Quantum Phenomena in the Lighting Design Industry for Music

Madhu Ashok

University of Rochester

The Institute of Optics

Research Paper Submitted to Nick Vamivakas on 4/30/2015

Abstract: Advancements in LED technology has made lighting fixtures more

efficient and affordable, while also broadening horizons for visualizations in live

music productions. LEDs, both as light sources and display devices, have

revolutionized lighting design and created an alternative to incandescent sources

—which are phasing out in industrial lighting. Lighting designers in the music

industry desire fixtures with a large color gamut, and with the introduction of the

blue LED this became possible with RGB systems. A blue light-emitting diode

consists of InGaN quantum wells between cladding layers of GaN, and with

advancements in epitaxial deposition these layers can be varied to change the

emitted wavelength of light. In this report we will be exploring the quantum

phenomena of the Nobel Prize winning blue LED, as well as the possibility for

quantum dot LEDs to be implemented in display technology in the future.

Introduction

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Lighting design in the music industry has been a growing passion of mine, so it seemed

appropriate to write on a topic which was relevant and interesting to me. As an avid live music

follower I have noticed the growth of LED technology in an industry previously dominated by

incandescent and discharge lamp sources. A most notable example is from my favorite band

Lotus, where lighting designer Scott Huston implements Clay Paky B-EYE K20 fixtures as the

centerpiece of his work [1].

Figure 1: Clay Paky B-EYE LED light fixtures used by designer Scott Huston from lotus.

These luminaires provide greater energy efficiency and an increased color pallet along with

breathtaking kaleidoscopic effects [1].

The advent of light-emitting diodes in the music industry, as well as lighting design as a whole,

can be attributed to the Nobel Prize winners Isamu Akasaki, Hiroshi Amano, and Shuji

Nakamura for developing efficient blue GaN LEDs—a crucial piece necessary to making white

light with LEDs [2]. Blue LED technology is based on doping gallium nitride with aluminum

and indium to create heterostructures and quantum well layers which optimize the emitted

output. Prior to advancements in crystal growth techniques such as Molecular Beam Epitaxy

(MBE) and Metalorganic Vapour Phase Epitaxy (MOVPE), achieving blue light’s lower

wavelength proved to be more sophisticated than its red and green predecessors developed in the

1950s [3].

By combining red, green, and blue LEDs, a variety of colors can be generated through

additive color mixing; thus revolutionizing lighting fixtures which were limited to color

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rendering through the use of filters. In addition, light-emitting diodes can last over 50,000 hours

and save energy by an order of ten to their incandescent counterparts. This technologies

efficiency is attributed to combinations of AlGaN/GaN, InGaN/GaN, and InGaN/AlGaN

quantum well layers.

Theory

A light-emitting diode is a directionally biased optoelectronic device that emits visible (or

non-visible) light when a voltage is applied across the semiconductor chip. Half of the chip is

doped with p-type impurities which have a net positive charge, and the other half is doped with

n-type impurities containing net negative charge. At the junction of P-type and N-type material

free electrons will move from the net negative material to the positive when a voltage is applied.

The color of light emitted from the diode depends on the materials used in the chip as well as the

layers of heterostructures and quantum wells [4].

Figure 2: Current flows from the p-type material to the n-type material, which causes electrons to

recombine with a hole from the p-type material and emit energy in the form of light (red arrows).

Quantum wells are a classification of heterostructures which are made by layering two or

more materials together at the atomic level. These structures, when made with two

semiconductors of varying bandgap energies, are crucial in optoelectronics for their precise and

efficient emission of wavelength specific light. Although a rough approximation with many

assumptions, these quantum wells can be modeled with a finite square potential well. Let us

perform the thought experiment of a layer of InGaN with thickness L z in between two “barrier”

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layers of GaN. The barrier potential height V o will represent a known potential difference

between the two materials seen by an electron, and each material will exhibit an electron

effective mass mGaN¿ and mInGaN

¿ which is tabulated.

Figure 3: Finite potential well model for the first two solutions of the Schrödinger time

independent equation. The quantum well consists of two barrier layers of GaN surrounding a

layer of InGaN.

An incident wave with known zero-point energy E (assuming E<V o) impinges the barrier

causing solutions to Schrödinger’s equations in the form:

ψ ( z )=G eκz , z←Lz

2ψ ( z )=Asin (kz )+Bcos (kz ) ,−

L z

2< z<

L z

2ψ ( z )=F e−κz , z>

Lz

2

k=√ 2 mInGaN¿ Eħ2 ,κ=√ 2 mGaN

¿ (V o−E)

ħ2

Using these three piecewise equations in conjunction with continuity of mass across the

boundaries (1

m¿∂ψ∂ z ) we can derive the solution set with respect to well thicknessLz [5]:

tan( k Lz

2 )=√ mInGaN¿

mGaN¿

κk

,cot ( k L z

2 )=−√mInGaN¿

mGaN¿

κk

The bandgap energy EBandgap refers to the energy difference between the valence and conduction

bands, and subsequently corresponds to an approximation of the photon energy of light emitted

by the semiconductor material (in this case the well material InGaN). We can use this

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information to solve for an appropriate zero point energy for a desired energy emitted from the

system EEmitted=EBandgap+E [6]. This model is important for understanding the basic quantum

mechanics behind modern LED technology due to the influence of layer thickness,

semiconductor material, and input energy on the emitted wavelength of light.

Prior to the use of gallium nitride, attempts were made using the semiconductor zinc

selenide. ZnSe initially attracted more interest with a bandgap energy of 2.7 eV (~460nm),

making it an ideal candidate for blue light. A blue ZnSe laser diode was successfully

demonstrated in 1991, but it proved to be unreliable for use in commercial applications due to its

instability. By the 1960s the red and green LEDs had already been invented leaving researchers

searching for a suitable material with a bandgap ideal for shorter wavelength light. Later that

decade Maruska used hydride vapor phase epitaxy (HVPE) to grow GaN with a bandgap of 3.3

eV (366nm). Gallium Nitride proved its own difficulties with a very high melting temperature

and equilibrium vapour pressure of nitrogen. Three decades passed after the red LED before the

blue LED could be commercialized [2].

It is important to note that heterostructures and quantum well layers for blue LEDs and

other multi-colored LEDs are significantly more complex, but have an underlying variability in

semiconductor layer material and layer thickness. With breakthroughs in crystal growth

techniques, the layer thickness can be varied on an atomic scale. Coupled with material

breakthroughs for doping GaN in a controlled manor, Akasaki and Nakamura’s research groups

were able to develop a blue LED with a double heterojunction of InGaN and AlGaN. Illustrated

in the figure below, a GaN buffer layer grown at low temperatures nucleates on the sapphire

substrate to accommodate the mismatch of thermal expansion coefficients between the two

materials. With further improvements on material quality and design features, in 1994 blue light

luminous intensity exceeded 1 candela, leading to the successful commercialization of the blue

LED [3].

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Figure 4: Nobel Prize winning design for blue LEDs using varying GaN and doped GaN layers

to create a suitable heterostructure for emitting blue light [3]

Creating a p-type layer from doping GaN to obtain control over its conductivity posed a

great challenge as well. Using a technique called low-energy electron-beam irradiation (LEEBI)

Akasaki and Amano managed to activate the doped acceptor with magnesium. These

breakthroughs allowed for the fabrication of a p-n junction in blue LEDs [2,3].

White Light and Multi-colored LEDs

The blue LED introduced the possibility for efficient white light either through excitation

of one or more phosphors, or through additive RGB mixing. The advantages of RGB-LEDs is in

the variability of the color point, which allowed for great breakthroughs in the lighting industry.

By combining three different colored InGaN and AlInGaP LEDs it is possible to obtain any color

within the CIE triangle shown below.

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Figure 5: 1964 CIE (u ,v) coordinate system for InGaN and AlInGaP Leds. By connecting three

LEDs to form a triangle one can illustrate the range of possible colors attainable through varying

the ratio of illumination [7].

This proved to be monumental for electronics with screens, and with light-emitting diodes

growing ever smaller their applications grew exponentially. As opposed to incandescent light

bulbs which produce around 16 lumens/watt, white LEDs can be as efficient as 300 lumens/watt.

LEDs are also environmentally safe and a mercury-free alternative white light source. Energy

used for lighting accounts for 20-30% of industrial energy consumption, and with breakthroughs

in LED technology which enable control over the color temperature to create pseudo sunlight,

Edison’s now obsolete incandescent lightbulb has found a suitable rival [8].

LED technologies continue to grow in search of a more powerful and efficient source

which can produce more colors and stable color temperatures. Alternative color mixing

approaches include the introduction of amber and white LEDs to create multi-chip LEDs coupled

to a microcontroller. Red, amber, green, and InGaN blue (RAGB) multi-chip white LEDs are

implemented to widen the range of possible colors as well as integrate the LEDs into one unit.

Green light in an RGB system has relatively lower intensity compared to red and blue, so with

the addition of amber (~600nm) yellow light can be obtained with higher accuracy as well as

raising the luminous efficacy of the system [white]. This approach is favorable in the lighting

design industry for music due to its increased control in color and the compact nature of the light

sources [9].

Lighting Design and the LED Revolution

With technological advances in production and efficiency of light-emitting diodes, major

stage lighting manufacturers have developed LED based lighting fixtures as alternatives to

tungsten or discharge moving lights. These new fixtures require less power, produce less heat,

and are lighter in weight, making them attractive for small scale lighting designers. As opposed

to traditional subtractive color mixing through the use of filters, color effects can be managed

with higher precision all in the functionality of a single fixture. Complex LED color mixing

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fixtures use RGBA (amber), RGBW (white), or in some cases up to 7 different LEDs such as the

ETC Selador [10].

Another advantage seen by lighting designers is the increased lifetime of LEDs which

can last from 50,000 to 100,000 hours. This makes the fixture virtually maintenance free, with

the exception that they depreciate in lumen output due to mechanical and thermal stress.

Traditional discharge moving lights require a new lamp after around 1000 hours of use, as well

as frequent maintenance due to the heat produced. LEDs are dimmed by using a high-frequency

pulse-width modulated supply which is an on board feature as opposed to an external dimmer

rack. Dimming in conjunction with on board color cycling and other color effects makes the

programming of a fixture less of a headache for lighting designers [4,10].

In a recent article from Lighting and Sound America, Mike Wood explores a

groundbreaking new LED fixture on the market by Martin Professional. Martin Professional is

famous for its discharge lamp moving head fixtures, but has recently produced a new line with

LED light sources, specifically the MAC Quantum Profile. This LED fixture houses an array of

90 white LEDs which are bolstered with high emission in the 420-460 nm range. Due to its high

quality spectral profile after a color correcting filter, this fixture can rival 700W discharge lamps

(standards in the music industry) by saving energy and producing significantly less heat. Much

like older discharge sources, the MAC Quantum Profile uses subtractive color filtering since the

color temperature after filtering was measured to be “warm white” at 3,309K [11]. Martin

Professional is highly regarded in the music industry, and their switch to LEDs as light sources

foreshadows big changes to come in the future.

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Figure 6: Spectral Intensity graphs for the MAC Quantum Profile moving head fixture by

Martin Professional. On the left is the spectral data of the white LED, and on the right is the

spectral data of the LEDs with a CTO (color temperature orange) correction filter [11].

LED light fixtures come in all shapes and sizes, and with a rising popularity of video

content in live music, lighting designers such as Michael Smalley of Bassnectar are

implementing large scale LED video panels. Smalley used an LED panel which wrapped 360

degrees around the stage so that the entire audience in the arena could view it. This is just an

isolated example of the versatility of LED lighting, especially when pixelated in arrays. With the

number of fixtures used in the average production rising, the demand for higher efficiency

lighting is increasing.

Moving Forward—Quantum-dot LEDs

Lighting technology has grown exponentially much like any other electronics field.

Researchers are looking to the atomic scale for producing light with higher efficiency and control

of emission spectrum. In an MIT lab quantum-dot organic light emitting devices (QD-OLED)

are being developed by sandwiching two organic thin films around a single layer of quantum

dots. By using organic molecules as an organic semiconductor to deliver an electrical charge to

quantum dots, bright and wavelength specific light can be produced. These devices can be made

into screens that are a fraction of an inch thick with the same brightness as LCD screens, making

them a suitable future competitor in the commercial electronics [12]. Quantum-dot LEDs, while

still in a research phase, may prove to hold a future in lighting design in the music industry.

References

[1] D. Barbetta, “All Eyes are on Clay Paky B-EYE Fixtures For Lotus Tour” Clay Paky.

<www.claypaky.it> (2011).

[2] Y. Nanishi, “Nobel Prize in Phyiscs: The Birth of the Blue LED” Nature Photonics 8 884-

886 (2014).[3] Class for Physics of the Royal Swedish Academy of Sciences “Efficient Blue Light-Emitting Diodes

Leading to Bright and Energy-Saving White Light Sources” The Royal Swedish Academy of Sciences.

(2014)

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[4] Cadena, Richard. Automated Lighting Second Edition. Oxon, UK: Focal Press, 2010. Print.

[5] D. Miller, Quantum Mechanics for Scientists and Engineers. New York: Cambridge

University Press, 2008. Print.

[6] D. Miller, “Optical Physics of Quantum Wells” Stanford University.

[7] S. Muthu, F. Schuurmans, and M. Pashley, “Red, Green, and Blue LEDs for White Light

Illumination” IEEE Xplore 8 2 (2002)[8] “Blue LEDs – Filling the world with new light” The Royal Swedish Academy of Sciences . (2014)

[9] J. H. Oh, J. R. Oh, H. Park, Y. Sung, and Y. Do, “New Paradigm of Multi-chip White LEDs:

Combination of an InGaN blue LED and Full Down-converted Phosphor-converted LEDs” Optics

Express 19 S3 (2011).

[10] Rob Sayer, “Pro Production: A Guide to LED Stage Lighting” Pro Sound Web. (2013)

[11] M. Wood. “Martin Professional MAC Quantum Profile” Lighting and Sound America. 12 26880

(2015).

[12] “Quantum-dot LED may be screen choice for future electronics” MIT News. (2002)

< http://newsoffice.mit.edu/2002/dot>

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