Plasmonics at the bottom: Nanoshells and Invisibility Cloaks

Plasmonics may have roles to play aside from computing.  Researchers Naomi Halas and Peter Nordlander at Rice University have developed structures called nanoshells that consist of a thin layer of gold coating (usually 10 nm thick) around the entire surface of a silica particle about (100 nm in diameter).  Placing this coated particle in an electromagnetic (EM) field generates electron oscillations in the gold shell; this is due to the coupling interaction between the fields on the shell's inner and outer surfaces.  Varying the silica particle size and the gold layer thickness changes the wavelength at which the particle resonantly absorbs energy.  In short, nanoshells can be designed to selectively absorb wavelengths from the blue end of the visible spectrum (approx. 300 nm), and down to the near infrared (about 10 microns).

Surprisingly enough, nanoshells are now being considered for cancer treatment.  In 2004, Halas, working with Rice colleague Jennifer West, injected plasmonic nanoshells into the bloodstream of mice with malignant tumors and found that the particles were nontoxic.  Moreover, the nanoshells tended to embed themselves in the rodents' cancerous tissues rather than healthy ones due to higher blood circulation in the fast-growing tumors.

Additionally, human and animal tissues are transparent to radiation at certain infared wavelengths.  When near-infrared laser light is aimed at the tumors through the mice's skin, the resonant absorption of energy in the embedded nanoshells raised the temperature of the cancerous tissues from 37 ºC to 45 ºC.

This photothermal heating killed the cancer cells while leaving the surrounding healthy issue intact.  In the mice treated with nanoshells, the tumors virtually disappeared within 10 days; in the control groups, the tumors grew unabated.  As of 2009, practitioners are seeking FDA approval for clinical trials of nanoshell therapy in patients with head and neck cancer.

Plasmonics may also revolutionize the lighting industry by making LEDs bright enough to compete with incandescent bulbs.  In the early '80s, researchers found that the plasmonic enhancement of the electric field at the metal-insulator boundary could make certain dyes more luminescent if placed near the metal's surface.  This kind of plasmonic enhancement can also raise the radiation rate of quantum dots and quantum wells (tiny semiconductor structures that absorb and emit light), which improves the efficiency and brightness of solid-state LEDs.  A recent collaboration between Caltech and Nichia Corporation in Japan demonstrated that coating the surface of a gallium nitride LED with dense arrays of plasmonic nanoparticles (made of gold, silver, or aluminum) could intensify the emitted light 14-fold. 

Additionally, plasmonic nanoparticles may one day lead to LEDs made of silicon.  Silicon-based LEDs would be much cheaper than conventional LEDs composed of gallium nitride or gallium arsenide, but such devices currently emit too little light.  Research has shown that coupling silver or gold plasmonic nanostructures to silicon-dot arrays could boost their photon intensity by 10 times.  Moreover, the frequency of the enhanced emissions can be tuned by adjusting the geometries of the nanoparticles.  Simulations have indicated that careful tuning of the plasmonic resonance frequency combined with precise control of the separation between the metallic particles and the semiconductor materials may allow emission rates to rise more than 100-fold, allowing silicon LEDs to shine just as brightly as traditional devices.

Then there's the plasmonic analogue to a laser.  Mark Stockman of Georgia State University and David Bergman of Tel Aviv University theorized a new device called a SPASER (surface plasmon amplification of stimulated emission of radiation).  They have suggested fabrication methods via semiconductor quantum dots and metal particles.  The radiative energy from the quantum dots would be transformed into plasmons, which would then be amplified in a plasmonic resonator.  The plasmons generated by a SPASER  would be much more tightly localized than a conventional laser beam; this would allow the device to operate at very low power and selectively excite very small objects.  The result: SPASERs may further sensitize spectroscopy to more phenomena and to more sensitive hazardous material-detectors for minute amounts of chemicals or viruses.

Speaking of excitement, plasmonics are a potential candidate for the proverbial invisibility cloak of Hollywood films.  What is taught again and again in freshman physics is the refractive index, which is the ratio of the speed of light in a vacuum to the speed of light in the material.  If the refractive index is made equal to that of air, the object is rendered nearly invisible.  For plasmonic materials, this could be done with two things:
1) Use radiation that is close to the resonant frequency of the structure; it would neither bend nor reflect light.  The light is absorbed, never to bounce back to the observer's eye.
2) Laminate the structure with a material that produces optical gain (amplifying the transmitted signal just as the resonator in a SPASER would); the resulting higher intensity would offset the absorption losses.  The structure is thus rendered invisible, at least to radiation in a selected range of frequencies.

In theory, plasmonic materials could render objects invisible.  In one proposal, the cloaking device would be a thick shell constructed of metamaterials, which exhibit unusual optical properties.  This shell could bend radiation around its central cavity, where a spaceship could be hidden.  A space telescope pointed at the shell would see only the galaxy behind it. 

Now, it's one thing to render an object invisible for a set frequency, but it's quite another to hide it for the entire visible spectrum.  Some physicists say it is possible.  John B. Pendry of Imperial College London and colleagues showed that a shell of metamaterials might, in theory, reroute the electromagnetic waves traveling through it, diverting them around a spherical shell, as seen in the figure above.

Invisibility cloaks may never see the light of day (ha!), but such ideas highlight the wealth of optical properties that inspire scientists working in plasmonics.  By investigating the sophisticated coupling between electrons unbounded to atoms, and electromagnetic waves, researchers have identified new possibilities for transmitting data in our microchips, illuminating our homes, and fighting cancer.  Indeed, there's still a lot of 'room' to explore in plasmonics.

Plasmonics at the bottom: shrinking wavelengths

Plasmonics may be older than you think.  Alchemists and glassmakers spent millennia taking advantage of plasmonic effects when they created stained-glass windows and colorful goblets that incorporated small metallic particles in the glass. By analogy, ignorance of modern biology did not stop Stone-Age humans from genetically engineering wolves and various plants (via selective breeding) to become dogs and farming crops.   Best known example of alchemical plasmonics is the Lycurgus cup, a Roman goblet dating from the 300s A.D.; now held in the British museum.  Thanks to plasmonic excitation of electrons in the metallic particles suspended in the glass, the cup absorbs and scatters blue and green light (relatively short wavelengths in the visible spectrum).  When viewed in reflected light, the plasmonic scattering gives the cup a greenish hue, but if a normal light source (emits white light) is placed inside the goblet, the glass appears red because it transmits only the longer wavelengths (red) and absorbs the shorter ones (green/blue).

Lycurgus Cup: a Roman goblet dating from the 4th century A.D.  It changes color because of the plasmonic excitation of metallic particles within the glass matrix.  When a light source is placed inside the normally green goblet, it looks red.

 Surface plasmon research began with a bang in the 1980s, as chemists studied the phenomenon using Raman spectroscopy: the scattering of laser light off a sample to determine its structure from molecular vibrations.  In 1989, Thomas Ebbesen, then at the NEC Research Institute in Japan, discovered that when he illuminated a thin but opaque gold film imprinted with millions of microscopic holes, the foil somehow transmitted more light than was expected from the number and size of the holes.  This phenomenon, now known as extraordinary optical transmission, was eventually found to be caused by surface plasmons that intensified the transmission of electromagnetic energy.

Then came the discovery of novel "metamaterials": materials in which electron oscillations can result in some weird optical effects.  Their optical effects are so complex (due to plasmons) they require astronomically powerful computers for accurate simulation, but studying them has become less daunting by novel methods for constructing nanoscale structures that allow researchers to build and test ultrasmall plasmonic devices and circuits.

In most circumstances, it would be unwise to use metallic structures to transmit light signals because metals are known for high optical losses (signal weakens after each bounce off the metallic surface until evanescence).  However, when a thin film is combined with an electrical insulator, optical losses decrease due to the electromagnetic (EM) field spreading into the insulating material, where there are no conducting electrons  to oscillate, thus no energy-dissipating collisions.  This property naturally confines plasmons to the metallic surface adjoining the insulator; e.g., in the the top portion of the figure below (PLANAR WAVEGUIDE), the surface plasmons propagate only in the thin plane at the interface.  Note that in this example, the insulator is air. 

With this planar structure acting as a waveguide, shepherding the EM waves along the metal-dielectric boundary, it may be useful in routing signals on a chip.  Most optical signals may attenuate rapidly in metals, but the exception here is a plasmon traveling in a thin-film metal waveguide, whose journey can last for several cm.  The plasmon signal can be made to last longer  if the waveguide employs an asymmetric mode,  meaning the EM energy disperses more intensely in a certain direction than others.  This mode pushes a greater portion of the EM energy away from the guiding metal film and into the surrounding insulator, thus delaying attenuation a little longer.  The EM fields at the top and bottom surfaces of the metal film are known to interact with each other, and this interaction can be manipulated by changing the thickness of the film, which manifests as different frequencies and wavelengths for the plasmons.  In fact, this has already been done in the late '90s by a Danish/Canadian collaboration; they created a planar plasmon waveguide like in the top of the figure, but the EM fields it generated were too large to convey signals through the nanoscale innards of a processor.

Plasmons can propagate through nanoscale wires, but they require more complex waveguide geometries than that of the planar form; plasmons can shrink the wavelength of the optical signal by squeezing it into a narrow space.  This is seen in the middle illustration of the above figure.  This device is called a plasmon slot waveguide; the plasmon wavelength changes with respect to the thickness of the insulator core in the middle.  This is capable of transmitting a signal 10s of microns long (1 micron = 1 µm ≈ diameter of a blood cell).  Now comes the juicy part: a Japanese researcher had managed to squeeze red light (wavelength of 651 nm) into a plasmon slot waveguide that was only 3 nm thick and 55 nm wide.  The wavelength of the surface plasmon propagating through the device was 51 nm, or 8% of the original red light wavelength.  But the frequency remains the same, which means information gets transmitted.  This striking ability to shrink the wavelength opens the floodgates for nanoscale plasmonic structures to perhaps replace purely electronic circuits containing wires and transistors. 

Mass-production of plasmon slot waveguides is another story, but it should be similar to lithography (used to imprint circuit patterns on silicon chips).  This process could mass-produce miniuscule plasmonic devices with arrays of narrow insulating stripes and gaps.  These arrays would guide the waves of positive and negative charges on the metal surface; their behavior is similar to alternating current traveling along an ordinary wire.  However, since the frequency of an optical signal is so much higher than that of an electrical signal (400,000 GHz vs. 60 Hz, respectively),  the plasmonic circuit can carry tons more data.  Additionally, electrons  don't travel from one end of a plasmonic circuit to another; rather, they clump together and spread apart (thus no net directional surface current).  This fact accounts for the device's immunity to resistance and capacitance effects that limit the data-carrying capacity of integrated circuits with electrical interconnects.