Bidirectional Optical Neural Networks Based on Free-Space Optics Using Lens Arrays and Spatial Light Modulator.

This paper introduces a novel architecture-bidirectional optical neural network (BONN)-for providing backward connections alongside forward connections in artificial neural networks (ANNs). BONN incorporates laser diodes and photodiodes and exploits the properties of Köhler illumination to establish optical channels for backward directions. Thus, it has bidirectional functionality that is crucial for algorithms such as the backpropagation algorithm. BONN has a scaling limit of 96 × 96 for input and output arrays, and a throughput of 8.5 × 1015 MAC/s. While BONN's throughput may rise with additional layers for continuous input, limitations emerge in the backpropagation algorithm, as its throughput does not scale with layer count. The successful BONN-based implementation of the backpropagation algorithm requires the development of a fast spatial light modulator to accommodate frequent data flow changes. A two-mirror-like BONN and its cascaded extension are alternatives for multilayer emulation, and they help save hardware space and increase the parallel throughput for inference. An investigation into the application of the clustering technique to BONN revealed its potential to help overcome scaling limits and to provide full interconnections for backward directions between doubled input and output ports. BONN's bidirectional nature holds promise for enhancing supervised learning in ANNs and increasing hardware compactness.

Scalable Optical Convolutional Neural Networks Based on Free-Space Optics Using Lens Arrays and a Spatial Light Modulator.

A scalable optical convolutional neural network (SOCNN) based on free-space optics and Koehler illumination was proposed to address the limitations of the previous 4f correlator system. Unlike Abbe illumination, Koehler illumination provides more uniform illumination and reduces crosstalk. The SOCNN allows for scaling of the input array and the use of incoherent light sources. Hence, the problems associated with 4f correlator systems can be avoided. We analyzed the limitations in scaling the kernel size and parallel throughput and found that the SOCNN can offer a multilayer convolutional neural network with massive optical parallelism.

A compact photonic resonator absorption microscope for point of care digital resolution nucleic acid molecular diagnostics.

Rapid, sensitive, and selective detection of nucleic acid biomarkers for health diagnostic applications becomes feasible for point of care scenarios when the detection instrument is inexpensive, simple, and robust. Here, we report the design, implementation, and characterization of a point of care instrument for photonic resonator absorption microscopy (PRAM) that takes advantage of resonant optical coupling between plasmonic gold nanoparticle tags and a photonic crystal (PC) surface. Matching the PC resonant wavelength to the gold nanoparticle's surface plasmon wavelength generates localized and efficient quenching of the PC resonant reflection intensity, resulting in the ability to clearly detect and count individual gold nanoparticles when they are captured on the PC surface. Surface-captured nanoparticles are observed by illuminating the PC at normal incidence with polarized light from a low-intensity red LED, and recording of PC reflected intensity on an inexpensive CMOS image sensor. A contrast limited adaptive histogram equalization (CLAHE) image processing algorithm was applied to derive counts of captured nanoparticles. The instrument is utilized in the context of an activate capture + digital counting (AC + DC) assay for a specific miRNA sequence, using nucleic acid toehold probes applied to gold nano-urchin (AuNU) nanoparticles to achieve 160 aM detection limits in a 30 min. assay.

Erratum: A compact photonic resonator absorption microscope for point of care digital resolution nucleic acid molecular diagnostics: publisher's note.

[This corrects the article on p. 4637 in vol. 12.].

Effects of metallic absorption and the corrugated layer on the optical extraction efficiency of organic light-emitting diodes.

The absorption of a metallic cathode in OLEDs is analyzed by using FDTD calculation. As the light propagates parallel to the layer, the intensity of E(z) polarization decreases rapidly. The intensity at 2.0 microm from the dipole is less than a quarter of that at 0.5 microm. The strong absorption by a cathode can be a critical factor when considering the increase of optical extraction by means of bending the optical layers. The calculation indicates that the corrugation of layers helps the guided light escape the guiding layer, but also increases the absorption into a metallic cathode. The final optical output power of the corrugated OLED can be smaller than that of the flat OLED. On the contrary, the corrugated structure with a non-absorptive cathode increases the optical extraction by nearly two times.

Analysis of a metallic nano-rod polarizer using finite-difference-time-domain method.

The polarization behavior of metallic nano-rods has been analyzed by means of the finite-difference-time-domain method. When the average spacing between the nano-rods is less than a half wavelength, the layer reflects the light polarized parallel to the nano-rods, as in a nano-slit. However, when the spacing is larger than a half wavelength, the metallic surface absorbs the light, polarized perpendicular to the rods, leading to a polarization switching. Multiple layers of nano-rods can make a polarizer with a high extinction ratio and good transmittance.

Analysis of polarization dependence of a nano-slit using finite-difference-time-domain method.

We analyzed the polarization dependent behavior of a nano-slit using a quasi-periodic finite-difference-time-domain (FDTD). In the simulation, the transmission decreases nearly to zero when the polarization is parallel to a slit, and the slit is narrower than a half wavelength. On the other hand, transmission of polarization perpendicular to the slit produces monotonic decrease with a decrease of the slit width. The polarization discrimination can be attributed to the different boundary conditions for two polarizations. According to a derived simple formula, the parallel polarization decays exponentially with the thickness of the slit when the width is smaller than a half wavelength. The exponential decay is verified by the simulation. In addition, the calculated transmission of aluminum nano-slit has a similar polarization behavior to that of a dielectric nano-slit.

Reliability in the oxide vertical-cavity surface-emitting lasers exposed to electrostatic discharge.

Electrostatic discharge(ESD) damage is known as a major source affecting the lifetime of oxide VCSEL. We investigated how ESD damage threshold voltage depends on the size, thickness, and composition of the oxide aperture by measuring the change of output power and reverse leakage current after ESD. ESD damage threshold voltage increased with the size of the oxide aperture, regardless of the thickness and the composition of the oxide aperture. However, damaged devices with thinner oxide layers showed relatively longer lifetime in the reliability test. The reliability data also showed that the VCSELs exposed to ESD have steeper power declines in reliability test than normal devices. This may be due to the defects formed in the active medium by ESD.

Analysis of mode stability in a concave mirror vertical-cavity surface-emitting laser with an oxide aperture.

We applied the Fox-Li resonator theory to analyze the mode stability of concave mirror surface-emitting lasers. The numerical modeling incorporates the oxide aperture in the simple classical cavity by adding a non-uniform phase shifting layer to the flat mirror side. The calculation shows that there is a modal loss difference between the fundamental mode and the competing modes. The amount of loss difference depends upon cavity length and the thickness of the oxide aperture. In addition to loss difference, modal gain difference plays a key role in discriminating between the fundamental mode and the higher order transverse modes. The modal gain difference heavily depends upon the size of the oxide aperture and the field intensity distribution. To summarize, the geometry of the concave cavity affects the mode profile and the unique field profile of each transverse mode makes a difference in both modal loss and gain. Finally, this leads to a side-mode suppression.

Electrically driven single-cell photonic crystal laser.

We report the experimental demonstration of an electrically driven, single-mode, low threshold current (approximately 260 microA) photonic band gap laser operating at room temperature. The electrical current pulse is injected through a sub-micrometer-sized semiconductor wire at the center of the mode with minimal degradation of the quality factor. The actual mode of interest operates in a nondegenerate monopole mode, as evidenced through the comparison of the measurement with the computation based on the actual fabricated structural parameters. As a small step toward a thresholdless laser or a single photon source, this wavelength-size photonic crystal laser may be of interest to photonic crystals, cavity quantum electrodynamics, and quantum information communities.

Transverse mode control by etch-depth tuning in 1120-nm GaInAs/GaAs photonic crystal vertical-cavity surface-emitting lasers.

Robust and tolerant single-transverse-mode photonic crystal GaInAs vertical-cavity surface-emitting lasers are fabricated and investigated. Triangular lattice patterns of rectangular air holes of various etch-depths are introduced in the top mirror. The stable single-transversemode operation is observed with a large margin of allowance in the etch depth (t = 2.5 +/- 0.6 microm). This stable mode selection mechanism is explained by the mode competition between the two lowest photonic crystal guided modes that are influenced by both the index guiding effect and the etchdepth dependent modal losses.

Evaluation scheme for the design of poweroptimized single mode vertical-cavity surfaceemitting lasers.

A very simple and efficient evaluation procedure is suggested for the design of power-optimized single mode VCSELs by reviewing the physical mechanisms that governs mode transition and simplifying the computation steps. In addition, the new structures are proposed and tested following the suggested evaluation procedure. As a result, the proposed design exhibits much better stability of the fundamental mode over a current range wider than the conventional one.