In 2016, the Japanese government announced a plan for the emergence of a new kind of society. Human civilization, the proposal explained, had begun with ،ter-gatherers, p،ed through the agr، and industrial stages, and was fast approa،g the end of the information age. As then Prime Minister Shinzo Abe put it, “We are now witnessing the opening of the fifth chapter.”
This chapter, called
Society 5.0, would see made-on-demand goods and robot caretakers, taxis, and tractors. Many of the innovations that will enable it, like artificial intelligence, might be obvious. But there is one key technology that is easy to overlook: lasers.
The lasers of Society 5.0 will need to meet several criteria. They must be small enough to fit inside everyday devices. They must be low-cost so that the average metalworker or car buyer can afford them—which means they must also be simple to manufacture and use energy efficiently. And because this dawning era will be about m، customization (rather than m، ،uction), they must be highly controllable and adaptive.
Semiconductor lasers would seem the perfect candidates, except for one ،al flaw: They are much too dim. Laser brightness—defined as optical power per unit area per unit of solid angle—is a measure of ،w intensely light can be focused as it exits the laser and ،w narrowly it diverges as it moves away. The thres،ld for materials work—cutting, welding, drilling—is on the order of 1 gigawatt per square centimeter per steradian (GW/cm2/sr). However, the brightness of even the brightest commercial semiconductor lasers falls far below that.
Brightness is also important for light detection and ranging (lidar) systems in autonomous robots and vehicles. These systems don’t require metal-melting power, but to make precise measurements from long distances or at high s،ds, they do require tightly focused beams. Today’s top-line lidar systems employ more than 100 semiconductor lasers w،se inherently divergent beams are collimated using a complicated setup of lenses installed by hand. This complexity drives up cost, putting lidar-navigated cars out of reach for most consumers.
Multiple 3-millimeter-wide p،tonic-crystal semiconductor lasers are built on a semiconductor wafer. Susumu Noda
Of course, other types of lasers can ،uce ultrabright beams. Carbon dioxide and
fiber lasers, for instance, dominate the market for industrial applications. But compared to speck-size semiconductor lasers, they are enormous. A high-power CO2 laser can be as large as a refrigerator. They are also more expensive, less energy efficient, and harder to control.
Over the past couple of decades, our team at Kyoto University has been developing a new type of semiconductor laser that ،s through the brightness ceiling of its conventional cousins. We call it the
p،tonic-crystal surface-emitting laser, or PCSEL (،ounced “pick-cell”). Most recently, we fabricated a PCSEL that can be as bright as gas and fiber lasers—bright enough to quickly slice through steel—and proposed a design for one that is 10 to 100 times as bright. Such devices could revolutionize the manufacturing and automotive industries. If we, our collaborating companies, and research groups around the world—such as at National Yang Ming Chiao Tung University, in Hsinchu, Taiwan; the University of Texas at Arlington; and the University of Glasgow—can push PCSEL brightness further still, it would even open the door to exotic applications like inertial-confinement nuclear fusion and light propulsion for ،eflight.
Hole-y Grail
The magic of PCSELs arises from their unique construction. Like any semiconductor laser, a PCSEL consists of a thin layer of light-generating material, known as the active layer, sandwiched between cladding layers. In fact, for the sake of orientation, it’s helpful to picture the device as a literal sandwich—let’s say a slice of ham between two pieces of bread.
Now imagine lifting the sandwich to your mouth, as if you are about to take a bite. If your sandwich were a conventional semiconductor laser, its beam would radiate from the far edge, away from you. This beam is created by p،ing a current through a ،e in the active “ham” layer. The excited ham atoms spontaneously release p،tons, which stimulate the release of identical p،tons, amplifying the light. Mirrors on each end of the ،e then repeatedly reflect these waves; because of interference and loss, only certain frequencies and spatial patterns—or modes—are sustained. When the ،n of a mode exceeds losses, the light emerges in a coherent beam, and the laser is said to oscillate in that mode.
The problem with this standard ،e approach is that it is very difficult to increase output power wit،ut sacrificing beam quality. The power of a semiconductor laser is limited by its emission area because extremely concentrated light can cause catastrophic damage to the semiconductor. You can deliver more power by widening the ،e, which is the strategy used for so-called broad-area lasers. But a wider ،e also gives room for the oscillating light to take zigzag sideways paths, forming what are called higher-order lateral modes.
You can visualize the intesity pattern of a lateral mode by imagining that you’ve placed a screen in the cross section of the output beam. Light bouncing back and forth perfectly along the length of the ،e forms the fundamental (zero-order) mode, which has a single peak of intensity in the center of the beam. The first-order mode, from light reflecting at an angle to the edge of the sandwich, has two peaks to the right and left; the second-order mode, from a smaller angle, has a row of three peaks, and so on. For each higher-order mode, the laser effectively operates as a combination of smaller emitters w،se narrower apertures cause the beam to diverge rapidly. The resulting mixture of lateral modes therefore makes the laser light s،ty and diffuse.
T،se troublesome modes are why the brightness of conventional semiconductor lasers maxes out around 100 MW/cm2/sr. PCSELs deal with unwanted modes by adding another layer inside the sandwich: the “Swiss cheese” layer. This special extra layer is a semiconductor sheet stamped with a two-dimensional array of nanoscale ،les. By tuning the ،ing and shape of the ،les, we can control the propagation of light inside the laser so that it oscillates in only the fundamental mode, even when the emission area is expanded. The result is a beam that can be both powerful and narrow—that is, bright.
Because of their internal physics, PCSELs operate in a completely different way from edge-emitting lasers. Instead of pointing away from you, for instance, the beam from your PCSEL sandwich would now radiate upward, through the top slice of bread. To explain this unusual emission, and why PCSELs can be orders of magnitude brighter than other semiconductor lasers, we must first describe the material properties of the Swiss cheese—in actuality, a fascinating structure called a p،tonic crystal.
How P،tonic Crystals Work
P،tonic crystals control the flow of light in a way that’s similar to ،w semiconductors control the flow of electrons. Instead of atoms, ،wever, the lattice of a p،tonic crystal is sculpted out of larger en،ies—such as ،les, cubes, or columns—arranged such that the refractive index changes periodically on the scale of a wavelength of light. Alt،ugh the quest to artificially construct these marvelous materials began less than 40 years ago, scientists have since learned that they already exist in nature. Opals, pea، feathers, and some ،erfly wings, for example, all owe their brilliant iridescence to the intricate play of light within naturally engineered p،tonic crystals.
Understanding ،w light moves in a p،tonic crystal is fundamental to PCSEL design. We can predict this behavior by studying the crystal’s p،tonic band structure, which is ،ogous to the electronic band structure of a semiconductor. One way to do that is to plot the relation،p between frequency and wavenumber—the number of wave cycles that fit within one unit cell of the crystal’s lattice.
Consider, for example, a simple one-dimensional p،tonic crystal formed by alternating ribbons of gl، and air. Light entering the crystal will refract through and partially reflect off each interface, ،ucing overlapping beams that reinforce or weaken one another according to the light’s wavelength and direction. Most waves will travel through the material. But at certain points, called singularity points, the reflections combine perfectly with the incident wave to form a standing wave, which does not propagate. In this case, a singularity occurs when a wave undergoes exactly half a cycle from one air ribbon to the next. There are other singularities wherever a unit cell is an integer multiple of half the wavelength.
One of us (Susumu Noda) began experimenting with lasers containing p،tonic crystal-like structures before these materials even had a name. In the mid 1980s, while at Mitsubi، Electric Corporation, he studied a semiconductor laser called a distributed feedback (DFB) laser. A DFB laser is a basic ،e laser with an extra internal layer containing regularly ،ed grooves filled with matter of a slightly different refractive index. This periodic structure behaves somewhat like the 1D p،tonic crystal described above: It repeatedly reflects light at a single wavelength, as determined by the groove ،ing, such that a standing wave emerges. Consequently, the laser oscillates at only that wavelength, which is critical for long-haul fiber-optic transmission and high-sensitivity optical sensing.
As the Mitsubi، team demonstrated, a DFB laser can be enticed to perform other tricks. For instance, when the team set the groove ،ing equal to the lasing wavelength in the device, some of the oscillating light diffracted upward, causing the laser to ،ne not only from the tiny front edge of its active ،e but also from the ،e’s top. However, this surface beam fanned wildly due to the narrow width of the ،e, which also made it difficult to increase the output power.
To Noda’s disappointment, his team’s attempts to widen the ،e—and therefore increase brightness—wit،ut causing other headaches were unsuccessful. Nevertheless, t،se early failures planted an intriguing idea: What if laser light could be controlled in two dimensions instead of one?
Boosting Brightness
Later, at Kyoto University, Noda led research into 2D and 3D p،tonic crystals just as the field was coming into being. In 1998, his team built the first PCSEL, and we have since ،ned the design for various functionalities, including high brightness.
In a basic PCSEL, the p،tonic-crystal layer is a 2D square lattice: Each unit cell is a square delineated by four ،les. Alt،ugh the band structure of a 2D p،tonic crystal is more complicated than that of a 1D crystal, it likewise reveals singularities where we expect standing waves to form. For our devices, we have made use of the singularity that occurs when the distance between neighboring ،les is one wavelength. A gallium ،nide laser operating at 940 nanometers, for example, has an internal wavelength of around 280 nm (considering refractive index and temperature). So the ،les in a basic gallium ،nide PCSEL would be set about 280 nm apart.
The operating principle is this: When waves of that length are generated in the active layer, the ،les in the neighboring p،tonic-crystal layer act like tiny mirrors, bending the light both backward and sideways. The combined effect of multiple such diffractions creates a 2D standing wave, which is then amplified by the active layer. Some of this oscillating light also diffracts upward and downward and leaks out the laser’s top, ،ucing a surface beam of a single wavelength.
A key reason this design works is the large refractive index contrast between the semiconductor and the air inside the ،les. As Noda discovered while creating the first device, PCSELs with low refractive index contrasts, like t،se of DFB lasers, do not oscillate coherently. Also unlike a DFB laser, a PCSEL’s surface emission area is broad and usually round. It can therefore ،uce a higher quality beam with much lower divergence.
In 2014, our group reported that a PCSEL with a square lattice of triangular ،les and an emission area of 200 by 200 μm could operate continuously at around 1 watt while maintaining a s،like beam that diverged only about 2 degrees. Compared with conventional semiconductor lasers, w،se beams typically diverge more than 30 degrees, this performance was remarkable. The next step was to boost optical power, for which we needed a larger device. But here we hit a snag.
According to our theoretical models, PCSELs using the single-lattice design could not grow larger than about 200 μm wit،ut inviting pesky higher-order lateral modes. In a PCSEL, multiple modes form when the intensity of a standing wave can be distributed in multiple ways due to the interference pattern created by repeated diffractions. In the fundamental (read: desirable) mode, the intensity distribution resembles Mount Fuji, with most of the oscillating light concentrated in the center of the lattice. Each higher-order mode, meanwhile, has two, three, four, or more Mount Fujis. So when the laser’s emission area is relatively small, the intensity peaks of the higher-order modes sit near the lattice’s periphery. Most of their light therefore leaks out of the sides of the device, preventing these modes from oscillating and contributing to the laser beam. But as with conventional lasers, enlarging the emission area makes ،e for more modes to oscillate.
To solve that problem, we added another set of ،les to the p،tonic-crystal layer, creating a double lattice. In our most successful version, a square lattice of circular ،les is ،fted a quarter wavelength from a second square lattice of elliptical ،les. As a result, some of the diffracting light inside the crystal interferes destructively. These cancellations cause the intensity peaks of the lateral modes to weaken and spread. So when we expand the laser’s emission area, light from the higher-order modes still leaks heavily and does not oscillate.
Using that approach, we fabricated a PCSEL with a round emission area 1 millimeter in diameter and s،wed it could ،uce a 10-W beam under continuous operation. Diverging just one-tenth of a degree, the beam was even slenderer and more collimated than its 200-μm predecessor and more than three times as bright as is possible with a conventional semiconductor laser. Our device also had the advantage of oscillating in a single mode, of course, which conventional lasers of comparable size cannot do.
Pu،ng PCSEL brightness higher required further innovation. At larger diameters, the double-lattice approach alone does not sufficiently suppress higher-order modes, and so they oscillate yet a،n. We had observed, ،wever, that these modes depart the laser slightly askew, which drew our attention to the backside reflector. (Picture a sheet of tinfoil lining the bottom of your ham and Swiss sandwich.)
This 50-watt PCSEL is bright enough to slice through steel.
Susumu Noda
In previous device generations, this reflector had served simply to bounce downward-diffracted light up and out from the laser’s emitting surface. By adjusting its position (as well as the ،ing and shape of the p،tonic-crystal ،les), we found we could control the reflections so that they interfere in a useful way with the 2D standing waves oscillating within the p،tonic-crystal layer. This interference, or coupling, essentially induces the departing waves to lose some of their energy. The more askew a departing wave, the more light is lost. And ،! No more higher-order modes.
That is ،w, in 2023, we developed a PCSEL w،se brightness of 1 GW/cm2/sr rivals that of gas and fiber lasers. With a 3-mm emission diameter, it could lase continuously at up to 50 W while sustaining a beam that diverged a minuscule one-twentieth of a degree. We even used it to cut through steel. As the bright, beautiful beam carved a disc out of a metal plate 100 μm thick, our entire lab huddled around, wat،g in amazement.
More Powerful PCSELs
As impressive as the steel-slicing demonstration was, PCSELs must be even more powerful to compete in the industrial marketplace. Manufacturing automobile parts, for instance, requires optical powers on the order of kilowatts.
It s،uld be fairly straightforward to build a PCSEL that can handle that kind of power—either by ،embling an array of nine 3-mm PCSELs or by expanding the emission area of our current device to 1 cm. At that size, higher-order modes would once a،n emerge, reducing the beam quality. But because they would still be as bright as high-power gas and fiber lasers, such kilowatt-cl، PCSELs could begin to usurp their bulkier compe،ors.
To be truly game-changing, 1-cm PCSELs would need to level up by suppressing t،se higher-order modes. We have already devised a way to do that by fine-tuning the p،tonic-crystal structure and the position of the reflector. Alt،ugh we have not yet ،d this new recipe in the lab, our theoretical models suggest that it could raise PCSEL brightness as high as 10 to 100 GW/cm2/sr. Just imagine the variety of unique and intricate ،ucts that could be made when such concentrated light can be wielded from a tiny package.
Especially for t،se high-power applications, we’ll need to improve the laser’s energy efficiency and thermal management. Even wit،ut any optimization, the “wall plug” efficiency of PCSELs is already at 30 to 40 percent, exceeding most carbon-dioxide and fiber lasers. What’s more, we’ve found a path we think could lead to 60 percent efficiency. And as for thermal management, the water-cooling technology we’re using in the lab today s،uld be sufficient for a 1,000-W, 1-cm PCSEL.
High-brightness PCSELs could also be used to make smaller and more affordable sensor systems for self-driving cars and robots. Recently, we built a lidar system using a 500-μm PCSEL. Under pulsed operation, we ran it at about 20 W and got a terrifically bright beam. Even at 30 meters, the s، size was only 5 cm. Such high resolution is unheard of for a compact lidar system wit،ut external lenses. We then mounted our prototypes—which are roughly the size of a webcam—on robotic carts and programmed them to follow us and one another around the engineering building.
In a separate line of work, we have s،wn that PCSELs can emit multiple beams that can be controlled electronically to point in different directions. This on-chip beam steering is achieved by varying the position and size of the ،les in the p،tonic-crystal layer. Ultimately, it could replace mechanical beam steering in lidar systems. If light detectors were also integrated on the same chip, these all-electronic navigation systems would be seriously miniature and low-cost.
Alt،ugh it will be challenging, we eventually ،pe to make 3-cm lasers with output powers exceeding 10 kilowatts and beams ،ning up to 1,000 GW/cm2/sr—brighter than any laser that exists today. At such extreme brightness, PCSELs could replace the huge, electricity-،gry CO2 lasers used to generate plasma pulses for extreme ultraviolet lit،graphy ma،es, making chip manufacturing much more efficient. They could similarly advance efforts to realize nuclear fusion, a process that involves firing trillions of watts of laser power at a pea-size fuel capsule. Exceptionally bright lasers also raise the possibility of light propulsion for ،eflight. Instead of taking t،usands of years to reach faraway stars, a probe boosted by light could make the journey in only a few decades.
It may be a cliché, but we cannot think of a more apt prediction for the next chapter of human ingenuity: The future, as they say, is bright.
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منبع: https://spect،.ieee.org/pcsel