One of the possible future architectural implementations of new computing devices is brain-inspired neuromorphic computers. Artificial synapse is one of the key neuromorphic computing elements. This work is devoted to the search for new bioinspired artificial synapse properties and the demonstration of already known neuromorphic properties on the original photoelectric synapse based on nanocrystalline ZnO film. Photoelectric synapse demonstrated basic neuromorphic properties: spike signals operation, the presence of short-term memory, long-term memory and paired-pulse facilitation. Artificial photoelectric synapse adaptation properties have been demonstrated in a series of experiments with different conductivity cutoff levels.
One of the promising future ways of computing is using principles similar to the human brain work mechanism. Neuromorphic photonics makes it possible to create computational elements with properties similar to the principles of the biological synapse. Neuromorphic computers can overcome the von Neumann bottleneck fundamental limitation of existing computing systems.
In a current study, we demonstrate a neuromorphic properties, observing on photoconductive structures based on nanocrystalline ZnO, WO3, In2O3 triggered by presynaptic light spikes with the 405nm wavelength. Photoconductive structures based on ZnO, WO3, In2O3 were deposited as a 100–200 nm thick film on the surface of the chip.
Excitatory post-synaptic current value was measured for different excitation pulse durations. The excitatory post-synaptic current caused by a pair of presynaptic light spikes was studied for different delay times between pulses. The ability of these structures to act as biological synapses like high-pass temporal filtering function was demonstrated by measuring post-synaptic current when exposed to a series of 30 consecutive presynaptic light spikes.
Our photoconductive semiconductor structures have two different relaxation mechanisms. Due to this, the structures possess short-term and long-term photoconductivity memory. To demonstrate the ability of our samples possesses long-term memory, we studied the semiconductor photoconductivity relaxation values after light exposure during 500 seconds. The memory level after light exposure were stored over an hour.
The studied photoconductive structures showed the presence of a spike reaction properties, the effect of amplitude and frequency filtering, short-term and long-term memory, and they are looking promising for use as elements of neuromorphic photonics.
Plasmonic band gap is a range of frequencies, within which, surface plasmon polaritons cannot propagate for any wavevector. Unfortunately the first plasmonic band gap cannot be observed directly in reflectance spectroscopy [1]. To detect it, biharmonic metal-air surface structuring is conventionally utilized [2,3]. However in this case experimental geometry is strictly limited to normal angle of incidence, which is not compatible with large range of applications.
In current work we introduce biperiodic plasmonic crystals. We experimentally demonstrate, that biperiodic structuring allows to tune band gap spectral-angular position.
Laser interference lithography (LIL) is a well-established technique for creating periodic planar nanostructures over a large surface area. LIL allows to precisely control the modulation period and depth and thus perfectly match diffraction coupling conditions and tune plasmonic band gap properties.
We used LIL experimental setup based on Lloyd interferometer. The radiation from the laser source (He-Cd, wavelength 325 nm, average power 14 mW) was spatially filtered and then formed interference pattern on the silicon wafer, covered with a thin layer of SU-8 2015. The structure period was defined by the incident angle on the interferometer. Modulation depth was defined by exposure time. By applying subsequent second exposure with another angle of incidence, we obtained biperiodic structure. Exposed samples were washed in corresponding developer, dried in air and later sputtered with 100 nm of aluminium.
We fabricated a set of biperiodic plasmonic crystals with different periods and modulation depths. The quality and geometrical parameters of biperiodic plasmonic crystals were monitored by scanning electron microscopy and atomic force microscopy. The appearance of plasmonic band gap was measured by spectral-angular polarisation spectroscopy. We experimentally determined the dependance of plasmonic band gap properties (width and position) on geometrical parameters of biperiodic plasmonic crystals. We also performed FDTD numerical simulations (Lumerical). The experimental results are in good agreement with numerical calculations.
[1] Raether, Heinz. [Surface Plasmons on Smooth and Rough Surfaces and on Gratings.], Springer Berlin Heidelberg, 91-105 (1988).
[2] Barnes, William L., et al. "Physical origin of photonic energy gaps in the propagation of surface plasmons on gratings." Physical Review B 54.9 (1996): 6227.
[3] Kocabas, Askin, S. Seckin Senlik, and Atilla Aydinli. "Plasmonic band gap cavities on biharmonic gratings." Physical Review B 77.19 (2008): 195130.
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