The above image is a segment of the famed stained glass windows in the medieval Sainte Chapelle on the Ile de la City in Paris, France. It’s featured here because Mark Stockman of Georgia State University specifically cited it as an early example of nanoplasmonics in his talk this afternoon on developing an attosecond nanoplasmonic-field microscope. Apparently, glaziers in medieval forges throughout Europe were the first nanotechnologists, producing colors with gold nanoparticles of different sizes.
In August, scientists at Queensland University of Technology made headlines when they announced that stained glass windows that are painted with nanoparticles of gold purify the air when they are lit up by sunlight. The electromagnetic field of the sunlight can couple with the oscillations of the electrons in the gold particles and creates a resonance which breaks apart pollutant molecules in the air. Maybe that’s why the windows were fresh in Stockman’s mind.
Plasmons are a surface phenomenon unique to metals. When light strikes a metallic surface — silver or gold, for example — it generates electron waves, called plasmons. Plasmon oscillations naturally generate local electric fields, and these become especially strong if the frequency of the excited light approaches the frequencies associated with a phenomenon called plasmon resonances.
(Historical footnote: Robert W. Wood, a physics professor at Johns Hopkins University, first observed these so-called “field emissions” — charged particles emitted from a conductor in an electric field — in 1897. This effect became the basis for field-emission microscopes, used to study atomic structure. Wood was also the first person to unwittingly record the energy lost as heat by plasmons skimming along the surface of metals in 1902, although he couldn’t explain the effect at the time. It took 40 years for Italian physicist Ugo Fano to provide an explanation: metals are not perfect conductors, as had been previously believed. Fano found that a conducting surface could guide light as a 2D surface wave (which is why plasmons are also known as two-dimensional light). Those waves absorb energy, which explains Wood’s anomalous observations of energy loss in the light reflected from metallic surfaces.)
The field of nanoplasmonics seeks to exploit these effects at the nanoscale for various applications, such as extracting light from LEDs and nano-antennae for photodetectors and solar cells. For instance, back in 2003, scientists at Los Alamos Natinal Laboratory developed a device called a “nanoscale flashlight”, which is how they learned that gold nanoparticles only absorb light above the plasmon resonance; below that threshold, it actually “transmitted” more light than was shone onto it — known as the nanoantenna effect.
Nanoplasmonic systems are also great for ultrasensitive sensing and detection. There are already nanoplasmonic-based immunoassays available, and a heart attack test is in clinical trials, as is an HIV test, according to Stockman. Home pregnancy kits are now a mass-market item. Nanoplasmonics also hold promise for a non-toxic thermal cancer therapy; the technique is currently in stage 3 clinical trials.
Of course, this is a tough thing to control at the nanoscale, where materials tend to behave somewhat differently than they would at the macroscale, but recent advances in coherent control and visualization of nanoplasmonics inspired Stockman to pursue an attosecond nanoplasmonic-field microscope (ANFM), capable of non-invasively capturing the ultrafast dynamics of surface plasmons, which tend to unfold in a few hundred attoseconds.
Stockman’s method combines photoelectron emission microscopy and attosecond streaking metrology to achieve this, along with spatial resolutions at the nanoscale. According to him, this approach is a valuable new means of probing such ultrafast nanolocalized fields in nanoplasmonic systems, and a boon to existing and potential applications.
Those medieval glaziers would no doubt be astounded to learn what materials science has made of their art. Who knew there could be a link between stained glass windows and advanced cancer therapies?