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EPFL researchers are pushing the limits of perovskite solar cell performance by exploring the best way to grow these crystals.

This is a Perovskite solar cell prototype. Credit: Alain Herzog / EPFL

This is a Perovskite solar cell prototype. Credit: Alain Herzog / EPFL

Michael Graetzel and his team found that, by briefly reducing the pressure while fabricating perovskite crystals, they were able to achieve the highest performance ever measured for larger-size perovskite solar cells, reaching over 20% efficiency and matching the performance of conventional thin-film solar cells of similar sizes. Their results are published in Science.

This is promising news for perovskite technology that is already low cost and under industrial development.

However, high performance in pervoskites does not necessarily herald the doom of silicon-based solar technology. Safety issues still need to be addressed regarding the lead content of current perovskite solar-cell prototypes in addition to determining the stability of actual devices.

Layering perovskites on top of silicon to make hybrid solar panels may actually boost the silicon solar-cell industry. Efficiency could exceed 30%, with the theoretical limit being around 44%. The improved performance would come from harnessing more solar energy: the higher energy light would be absorbed by the perovskite top layer, while lower energy sunlight passing through the perovskite would be absorbed by the silicon layer.

From dye solar cells to perovskite

Graetzel is known for his transparent dye-sensitized solar cells. It turns out that the first perovskite solar cells were dye-sensitized cells where the dye was replaced by small perovskite particles.

His lab’s latest perovskite prototype, roughly the size of an SD card, looks like a piece of glass that is darkened on one side by a thin film of perovskite. Unlike the transparent dye-sensitized cells, the perovskite solar cell is opaque.

How to make a perovskite solar cell

To make a perovskite solar cell, the scientists must grow crystals that have a special structure, called “perovskite” after Russian mineralogist Lev Perovski who discovered it.

The scientists first dissolve a selection of compounds in a liquid to make some “ink”. They then place the ink on a special type of glass that can conduct electricity. The ink dries up, leaving behind a thin film that crystallizes on top of the glass when mild heat is applied. The end result is a thin layer of perovskite crystals.

The tricky part is growing a thin film of perovskite crystals so that the resulting solar cell absorbs a maximum amount of light. Scientists are constantly looking for smooth and regular layers of perovskite with large crystal grain size in order to increase photovoltaic yields.

For instance, spinning the cell when the ink is still wet flattens the ink and wicks off some of the excess liquid, leading to more regular films. A new vacuum flash technique used by Graetzel and his team also selectively removes the volatile component of this excess liquid. At the same time, the burst of vacuum flash creates seeds for crystal formation, leading to very regular and shiny perovskite crystals of high electronic quality.

A research group at Tohoku University’s WPI-AIMR has succeeded in finding the origin and the mechanism of ferromagnetism in Mn-doped GaAs. The discovery is significant as it will accelerate the development of the spintronic element.

GaAs, like silicon, is a well-known semiconductor commonly used in high-speed electronic devices and laser diodes.

When manganese (Mn) atoms are doped into a GaAs crystal ((Ga,Mn)As), the crystal exhibits characteristics and properties of both the semiconductor and magnet (Fig. 1). Since it is possible to use an electric field to control the magnetism in (Ga,Mn)As, Mn-doped GaAs has been a key material in spintronic devices and a significant contributor to the development of spintronics technology.

Fig.1: Crystal structure of (Ga,Mn)As. Mn ions substituted for Ga have a magnetic moment, and the magnetic moment of each Mn ion aligns along the same direction when (Ga,Mn)As becomes a ferromagnet. Credit: Seigo Souma

Fig.1: Crystal structure of (Ga,Mn)As. Mn ions substituted for Ga have a magnetic moment, and the magnetic moment of each Mn ion aligns along the same direction when (Ga,Mn)As becomes a ferromagnet. Credit: Seigo Souma

However, although it has been 20 years since that discovery, the mechanism of ferromagnetism in (Ga,Mn)As is still not widely understood or well explained. There remains fierce debate and confusion, leading to obstacles preventing the progress and further development of spintronics technology.

The researchers at Tohoku University, led by Profs. H. Ohno and T. Takahashi, have succeeded in directly observing the electronic states which participate in creating the ferromagnetism by photoemission spectroscopy. They found that doped Mn atoms extract electrons from As atoms, leaving “holes” (empty states of electrons) in the As orbital. This then causes the ferromagnetism in (Ga,Mn)As.

“This finding resolves the long-standing problem in the mechanism of ferromagnetism in (Ga,Mn)As,” says researcher Seigo Souma. “It also accelerates the materials engineering of magnetic semiconductors, as well as the tunable controlling of spin states in spintronic devices. This is very significant result and we’re excited about the potential it represents.”

A group of scientists from Hong Kong University of Science and Technology; the University of California, Santa Barbara; Sandia National Laboratories and Harvard University were able to fabricate tiny lasers directly on silicon — a huge breakthrough for the semiconductor industry and well beyond.

For more than 30 years, the crystal lattice of silicon and of typical laser materials could not match up, making it impossible to integrate the two materials — until now.

As the group reports in Applied Physics Letters, from AIP Publishing, integrating subwavelength cavities — the essential building blocks of their tiny lasers — onto silicon enabled them to create and demonstrate high-density on-chip light-emitting elements.

To do this, they first had to resolve silicon crystal lattice defects to a point where the cavities were essentially equivalent to those grown on lattice-matched gallium arsenide (GaAs) substrates. Nano-patterns created on silicon to confine the defects made the GaAs-on-silicon template nearly defect free and quantum confinement of electrons within quantum dots grown on this template made lasing possible.

The group was then able to use optical pumping, a process in which light, rather than electrical current, “pumps” electrons from a lower energy level in an atom or molecule to a higher level, to show that the devices work as lasers.

“Putting lasers on microprocessors boosts their capabilities and allows them to run at much lower powers, which is a big step toward photonics and electronics integration on the silicon platform,” said professor Kei May Lau, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology.

Traditionally, the lasers used for commercial applications are quite large — typically 1 mm x 1 mm. Smaller lasers tend to suffer from large mirror loss.

But the scientists were able to overcome this issue with “tiny whispering gallery mode lasers — only 1 micron in diameter — that are 1,000 times shorter in length, and 1 million times smaller in area than those currently used,” said Lau.

Whispering gallery mode lasers are considered an extremely attractive light source for on-chip optical communications, data processing and chemical sensing applications.

“Our lasers have very low threshold and match the sizes needed to integrate them onto a microprocessor,” Lau pointed out. “And these tiny high-performance lasers can be grown directly on silicon wafers, which is what most integrated circuits (semiconductor chips) are fabricated with.”

In terms of applications, the group’s tiny lasers on silicon are ideally suited for high-speed data communications.

“Photonics is the most energy-efficient and cost-effective method to transmit large volumes of data over long distances. Until now, laser light sources for such applications were ‘off chip’ — missing — from the component,” Lau explained. “Our work enables on-chip integration of lasers, an [indispensable] component, with other silicon photonics and microprocessors.”

The researchers expect to see this technology emerge in the market within 10 years.

Next, the group is “working on electrically pumped lasers using standard microelectronics technology,” Lau said.

In our computer chips, information is transported in form of electrical charge. Electrons or other charge carriers have to be moved from one place to another. For years scientists have been working on elements that take advantage of the electrons angular momentum (their spin) rather than their electrical charge. This new approach, called “spintronics” has major advantages compared to common electronics. It can operate with much less energy.

However, it is difficult to create such a spin current, which is required in spintronics. In the journal Physical Review Letters, physicists from TU Wien (Vienna) have now proposed a new method to produce gigantic spin currents in a very small period of time. The secret is using ultra short laser pulses.

A laser pulse hits nickel (green). Spin-up-electrons (red) change into silicon (yellow). Electrons with both spin-orientations change back from silicon into nickel. Credit: TU Wien

A laser pulse hits nickel (green). Spin-up-electrons (red) change into silicon (yellow). Electrons with both spin-orientations change back from silicon into nickel. Credit: TU Wien

Magnets and semiconductors

For every electron, two different spin-states are possible; they are called “spin up” and “spin down”. The electron spin is responsible for ferromagnetism: when many electron spins in a metal are aligned, they can collectively create a magnetic field. Therefore, using ferromagnets to create spin flux seems like a straightforward idea. “There have been attempts to send an electric current through a combination of magnets and semiconductors,” says Professor Karsten Held (TU Wien). “The idea is to create a flux of electrons with uniform spin, which can then be used for spintronic circuits. But the efficiency of this method is very limited.”

Karsten Held and Marco Battiato found another way. In computer simulations, they analysed the behaviour of electrons in a thin layer of nickel when it is attached to silicon and hit with ultra short laser pulses. “Such a laser pulse has an overwhelming effect on the electrons in nickel,” says Marco Battiato. They are swept away and accelerated towards the silicon.

An electric field builds up at the interface between nickel and silicon, which stops the current. Electrons still keep on migrating between the nickel layer and silicon, but the motion in both directions cancel each other, there is no net charge transfer.

Spin up and spin down

But even when no electric charge is transported, it is still possible to transport spin. “In the nickel layer, there are both spin-up electrons as well as spin-down electrons,” says Karsten Held. “But the metal atoms influence both kinds of electrons in different ways. The spin-up electrons can move rather freely. The spin-down electrons however have a much higher probability of being scattered at the nickel atoms.”

When the electrons are scattered, they change their direction and lose energy. Therefore, the majority of the electrons which do make it all the way to the nickel-silicon interface are spin-up electrons. Electrons which move in the opposite direction have equal probabilities of being in the spin-up or spin-down state.

This spin-selective effect leads to a dominance of spin-up electrons in the silicon. This means that a spin current has been injected into the silicon without creating a charge current. “Our calculations show that this spin-polarization is extremely strong — much stronger than we could create with other methods,” says Marco Battiato. “And this spin flux can be created in femtoseconds.” Time is of the essence: today’s computer processors operate with gigahertz frequencies. Billions of operations per second are possible. Even higher frequencies in the terahertz range can only be reached with extremely fast elements.

So far, the method has only been tested in computer simulations. But Battiato and Held are already working with experimentalists who want to measure this laser-triggered spin flux. “Spintronics has the potential to become a key technology of the next few decades,” says Held. “With our spin injection method there is now finally a way to create ultrafast, extremely strong spin currents.”

Along with being a “girl’s best friend,” diamonds also have remarkable properties that could make them ideal semiconductors. This is welcome news for electronics; semiconductors are needed to meet the rising demand for more efficient electronics that deliver and convert power.

The thirst for electronics is unlikely to cease and almost every appliance or device requires a suite of electronics that transfer, convert and control power. Now, researchers have taken an important step toward that technology with a new way to dope single crystals of diamonds, a crucial process for building electronic devices.

“We need the devices to manipulate the power in the way that we want,” said Zhengqiang (Jack) Ma, an electrical and computer engineering professor at the University of Wisconsin-Madison. He and his colleagues describe their new method in the Journal of Applied Physics, from AIP Publishing.

For power electronics, diamonds could serve as the perfect material. They are thermally conductive, which means diamond-based devices would dissipate heat quickly and easily, foregoing the need for bulky and expensive methods for cooling. Diamond can also handle high voltages and power. Electrical currents also flow through diamonds quickly, meaning the material would make for energy efficient devices.

But among the biggest challenges to making diamond-based devices is doping, a process in which other elements are integrated into the semiconductor to change its properties. Because of diamond’s rigid crystalline structure, doping is difficult.

Currently, you can dope diamond by coating the crystal with boron and heating it to 1450 degrees Celsius. But it’s difficult to remove the boron coating at the end. This method only works on diamonds consisting of multiple crystals stuck together. Because such polydiamonds have irregularities between the crystals, single-crystals would be superior semiconductors.

You can dope single crystals by injecting boron atoms while growing the crystals artificially. The problem is the process requires powerful microwaves that can degrade the quality of the crystal.

Now, Ma and his colleagues have found a way to dope single-crystal diamonds with boron at relatively low temperatures and without any degradation. The researchers discovered if you bond a single-crystal diamond with a piece of silicon doped with boron, and heat it to 800 degrees Celsius, which is low compared to the conventional techniques, the boron atoms will migrate from the silicon to the diamond. It turns out that the boron-doped silicon has defects such as vacancies, where an atom is missing in the lattice structure. Carbon atoms from the diamond will fill those vacancies, leaving empty spots for boron atoms.

This technique also allows for selective doping, which means more control when making devices. You can choose where to dope a single-crystal diamond simply by bonding the silicon to that spot.

The new method only works for P-type doping, where the semiconductor is doped with an element that provides positive charge carriers (in this case, the absence of electrons, called holes).

“We feel like we found a very easy, inexpensive, and effective way to do it,” Ma said. The researchers are already working on a simple device using P-type single-crystal diamond semiconductors.

But to make electronic devices like transistors, you need N-type doping that gives the semiconductor negative charge carriers (electrons). And other barriers remain. Diamond is expensive and single crystals are very small.

Still, Ma says, achieving P-type doping is an important step, and might inspire others to find solutions for the remaining challenges. Eventually, he said, single-crystal diamond could be useful everywhere — perfect, for instance, for delivering power through the grid.

GLOBALFOUNDRIES today announced a next-generation radio-frequency (RF) silicon solution for its Silicon Germanium (SiGe) high-performance technology portfolio. The technology is optimized for customers who need improved performance solutions for automotive radar, satellite communications, 5G millimeter-wave base stations and other wireless and wireline communication network applications.

GLOBALFOUNDRIES’ SiGe 8XP technology is the latest extension to the company’s 130nm high-performance SiGe family and enables customers to develop RF solutions that deliver even faster data throughput, over greater distances, while consuming less power. The advanced technology offers an improved heterojunction bipolar transistor (HBT) performance with lower noise figure, higher signal integrity, and up to a 25 percent increase in maximum oscillation frequency (fMAX) to 340GHz compared to its predecessor, SiGe 8HP.

The complexity and performance demands of high bandwidth communication systems operating in the mmWave frequency bands have created the need for higher performance silicon solutions. This creates opportunities for high-performance SiGe solutions in the RF front end of 5G smartphones and other mmWave phased array consumer applications in addition to the current applications that depend on SiGe for high performance, such as the communications infrastructure base stations, backhaul, satellite and fiber optic networks.

“5G networks promise to bring a new level of innovation to RF SOC design to support high bandwidth data delivery and meet the demands for increased data rates and low latency applications,” said Dr. Bami Bastani, senior vice president of GLOBALFOUNDRIES RF business unit. “GLOBALFOUNDRIES’ SiGe 8HP and 8XP technologies offer an outstanding balance of performance, power, and efficiency that enable customers to develop differentiated RF solutions in next-generation mobile and infrastructure hardware.”

“GLOBALFOUNDRIES’ SiGe technology leadership and comprehensive PDKs enable our designers to develop performance-optimized, differentiated millimeter wave solutions quickly,” said Robert Donahue, Anokiwave CEO. “Utilizing SiGe 8XP allows us to take performance to even higher levels in future-ready mmWave solutions designed to help providers stay ahead of the demands for reliable connectivity, from anywhere, while handling exploding volumes of mobile data traffic.”

With tomorrow’s 5G deployments poised to drive a proliferation of base stations with smaller cell areas, SiGe 8HP and 8XP are designed to help offer a balance of value, power output, efficiency, low noise, and linearity at microwave and millimeter-wave frequencies for differentiated RF solutions in next-generation mobile infrastructure hardware and smartphone RF front ends. GLOBALFOUNDRIES’ SiGe 8HP and 8XP high-performance offerings enable chip designers to integrate significant digital and RF functionality while exploiting a more economical silicon technology base compared to gallium arsenide (GaAs) and higher performance than CMOS.

In addition to high performance transistors for efficient operation at mmWave frequencies, SiGe8HP and 8XP introduce technology innovations that can reduce the die size and enable area-efficient solutions. A new Cu metallization feature provides improved current carrying capabilities with five times the current density at a 100C, or up to 25 degrees C higher operating temperature at the same current density compared to standard Cu lines. In addition, GLOBALFOUNDRIES’ through-silicon-via (TSV) interconnect technology is available.

A new, high-pressure technique may allow the production of huge sheets of thin-film silicon semiconductors at low temperatures in simple reactors at a fraction of the size and cost of current technology. A paper describing the research by scientists at Penn State University publishes on May 13, 2016, in the journal Advanced Materials.

“We have developed a new, high-pressure, plasma-free approach to creating large-area, thin-film semiconductors,” said John Badding, professor of chemistry, physics, and materials science and engineering at Penn State and the leader of the research team. “By putting the process under high pressure, our new technique could make it less expensive and easier to create the large, flexible semiconductors that are used in flat-panel monitors and solar cells and are the second most commercially important semiconductors.”

Thin-film silicon semiconductors typically are made by the process of chemical vapor deposition, in which silane — a gas composed of silicon and hydrogen — undergoes a chemical reaction to deposit the silicon and hydrogen atoms in a thin layer to coat a surface. To create a functioning semiconductor, the chemical reaction that deposits the silicon onto the surface must happen at a low enough temperature so that the hydrogen atoms are incorporated into the coating rather than being driven off like steam from boiling water. With current technology, this low temperature is achieved by creating plasma — a state of matter similar to a gas made up of ions and free electrons — in a large volume of gas at low pressure. Massive and expensive reactors so large that they are difficult to ship by air are needed to generate the plasma and to accommodate the large volume of gas required.

“With our new high-pressure chemistry technique, we can create low-temperature reactions in much smaller spaces and with a much smaller volume of gas,” said Badding. “The reduced space necessary allows us, for the first time, to create semiconductors on multiple, stacked surfaces simultaneously, rather than on just a single surface. To maximize the surface area, rolled-up flexible surfaces can be used in a very simple and far more compact reactor. The area of the resulting rolled-up semiconducting material could, upon further development, approach or even exceed a square kilometer.”

Samsung Electronics Co. Ltd. today recognized five of the best and brightest computer science and engineering students in the U.S. as it announced the inaugural class of the Samsung PhD Fellowship. Each student will receive a Fellowship award of $50,000 as well as mentorship to support their ground-breaking research.

The new PhD Fellowship program rewards those who dare to innovate. Jointly sponsored by Samsung Semiconductor and the Samsung Strategy and Innovation Center (SSIC), the program recognizes outstanding Ph.D. students working in five areas: Software and Memory System Solutions for Data Centers; Low-Power CPU and System IP Architecture and Designs; Advanced Semiconductor Devices, Materials and Simulation; Internet of Things; and Smart Machines.

Samsung launched the Fellowship program with a call for partner universities to nominate outstanding students working on the above topics. Twelve of the best-qualified nominees were selected as Finalists and invited to showcase events at the new headquarters in Silicon Valley or at the Samsung Austin R&D Center. Each student Finalist presented his or her research proposal to an audience of Samsung engineers, Lab directors, and innovation leaders and met many of them for interviews as well. Following these events, the five Fellows were selected from this terrific group.

Each Fellow will be connected to an engineer from one of the Samsung Semiconductor or SSIC Labs in Silicon Valley or Austin. This mentor will provide an industry perspective on their research and will invite the student to join Samsung for an internship.

“We are thrilled to be supporting these outstanding students through our Fellowship program. Samsung strives to be a leader in the creation of new technology, and a great way to do that is by supporting basic research and PhD training,” said Stefan Heuser, VP of Operations and Innovation for SSIC. “We were very impressed by the students nominated by the universities—all of them have made an impact in key areas of research. The Finalists were an even stronger group, and we are confident that they will become leaders in their fields. But the five Fellows are truly exceptional, and we look forward to working with them in the coming year. We thank the universities and all of the student nominees for their efforts and for their interest in our program.”

Following are the five Samsung PhD Fellows for 2016-2017:

  • Dinesh Jayaraman, “Embodied Learning for Visual Recognition”
    Nominating professor: Kristen Grauman, University of Texas at Austin
  • Jiajun Wu, “Computational Perception of Physical Object Properties”
    Nominating professors: William Freeman and Joshua Tenenbaum, MIT
  • Joy Arulraj, “Rethinking Database Systems for Next-Generation Memory Technologies and Real-Time Analytics”
    Nominating professor: Andy Pavlo, Carnegie Mellon University
  • Niranjini Rajagopal, “Sensor Fusion and Automatic Infrastructure Mapping for Indoor Localization Systems”
    Nominating professors: Anthony Rowe and Bruno Sinopoli, Carnegie Mellon University
  • Wooseok Lee, “Exploring Future Mobile Heterogeneous Multicore System Architectures”
    Nominating professors: Lizy John and Andreas Gerstlauer, University of Texas at Austin

Nominations for next year’s PhD Fellowship program will open in September 2016. Additional information about the Fellowship program can be found at: http://www.samsung.com/us/labs/fellowship/index.html

Best of both worlds


May 9, 2016

More, faster, better, cheaper. These are the demands of our device-happy and data-centered world. Meeting these demands requires technologies for processing and storing information. Now, a significant obstacle to the development of next-generation device technologies appears to have been overcome, according to researchers from the University of Tokyo (Japan), Tokyo Institute of Technology (Japan) and Ho Chi Minh University of Pedagogy (Vietnam).

Specializing in the emerging field of semiconductor spintronics, the team has become the first to report growing iron-doped ferromagnetic semiconductors working at room temperature — a longstanding physical constraint. Doping is the practice of adding atoms of impurities to a semiconductor lattice to modify electrical structure and properties. Ferromagnetic semiconductors are valued for their potential to enhance device functionality by utilizing the spin degrees of freedom of electrons in semiconductor devices.

“Bridging semiconductor and magnetism is desirable because it would provide new opportunities of utilizing spin degrees of freedom in semiconductor devices,” explained research leader Masaaki Tanaka, Ph.D., of the Department of Electrical Engineering & Information Systems, and Center for Spintronics Research Network, University of Tokyo. “Our approach is, in fact, against the traditional views of material design for ferromagnetic semiconductors. In our work, we have made a breakthrough by growing an iron-doped semiconductor which shows ferromagnetism up to room temperature for the first time in semiconductors that have good compatibility with modern electronics. Our results open a way to realize semiconductor spintronic devices operating at room temperature.”

The researchers discuss their findings this week in Applied Physics Letters, from AIP Publishing. The researchers’ maverick move challenged the prevailing theory that predicted a type of semiconductor known as “wide band gap” would be strongly ferromagnetic. Most research focuses on the wide band gap approach. “We instead chose narrow-gap semiconductors, such as indium arsenide, or gallium antimonide, as the host semiconductors,” Tanaka said. This choice enabled them to obtain ferromagnetism and conserve it at room temperature by adjusting doping concentrations.

Investigators have long envisioned bridging semiconductors and magnetism to create new opportunities of utilizing spin degrees of freedom and harnessing electron spin in semiconductors. But until now, ferromagnetic semiconductors have only worked under experimental conditions at extremely low, cold temperatures, typically lower than 200 K (-73oC), which is much colder than the freezing point of water, 273.15 K. Here, K (Kelvin) is a temperature scale which, like the Celsius (oC) scale, has 100 degrees between boiling (373.15 K = 100oC) and freezing (273.15 K = 0oC) of water.

Potential applications of ferromagnetic-semiconductors include designing new and improved devices, such as spin transistors.

“Spin transistors are expected to be used as the basic element of low-power-consumption, non-volatile and reconfigurable logic circuits,” Tanaka explained.

In 2012, the team postulated that using iron as magnetic doping agents in semiconductors would produce performance advantages not seen in the more frequently studied manganese class of dopants.

Skeptics doubted this approach, but the team continued and successfully created a ferromagnetic semiconductor known as “n-type.”

“This was thought impossible by almost all leading theorists,” Tanaka noted. “They predicted that such n-type ferromagnetic semiconductors cannot retain ferromagnetism at temperatures higher than 0.1 K. We demonstrated, however, many new functionalities, such as the quantum size effect and the ability to tune ferromagnetism by wave function manipulation.”

On a practical level, the team continues its research with the goal of applying iron-doped ferromagnetic semiconductors to the field of spintronic device innovation. On a theoretical level, the team is interested in re-evaluating conventional theories of magnetism in semiconductors. “Based on the results of many experimental tests, we have proven that ferromagnetism in our iron-doped semiconductor is intrinsic,” Tanaka said.

A group of scientists from ITMO University in Saint Petersburg has put forward a new approach to effective manipulation of light at the nanoscale based on hybrid metal-dielectric nanoantennas. The new technology promises to bring about a new platform for ultradense optical data recording and pave the way to high throughput fabrication of a wide range of optical nanodevices capable of localizing, enhancing and manipulating light at the nanoscale. The results of the study were published in Advanced Materials.

Selective laser exposure to create hybrid nanostructures. Credit: ITMO University

Selective laser exposure to create hybrid nanostructures. Credit: ITMO University

Nanoantenna is a device that converts freely propagating light into localized light – compressed into several tens of nanometers. The localization enables scientists to effectively control light at the nanoscale. This is one of the reasons why nanoantennas may become the fundamental building blocks of future optical computers that rely on photons instead of electrons to process and transmit information. This inevitable replacement of the information carrier is related to the fact that photons surpass electrons by several orders of magnitude in terms of information capacity, require less energy, rule out circuit heating and ensure high velocity data exchange.

Until recently, the production of planar arrays of hybrid nanoantennas for light manipulation was considered an extremely painstaking process. A solution to this problem was found by researchers from ITMO University in collaboration with colleagues from Saint Petersburg Academic University and Joint Institute for High Temperatures in Moscow. The research group has for the first time developed a technique for creating such arrays of hybrid nanoantennas and for high-accuracy adjustment of individual nanoantennas within the array. The achievement was made possible by subsequently combining two production stages: lithography and precise exposure of thenanoantenna to a femtosecond laser – ultrashort impulse laser.

The practical application of hybrid nanoantennas lies, in particular, within the field of ultradense data recording. Modern optical drives can record information with density around 10 Gbit/inch2, which equals to the size of a single pixel of a few hundred nanometers. Although such dimensions are comparable to the size of the nanoantennas, the scientists propose to additionally control their color in the visible spectrum. This procedure leads to the addition of yet another ‘dimension’ for data recording, which immediately increases the entire data storage capacity of the system.

Apart from ultradense data recording, the selective modification of hybrid nanoantennas can help create new designs of hybrid metasurfaces, waveguides and compact sensors for environmental monitoring. In the nearest future, the research group plans to focus on the development of such specific applications of their hybrid nanoantennas.

The nanoantennas are made of two components: a truncated silicon cone with a thin golden disk located on top. The researchers demonstrated that, thanks to nanoscale laser reshaping, it is possible to precisely modify the shape of the golden particle without affecting the silicon cone. The change in the shape of the golden particle results in changing optical properties of the nanoantenna as a whole due to different degrees of resonance overlap between the silicon and golden nanoparticles.

“Our method opens a possibility to gradually switch the optical properties of nanoantennas by means of selective laser melting of the golden particles. Depending on the intensity of the laser beam the golden particle will either remain disc-shaped, convert into a cup or become a globe. Such precise manipulation allows us to obtain a functional hybrid nanostructure with desired properties in the flicker of a second,” comments Sergey Makarov, one of the authors of the paper and researcher at the Department of Nanophotonics and Metamaterials of ITMO University.

Contrary to conventional heat-induced fabrication of nanoantennas, the new method raises the possibility of adjusting individual nanoantennas within the array and exerting precise control over overall optical properties of the hybrid nanostructures.

“Our concept of asymmetric hybrid nanoantennas unifies two approaches that were previously thought to be mutually exclusive: plasmonics and all-dielectric nanophotonics. Our hybrid nanostructures inherited the advantages of both approaches – localization and enhancement of light at the nanoscale, low optical losses and the ability to control the scattering power pattern. In turn, the use of laser reshaping helps us precisely and quickly change the optical properties of such structures and perhaps even record information with extremely high density,” concludes Dmitry Zuev, lead author of the study and researcher at the Department of Nanophotonics and Metamaterials of ITMO University.