Sony has been doing research and development of semiconductor lasers since over 40 years ago, developing laser technologies for optical discs along the way including CDs, DVDs and Blu-ray discs. The general semiconductor lasers, called edge-emitting lasers, emit light parallel to the semiconductor wafer surface.
Our R&D efforts on the Vertical-Cavity Surface-Emitting Laser (VCSEL) began around 2000. This laser emits light in a direction perpendicular to the wafer surface. It has some advantages over the edge emitting laser: more compact size, lower power consumption and easier two-dimensional arrangement.
VCSELs emit near-infrared laser have been widely used in present days. On the other hand, however, blue and green VCSELs with emission in the visible light range are difficult to be developed. No manufacturer has yet succeeded in bringing such VCSELs to market. In these circumstances, Sony is developing blue and green VCSELs using a unique structure, with an eye on the widening range of laser applications. Leveraging the features of the VCSEL, compact size and low power consumption, we create applications such as compact, lightweight mobile displays. We also take advantage of the ease of two-dimensional arrangement to produce high-output light sources, which can be applied to ultra-large projectors as well as to automotive headlamps that can control illumination patterns like a display. We think that these efforts will lead to the creation of new technologies.
Particularly important are mobile applications. Mobile displays that enable virtual reality, augmented reality and mixed reality, such as PlayStation VR and Google Glass, are now attracting attention. However, the realization of smart glass and other wearable technologies has been hindered in part by the lack of ultra-small low-power lasers suitable for compact low-power mobile displays. Concretely, the existing glasses-type displays typically have a large light source that consumes much electricity and a larger than normal battery that powers the light source attached to their outer side and connected with wires.
By contrast, the VCSEL technology we are currently developing reduces both the device size and power consumption by double digits. The technology is expected to allow the light source and battery to be built into a single eye glass frame. The visible light VCSEL should make a truly smart compact wearable display a reality.
One feature of the VCSEL is that it can limit the optical output when designed to be driven by a high current. When we create a type of display that emits laser light directly to the retina, it is nice to be able to design the laser so that its optical output does not exceed a certain level. That gives customers a sense of safety. Realizing VCSELs with emission in the visible light range helps create diverse kinds of mobile displays.
Here is how the VCSEL works. First, let’s look at an example of a widely used VCSEL with emission in the near-infrared wavelength region (800 to 1000 nm). The above diagram shows the structure of a general near-infrared VCSEL. In the VCSEL, a region that emits light, called the active layer, is sandwiched between two highly reflective upper and lower mirrors, which are typically with reflectance of 99% or more. The highly reflective mirror, generally known as a distributed bragg reflector, is formed by alternating layers of materials with varying refractive indices. Laser oscillation occurs when the light generated by the active layer resonates between the two highly reflective mirrors.
To create a high-performance VCSELs, we need three things:
The infrared VCSEL meets these requirements by performing crystal growth on gallium arsenide (GaAs) substrate. It is easy for a GaAs-based VCSEL to obtain a structure that efficiently concentrates light and electricity to the central part of the device, among the three requirements mentioned above. This approach involves forming an aluminum arsenide layer in the layered structure inside the device and oxidizing that layer laterally from outside to inside of the device, ultimately leaving only the central part of the device unoxidized. This unoxidized part acts as a lens, which can concentrate light to the center. Also, the oxidized part is insulated, enabling electric current to be concentrated to the central part as with light.
However, applying the VCSEL technology to the visible light range requires using materials other than gallium arsenide. Gallium nitride (GaN) is often used to create green and blue light emitters. Having developed semiconductor lasers for Blu-ray, Sony had the knowledge for forming high-quality light-emitting layers made of gallium nitride. However, we were not able to find any process for constructing a structure that would concentrate light in GaN-based materials.
It was in 2015 that we first presented GaN-based VCSEL technology at an academic conference. At the time, we were seeking to develop a planar mirror structure similar to that used in GaAs-based VCSELs. However, increasing efficiency by confining light remained difficult. Finding the way to form a light confinement structure inside the device was a challenge for all engineers who were engaged in the development of green and blue VCSELs in the world.
To overcome this challenge, we made a major change of course in 2016. We thought that using a lens-shaped curved mirror as a light reflector, instead of a planar mirror, would concentrate light to the central part of the device. Research and development personnel working on semiconductor lasers had taken it for granted that VCSELs used planar mirrors. We started by changing that concept drastically.
This out-of-the-box idea came from one of the tools we frequently used in our lab - optical fibers. Optical fiber refers to a glass filament that carries light along its length. Graded index (GI) optical fiber having a parabolic refractive index distribution, in particular, not only carries light but also reflects light back into its core. This prevents light from leaking when the fiber is bent. Creating a structure having a parabolic distribution similar to that of the GI fiber in the VCSEL should make it possible to concentrate light to the central part of the laser. We thought that, in order to do so, we might as well use a parabolic lens as one of the light reflector mirrors.
The idea was quite unconventional, and we could not have realized it without the Sony R&D Center’s culture open to novel ideas not found in textbooks on semiconductor lasers. Sony’s accumulated R&D efforts and teamwork also helped when we implemented the concave lens. The R&D engineers of the semiconductor laser team had no experience in creating a lens on a semiconductor. But, at the Sony R&D Center, we have the image sensor team working next to us. With imaging sensors, it is common to create a lens on a silicon wafer to ensure efficient concentration of light onto the sensor. While we work in different departments, we interact with each other through business training sessions and other occasions. So, we obtained the necessary information from the image sensor team and refined the obtained know-how. This way, we ultimately succeeded in establishing a technique for creating a high-quality lens structure for curved mirror for gallium nitride-based VCSELs.
And we implemented a new type of VCSEL that uses a curved mirror as one of its resonator mirrors. In this VCSEL, the curved mirror concentrates light to the center, while electric current is concentrated by ion implantation. In 2019, we successfully demonstrated the capability of the blue VCSEL to support the world’s smallest oscillation threshold of 0.25 mA. In 2020, we also implemented the world’s first green VCSEL capable of operating at room temperature while supporting the smallest oscillation threshold as with the blue VCSEL. These green and blue VCSELs are attracting attention both inside and outside the company, receiving not only Sony’s internal awards but awards from external organizations as well.
We expect that, as green and blue VCSELs are utilized in practical use in addition to red VCSELs, their application to displays will further increase. There are many potential applications, including high-brightness projectors and wearable devices like smart glasses. They span a wide range of fields from lighting and machining to healthcare. The practical use of visible light VCSELs will most likely promote the spread of smart glasses.
Moreover, there is a prospect that advances in VCSEL technology will enhance the accuracy of position determination systems such as the global positioning system (GPS). This is because a GaN-based VCSEL can emit light in the ultraviolet region and a highly accurate clock can be created using it. In the GPS and other position determination systems, terrestrial stations receive time information from atomic clocks on satellites and calculate the distance from each of the satellites based on the time differences to obtain location information. We think that, if ultraviolet VCSELs enhance the accuracy of the clocks on terrestrial stations, it will be possible to increase the GPS positioning accuracy from several meters to several millimeters in the future. Obtaining more accurate location information will increase the chance for entirely new applications to be developed.
Sony has a long history in the research and development of semiconductor lasers. We have a great deal of know-how and a superb experiment environment as well as many excellent experts. In addition, our organization encourages us to take on new challenges, which I think appeals to younger researchers as well. The use of semiconductor lasers is not limited just to new displays mentioned herein. Depending on the wavelength they use, semiconductor lasers could be used as sensing light sources that support autonomous driving, potentially contributing to realizing a safer society. I hope that our work draws interest from younger researchers willing to tackle the challenge of developing new types of lasers.
Sony gives us opportunities to train ourselves. Getting transferred to the lab from a business division, I had little research experience. I got to build my experience as I learned from many experts with doctorates and put together papers. Sony wants us to take on new challenges continuously, which has led to the success in developing green and blue VCSELs. I want to see younger researchers join us for even greater achievements.
