A long time ago, in a place not so far away, I used to believe that future lasers and optics would have knocked into a tin. Yet silicon electronics still dominate all forms of computation and integrated circuit technology.
For optics to really be useful, it needs to be based on the same technology as complementary metal oxide semiconductors (CMOS). Therein, the CMOS compatible are no light sources that are.
Silicon's dark secret
Light-emitting diodes are as chips as they are, but they are not 't made from silicon. Why? Because silicon does not like to emit light.
Semiconductors at a general level are all the same. Electrons can exist in the valence band, where they are stuck to a local atom, where they are free to move around.
It needs to acquire more energy than the energy difference between the valence and conduction bands. When an electron drops from the conduction band to the valence band, it must be loose that energy. In a light-emitting diode, electrons drop out of the conduction band by losing energy in the form of light.
In silicon, electrons have a photon and a sound vibration (a phonon) at the same time to reach the valence band. Instead, the electron typically finds a way to lose the energy without a photon, so no light comes out.
Buzzing the light out of silicon
To overcome this problem, a team of researchers has decided to scrape the light out of silicon , Imagine a middle-aged electron, in a middle class bit of silicon. Suddenly, out of nowhere, a boy-racer electron roars along the silicon surface, scaring out all the extraneous weight out of the silicon-bound electron. What is the result? Our perturbed electron emits its outrage in a storm of light and fury.
In slightly more detail: Then, the researchers fire electrons along the surface so they cross the bars. As the speeding electron approaches a bar, the other electrons in the silicon get driven away from the surface. As the speeding electron moves on, the electrons relax back to the surface in a series of oscillations that result in light emission.
Why bars? Why not a flat surface? Well, each bar produces light independently. Where we see the light (at a distance), we see only the product of the mixture, or interference, between all the bars. By separating emission locations and choosing the energy of the speeding electrons, we select certain colors of light to dominate the emission spectrum.
In the end, the result is a structure that can be broadly tuned of wavelengths. Even better, with improved control of the electron beam and feedback, it may be possible to make a free electron laser, except the electrons are not entirely free-they're in silicon.
The clarity and cleanliness of silicon
trick. It is well known that shooting at an electron beam along a structured metal surface will result in light, so what makes this special? The researchers intuited two advantages that silicon would have over metal.
First, metals are a bit messy. Yes, they have lots of electrons that are free and, therefore, easy to drive around. But those electrons thus collide with each other and the surrounding nuclei, so they lose their energy in many different ways. In the end, only a tiny fraction of the energy is imparted to the electrons is emitted as light. But in silicon, electrons are all bound to a local nucleus. Radiation is emitted a bit more easily.
The other is not transparent, while silicon is.
These advantages give the researchers hope that higher efficiencies (on the order of 1
Nature Communications, 2019, DOI: 10.1038 / s41467-019-11070-7 (About DOIs)