Dual-color lasers could lead to cheap and efficient LED lighting

May 03, 2013

(Phys.org) —A new semiconductor device capable of emitting two distinct colours has been created by a group of researchers in the US, potentially opening up the possibility of using light emitting diodes (LEDs) universally for cheap and efficient lighting.

The proof-of-concept device, which has been presented today, May 3, in Semiconductor Science and Technology, takes advantage of the latest nano-scale materials and processes to emit green and red light separated by a wavelength of 97 nanometres—a significantly larger bandwidth than a traditional semiconductor.

Furthermore, the device is much more energy efficient than traditional LEDs as the colours are emitted as lasers, meaning they emit a very sharp and specific spectral line—narrower than a fraction of a —compared to LEDs which emit colours in a broad bandwidth.

One of the main properties of semiconductors is that they emit light in a certain , which has resulted in their widespread use in LEDs. The wavelength range in which a given semiconductor can emit light—also known as its bandwidth—is typically limited in the range of just tens of nanometres. For many applications such as lighting and illumination, the wavelength range needs to be over the entire visible spectrum and thus have a bandwidth of 300 nm.

Single cannot emit across the entire and therefore need to be 'put' together to form a collection that can cover the entire range. This is very expensive and is, to a large extent, the reason why semiconductor LEDs are not yet used universally for lighting.

In this study, the researchers, from Arizona State University, used a process known as to create a 41 -long nanosheet made from and powders, using silicon as a substrate.

Lead author of the study, Professor Cun-Zheng Ning, said: "Semiconductors are traditionally 'grown' together layer-by- layer, on an atom-scale, using the so-called epitaxial growth of crystals. Since different semiconductor crystals typically have different lattice constants, layer-by-layer growth of different semiconductors will cause defects, stress, and ultimately bad crystals, killing light emission properties."

It is because of this that current cannot have different semiconductors within them to generate red, green and blue colours for lighting.

However, recent developments in the field of nanotechnology mean that structures such as nanowires, nanobelts and nanosheets can be grown to tolerate much larger mismatches of lattice structures, and thus allow very different semiconductors to grow together without too many defects.

"Multi-colour light emission from a single nanowire or nanobelt has been realized in the past but what is important in our paper is that we realized lasers at two distinct colours. To physically 'put' together several lasers of different colors is too costly to be useful and thus our proof-of concept experiment becomes interesting and potentially important technologically.

"In addition to being used for solid state lighting and full color displays, such technology can also be used as light sources for fluorescence bio and chemical detection," continued Professor Ning.

Explore further: Model-based highly efficient and highly reliable development process for digital controlled power supply units

More information: 'Simultaneous two-colour lasing in a single CdSSe hetereostructure nanosheet' by F Fan et al. 2013 Semicond. Sci. Technol. 28 065005. iopscience.iop.org/0268-1242/28/6/065005/article

Related Stories

New alloys key to efficient energy and lighting

Mar 22, 2010

A recent advance by Arizona State University researchers in developing nanowires could lead to more efficient photovoltaic cells for generating energy from sunlight, and to better light-emitting diodes (LEDs) that could replace ...

Conquering LED efficiency droop

Apr 30, 2012

WASHINGTON, April 30--Like a coffee enthusiast who struggles to get a buzz from that third cup of morning joe, light-emitting diodes (LEDs) seem to reach a point where more electricity no longer imparts the ...

Zeroing in on the elusive green LED

Apr 25, 2011

Researchers at Rensselaer Polytechnic Institute have developed a new method for manufacturing green-colored LEDs with greatly enhanced light output.

Berkeley Researchers Light Up White OLEDs

Apr 06, 2010

(PhysOrg.com) -- Light-emitting diodes, which employ semiconductors to produce artificial light, could reduce electricity consumption and lighten the impact of greenhouse gas emissions. However, moving this ...

Recommended for you

Drivebot aims to touch driver bases for safety, savings

3 hours ago

Five Thailand-based engineers have developed a dongle device that serves as a fitness tracker for cars and have turned to Indiegogo to raise funds for bringing it forward. The attraction is that it is a simple ...

HP announces Sprout—a truly innovative workstation

5 hours ago

Hewlett-Packard Co has announced the development of a new kind of computer workstation—one that combines the power of a desktop computer with 3D scanning and projection—and adds a second display surface ...

Q&A: 'Interstellar' filmmaker Nolan on his robots

5 hours ago

In his secrecy-shrouded sci-fi extravaganza "Interstellar," filmmaker Christopher Nolan isn't just taking audiences to outer space. He's also sending a couple of robots along for the ride—and they're just ...

User comments : 19

Adjust slider to filter visible comments by rank

Display comments: newest first

italba
2 / 5 (4) May 03, 2013
Not laser, LED! Please correct the title!
PPihkala
5 / 5 (1) May 03, 2013
"Multi-colour light emission from a single nanowire or nanobelt has been realized in the past but what is important in our paper is that we realized lasers at two distinct colours. To physically 'put' together several lasers of different colors is too costly to be useful and thus our proof-of concept experiment becomes interesting and potentially important technologically.

Here they write laser, not LED.
Eikka
3.4 / 5 (5) May 03, 2013
The narrower the spectrum peaks, the worse the light quality. It doesn't matter that you can make two peaks or ten peaks, it's still not a continuous spectrum and you can't distinguish colors properly under that sort of light. Real objects are percieved by how they subtract color out of the ambient light, so all wavelenghts of visible light must be present before you can properly see the color of an object. Otherwise your ability to distinguish hues gets reduced to a small palette; all things yellow start to look the same yellow, all things green the same green, etc. with differences mostly in saturation and lightness.

That's why the LED lighting industry is using phosphors to fill in the missing wavelenghts.
italba
1 / 5 (2) May 03, 2013
@PPihkala: "the colours are emitted as lasers": Colours as lasers (narrow bandwidth or even single wavelength) does not mean these devices are lasers! Laser light must be phase coherent!
italba
1 / 5 (2) May 03, 2013
@Eikka: Cone eyes receptors are wide band, with a slightly preference for its own colour. You will see a single object "yellow" because the light it emits (or reflect, it's the same) stimulates equally the red and the green cones and does not stimulate the blue ones. You can't distinguish between a stronger narrow band pure yellow, a wide band one, or two separate but mixed red and blue lights. That's the way television and colour print works.
Tektrix
not rated yet May 03, 2013
@PPihkala: " Laser light must be phase coherent!


Nope. For instance, the coherence length of most diode lasers is less than a cm. The light is still laser light past that point.
italba
1 / 5 (1) May 03, 2013
@Tektrix: From the "laser" page of Wikipedia, fundamentals: "Lasers are distinguished from other light sources by their coherence." How do you describe laser light?
Tektrix
not rated yet May 03, 2013
@ italba: Trick question: Is a single photon coherent?
italba
1 / 5 (1) May 03, 2013
@Tektrix: Are you learning quantum physics?
Eikka
not rated yet May 03, 2013
You can't distinguish between a stronger narrow band pure yellow, a wide band one, or two separate but mixed red and blue lights. That's the way television and colour print works.


Yes and no. Print and television work by transmitting only certain wavelenghts to your eyes to create pure or mixed colors, whereas real objects interact with all wavelenghts of light.

What would a yellow object look like under red and green light, but no wavelenghts in between? Well, it could look pretty dark if it doesn't reflect much of either, and it doesn't necessarily reflect the red and the green in the right proportions to appear yellow!

There's not one green or one yellow color, or one blue or one red. The object responds to a spectrum of wavelenghts, so two yellow objects would appear the same color but different lightness under the narrowband light because they both reflect some of that wavelenght, whereas the wideband light reveals the proper weighting of spectrum between red/green

italba
1 / 5 (1) May 03, 2013
For our eyes (and our brain) something that stimulate both our red and green cone receptors
IS yellow, it could appear slightly darker or lighter but our brain will compensate. You have to seek very hard for something that will reflect only yellow but not red or green. Holograms, for instance, or jewels. If you have a jewellery store, you'll probably best light it up with halogen lamps, but for home or outdoor lighting you can hardly find any difference with tree monochrome (R, G, B) light sources.
LightEmma
1 / 5 (1) May 04, 2013
LED lights should be the future of lighting. Aside from using less energy and being friendly to the eyes, it gives a home a very modern and sophisticated feel. You really do not have to buy new lamps and pendants. Replacement bulbs like these are widely available tinyurl.com/a4cnh5k. Should help you save big time!
Eikka
not rated yet May 05, 2013
For our eyes (and our brain) something that stimulate both our red and green cone receptors
IS yellow, it could appear slightly darker or lighter but our brain will compensate.


It can also appear more or less saturated (pure) depending on how much it also stimulates the blue receptors, which are also overlapping to some extent with the green receptors. You basically have three different sensations: hue, saturation and lightness.

And there's not one yellow hue either. People can distinguish pure monochromatic light with a difference of about 1 nm in wavelenght, so between 570–590 nm there's actually about 20 different colors that people would consider to be yellow, and distinguish as separate colors.

Your monochromatic light source with a narrow spectrum peak will only show one of them, because it doesn't reveal the true weighting of the spectrum. The object may return more 570nm yellow than 590 nm yellow, but your lightsource has just 560 nm light so that's what you see.
Eikka
not rated yet May 05, 2013
For clarification:

Hue is the "center of mass" of the spectrum that you see. It's the eye's sensation of what appears to be the peak wavelenght of the distribution of wavelenghts. That's why it averages red and green into yellow.

There's also hues that do not have a wavelenght, like pink and magenta, which come about when you remove green and get a double peaked distribution. The eye is not fooled to think that it's green, because it can sense that there is no green, so your brain invents a pair of colors that don't really exist.

Saturation is the sensation of the narrowness of the distribution. Colors are sensed more saturated when they stimulate only one or two kinds of receptors. A wider distribution appears more white or grey.

Lightness is how dark or light the color appears - how much light it returns overall.

Hue and saturation together give you the ability to distinguish about ten million different "chromaticities", which is roughly equivalent to saying "colors".
italba
1 / 5 (1) May 05, 2013
@Eikka: Sorry, your analysis is too much theoretical. We have no hue or saturation sensors in our eyes just three kind of luminosity sensors, with peak sensitivity for red, green or blue colours. We see the different colours mostly with our brain, there are dozen of optical tricks that will let you see different colours for the same one. We call an object "yellow" and we see it yellow, under the full sun light or the last sunset shade. The HSL (or HSV, or HSI) are just MODELS, three-dimensional maps of the colours world, but do not maps well our colours sensitivity. For instance, we can see much more shades of green than red, but there is no such difference in HSV model. Lastly, have a look at this http://popsci.typ...las.html report of the Mitsubishi laser colour tv. The writer is mostly impressed by the colour quality!
Eikka
1 / 5 (1) May 05, 2013
italba, you are still confusing the fact that televisions can simulate all colors with a limited set of colors because that's all they have to deal with. They're sending light directly to your eye at the right wavelenghts to fool you, so they don't have to deal with distributions and spectrums.

In lighting a room, the continuity of the spectrum becomes important because real objects don't care what your eyes are sensitive to. What I'm talking about is that there's different yellows, some more green, some more red, some combinations of red and green with no actual yellow, and without a full spectrum of wavelenghts between red and green you cannot see the difference.

Under monochromatic yellow light, you couldn't tell a ripe banana from a slightly raw banana because they'd both look like that yellow.

We call an object "yellow" and we see it yellow, under the full sun light or the last sunset shade.


That's a completely different effect.
italba
1 / 5 (1) May 05, 2013
As I said before, pure monochrome object are very rare in nature. Normal object reflects or transmits a broad spectrum of frequencies, with one or more small peaks that we call colour. That's why a normal object will appear just the same under a wide spectrum light or three (NOT ONE!) red, green and blue narrow spectrum or monochrome lights. We have not spectrum analyzers in our eyes, we cannot see "holes" in the spectrum of the light reflected by objects. We can only distinguish how much green, red and blue there is in that light. We will probably see in a different way only objects that break up the spectrum of light, like crystals or same insect's exoskeleton.
hb_
not rated yet May 24, 2013
@Eikka: Cone eyes receptors are wide band, with a slightly preference for its own colour. You will see a single object "yellow" because the light it emits (or reflect, it's the same) stimulates equally the red and the green cones and does not stimulate the blue ones. You can't distinguish between a stronger narrow band pure yellow, a wide band one, or two separate but mixed red and blue lights. That's the way television and colour print works.


A colored surface that does not have a color in the emitted color of the lasers will be percieved as black. And, even if the surface has a broad-band emitting profile, that profile might have a strongly wave-length dependent amplitude. So no, you do need broadband light for true color rendering.

Also, the spectral sensitivities of the cones overlap, so you can distinguish between the different ways of mixing the same average wave-length. The ratio of the response of the three different cones will differ from the two cases.
Eikka
1 / 5 (1) May 29, 2013
Normal object reflects or transmits a broad spectrum of frequencies, with one or more small peaks that we call colour.


The entire visible spectrum of the object matters when the eye is measuring it - not just the peaks. It's the shape of the spectrum that gives the object its distinct hue, because each cone in the retina samples the spectrum over a range of wavelenghts and effectively sums them up. With narrow band RGB light, you're only measuring the spectra at very distinct points, messing up the whole process, which has an effect of making it difficult to tell one color from another when the difference is small. Maybe one has a slightly bigger bump at blue-green but since your lightsource doesn't have that wavelenght, you can't tell.

Shining a light at the object at one single, or only a couple distinct wavelenghts just reveals how much of those wavelenghts the object reflects, but doesn't reveal the true "weight" of its whole spectrum as the eye would sense it.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.