(Issue: May 2017)
What is Next After LEDS?
By Jack Curran
For this issue, I have been asked to look into my crystal ball and ponder what comes next after LED technology. Are there new technologies waiting in the wings ready to take their place as the replacement for LED lighting sources? Before we dive into that, I think it is important to consider how rapidly LEDs have taken over the lighting industry and displaced the former king of the hill, the incandescent lamp which was the most popular light source for almost 130 years.
A Brief History of LEDs
The first visible LEDs were developed in the early 1960s by Nick Holonyak at GE. Those early LEDs had a light output of only 0.001 lumens, barely enough to act as a signaling device. However, by the mid-1990s the output of AlInGaP LEDs had advanced enough to replace the incandescent lamps used in signaling devices such as traffic and railroad signals as well as aviation obstruction lights used on radio transmission towers.
Also in the late-1990s, Shuji Nakamora -- working at Nichia -- developed the blue LED, which subsequently led to the white LED (a combination of blue die and yellow phosphor). By 2010, reasonably priced white LED A19 lamps began to make an appearance, and today have pretty much replaced the standard incandescent lamp.
So looking along this timeline, although it has been about 55 years since the first visible LED made an appearance in a laboratory, it has only taken about seven years for the descendant of those first LEDs to move from market introduction to industry standard. I will not attempt to predict what will happen in the next half-century, but I will make some observations as to what technologies I think will have an effect over the next decade. There are two technologies that I feel should be discussed when thinking about the future.
The first is the topic of Quantum Dots, and although not quite a new light source, they are allowing major improvements in traditional phosphor converted LED technology. Quantum dots are extremely small particles on the order of a couple of nanometers (10-9 meters). Their small size gives rise to optical and electrical properties that are typically only seen at the atomic level and follow quantum mechanical rules. For example, particle wave functions (which particles exhibit at quantum level) are restricted to certain energy levels. Quantum dots exhibit properties that are between those of bulk semiconductor materials and molecules. Optoelectrical properties change as functions of both size and shape of the quantum dots.
When quantum dots are formed from phosphor materials, their emission properties are such that they will emit a very narrow band of wavelengths when excited. This allows the phosphor to be tuned to specific wavelengths. For example, dots with larger sizes will emit light more toward the red and yellow portion of the spectrum, while smaller dots will emit in the shorter wavelengths of green and blue.
Why is all this important to the future of lighting? For one reason, by tightly controlling the emission spectra of the LED output, higher efficacies can be achieved. For example, with traditional phosphors, a large portion of the output spectra consists of the longer wavelengths, which does not add to the quality of the light produced, but does decrease the efficacy. Using quantum dot techniques for phosphors, manufacturers are able to achieve improvements in efficacy while also creating light output with higher CRIs due to the distribution of phosphors. While more expensive than traditional YAG phosphors, Quantum Dot phosphors are becoming more popular as manufacturers strive to improve light output by narrowing the emission spectra of the phosphors. Also, by controlling the size of the phosphor particles, manufacturers can easily tune the color of the phosphor.
Laser Diode Light Sources
Most of us in the lighting industry have at least heard of lasers, maybe from a college class or familiarity with fiber optic communications or DVD players. Laser stands for Light Amplification by Stimulated Emission of Radiation. What follows is a brief description of how lasers function: electrons are pumped to a high-energy state and remain there until stimulated by photons. These photons interact with the electrons in the outer shell of the atoms making up the active layer of the diode triggering them to return to their ground states. Just as with an LED, as these electrons return to their ground states, they get rid of their excess energy by emitting photons during the transition to the lower state. Those photons are emitted such that their phase is the same as the stimulating photon. The resultant emitted photons are all in phase with each other. This means that their electromagnetic peaks and valleys are all lined up with each other (a state called coherent).
Two advantages of coherent light are: the bandwidth of the light is very narrow, centered around a particular wavelength; the coherency limits the dispersion of the light, meaning the beam of light remains tightly focused over large distances (hence laser’s use in fiber optic communications, long distance transmissions, measuring the distance from the earth to the moon, etc.
One way you can create a solid-state laser is to produce a crystal where the active region is located between two reflective surfaces. This is called a bulk laser diode. A second method (and the more common method in use today) is called a quantum well laser diode. Just as with the quantum dots above, the quantum well is created by limiting the size of the active region that in turn limits the possible wave functions associated with the emitted photons. As shown in Figure 1 above, the quantum well region is the active region where the photons are emitted and where the light of the device is concentrated. The light is emitted out the sides of the diode.
What if you replace the traditional LED in a white light with a solid-state laser diode emitting blue light? If the blue laser light shines on a yellow phosphor coating you will get down-converted yellow light which when mixed with the blue light will produce white light, just like with an LED device. However, the intensity of light produced can be thousands of times higher than that that of the typical LED device1 as shown in Figure 2. At present laser diode devices are much more expensive that similar LEDs. However, with more applications requiring laser diodes as a light source, for example automotive applications, the cost should come down significantly, allowing their use in many traditional lighting applications.
The narrow wavelength distribution for laser diode devices can be an advantage when considering issues such as circadian effects of blue light. Figure 3 shows the much narrower distribution possible for laser diodes rather than the traditional LED2.
If one is trying to limit the amount of blue light in the shorter wavelength region, the sharper cutoff of the laser diode is a major advantage when trying to avoid any disruptions to the circadian cycle of building occupants.
Another advantage of laser diodes versus LEDs is obvious when using them in LiFi communication systems. As you may be aware, a number of companies are exploring the approach of using the ability to switch light on and off to send binary light signals to devices, known as LiFi. Retailers, for example are using LiFi to transmit sales and specials messages to shoppers within their stores, often tailored to the area of the store where the shopper is located. Traditional WiFi (radio wave signals) has two drawbacks: spectrum limitations (the amount of data you can transmit on the frequencies that make up the radio spectrum; security as radio transmissions can be relatively easy to intercept and hack. Using light to transmit data can provide orders of magnitude increase in data carrying capability as compared to traditional radio frequency WiFi. In addition, since light does not penetrate walls (wood, plaster, metal, etc.), transmissions are much more secure.
One concern with lasers of any type is the high intensity of the light beam. Many devices that use lasers internally have locks designed to turn off the power to the laser if the device is opened while powered. In the case of a laser diode used for illumination, if the source is aimed at the phosphor then the laser light is scattered and no longer coherent when emitted from the device, making is safer. However, some sort of interface lock would still be required just in case the phosphor would be displaced allowing the laser light to reflect out of the device.
I have limited this discussion to technologies that directly affect the production of light (in terms of sources). There are many other technologies that will have a major impact on the lighting industry such as sensors, Power over Ethernet (PoE), transmitters used as part of the Internet of Things, just to name a couple. Those are subjects for another day, but by all means stay current on these areas as well because they have the potential to change the entire nature (and possibly players) for the lighting industry.
1 Innovations in Solid State Lighting, Nakamura, DenBaars, Speck, R&D Workshop, February 2, 2017.
2 Laser Diode + Phosphor [LDP] for Highly Directional, Solid-State Lighting, Raring, Rudy, McClaurin, Trottier, DOE SSL R&D Workshop, February 2, 2017.
Jack Curran is President of LED Transformations, LLC. Jack is a regular LM&M contributor. You may reach Jack at firstname.lastname@example.org.