ASU researchers show first monolithic white laser

Illustration of the nanosheet with three parallel segments, each supporting laser action in one of three elementary colours. When the total field is collected, a white colour emerges.

Researchers at Arizona State University have shown that a monolithic semiconductor laser can emit over the full visible colour spectrum, to produce the first white laser.

Their multi-segment semiconductor nanosheet, based on a quaternary alloy of ZnCdSSe, simultaneously lases in the red, green and blue. This is made possible by a novel nanomaterial growth strategy that enables separate control of the composition, morphology and therefore bandgaps of the segments.

The approach means that the nanolaser can be dynamically tuned to emit over the full visible-colour range, covering 70 percent more perceptible colours than the most commonly used illuminants.

The researchers, engineers in ASU's Ira A. Fulton Schools of Engineering, published their findings in the July 27 advance online publication of the journal Nature Nanotechnology. Cun-Zheng Ning, professor in the School of Electrical, Computer and Energy Engineering, wrote the paper: 'A monolithic white laser' with doctoral students Fan Fan, Sunay Turkdogan, Zhicheng Liu and David Shelhammer.

Realising a monolithic white laser has been challenging because of intrinsic difficulties in achieving epitaxial growth of the mismatched materials required for different colour emission.

The main difficulty lies in the way light emitting semiconductor materials are grown and how they work to emit light of different colours. Typically a given semiconductor emits light of a single colour - blue, green or red - that is determined by a unique atomic structure and energy bandgap.

The "˜lattice constant' represents the distance between the atoms. To produce all possible wavelengths in the visible spectral range you need several semiconductors of very different lattice constants and energy bandgaps.

"Our goal is to achieve a single semiconductor piece capable of laser operation in the three fundamental lasing colours. The piece should be small enough, so that people can perceive only one overall mixed colour, instead of three individual colours," said Fan.

"The key obstacle is an issue called lattice mismatch, or the lattice constant being too different for the various materials required," Liu said. "We have not been able to grow different semiconductor crystals together in high enough quality, using traditional techniques, if their lattice constants are too different."

Ideally a single semiconductor structure would emit all the colours. Ning and his graduate students turned to nanotechnology to achieve this. At nanometer scale, larger mismatches can be tolerated than in traditional growth techniques for bulk materials. High quality crystals can be grown even with large mismatch of different lattice constants.

Six years ago, under US Army Research Office funding, they demonstrated that one could indeed grow nanowire materials in a wide range of energy bandgaps so that colour tunable lasing from red to green can be achieved on a single substrate of about 1cm long. Later on they realised simultaneous laser operation in green and red from a single semiconductor nanosheet or nanowires.

Blue, necessary to produce white, proved to be a greater challenge with its wide energy bandgap and very different material properties. 

After exhaustive research, the group finally came up with a strategy to create the required shape first, and then convert the materials into the right alloy contents to emit the blue colour. Turkdogan said: "To the best of our knowledge, our unique growth strategy is the first demonstration of an interesting growth process called dual ion exchange process that enabled the needed structure."

This strategy of decoupling structural shapes and composition represents a major change of strategy and an important breakthrough that finally made it possible to grow a single piece of structure containing three segments of different semiconductors emitting all needed colours and the white lasers possible. Turkdogan said that, "this is not the case, typically, in the material growth where shapes and compositions are achieved simultaneously."

The essence of this technique is to manipulate the position of the substrate along the axial temperature gradient in the reactor to optimise the substrate temperature for the desired alloy composition. So, rather than attempt to directly grow a ZnS-rich segment next to the CdS- and CdSe-rich segments, the team optimised the growth sequence and conditions such that CdSe nanosheet growth was followed by a dual-ion exchange reaction, a mechanism that has not been reported previously, to finally achieve a ZnS-rich nanosheet structure.

By independently controlling the optical pumping power to each segment the team has demonstrate full-colour tunable lasing over the entire triangular colour gamut, and white colour lasing in particular. The wavelength spans 191nm and is the largest ever reported for a monolithic structure.

While this first proof of concept is important, significant obstacles remain 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. For the present demonstration, the researchers had to use a laser light to pump electrons to emit light. This experimental effort demonstrates the key first material requirement and will lay the groundwork for the eventual white lasers under electrical operation.

The researchers say that the technological advance puts lasers one step closer to being a mainstream light source and potential replacement or alternative to LEDs. Lasers are brighter, more energy efficient, and can potentially provide more accurate and vivid colours for displays like computer screens and televisions. Ning's group has already shown that their structures could cover as much as 70 percent more colours than the current display industry standard.

Another important application could be in the future of visible light communication in which the same room lighting systems could be used for both illumination and communication. The technology under development is called Li-Fi for light-based wireless communication, as opposed to the more prevailing Wi-Fi using radio waves. Li-Fi could be more than 10 times faster than current Wi-Fi, and white laser Li-Fi could be 10 to 100 times faster than LED based Li-Fi currently still under development.