The global biophotonics market is estimated to reach $91.31 billion by 2024, according to a report by Grand View Research, San Francisco, CA. The developments in optical technologies, increasing investigations by researchers, and mounting demand for early diagnosis are anticipated to drive market growth. According to the report, biophotonics is “poised to renovate the healthcare sector worldwide.”

A schematic of a nanowire photoniccrystal hybrid laser fabricated by nanoprobe manipulation. (Credit: Takiguchi et al.)

The increasing geriatric population, rising frequency of chronic diseases, growing government initiatives, increasing healthcare amends, and incorporation of IT into healthcare applications are expected to “remarkably fuel the biophotonics market,” notes the report. Healthcare manufacturers are increasingly incorporating photonic components into their instruments to enhance value and improve accuracy and sensitivity. The report says that this is also projected to “create a major impact on the biophotonics market in the next few years.”

The report found that the complexity of the biophotonics technology and the high price of biophotonics-based instruments, along with slow commercialization rate and reluctance to accept new treatments, may hamper market growth somewhat. However, it also found that the emergence of nanotechnology, coupled with the increasing demand for home-based point-of-care devices, is expected to spur the market growth in the near future.

New developments in nanotechnology-based photonics are enabling advancements in medical lasers and photonics-based medical devices. This article looks at a sampling of some of those developments as well as some of the hurdles they face in reaching commercialization.

Crystal and Nanowire Combo

A holographic image created by laser light passing through a metasurface. (Credit: Wang et al. ©2016 American Chemical Society; here)

Contrary to the tremendous success story of electronic integration, photonic integration is still in its infancy. One the most serious obstacles it faces is the need to use a variety of materials to achieve different functions — unlike electronic integration. To complicate matters further, many of the materials required for photonic integration aren't compatible with silicon integration technology.

Attempts so far to place a variety of functional nanowires within photonic circuits to reach desired functionalities have shown that, while entirely possible, nanowires tend to be too small to effectively confine light. While bigger nanowires can improve the light confinement and performance, it increases both energy consumption and device footprint — both of which are considered fatal when it comes to integration.

Addressing this problem, a group of NTT Corp. researchers (Kanagawa, Japan) came up with an approach that involves combining a subwavelength nanowire with a photonic crystal platform, which they report the journal APL Photonics. Photonic crystals — artificial structures whose refractive index is periodically modulated — are at the heart of their work.

“A small local refractive index modulation of a photonic crystal produces strong light confinement that leads to ultra-high-quality optical nanoresonators,” says Masaya Notomi, a senior distinguished scientist for NTT Basic Research Laboratories. “We make full use of this particular feature in our work.”

In 2014, this same group demonstrated that it was possible to strongly confine light in a subwavelength nanowire with a diameter of 100 nm by placing it on a silicon photonic crystal. At that time, “it was a preliminary demonstration of the confinement mechanism, but with our present work we've successfully demonstrated subwavelength nanowire device operation on a silicon platform by using this method,” Notomi says.

In other words, while a subwave-length nanowire can't become a resonator with strong light confinement on its own, when placed on a photonic crystal, it causes the refractive index modulation needed to generate the light confinement.

“For our work, we carefully prepare a III-V semiconductor nanowire with sufficiently large optical gain and place it within a slot of a silicon photonic crystal by using the ‘nanoprobe manipulation technique,’ which results in an optical nanoresonator,” says Masato Takiguchi, the paper's lead author and a researcher working within Notomi's group at NTT Basic Research Laboratories. “With a series of careful characterizations, we've demonstrated that this subwavelength nanowire can exhibit continuous-wave lasing oscillation and high-speed signal modulation at 10 Gbps.”

To use nanowire lasers for photonic integration, three essential requirements must be met. “First, a nanowire should be as small as possible for sufficiently strong light confinement, which ensures an ultrasmall footprint and energy consumption,” Takiguchi says. “Second, a nanowire laser must be able to generate high-speed signals. Third, the lasing wavelength should be longer than 1.2 μm to avoid absorption in silicon, so it's important to create subwavelength nanowire lasers at optical communication wavelengths — 1.3–1.55 μm — capable of highspeed signal modulation.”

In fact, previous demonstrations of nanowire-based lasers “have all been at wavelengths shorter than 0.9 μm, which can't be used for silicon photonic integrated circuits — except a pulsed lasing demonstration of relatively thick micron-wire lasers at 1.55 μm,” Notomi says. This is presumably because the material gain is smaller at longer wavelengths, which makes it difficult for thin nanowires to achieve lasing.

Beyond this, “zero demonstrations of high-speed modulation by any types of nanowires have materialized,” he notes. This is also due to the small gain volume. “With our present work, we've solved these problems by combining a nanowire and a silicon photonic crystal,” Notomi says. “Our result is the first demonstration of continuous-wave lasing oscillation by a subwavelength nanowire, as well as the first demonstration of high-speed signal modulation by a nanowire laser.”

The group was able to achieve 10-Gbps modulation, which is comparable to conventional, directly modulated high-speed lasers used for optical communications.

“This proves that nanowire lasers show promise for information processing — especially photonic integrated circuits,” Notomi says. The most promising application for the group's present work is nanowire-based photonic integration circuits, for which they'll use various different nanowires to achieve different functionalities — such as lasers, photo-detectors, and switches in silicon photonic integrated circuits.

“It's expected that processors equipped with an on-chip photonic network will be needed within about 15 years, and nanowire-based photonic integration will be one possible solution,” Notomi says. In terms of lasers, the group's next target is to integrate nanowire lasers with input/output waveguides.

“Although this type of integration has been a difficult task for nanowire-based devices, we expect it will be much easier in our platform because the photonic crystal platform is intrinsically superior in terms of the waveguide connection,” Takiguchi says. “We'll aim for room temperature current-driven lasing as well.”

The group also plans to use the same technique to create “photonic devices other than lasers by choosing different nanowires,” Takiguchi says. “We want to demonstrate that we're able to integrate a number of photonic devices by having different functionalities on a single chip.”

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Dielectric Nanophotonics

Photonic crystal lasers on paper substrates. (Credit: KAIST)

Arseniy Kuznetsov of the Data Storage Institute in Singapore focuses his research in the field of dielectric nanophotonics, which studies the behavior and use of light in nanoscale projects. Among other things, his team is working on developing a new concept of low-loss dielectric nanoantennas. The concept can be used in several industries, including creating improved medical devices, applying it in virtual reality and augmented reality, and developing holographic displays.

Kuznetsov received Britain's IET AF Harvey Prize for his research in this field. According to Sir John O'Reilly, chair of the IET's selection committee for the prize, Kuznetsov's research may lead to explosive growth in this field, creating new products based on nanophotonics. Kuznetsov says the award demonstrates the success of his research in the past several years, as well as the field's promising future.

To effectively control light, its electric and magnetic components must be controlled simultaneously and independently. Yet, there is a problem: the magnetic response of natural materials at optical frequencies is often weak, and photonic devices mainly work with the electric part of the optical wave. Dielectric nanophotonics aims to solve this problem because it allows controlling both electric and magnetic resonances.

How does it work? According to Kuznetsov, most structures with a magnetic response contain metal elements and have high losses at optical frequencies. This naturally limits their performance. Plasmonic nanoantennas are structures based on metals — gold, aluminum, silver, and copper. They are almost ideal for working in the radiofrequency band, but are not good at controlling light on a nanoscale.

These limitations affect applications such as sensing, which is used in both biology and medicine. Almost every biochemical laboratory has devices based on surface plasmon resonance that can be used to monitor a chemical reaction on a nanoscale. The plasmon sensors overheat due to losses, which limits the use of sensing. For example, it is difficult to apply it to small concentrations of temperature-sensitive proteins.

Dielectric nanoantennas can solve this problem. Dielectric nanoantennas are structures based on materials with low losses such as silicon or gallium arsenide. Such materials react to both the electric and the magnetic parts of the wave. In addition, depending on the shape of the particle, responses can be varied, which gives much greater flexibility in controlling light.

In collaboration with the Data Storage Institute, the Metamaterials Laboratory at ITMO University (St. Petersburg, Russia) also conducts research on the optical properties of dielectric silicon metasurfaces that can conduct light in 2D on a potential optical chip. Other research focuses on detection of hot spots in the magnetic fields of dielectric structures. This research will make it possible to more effectively detect substances that are sensitive to the magnetic field at optical frequencies. Current projects involve research on luminescence amplification, nonlinear dielectric antennas, and hybrid nanoantennas and metasurfaces.

The next challenge in the field of dielectric nanophotonics will be creating tunable structures, the ability to dynamically control the optical properties of each nanoparticle separately. These developments will enable the creation of a display that will show 3D images. Future research in dielectric nanophotonics will allow the development of tunable flat optics, reducing the thickness of existing lenses to just a few hyperfine layers with dynamically controllable optical properties.

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Photonic Nanocavities

Photonic crystal resonator laser and refractive index sensor operating on paper substrates. (Credit: KAIST)

Researchers have developed a semiconductor photonic nanocavity laser that can operate on a paper substrate. Prof. Yong-Hoon Cho and his team in the Department of Physics at KAIST (Daejeon, Korea) hope that this novel method, which involves transferring nanosized photonic crystal particles onto a paper substrate with high absorptiveness, will enable the diagnoses of various diseases by using low-cost semiconductor sensors.

Photonic crystals, which utilize light as a medium to provide high bandwidths, can transfer large amounts of information. Compared with their electronic counterparts, photonic crystals also consume less energy to operate.

Normally, semiconductor photonic particles require substrates, which play only a passive role in the assembly and endurance of individual, functional photonic components. These substrates, however, are bulky and environmentally hazardous because they are made up of non-biodegradable materials.

The research team overcame these two shortcomings by replacing a semiconductor substrate with standard paper. The substrate's mass was reduced considerably, and because paper is made from trees, it degrades. Paper can be easily and cheaply acquired, which drastically reduces the unit cost of semiconductors. In addition, paper possesses superior mechanical characteristics. It is flexible and can be repeatedly folded and unfolded without tearing. These are traits that researchers have long sought for existing flexible substrates.

The team used a microsized stamp to detach photonic crystal nanobeam cavities selectively from their original substrate and transfer them onto a new paper substrate. Using this technique, the team removed nanophotonic crystals that had been patterned (using a process of selectively etching circuits onto a substrate) onto a semiconductor substrate with a high degree of integration and realigned them as desired on a paper substrate.

In this research, the nanophotonic crystals that the team combined with paper were 0.5 μm in width, 6 μm in length, and 0.3 μm in height — about one-hundredth of the width of a single hair (0.1 mm).

The team also transferred their photonic crystals onto paper with a fluid channel, which proved that it could be used as a refractive index sensor. Since photonic crystal particles have high sensitivity, they are highly suitable for applications such as sensors. Current commercial pregnancy diagnosis kits use paper with high absorptiveness.

Cho says that by using paper substrates, this technology can greatly contribute to the rising field of producing environmentally friendly photonic particles and that combining inexpensive paper and high-performance photonic crystal sensors enables them to keep costs low while designing technologies with high performance.

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This article was compiled by Sherrie Trigg, Editor and Director of Medical Content for Medical Design Briefs.