It is more luminous and energy efficient than LEDs. White lasers look to be the future in lighting and light-based wireless communication
Lasers came on the scene in 1960. They used in many applications. One characteristic of the technology has proven unattainable. No one has been able to create a laser that beams white light.
Researchers at Arizona State University have solved the puzzle. Semiconductor lasers are capable of emitting over the full visible color spectrum. This is necessary to produce a white laser.
The researchers have created a novel nanosheet. A thin layer of semiconductor measures roughly one-fifth of the thickness of a human hair in size. A thickness that is roughly a thousandth of the thickness of a human hair supports laser action.
It comes in one of three elementary colors. The device is capable of lasing in any visible color. It is tunable from red, green to blue, or any color. A white color emerges at the end.
The findings of SU’s Ira A. Fulton School of Engineering published on July 27 in the journal Nature Nanotechnology. The technological advance puts lasers one step closer to being a mainstream light source. They are a potential replacement or alternative to light emitting diodes (LEDs).
Lasers are brighter and more energy efficient. They can provide more accurate and vivid colors for display. This comes handy in case of computer screens and televisions. The structures could cover as much as 70 percent more colors. This is ahead of the current display industry standard.
Another important application could be in the future of visible light communication. The same room lighting systems could be fit for both illumination and communication. The technology under development is Li-Fi, which stands for light-based wireless communication.
This is opposed to the more prevailing Wi-Fi using radio waves. Li-Fi could be more than 10 times faster than current Wi-Fi. White laser Li-Fi could be 10 to 100 times faster than LED based Li-Fi. This is currently still under development.
The concept of white lasers first seems counter-intuitive. The light from a typical laser contains exactly one color. This is a specific wavelength of the electromagnetic spectrum. It is not a broad-range of different wavelengths. White light is viewed as a complete mixture of the wavelengths of the visible spectrum.
There is the typical LED-based lighting. A blue LED has phosphor materials coated on it. This converts the blue light to green, yellow and red light. This mixture of colored light is perceived by humans as white light. It is for general illumination.
Sandia National Labs in 2011 produced high-quality white light from four separate large lasers. The human eye is as comfortable with white light generated by diode lasers as with that produced by LEDs.
This pioneering proof-of-concept demonstration is impressive. The independent lasers are not for room lighting or in displays. A single tiny piece of semiconductor material emits laser light in all colors. Or it emits white light.
Semiconductors are used for computer chips or for light generation in telecommunication systems. They have interesting optical properties. They are used to make lasers and LEDs. This is because they can emit light of a specific color when a voltage runs through them.
The most preferred light emitting material for semiconductors is indium gallium nitride. Other materials such as cadmium sulfide and cadmium selenide may be used.
The main challenge lies in the way light emitting semiconductor materials are grown. They emit light of different colors. A semiconductor emits light of a single color. Blue, green or red color comes via a unique atomic structure and energy band gap.
The “lattice constant” represents the distance between the atoms. To produce all possible wavelengths in the visible spectral range you need several semiconductors. These have different lattice constants and energy band gaps.
The goal is to achieve a single semiconductor piece capable of laser operation. The three fundamental lasing colors are the goal. The piece should be small enough. This is so that people can perceive only one mixed color, instead of three individual colors. But it is not easy.
The key obstacle is an issue called lattice mismatch. The lattice constant is too different for the various materials required. They have not been able to grow different semiconductor crystals. The quality is not high enough. The traditional techniques have failed.
The most desired solution would be to have a single semiconductor structure. It would emit all needed colors. The researchers have turned to nanotechnology to achieve their milestone.
The key is that at nano-meter scale larger mismatches can be better tolerated. Traditional growth techniques for bulk materials is not an option. High quality crystals can be grown even with large mismatch of different lattice constants.
Recognizing this unique possibility early on, the distinctive properties of nano-materials got utilized. These included nano-wires or nano-sheets. But that was more than 10 years ago. Research into various nano-materials shows that you could push the limits of their advantages. This will allow exploration of the high crystal quality growth of dissimilar materials.
Six years ago, growth of nano-wire materials took place. This occurred in a wide range of energy band gaps. The color tunable lasing from red to green can occur on a single substrate of about one centimeter long.
The result is simultaneous laser operations. And they were in green and red from a single semiconductor nano-sheet or nano-wires. These achievements triggered the effort to push the envelope. This was to see if a single white laser is ever possible.
Blue is necessary to produce white. This proved to be a greater challenge. It's due to energy band gap and different material properties.
A struggle lasting two years grew blue emitting materials in nano-sheet form. This led to white lasers.
The group finally came up with a strategy to create the required shape. And then they converted the materials into the right alloy contents to emit the blue color. The unique growth strategy is the first demonstration of an interesting process. It is called dual ion exchange process.
This strategy of decoupling structural shapes and composition represents a major change of strategy. It is an important breakthrough. Finally, it is possible to grow a single piece of structure. And it contains three segments of different semiconductors.
This first proof of concept is important. But significant obstacles remain. We have to make such white lasers applicable for real-life lighting or display applications. One of crucial next steps is to achieve the similar white lasers under the drive of a battery.
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The researchers had to use a laser light to pump electrons to emit light. This experimental effort demonstrates the key first material requirements. And it will lay the groundwork for the eventual white lasers under electrical operation.