Quantum Wells as Light Sources
Abstract
A quantum well is an extremely thin layer of low-bandgap
semiconductor material sandwiched between thicker layers
of high-bandgap material. The narrowness of this well creates
discrete energy levels and changes electron-hole transport
through the semiconductor. Utilizing quantum wells in
modern light sources advances laser technology by widening
the range of precise, achievable wavelengths. This optimizes
wavelength control and tuning, increases modulation
speeds, lowers energy loss, and reduces the overall system
size. This research project examines the physics behind
quantum wells and the importance of these advancements in
modern technology.

MacKenzie Byrd
What is a Quantum Well?
A quantum well is created when a thin layer of low-bandgap
semiconductor material (tens of nanometers wide) is
sandwiched between two layers of higher-bandgap material.
This makes a “well”, where the high energy materials sit on
either side of the low energy material. Electrons from the nside of the semiconductor and holes from the p-side get
pushed into the well and trapped when voltage is applied.
Due to attractive forces, the electrons and holes form
excitons, which then recombine to produce photons. The
narrowness of the well increases chances of recombination,
which generates more light than would occur without a
quantum well.

A semiconductor is a crystalline “insulator” with a small band
gap. It is compromised of a valence band, which is normally
filled with electrons at low temperatures, and a conduction
band, normally empty of electrons at lower temperatures.
The area between these bands is called a band gap, the size
of which is directly proportional to the amount of energy
needed for an electron to break free from the valence band
and jump across to the conduction band. When an electron
moves in this way, it leaves behind a positively charged “hole”
behind in the valence band. Additionally, “doping” the
semiconductor with specific impurities adds electrons or
holes to the valence or conduction bands—this is called a pn junction.

References
Miller, D. A. B. (1996). Institute of Physics.
R. Harris, Modern Physics, 2nd ed. (Pearson, Boston, 2008).

In lasers, both ends of the semiconductor are cleaved, acting
as mirrors for the photons to bounce between. These
photons energize free electrons to combine with free holes,
creating more photons in a process known as stimulated
emission. Because of this, the threshold current and the
power supplied to create photons is lower than in lasers
without quantum wells—these lasers are more efficient.
Additionally, an optical waveguide surrounding the quantum
well increases optical gain through total internal reflection of
light, amplifying fiber optic signals. Quantum well lasers also
react faster to changes in current, resulting in higher
modulation speeds (tens of GHz). Finally, the use of quantum
wells in lasers greatly decreases temperature sensitivity,
leading to better performance, stability, and reliability.

Figure 3. A green quantum well laser, which could be used in many of the
applications listed below, including the observation of constellations in
astronomy or other demonstrations.

Figure 1. A diagram of the combination of semiconductors to create a quantum
well, where the valence band is represented by the bottom line, and the
conduction band is represented by the top line (in all depictions).

Semiconductor Physics

Quantum Wells in Laser Technology

Figure 2. An illustration of the structure of a Gallium Arsenide (GaAs) quantum
well, with the corresponding energy diagram on the right. Note that rotating the
energy diagram 90 degrees shows a depiction of the GaAs quantum well
comparable to Figure 1.

Physics of Quantum Wells
The energy equation for a very deep well is:
𝑛 2 𝜋 2 ℏ2
𝐸𝑛 =
, 𝑛 = 1, 2, 3, …
2
2𝑚𝐿
Where n is an integer and L is the width of the quantum well.
Because energy in the well is quantized, and the following
can be applied:
ℎ𝑐
1
𝐸=
𝐸~ 2
𝜆
𝐿
From these two expressions, it can be seen that increasing
the width of the well L, decreases the energy E of the photons
and increases the wavelength, emitting longer wavelength
light. Inversely, if L is decreased, E is increased and
wavelength is decreased, emitting shorter wavelength light.
The quantization of this energy produces well-defined
wavelengths.

Applications of Quantum Well Lasers
Quantum well lasers are used everywhere in modern day
technology. Applications include:
• Fiber optics: requires specific wavelengths of light for
transmission through fiber, making quantum well lasers
valuable. This is the backbone of the internet and other
telecommunications—without quantum well lasers, highspeed internet would not exist.
• Laser pointers: used for presentations, construction
alignment, and astronomy observation.
• Barcode scanners: used to optimize asset management
and logistics in retail, package delivery, and manufacturing
• Medical imaging: non-invasive techniques for visualization
of tumors, blood vessels, and other tissues.
• LiDAR: a remote distance sensing technology used for
topography, autonomous vehicles, and facial recognition
• Photonic integrated circuits (PICs): the future of computing
technology. PICs are integral to the development of AI,
autonomous vehicles, and 5G/6G infrastructures.
This project was done in Dr. Mukherjee’s Modern Physics class