Month: June 2019

Some of us are anxiously awaiting the photonics revolution, where photons will band together to overthrow the tyranny of electrons. One of the perpetual problems that’s slowing up the revolution is light sources: you need to have a laser that is coupled to a circuit that is going to do something. All of this has to be on a tiny scale that can compete with electronics. It shouldn’t consume much power either.

That’s a hard collection of hurdles to clear. But there is a loophole: a photonic circuit could have the sort of application that electronic devices don’t do very well.

Now, researchers have used that loophole as an excellent excuse to do a very cool experiment in powering lasers. The researchers showed how to power a laser with a laser—something that anyone with a modicum of laser physics knowledge can do. However, this is quite different: the powering laser changes the electronic structure of the medium to trick it into lasing.

Return of the band gap

Before we jump into the details, let’s take a quick trip down population inversion lane to visit the semiconductor laser. From our perspective, a semiconductor consists of electrons that are trapped in the valence band (where they cannot move) and electrons in the conduction band (where they can move). There’s an energy gap between the highest energy electron in the valence band and the lowest energy electron that can conduct.

For an electron to go from the valence band to the conduction band, it has to gain at least enough energy to cross the gap. For an electron to return from the conduction band to the valence band, it has to lose enough energy to cross the gap. It can make these transitions by absorbing or emitting a photon of light.

To get a semiconductor laser, electrons have to be excited from valence band to the conduction band so that they emit light as they lose energy and return to the valence band. On its own, this will get you a light emitting diode. To turn that into a laser, you have to achieve population inversion, which means that you have to have more electrons in the conduction band than in the valence band.

Once that is achieved, stimulated emission can take over. The presence of a light field drives the electrons in the conduction band to emit light of the same color and traveling in the same direction as the field.

Getting a semiconductor laser to go is really easy with a battery: you just supply current at the right voltage and you’re done. But you don’t have to use a battery. You can also supply light with photons at an energy greater than the gap. Let’s say that your semiconductor material will naturally emit red light as electrons cross the gap. If you pump in green light, the green photons will excite electrons. The overly-excited electrons will bounce around like toddlers mainlining sugar, losing energy as they go, until they hit the bottom of the conduction band. At this point, the electron will emit a photon and return to the valence band.

Put in sufficient green light, and you will get a red-light emitting laser.

Bending the band gap

Now, imagine that you have an entire stack of lasers that you can’t hook up to wires, so you want to power it with light. Your green laser is not going to work. The problem is  the top laser on the stack will absorb almost all the light. If you turn the power of the green laser up to force more light through, it is going to burn the top laser before it powers the next laser in the stack.

This is where the researchers’ new work comes in. Let’s return to our picture of a semiconductor. All the electrons are sitting at low energy, trapped in the valence band. Instead of directly absorbing energy, electrons can also tunnel through barriers to get to the conduction band. For this to work, the energy difference between the conduction band and the valence band needs to be reversed. This can be done by applying a very large electric field to the semiconductor. The field raises the energy of the valence band and drops the energy of the conduction band, allowing electrons to jump to the conduction band. 

The researchers apply that field using a very bright laser, which works because light has an electric field. When the light is turned on, the electrons can tunnel from what is now a high energy valence band to the low energy conduction band. After the laser is switched off, the conduction band energy increases, dragging the electrons with it.

Suddenly, the semiconductor finds itself with lots of electrons at high energy wanting to emit light. Hence, a laser is born.

Wireless lasers

The cool thing about this is that the color of the light used to drive the laser doesn’t matter much. The only conditions is that the photon energy should be much lower than the energy required to excite an electron to the conduction band. That means you can choose a color that suits the application (e.g., choosing light that is transparent to the material in which the photonic circuit is embedded). Mostly, you don’t have to worry about which color of light suits the semiconductor material.

It also works best on tiny lasers, like nanowires of Zinc Oxide (this is the material the researchers used). These are the types of lasers that are best suited to photonics applications that need to use inert materials—oxides are very unreactive—and need to be small. Finally, because the lasers only turn on when the light focuses on them, you can switch between different lasers by shifting the point of focus (admittedly, the lasers need to be spaced by quite a large distance in this case).

So, where will these be used? I’ve no idea at this point, and I don’t really care—I just love the physics. More seriously, it takes a very bright light to turn a laser on like this (think ~1TW/cm²), so the applications will certainly be niche.

Nano Letters, 2019, DOI: 10.1021/acs.nanolett.9b00510  (About DOIs)

When University of Maryland physicist Timothy Koeth received a mysterious heavy metal cube from a friend as a birthday gift several years ago, he instantly recognized it as one of the uranium cubes used by German scientists during World War II in their unsuccessful attempt to build a working nuclear reactor. Had there been any doubt, there was an accompanying note on a piece of paper wrapped around the cube: “Taken from Germany, from the nuclear reactor Hitler tried to build. Gift of Ninninger.”

Thus began Koeth’s six-year quest to track down the cube’s origins, as well as several other similar cubes that had somehow found their way across the Atlantic. Koeth and his partner in the quest, graduate student Miriam “Mimi” Hiebert, reported on their progress to date in the May issue of Physics Today. It’s quite the tale, replete with top-secret scientific intrigue, a secret Allied mission, and even black market dealers keen to hold the US hostage over uranium cubes in their possession. Small wonder Hollywood has expressed interest in adapting the story for the screen.

Until quite recently, Koeth ran the nuclear reactor program at UMD, which is how he met his co-author. Hiebert is completing a PhD in materials science and engineering, specializing in the study of historical materials in museum collections (glass in particular) and the methods used to preserve them, using the reactor facility for neutron imaging of a few samples. Koeth told her about his research into his cube’s origins, and she started collaborating with him as a side project.

A quest for cubes

So far they have tracked down ten cubes around the US. For instance, the Smithsonian Institute had a German uranium cube in storage. “We wound up in a warehouse that looked like the final scene in Raiders of the Lost Ark, wooden crates from floor to ceiling,” said Koeth. “And in one of those crates there was another German cube.” There was also a piece of uranium from the original Chicago Pile-1—the first sustained nuclear chain reaction achieved by US physicists. They tracked a third cube to Harvard University, where it regularly gets passed around to students in introductory physics classes as a curiosity. (The cubes are only slightly radioactive and don’t pose a health concern, according to Koeth. Since uranium is so dense, “It winds up shielding itself,” he said. “The radiation you measure from it is only coming from the surface.”)

Underpinning the Manhattan Project in the US was the fear that German scientists under Adolf Hitler’s Nazi regime would beat the Allies to a nuclear bomb. The Germans had a two-year head-start, but according to Koeth, “fierce competition over finite resources, bitter interpersonal rivalries, and ineffectual scientific management” resulted in significant delays in their progress toward achieving a sustained nuclear reaction. German nuclear scientists were separated into three isolated groups based in Berlin (B), Gottow (G), and Leipzig (L).

Renowned physicist Werner Heisenberg headed up the Berlin group, and as the Allied forces advanced in the winter of 1944, Heisenberg moved his team to a cave under a castle in a small town called Haigerloch—now the site of the Atomkeller Museum. That’s where the group built the B-VIII reactor. It resembled an “ominous chandelier,” per Koeth, because it was composed of 664 uranium cubes strung together with aircraft cable and then submerged in a tank of heavy water shielded by graphite to prevent radiation exposure.

As the German scientists were racing against time, Manhattan Project lead Lieutenant General Leslie Groves kicked off a covert mission dubbed “Alsos,” with the express purpose of gathering information and materials related to Germany’s scientific research. When the Allied forces closed in at last, Heisenberg took apart the B-VIII experiment and buried the uranium cubes in a field, ferreting away key documentation in a latrine. (Pity Samuel Goudsmit, the poor physicist who had to dig those out.) Heisenberg himself escaped by bicycle, carrying a few cubes in a backpack.

Koeth has been interested in physics in general, and nuclear physics in particular, since he was a young boy. “My parents will tell you they tried taking me to Toys R Us at age four and I just cried until we went to Radio Shack,” he said. When he was eight, an uncle gave him a copy of Richard Rhodes’ seminal history, The Making of the Atomic Bomb, and a nuclear physicist was born. So he knew a little about the history of the cubes, and his first question when he received one as a gift was, ‘What happened to the other cubes?”

He initially assumed all the uranium cubes would have been confiscated after the Nazi defeat and sent to the uranium processing facility at Oak Ridge in the US, to be used to fuel an atomic bomb. But a historian told him that by April 1945, the US had plenty of feedstock material and wouldn’t have needed the extra uranium. So he wondered if someone might have handed them out as souvenirs, perhaps to serve as paperweights.

There is no record of the cubes entering the US, but Koeth and Hiebert reasoned they might be able to determine if there was a common source for all the recipients of the cubes they’ve tracked so far—a “patient zero” responsible for distributing them. Koeth’s cube had come with that note as a clue. Now he just had to figure out who “Ninninger” had been.

It turned out the last name had an extra “N.” Koeth found a War Department memo dated February 24, 1945, stating that “Robert D. Nininger, Second Lieutenant, has been appointed Accountability Property Officer for the Murray Hill area.” That area was part of the feed materials network for the Manhattan Project. That meant he was in charge of all uranium for that part of the network, said Koeth. Nininger turned out to be a geologist by training and had even written a book on minerals for atomic energy.

As Heisenberg himself reported, the German scientists’ final experiment failed because the amount of uranium in the cubes was insufficient to trigger a sustained nuclear reaction. But Heisenberg was confident that “a slight increase in its size would have been sufficient to start off the process of energy production.” A model described in a 2009 paper bears that out, showing that the group would only have needed 50 percent more uranium cubes to get the design to work.

“If the Germans had pooled rather than divided their resources, they would have been significantly closer to creating a working reactor.”

During their quest, Koeth and Hiebert uncovered a box of declassified documents about German uranium in the National Archives and discovered there were about 400 other uranium cubes from a separate reactor experiment by the Gottow group. “The combined inventory would have been more than enough to have achieved criticality in the B-VIII reactor,” the authors concluded. So Germany’s secretive, isolationist approach actually hampered their nuclear program, because the two groups weren’t sharing information or resources. That said, it still might not have changed the course of the war in favor of the Axis powers, since the US Manhattan Project was fairly well advanced by then.

“Many contributing factors were likely involved in the resulting sequence of events,” the authors write. “Yet the revelation of the existence of the additional cubes makes it clear that if the Germans had pooled rather than divided their resources, they would have been significantly closer to creating a working reactor before the end of the war.”

For Koeth and Hiebert, the cubes “represent a bygone era in science” and supply crucial context to this vital period in physics history, along with other forgotten objects. “Perhaps most importantly, the story of the cubes is a lesson in scientific failure, albeit a failure worth celebrating,” they wrote.

DOI: Physics Today, 2019. 10.1063/PT.3.4202  (About DOIs).