Nanophotonics / Nano-Optics (Part I)
Fundamentals
Nanophotonics is one of the technologies of the future. It may seem that nanophotonics is high technology only suitable for industrial and computer applications, however when this technology is sufficiently developed it will have its influence in all areas of people's daily lives.
Nanophotonics is the science that deals with the study of the interactions between matter and light at the nanometer scale, as well as the fabrication of nanostructured materials that process light waves. The manufacture of nanostructured materials is the science and engineering that goes by the name of nanotechnology. Nanophotonics is therefore a nanotechnology that is based on photonics as a medium.
Ever since Richard Feynman established that the laws of physics do not preclude manipulating things atom by atom, and that was 50 years ago (29 December 1959 at the annual conference of the American Physical Society at the California Institute of Technology), the search has been on for ways of being able to design materials atom by atom. In fact, nanostructured materials (NEMs) have already been used in practical applications, such as the gold nanoparticles used for the red color of the stained glass windows in the Gothic cathedrals of Europe, perhaps the first application of nanotechnology, or the silver nanoparticles used in photographic films. Currently, research in the field of nanostructured materials has grown to a high level and its applications cover practically all disciplines, which is why nanotechnology has become an interdisciplinary science. This diversity of applications offered by nanostructured materials is what has aroused so much interest in society and the scientific community. MEMs are those materials in which at least one of their dimensions is in the range of 1-100nm. One nanometer is 0.000000001 meters, i.e. one millimeter has one million nanometers. At the nanometer scale, materials exhibit electrical, magnetic, mechanical and optical properties that are totally different from materials at the micrometer or millimeter scale, also called bulk materials. The properties of bulk (millimeter) materials are dominated by a bulk effect while those of NEMs are dominated by surface effects.
As mentioned above, nanophotonics, a fusion of nanotechnology and photonics, studies the optical properties of nanostructured systems and the interaction between light and matter at the nanometer scale (nanoscopic level). As we have mentioned, the optical properties of MEMs are dominated by surface effects, so that by controlling the size and shape of the nanostructures and interacting an optical signal on them, results can be obtained that can be applied in different fields, such as biology, medicine, photodetectors, processors, sensors, solar cells, aeronautics, etc. Nanostructures can be classified into three types: dielectric, semiconductor and metallic.
One of the most interesting results of these semiconductor nanostructures is the ability to tune the emission wavelength, so that with a single material and varying the size of the nanoparticle we obtain the wavelengths of the emitted signal. This is really relevant in the world of optical communications because new wide bandwidth optical amplifiers can be designed, since each nanoparticle works as an amplifier and by selecting the appropriate diameter of the particles we are able to determine the bandwidth of the amplifier. But one of the applications that has generated the greatest expectation is the detection of a wide variety of compounds by fluorescent coloring of nanoparticles with emission in the visible region of the spectrum, in particular the detection of cancer cells.
Dielectric nanoparticles or nanocrystals are oxides with a very wide forbidden energy band and therefore we need high pumping energies to obtain emissions that are generally weak, however if various components are properly combined we achieve excellent light emitters of high stability.
Finally, metallic nanostructures, also called plasmons, have the ability to scatter and absorb incident light. In this case the metaldielectric boundary at the nanoscale produces important changes in the optical properties, and when we inject an optical signal into the nanostructure, resonance bands (known as localized plasmons) generated by the oscillation of the surface electrons are produced. The wavelength at which the resonance is obtained is called the plasmon absorption band. One of the applications of this nanostructure is surface-enhanced Raman spectroscopy, which achieves strong amplification of the Raman spectrum of a component close to the metal surface.
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!).
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.
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 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!
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