| Dec 19, 2024 |
|
(Nanowerk News) In everyday life, light is a wave as is apparent from the colorful iridescence of opal gemstones, or from oil films on water puddles. We also take it for granted that light travels in all directions. Recently, however, scientists from the Universities of Twente, Copenhagen, and Iowa have forced light to travel in very different and unusual ways. Using nifty engineered chip-nanostructures made in the Twente MESA+ Nanolab, they observed that photons hop in a crystal lattice from one site to another, similar to how pieces move on a Go or checkers board, yet in 3D! Even stranger, the photons hop only in perpendicular x-y-z directions.
|
|
The findings have been published in Physical Review Research (“Observation of Cartesian light propagation through a three-dimensional cavity superlattice in a silicon photonic band gap crystal”).
|
|
This novel dance of light is called “Cartesian light”, after French philosopher René Descartes who first described x-y-z coordinate systems in space. Cartesian light forms a basic building block for novel 3D photonic circuits and exotic quantum photonics on-chip that control vast amounts of data at high speeds. Cartesian light may also be forced to a complete stop and localized as predicted by Nobelist Philip Anderson.
|
 |
| Figure 1. Left: Schematic of several possible paths for photons (yellow) that hop in Cartesian directions in a 3D cavity superlattice. At the end of their Cartesian dance, the photons exit from the top surface, whereas they entered from the front (lower right). Each cavity is indicated as a red sphere, and the underlying band gap crystal is shown as a 3D mesh with small blue spheres indicating the crystal’s unit cells. Right: SEM image of a 3D cavity superlattice made from silicon in the MESA+ Nanolab in Twente. The nanostructure is a 3D photonic band gap crystal, that consists of pores (r= 160 nm) that that are precisely aligned and etched in the +X and -Z directions. Once every 5 pores, smaller pores are created (r’ = r/2 = 80 nm); at the intersections of each pair of such pores, a cavity appears (red circles at Left.) By sending light from the front of the superlattice reflectivity is collected or scattering from the top. (Image: University of Twente)
|
|
In everyday life, our eyes detect the presence of light as brightness including colors that provide information about our environment. In detail, light consists of oscillations of the electric and magnetic fields. These oscillations are superfast, about 1014 or one million times one million times one hundred times per second, much faster than can be detected, hence we see it as a brightness.
|
|
When light waves that originate from multiple sources converge at one point, they reveal interference. When the arriving waves are in sync, the interference is constructive: their summed intensity is much greater than the intensities from the individual sources. When the waves oscillate counter to each other, the interference is destructive: their summed intensity is much smaller and can even completely vanish.
|
|
Light waves are described by the Maxwell equations, named after British scientist James Clerk Maxwell. Since these equations have been extensively studied, it is rare to see light perform a different kind of motion. Yet this is exactly what the team achieved.
|
|
First, the team fabricated 3D photonic crystals from silicon. The crystals consists of pores etched into the silicon. The team mastered the challenge to align and etch pores in two perpendicular directions, using methods similar to those in the chip industry. First author Manashee Adhikary proudly explains: “Our crystals possess a 3D photonic band gap. This means that for a range of colors or optical frequencies, light is completely forbidden to exist inside the crystal due to interference. A photonic band gap is the analogy of a gap in silicon, that is ubiquitous in our computer chips.”
|
 |
| Figure 2. Camera images of the XY-front surface of a superlattice at two frequencies: ω = 7386 cm−1 at the center of the superlattice peak (left) and at 7174 cm−1 away from the superlattice peak (right). The crystal surface is illuminated by a separate LED. The scale bar is 5 µm (top left). The red arrow in each panel points to the incident focus that was kept constant. (Image: University of Twente)
|
|
Next, the team introduced resonant cavities in their crystals, by etching smaller pores on purpose once every so often (see Fig. 1). At the crossings of perpendicular smaller pores, there is excess silicon that confines light and forms a cavity: a tiny volume where photons are confined since they are forbidden elsewhere throughout the photonic band gap crystal.
|
|
Co-author Marek Kozoň explains: “We realized many cavities regularly spaced in a large crystal lattice, called a superlattice. Hence, photons confined in one cavity can hop to a neighboring cavity. Hopping means that photons do not travel in their usual “wavy” sense, but they tunnel through the forbidden crystal in between two neighboring cavities. Since the cavities have a Cartesian arrangement in the superlattice, we thus force the photons to hop in Cartesian directions, in simple terms: photons dance in perpendicular steps. In our original theory paper, this peculiar motion of photons was therefore called: Cartesian light!”
|
|
The team performed optical studies, complemented by theoretical calculations, to find evidence for Cartesian light. Figure 2 shows camera images of the XY-front surface of one cavity superlattice where the cavities are interspaced by 3 unit cells. The researchers send polarized light at two different frequencies (or colors). The left panel pertains to light with the correct color of the Cartesian light, the right panel for detuned light.
|
|
Ravitej Uppu explains: “When we send in the correct frequency, we see that light spreads over multiple defect-cavity sites, which is only possible when the light hops from cavity to cavity. Conversely, when we send in detuned light, we only see a faint signal near the one cavity that is then addressed. We were very happy since the difference is really dramatic!”
|
|
“What is this good for” is usually on the minds of of non-specialists. Willem Vos is ebullient about the results: “Multiple exciting new phenomena and even applications are now within reach. Firstly, our COPS team has always been enamored by the phenomenon Anderson localization, named after famous American scientist Philip Anderson, where light is completely stopped, amazingly as a result of intentional disorder. One can perceive our superlattices as a highly tailored form of disorder. For years our group pursued localization, and with our superlattices we see a major breakthrough. And since our structures are made from silicon, combining Anderson localized light with CMOS electronics is suddenly very much on the cards. Secondly, we perceive our 3D cavity superlattice as a 3D network where information encoded as optical bits may be processed in much richer ways than possible with current on-chip technology. And in addition to being feasible with classical light pulses, there’s nothing stopping us from applying this novel platform to single photons to encode quantum information!”
|
