Toward All-Optical Artificial Neural Networks

Training an artificial neural network for a specific task can be a computationally intensive and energy-consuming feat. Researchers at a U.S. university have demonstrated that such training can be accomplished on a silicon photonic chip.

A new way to train

In previous experiments on optical neural networks, other researchers performed the network training on a traditional computer and then transferred the results onto a photonic chip. Here, the Stanford group performed the algorithm physically by propagating an error signal through the circuits of the chip. According to Hughes, this method “should make training of optical neural networks far more efficient and robust.”

For hardware, the Stanford team used a silicon photonic architecture similar to a programmable processor describe last year at the Massachusetts Institute of Technology, USA. Basically, it’s a mesh of tiny, tunable Mach-Zender interferometers. For software, the researchers derived the algorithm from the mathematics of the optical circuit, going all the way back to Maxwell’s equations.


The “teaching” of the network involves sending a laser pulse one way through the optical circuit, measuring how the signal was changed from the predicted signal, then adjusting the circuit and sending the optical signal back. Based on the received signal, the artificial neural network adjusts itself by tweaking its circuitry via optical phase shifters. This tuning happens by “applying an electrical voltage to a heating element on the chip’s surface,” says Hughes, “which changes the optical properties of the waveguide slightly.” Tiny photodetectors near the phase shifters measure the intensity of the signal passing through the chip, giving the algorithm the gradient information needed for training and optimization.

For more information: Optica, doi:10.1364/OPTICA.5.000864

Could Excitons Aid Optical-Electronic Interconnects?

A nagging efficiency bottleneck in today’s communications networks is the need to convert between the optical signals that transmit data over long distances, and the electrical signals used in data processing. One potential solution lies in devices that manipulate not electrons or photons, but “excitons”—the bound electron-hole pairs formed when photons excite electrons in a semiconductor. But thus far, the “excitonic” devices demonstrated using bulk semiconductor materials have had to operate at frigid temperatures, a disadvantage that has held back practical applications. Now, a Swiss-Japanese research team has used an ingenious stack of 2-D materials to develop a key component for practical excitonics: an excitonic transistor that can operate at room temperature.

The heterostructure difference

At the heart of the system are layers of two atomically thin TMDs, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2). Because of the differing band structure of the two materials, when an exciton is created in the heterostructure (for example, by absorption of a photon), the electron tends to reside in the MoS2 layer, while the hole stays in the WSe2 layer. The result is a system in which the exciton “lives” not in a single 2-D material layer, but between the two layers.

Such an interlayer exciton, it turns out, has a spatial separation between the electron and the hole that’s large enough to allow the exciton to survive 100 times longer than it would in a single 2-D material layer. Yet the exciton can still exist and thrive at room temperature. Further, the two-layer structure means that the exciton has a built-in out-of-plane dipole moment. That means it can be manipulated and controlled by an electric field and voltage bias in ways that would be impossible with excitons in a single 2-D layer.

Graphene gates

The team found that the interlayer excitons were sufficiently long-lived to diffuse across a distance as long as five microns within the structure before recombining and emitting light. Further, the flux of excitons could be controlled and manipulated electrically by applying different voltage biases using the graphene electrodes, in transistor-like fashion.

For more information: Nature, doi: 10.1038/s41586-018-0357-y



Gold Nanoparticles Speed Up Photocatalysis

Boosting renewable energy and combating climate change are crucial scientific goals, but one common roadblock is how to effectively store solar energy once it’s been harvested. One way around that roadblock is hydrogen.

Researchers from Rutgers University, USA, have now found a nanotech-driven way to dramatically boost the efficiency of photocatalysis.

Boosting photocatalysis

TiO2 is a desirable semiconductor for photocatalysis because of its abundance, low cost, stability, and well-aligned valence and conduction bands. But because of its large band gap, TiO2 can use only UV radiation to drive water-splitting reactions. The Rutgers team, led by engineering professor Laura Fabris, wanted to see if there were ways to tap a larger slice of the solar spectrum.That energy concentration results in a local surface plasmon resonance (LSPR), with intense local electric fields. Those intense fields, in turn, can enhance the formation of excited or “hot” electrons that can boost photocatalysis rates in the semiconductor.

A (nano)star is born

The researchers’ modeling suggested that the long spikes of star-shaped nanoparticles, which generate intense electric fields at their tips when illuminated with visible and near-infrared (NIR) radiation, were the most promising candidates for plasmonic hot-electron generation under sunlight. The team developed a method for synthesizing such nanostars without a surfactant, which allowed a shell of TiO2 to be grown directly onto the gold surface, facilitating efficient electron transfer between the nanostars and the semiconductor. An added bonus is that, since their shape can be modified to change the number of points and dimensions, nanostars allow for predictable tunability of the LSPR bands from the visible to the NIR.

Low-temperature process

The researchers found that a simple, low-temperature, sol-gel approach worked best to synthesize and tune the TiO2-coated nanostars. The low temperature preserves the delicate morphology of the particles. “We were also able to use very low temperature synthesis to coat these gold particles with crystalline titanium, Fabris explained in a press release. “I think both from the materials perspective and the catalysis perspective, this work was very exciting all along.”

For more information:  doi: 10.1016/j.chempr.2018.06.004


Sustainable Energy from Bacteria

Adopting solar power can be tricky, and expensive, especially in regions where cloudy skies are the norm, such as parts of Canada and Northern Europe. Now, researchers at the University of British Columbia (UBC), Canada, have devised a cheap, sustainable solar cell that relies on bacteria to convert light to energy, even in an overcast environment .

Going biogenic

The UBC team took a more affordable and greener route that bypassed the extraction process altogether. First, the team genetically engineered E. coli cells to synthesize lycopene, a photosensitive pigment that absorbs light in the 380-to-520-nm range. Then, the researchers coated the bacteria with a layer of TiO2 nanoparticles, which acts as a semiconductor. Finally, the group applied the mixture to a conductive glass surface to act as the anode (along with an I/I3electrolyte and a graphite cathode) in a dye-sensitized solar cell.

Measuring up

The researchers recorded an open-circuit potential of 0.289 V, a short-circuit current of 0.19 mA and a corresponding short-circuit density of 0.686 mA cm−2 (an improvement on the 0.362 mA cm−2 achieved by others in the field). The UBC team suggests that this method for fast and efficient synthesis of a new class of bio-hybrid photovoltaic materials directly addresses the need for reducing the manufacturing cost of biogenic solar cells. However, the team notes that there is room for improvement. Efficiency could increase through ordered deposition of the biogenic material, use of platinum as the counter electrode, minimizing dark currents, using MOF complexes as photoactivators, better matching of electrolytes and use of more light-sensitive dyes.

For more information: doi: 10.1002/smll.201800729

A Step Toward Practical Plasmonic Chips?

Optoelectronics researchers in Russia have proposed a new design for a fast plasmonic chip, with the potential to dramatically cut the large energy losses that have typically blocked practical use of such devices.

Plasmonic components on integrated circuits—in which energy from light is concentrated into surface plasmon polaritons (SPPs), sub-wavelength electromagnetic oscillations that can propagate along a metal-dielectric interface—have significant promise for enabling large-scale integration in nanoscale optoelectronic chips and devices. That’s because SPPs offer the potential for breaking the diffraction limit imposed by the micrometer-scale wavelength of light in conventional waveguides, and allowing for the nanometer-scale integration common in electronic chips.

But there’s a catch: SPP propagation requires a metal interface, and that means that the electric field attenuates quickly through absorption in the metal—dropping off, according to Fedyanin, a billion times at distances of around a millimeter. And, while it’s possible to compensate for these losses by pumping additional energy into the system, the optical pumping schemes demonstrated thus far to do so have required a large, impractical energy input.

The added insulating layer helps to suppress leakage current and ohmic losses in the metal layer. And, when a forward bias voltage is applied, it allows a sufficient concentration of electrons near the semiconductor-insulator-metal interface to create a population inversion in the semiconductor and provide optical gain for the plasmonic mode propagating in the waveguide—amplification that compensates for SPP propagation losses.

In numerical models of the geometry, using a hypothetical system with gold as the metal layer, hafnium dioxide as the insulator, and the p-type semiconductor indium arsenic, the team calculated that the system could fully compensate for SPP propagation losses “at a current density of only 2.6 kA/cm2.” Replacing gold with copper, which significantly increases the minority-carrier injection efficiency, dropped the required current density to 0.8 kA/cm2. “Such an exceptionally low value,” the study concludes, “demonstrates the potential of electrically pumped active plasmonic waveguides and plasmonic nanolasers for future high-density photonic integrated circuits.”

For more information: doi: 10.1364/OE.23.019358

Enhancing Surface Sensing Sensitivity of Metallic Nanostructures using Blue-Shifted Surface Plasmon Mode and Fano Resonance

Improving surface sensitivities of nanostructure-based plasmonic sensors is an important issue to be addressed. Among the SPR measurements, the wavelength interrogation is commonly utilized. We proposed using blue-shifted surface plasmon mode and Fano resonance, caused by the coupling of a cavity mode (angle-independent) and the surface plasmon mode (angle-dependent) in a long-periodicity silver nanoslit array, to increase surface (wavelength) sensitivities of metallic nanostructures. It results in an improvement by at least a factor of 4 in the spectral shift as compared to sensors operated under normal incidence. The improved surface sensitivity was attributed to a high refractive index sensitivity and the decrease of plasmonic evanescent field caused by two effects, the Fano coupling and the blue-shifted resonance. These concepts can enhance the sensing capability and be applicable to various metallic nanostructures with periodicities.

Optical setup and optical properties of 900-nm-period Ti/Ag capped nanoslits with normal and oblique-angle incidence. (a) Optical setup for measuring angular transmission spectra. (b) Schematic configuration depicts the geometrical parameters of capped nanoslits with a 10-nm-thick titanium and 60-nm-thick silver film and the direction of the TM-polarized incident light. (c) Measured angular transmission diagram in air for 900-nm-period capped nanoslit arrays with a Ti/Ag film. The color dashed lines show the theoretical resonance wavelengths for the SPR mode. (d) Measured transmission spectra in air at 0° and 35° for 900-nm-period capped nanoslit arrays with a Ti/Ag film.

For more information:

Researchers simulate simple logic for nanofluidic computing

Invigorating the idea of computers based on fluids instead of silicon, researchers at the National Institute of Standards and Technology (NIST) have shown how computational logic operations could be performed in a liquid medium by simulating the trapping of ions (charged atoms) in graphene (a sheet of carbon atoms) floating in saline solution. The scheme might also be used in applications such as water filtration, energy storage or sensor technology.

Invigorating the idea of computers based on fluids instead of silicon, researchers at the National Institute of Standards and Technology (NIST) have shown how computational logic operations could be performed in a liquid medium by simulating the trapping of ions (charged atoms) in graphene (a sheet of carbon atoms) floating in saline solution. The scheme might also be used in applications such as water filtration, energy storage or sensor technology.

NIST’s ion-based transistor and logic operations are simpler in concept than earlier proposals. The new simulations show that a special film immersed in liquid can act like a solid silicon-based semiconductor.

The NIST molecular dynamics simulations focused on a graphene sheet 5.5 by 6.4 nanometers (nm) in size and with one or more small holes lined with oxygen atoms. These pores resemble crown others electrically neutral circular molecules known to trap metal ions.

In the NIST simulations, the graphene was suspended in water containing potassium chloride, a salt that splits into potassium and sodium ions. The crown ether pores were designed to trap potassium ion, which have a positive charge.

Applying voltages of less than 150 mV across the membrane turns “off” any penetration. Essentially, at low voltages, the membrane is blocked by the trapped ions, while the process of loose ions knocking out the trapped ions is likely suppressed by the electrical barrier. Membrane penetration is switched on at voltages of 300 mV or more. As the voltage increases, the probability of losing trapped ions grows and knockout events become more common, encouraged by the weakening electrical barrier. In this way, the membrane acts like a semiconductor in transporting potassium ions.

More information: Alex Smolyanitsky et al. Aqueous Ion Trapping and Transport in Graphene-Embedded 18-Crown-6 Ether Pores, ACS Nano (2018). DOI: 10.1021/acsnano.8b01692

Researchers taking optical device out of the lab and into the clinic to detect cancer at its earliest stages

In a paper published in Nature Scientific Reports, a team of researchers at Worcester Polytechnic Institute (WPI) has demonstrated how a device that uses beams of light to grip and manipulate tiny objects, including individual cells, can be miniaturized, opening the door to creating portable devices small enough to be inserted into the bloodstream to trap individual cancer cells and diagnose cancer in its earliest stages.

The technique, known as optical tweezers, uses optical beams of laser light to create an attractive force field that can hold, or trap, small objects in place without physical contact. Traditional optical tweezers focus the light with a large and expensive lens, which makes the device bulky and susceptible to environmental fluctuations. These limitations make optical tweezers impossible to use outside the lab.

“Currently, to test for cancer, you must wait until there’s a visible tumor or a sufficient volume of cancerous cells in a blood sample,” he said. “By that time, the cancer may be advanced. But cancer starts with single cells. If doctors could separate those cells from among millions of blood cells, we could detect cancer much sooner—at a point where it’s not visible using other techniques. This could advance diagnoses by months or even years and make treatment much more successful.”


Read more at:

Our new paper in optics communication

Congratulations for the publication of paper” Highly Sensitive Biochemical sensor based on Nanostructured Plasmonic Interferometer” , by Khajemiri , S. M. Hamidi , Om. K. Suwal.

We propose a novel plasmonic interferometric sensor with a slit and surrounding rectangular grooves array on an optically thick gold film for biochemical sensing. We did finite-difference time-domain (FDTD) simulation for design optimization and analytical calculation for characterization of sensitivity in the proposed sensor. Our interferometer is functional for visible to near infrared region with maximum sensitivity of 500 nm/RIU and figure of merit 1933 at 741 nm wavelength. The peak intensity and wavelength change in different refractive indices. In conclusion, the results obtained in the present study indicate the potential of the proposed plasmonic interferometer as a low cost, compact, and label-free high-throughput device.



Surface Plasmon-Mediated Nanoscale Localization of Laser-Driven sub-Terahertz Spin Dynamics in Magnetic Dielectrics

We report spatial localization of the effective magnetic field generated via the inverse Faraday effect employing surface plasmon polaritons (SPPs) at Au/garnet interface. Analyzing both numerically and analytically the electric field of the SPPs at this interface, we corroborate our study with a proof-of-concept experiment showing efficient SPPdriven excitation of coherent spin precession with 0.41 THz frequency. We argue that the subdiffractional confinement of the SPP electric field enables strong spatial localization of the SPP-mediated excitation of spin dynamics. We demonstrate two orders of magnitude enhancement of the excitation efficiency at the surface plasmon resonance within a 100 nm layer of a dielectric garnet. Our findings broaden the horizons of ultrafast spin-plasmonics and open pathways toward nonthermal optomagnetic recording on the nanoscale.

KEYWORDS: Ultrafast spin dynamics, surface plasmonpolariton, inverse Faraday effect, rare-earth iron garnet, nonlinear optics, Magnetoplasmonics


Effective static magnetic field induced by a propagating SPP at the Au/magnetic garnet interface.