compared to the composite material.