Basic Research

Introducing cutting-edge technology for realizing concept that will bring innovation to society.

Basic Research

Introducing cutting-edge technology for realizing concept that will bring innovation to society.

What is a photonic crystal? The reason why photonics-electronics convergence technology is needed, and the details of research

In this, the first article of a new series, we will introduce a material called a "photonic crystal". This word is probably unfamiliar to most people, so we will begin by explaining what a photonic crystal is and why research on photonic crystals is necessary, then explain the findings of research actually conducted by NTT.

What is a photonic crystal?

A photonic crystal is a structure in which the refractive index changes periodically, making it possible to confine light in a small area and improve interaction between light and material. Using nanofabrication technology, semiconductors can be microfabricated to create light-manipulating structures from these crystals.
Using this structure, we have been able to realize various phenomena not possible with ordinary materials, such as extremely strong light confinement, slow light states, and unusual negative refraction phenomena. We have also succeeded in greatly reducing the size and energy consumption of optical devices such as optical memory using these properties, and we can now see the path to full-scale optical integration.

Why is it called a "photonic crystal"?

Why is this artificial periodic structure called a photonic crystal? To understand this, we will first consider a normal solid state crystal. In normal substances, atoms are arranged periodically to form a crystal, and the electrical characteristics are determined by its periodicity. Periodic potential works on the electrons existing in the crystal. It is known that electrons moving in periodic potential behave differently from the electrons in free space. As a result, there are substances existing in nature that have various electrical properties, such as being conductors, insulators, and semiconductors. In other words, substances with various electrical properties in nature exist due to the periodicity of crystals.

This behavior is due to the quantum mechanical wave nature of electrons, so the same effect can be expected for light with the same wave motion. However, the period of naturally formed crystals is about 0.1 nm, which is incredibly small compared to the wavelength. Because it is 3 digits smaller, light cannot sense this periodicity. As a result, ordinary substances are all light conductors (or absorbers), and there are no "light insulators" that neither absorb light nor conduct it. By artificially creating a periodic structure of a size suitable for the wavelength of light (typically around 200 to 400 nm), it is possible to realize metamaterials with optical properties that are impossible in ordinary materials. For example, it is possible to create a "light insulator" that does not allow any light to pass through. This marvelous structure is a photonic crystal.
 

Fig. 1 Photonic Crystal
Fig. 1 Photonic Crystal

Why photonic crystals are needed

In order to further develop computing technology in the future, one current major issue is how to overcome the performance limitation of CMOS (Complementary Metal Oxide Semiconductor) technology. Computing platforms based on CMOS electronic circuit technology have been able to process large amounts of information by improving circuit performance in accordance with Moore's Law. However, due to the limitation on microfabrication and integration density, processing with electronic circuits is approaching its limits in terms of speed and energy consumption.

In these circumstances, parallel CPUs have increased computing power, but in recent years, the performance of answer delay has reached a ceiling and formed a bottleneck in information processing. However, in the future world of security, online processing with low latency and high-speed event processing will be required more and more for autonomous driving, disaster prediction, traffic control, and solutions will be needed for this. In other words, technological innovation of information processing will require miniaturization and energy saving in elements that convert optical signals into electrical signals and vice versa, as well as realizing high-density photoelectric conversion interfaces.
NTT is trying to solve this problem using photonic crystal technology, which we have researched for many years.

Breakthroughs in photonics-electronic convergence with photonic crystals

Recent advances in semiconductor microfabrication technology have made production possible, and research is actively being done on various types of photonic crystals. With photonic crystals, we have confirmed basic operation with low power consumption in various optical devices, including optical switches, lasers, optical memory, and laser light sources. Furthermore, photonic crystals have proven to be indispensable for reducing power consumption of devices that convert optical signals to electric signals and vice versa, such as nano-receivers and nano-optical modulators.

We at NTT have been researching nanophotonics technology for 20 years, and about a decade ago, we made progress with research on optical switches used to manipulate light with light. As a result, we found that using photonic crystals reduces power consumption, and we have come to see how photonic crystals could be used in industrial technology. Furthermore, in the last 4 to 5 years, we have made progress with research on photonics-electronic integration using photonic crystals. As a result, we have found that photonics-electronics integration is possible with extremely low capacitance, and the research has progressed. NTT has succeeded in using photonic crystals to integrate a photoelectric conversion element with the smallest electric capacity in the world. In April 2019, we announced that we had realized an optical modulator and optical transistor that operate with the lowest energy consumption in the world. We have great expectation that if we can realize a circuit based on this, it will be possible to realize a high-speed computing platform with ultra-low energy consumption, the likes of which has never existed before.

In 2019, NTT announced the "IOWN Concept," a communication platform that will support the smart world of the near future, and we are now conducting research and development with the aim of realizing IOWN by about 2030. We have positioned photonics-electronics convergence technology as a key technology of APN (all-photonics network), which could be called the foundation that supports the IOWN concept.
 

Fig. 2. Shortening the distance of optical transmission technology and development of photonics-electronics convergence information processing
Fig. 2. Shortening the distance of optical transmission technology and development of photonics-electronics convergence information processing

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