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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