ASU researchers demonstrate the world's first white lasers


July 28, 2015

More luminous and energy efficient than LEDs, white lasers look to be the future in lighting and light-based wireless communication

While lasers were invented in 1960 and are commonly 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. white laser This schematic illustrates the novel nanosheet with three parallel segments created by the researchers, each supporting laser action in one of three elementary colors. The device is capable of lasing in any visible color, completely tunable from red, green to blue, or any color in between. When the total field is collected, a white color emerges. Photo by: ASU/Nature Nanotechnology Download Full Image

Researchers at Arizona State University have solved the puzzle. They have proven that semiconductor lasers are capable of emitting over the full visible color spectrum, which is necessary to produce a white laser.

The researchers have created a novel nanosheet – a thin layer of semiconductor that measures roughly one-fifth of the thickness of human hair in size with a thickness that is roughly one-thousandth of the thickness of human hair – with three parallel segments, each supporting laser action in one of three elementary colors. The device is capable of lasing in any visible color, completely tunable from red, green to blue, or any color in between. When the total field is collected, a white color emerges.

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, authored the paper, “A monolithic white laser,” with his doctoral students Fan Fan, Sunay Turkdogan, Zhicheng Liu and David Shelhammer. Turkdogan and Liu completed their doctorates after this research.

The technological advance puts lasers one step closer to being a mainstream light source and potential replacement or alternative to light emitting diodes (LEDs). Lasers are brighter, more energy efficient, and can potentially provide more accurate and vivid colors for displays like computer screens and televisions. Ning’s group has already shown that their structures could cover as much as 70 percent more colors 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.

“The concept of white lasers first seems counterintuitive because the light from a typical laser contains exactly one color, a specific wavelength of the electromagnetic spectrum, rather than a broad-range of different wavelengths. White light is typically viewed as a complete mixture of all of the wavelengths of the visible spectrum,” said Ning, who also spent extended time at Tsinghua University in China during several years of the research.

In typical LED-based lighting, a blue LED is coated with phosphor materials to convert a portion of the blue light to green, yellow and red light. This mixture of colored light will be perceived by humans as white light and can therefore be used for general illumination.

Sandia National Labs in 2011 produced high-quality white light from four separate large lasers. The researchers showed that the human eye is as comfortable with white light generated by diode lasers as with that produced by LEDs, inspiring others to advance the technology.

“While this pioneering proof-of-concept demonstration is impressive, those independent lasers cannot be used for room lighting or in displays,” Ning said. “A single tiny piece of semiconductor material emitting laser light in all colors or in white is desired.”  

Semiconductors, usually a solid chemical element or compound arranged into crystals, are widely used for computer chips or for light generation in telecommunication systems. They have interesting optical properties and are used to make lasers and LEDs because they can emit light of a specific color when a voltage is applied to them. The most preferred light emitting material for semiconductors is indium gallium nitride, though other materials such as cadmium sulfide and cadmium selenide also are used for emitting visible colors.

The main challenge, the researchers noted, lies in the way light emitting semiconductor materials are grown and how they work to emit light of different colors. Typically a given semiconductor emits light of a single color – 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 colors. The piece should be small enough, so that people can perceive only one overall mixed color, instead of three individual colors,” said Fan. “But it was not easy.”

“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.”

The most desired solution, according to Ning, would be to have a single semiconductor structure that emits all needed colors. He and his graduate students turned to nanotechnology to achieve their milestone.

The key is that at nanometer scale larger mismatches can be better tolerated than in traditional growth techniques for bulk materials. High quality crystals can be grown even with large mismatch of different lattice constants.

Recognizing this unique possibility early on, Ning’s group started pursuing the distinctive properties of nanomaterials, such as nanowires or nanosheets, more than 10 years ago.  He and his students have been researching various nanomaterials to see how far they could push the limit of advantages of nanomaterials to explore the high crystal quality growth of very dissimilar materials.

Six years ago, under U.S. Army Research Office funding, they demonstrated that one could indeed grow nanowire materials in a wide range of energy bandgaps so that color tunable lasing from red to green can be achieved on a single substrate of about one centimeter long. Later on they realized simultaneous laser operation in green and red from a single semiconductor nanosheet or nanowires. These achievements triggered Ning’s thought to push the envelope further to see if a single white laser is ever possible. 

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

“We have struggled for almost two years to grow blue emitting materials in nanosheet form, which is required to demonstrate eventual white lasers, ” said Turkdogan, who is now assistant professor at University of Yalova in Turkey.

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 color. 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 colors 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.”

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. 

Sharon Keeler

associate director, Ira A. Fulton Schools of Engineering

480-727-5618

Pakistani women at ASU provide glimpse of culture 'Beyond the Hijab'


July 28, 2015

When Tehreem Aurakzai stepped off the plane at Phoenix Sky Harbor airport, she didn’t fully appreciate her new role as “cultural representative.”

A scholar on a U.S. State Department exchange, Aurakzai was in Arizona to research American literature and culture. What she hadn’t planned was having to defend the reputation of her home country of Pakistan, which many Americans view as a strife-ridden hotbed of terrorism. Panelists at the March 26 'Beyond the Hijab' event Panelists at the March 26, "Beyond the Hijab" event (left to right) were moderator Neal A. Lester, Foundation Professor of English and director of Project Humanities at ASU; Zahra Hamdani; Mahwish Khan; Kanza Javed; Tehreem Aurakzai; and Aisha Usman. “If we are educating a woman, we are educating a nation,” Usman has said. Photo by: Brian Bossert Download Full Image

Aurakzai knows a different view of her home country. Pakistan has an active civil society, with many strong women leaders who are key to combating extremism in the Islamic South Asian Republic. It is also a place of rich culture, history and diversity. Aurakzai worked to convey Pakistan’s complexity to students and faculty she met at Arizona State University and in the surrounding community.

“Terrorism is everywhere because we are living in the age of violence," she said. "It doesn’t happen only in Pakistan.”

Aurakzai believes that to combat negative views of another culture, “The best way is to visit the country itself, meet people and experience on your own.”

Aurakzai is a lecturer in the English department at Kinnaird College for Women in Lahore, Pakistan. She and four other scholars from Kinnaird – Zahra Hamdani, Kanza Javed, Mahwish Khan and Aisha Usman – spent the spring 2015 semester at ASU funded by a grant from the U.S. Embassy in Islamabad, Pakistan.

The three-year project, “Globalizing Research and Teaching of American Literature: A University Partnership between ASU and Kinnaird College (Lahore),” aims to create an academic, research and knowledge exchange that will help to empower Pakistani women in academia and in society. Faculty and staff from ASU’s Department of English and the Center for the Study of Religion and Conflict co-direct the project and participated in the first wave of exchanges to Pakistan in fall 2014 while ASU hosted two scholars from Pakistan.

The newest Pakistani cohort arrived in January 2015 and participated in classes, attended cultural events, met with mentors and gave presentations of their own research and scholarship during the semester-long ASU visit.

One course that two Pakistani scholars attended was professor Melissa Pritchard’s English 594: Creative Writing – Fiction class. Not only did the women gain insight on trends in the field of contemporary American literature, their very presence in the classroom was a learning experience for their U.S. counterparts.

“Having students from overseas in ASU classes brings new perspectives that many students haven’t considered,” said Carolyn Forbes, assistant director of the ASU Center for the Study of Religion and Conflict and project manager for the exchange.

“These sorts of exchanges shape new research questions that lead to new ways of teaching the material.”

The ASU hosts, which in addition to Forbes included English faculty Deborah Clarke, Claudia Sadowski-Smith and Neal A. Lester, as well as Hardt-Nickachos Chair in Peace Studies Yasmin Saikia, provided opportunities for the Pakistani scholars to be immersed in multi-ethnic literatures of the U.S. In particular, the group focused on introducing the unique culture of the American Southwest.

“We built in to the grant a variety of cultural experiences that we thought would enhance their understanding of the literature they were reading,” Forbes said. “For example, we visited Santa Fe where they were able to tour one of the pueblos in the area and explore the intermingling of Native, Hispanic and Anglo culture.”

Forbes said that she and her colleagues did their best to minimize culture shock for the participants. One boon: Forbes found a house to rent, so the Pakistani women were all able to stay together.

“The house was within walking distance of campus, and this sort of living experience more closely resembled the way families live in Pakistan. The neighbors also did a lot of community activities together and this turned out to be a real plus since these sort of social interactions are also more common in Pakistan,” Forbes said.

The five women participated in a public panel discussion at ASU on March 26 called “Beyond the Hijab: Pakistani Women’s Perspectives.” Each scholar shared her introduction to American culture and focused on dispelling some of the preconceived notions of Pakistani women.

During the discussion, panelist Aisha Usman, a member of the English Literature faculty at Kinnaird, addressed a main area of concern – media's role in shaping the perceptions of women in Muslim countries.

Usman sees the media as emphasizing the stereotype that women in Pakistan aren’t able to fill leadership roles. But, she pointed out, Kinnaird College is a women’s college where most of the faculty are women. She also revealed that much of Pakistan’s higher education is co-educational, and women have provided strong leadership in that sector.

“Women in Pakistan do face difficult situations,” Usman said. “But they are also empowered and have minds of their own.” 

Pakistani women are typically charged with providing structure and early learning opportunities for their children, so it is with this in mind that Usman quips, “If we are educating a woman, we are educating a nation.”

As hoped, the exchange has affected the Pakistani women’s own perspectives. Aurakzai says that she now has a global mindset and will apply newly learned teaching skills when she goes back to her country.

“I grew as a person, and this was very intellectually stimulating experience,” she said. “I became an English professor because of the opportunities it provides to enter into others’ experiences. This is the spirit at the heart of this exchange.”

Each of the scholars will return to the U.S. this fall to present academic papers based on research they did while at ASU. Several ASU faculty will travel to Pakistan in the fall, and the next cohort of Kinnaird scholars is due to arrive at ASU in spring 2016.

Written by Paulina Iracka and Kristen LaRue

Kristen LaRue

communications specialist, Department of English

480-965-7611