An Introduction to Nanophotonics

Hannah Le
Hannah Le
Oct 1, 2018 · 8 min read

Close your eyes for five seconds. What do you see? Now open your eyes again. What do you see? The answer is simple: light.

You may find that light is a fundamental existence in life, and there is nothing a human being can possibly do with his bare hands to intervene with it. Yet researchers and scientists have been studying the nature of light and how to not only observe, but also to manipulate and engineer light over the past hundred years, giving birth to a field known as nanophotonics.

So… what is nanophotonics?

Think of nanophotonics as the study of understanding and engineering light at a very, very small scale, or also known as a nanometer scale. As our human brains are not very good at conceptualizing how small a nanometer is, let’s put things into perspective: The average human head has around 100,000 hair follicles, and each hair follicle can grow 20 hair. Quick maths, in total, you can have 100,000 times 20 equals to two million hair strands. A nanometer is essentially a hundredth of a human hair or 1/200 millionth of your head!

On the other hand, the word photonics in nanophotonics basically refer to photons, the building block of light.

What’s really interesting about nanophotonics is by understanding how these photons behave on a nanoscale, we can start controlling and manipulating their interactions, giving rise to ingenious inventions, such as a better way for cancer imaging or a potential solution for room-temperature quantum computers!

Ever since Richard Feymann’s announced lecture in 1959, researchers and scientists have been studying how once light is squeezed down into a nanometer scale, odd behaviors can occur and completely challenge the way we perceive our physical world. By demystifying these odd behaviors, we can have the power to break the limits of current technology and create superior photonics devices.

How nanophotonics can impact our world!

Quantum optics

The opportunity in nanoscale quantum optics lies in developing components for quantum communication and quantum computing. Quantum cryptography is the science of using quantum mechanical properties to perform cryptographic tasks and provides an intrinsically secure and unbreakable code. It has, in fact, been demonstrated both inside and outside the lab up to modest video speeds and over modest distances (around tens of km!).

On the other hand, quantum computing has only been demonstrated in the lab, using complicated systems at operate at low-vacuum temperature. And the potential of quantum computers is enormous! These computers promise an exponential speed-up of important processes, allowing them to have massive parallel computation to solve important problems.

What’s fascinating is s nanophotonics can help actualize these technologies by making them readily available for room temperature operation! In fact, scientists at MIT recently developed a new photonic device, using a silicon crystal with distinctive patterns etched into it, to enable photon-photon interactions at room temperature.

Photonic device

In quantum computers, there is a strange physical property called superposition, where a quantum particle can occupy two contradictory states at the same time. For instance, the spin, or magnetic orientation in space, of an electron can be up and down at the same time. Similarly, the polarization of a single photon can be horizontal and vertical at the same time. If a string of qubits (the quantum analog of bits for classical computers) is in superposition, it can canvass solutions to a problem simultaneously, leading to the promise of incredible speedups.

As protons aren’t susceptible to interactions with the environment, they are great at maintaining superposition, but for the same reason, they are difficult to control. This is where the photonic device comes in: If a single photon enters their device, it will pass through unimpeded. But if two photons — in the right quantum states — try to enter the device, they’ll be reflected back. Thus the quantum state of one proton can be thought of as controlling the quantum state of the others.

Photonic nanomaterials

Another exciting example for photonic nanomaterials is quantum dots (QDs). As the name implies, think of quantum dots as incredibly small matter that is concentrated in a single dot. In other words, if you set the constraints for a nanomaterial, put it inside an imaginary box and confine it in all three dimensions, you will get a sphere that ranges a few nanometers.

Thus, quantum dot is an epitome of such sphere in the field of nanotechnology and is zero-dimensional. They are composed of semiconducting materials such as silicon or Cd, meaning that they are neither strictly an insulator or conductor but chemically behave like both. Due to their atom-like behaviors, QDs are often used for special optical properties that can be employed for fabricating optical probes for biological and medical imaging.

One promising application of QDs is they can revolutionize the way we diagnose and treat cancer today. QD molecular imaging introduces new way of seeing biologic processes at work within cells and in small animals in real time, which is in itself an incredible feat.

Quantum dots emitting different wavelengths are visible after injection into a mouse.

To put things into perspective, let’s conceptualize the magnitude of the human genome — the cookbook of our life. Humans have approximately 40,000 genes. A large group of these genes operate at every single moment, in every cell of our body, in very complicated ways. As Weiss, a researcher at Jonsson Cancer Center and UCLA, has exquisitely described:

“By color encoding a subset of proteins in the cell with different color quantum dots, we can follow molecular circuitry, the dynamic rearrangement of the molecular interactions and interactions that re-program cells to gain and lose function in disease — in short, oversee the ‘molecular dance’ that defines life itself.”

A representation of quantum dot based single cell imaging cytometry for the determination of breast cancer subtypes. Biopsied tissues or primary cells from breast tumors were treated with four different quantum dot-biomarker (EGFR1, HER2, ER, and PR) conjugates and excited by UV light.

A similar technique can be used for cancer patients, who can be injected with a cocktail of QDs that would “label” cancerous cells. Once they’re gathered at the tumor site, the positrons emitted from the QDs could be imaged with PET scanner which would indicate the presence and location of a tumor. An “optical barcode” of the different colored qdots could help doctors identify tumor type and stage by allowing them to see differing levels of various tumor markers. QDs imaging can potentially replace the lengthy and costly process of how we are currently treating cancer today!

The Superpowers of Light: Virtual Photons

In classical mechanics, light is described as coupled electric and magnetic fields propagating through space as a traveling wave.

However, this wave theory is not sufficient to explain the properties of light at very low intensities or at a nanoscale. Thus, people have turned to quantum theory describes light as consisting of discrete packets of energy, known as photons.

As light behaves as both waves and particles physicists have merged these two classical theories together and introduced a more comprehensive one, often known as quantum electrodynamics (QED).

Let’s break it down and step-by-step understand what QED talks about!

In classical mechanics, we become aware of field forces, such as gravitation force or electromagnetic force. The key thing we need to understand is that these fields impart forces on the object, causing it to accelerate. For example, the Earth has a gravitational field, which causes the apple to experience a gravitational force downwards and fall from the tree.

Quantum theory redefines these field forces by describing forces as the interactions between particles. Instead of force being mediated by light, Richard Feynman hypothesized that it is mediated by something called virtual photons.

In the Chalmers scientists’ experiments, virtual photons bounce off a “mirror” that vibrates at a speed that is almost as high as the speed of light. The round mirror in the picture is a symbol, and under that is the quantum electronic component (referred to as a SQUID), which acts as a mirror. This makes real photons appear (in pairs) in vacuum

Now, these virtual particles are actually quite interesting: They are able to come in and out of existence for brief instants. Due to this property, they are considered as virtual, since they don’t exist with the same permanence as ordinary particles that makeup matters. Once these virtual particles interact with charged particles, like photons or electrons, they cause these particles to bounce off and change direction, as if they were affected by a force. In short, force is replaced by interactions between virtual and real particles.

Although it sounds absolutely absurd that a particle can pop out of nowhere, virtual particles can explain many phenomena in nanoscale photonics.

Casimir-Polder effect

First, the Casimir-Podler effect, which basically refers to the attraction between two objects should they come within 100 nm of each other. The two objects were placed in a vacuum, which contains empty space.

Surprisingly, however, the empty space isn’t really empty. It roils and boils with something known as quantum fluctuations, occasionally spitting out pairs of “virtual” elementary particles. Think of it as waves of particles appearing and vanishing all the time. These virtual particles annihilate and disappear back into the quantum vacuum so quickly that the apparent violation of energy conservation incurred by their creation can’t be observed directly.

Strong Casimir force reduction through metallic surface nanostructuring

Although practical applications of the Casimir effects have not been found, it is important for MEMS researchers and manufacturers to keep in mind to design effective nanoscale devices that we use in, say computer chips, today!

Optical binding

Another important way of controlling light is through optical binding. When two walls of nanoparticles interact with a plane wave, the optical binding force occurs. As a result, the locations of the two nanoparticles become fixed.

What happens consequently is a potential well occur, trapping more nanoparticles and making the structure more stable. Compared to microscale particles, stronger trapping forces are required to overcome the thermal forces for fine control of objects at the nanoscale. Thus, optical binding has potential applications in trapping small particles and assembling nanostructures.

Key Takeaways

  • Nanophotonics is the study of understanding and engineering light at a nanometer scale.
  • By understanding how these photons behave on a nanoscale, we can start controlling and manipulating their interactions, leading to inventions like a new way for cancer diagnosis and treatment or photonic quantum computing.
  • Quantum electrodynamics and virtual photons explain some important behaviors of particles at the nanoscale. Two examples are the Casimir effect and optical binding, which play an important role in MEMS devices.

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