Historical research guide · 0431 · 2024
Casimir repulsion with biased semiconductors
Benjamin Spreng · Calum Shelden · Tao Gong · Jeremy N. Munday
In one minute
The Casimir force normally pulls. Two surfaces held close together in vacuum are pressed toward each other by the fluctuations of the field around them, and in very small machines that pull is the stiction that jams moving parts. Benjamin Spreng, Calum Shelden, Tao Gong and Jeremy Munday show how to reverse it with a battery. Put a modest forward bias across a semiconductor and the light it radiates stops being ordinary thermal light: its photons carry a chemical potential set by the applied voltage. Those extra modes stream out of the semiconductor and press on whatever is nearby. The team adds that push to the ordinary Casimir pull for real materials at room temperature and finds the total changes sign — gallium arsenide facing gold turns repulsive beyond a few hundred nanometres, and the crossover moves closer in as the voltage rises. For the sphere-and-plate geometry every Casimir laboratory already owns they predict tens to hundreds of piconewtons of repulsion.
Connection
Chapter 2 treats the vacuum as a real medium whose forces are measurable, and this paper turns the sign of those forces into a knob you can turn with a power supply rather than a knob you set once by choosing materials. Chapter 6’s vacuum-force devices need exactly that: a repulsive branch means levitation instead of stiction, and a bias means the branch can be switched on and off while the device runs.
From the source
The ingredient that makes the force reversible is settled physics being put to a new use: apply a bias across a semiconductor and the electromagnetic fluctuations inside it acquire a non-zero chemical potential proportional to the applied voltage, so the semiconductor emits more than a body at its own temperature would. That photon chemical potential was predicted by Landsberg and Würfel, confirmed on gallium arsenide light-emitting diodes by Feuerbacher and Würfel, and used for near-field photonic cooling.Introduction, final paragraphs, with references 34 to 37 and 40
For two parallel plates — biased gallium arsenide with a 1.43 electronvolt bandgap facing gold at 300 kelvin — the net Casimir pressure is attractive at short range and repulsive further out, and the crossover walks inward as the bias rises: 850, 540, 400 and 380 nanometres from the smallest bias to the largest. Both the size of the far-field repulsion and the amplitude of its oscillations grow with the bias.Section 2.B Application, Figure 1a
The mechanism is separated term by term rather than asserted. The repulsion and its oscillations come almost entirely from the propagating-wave contribution, carried by modes in a narrow band around the bandgap, and the oscillation peaks line up with the Fabry-Pérot resonant separations of the gap; the equilibrium Casimir pressure stays attractive at every separation and takes over close in, so the change of sign is the crossing of those two curves.Section 2.B Application, Figure 1b
The effect is not a single lucky material. At a fixed bias of 0.95 of the bandgap the far-field repulsive pressure on the gold plate is 3.6 millipascal for gallium arsenide and 3.5 for indium phosphide, then 0.7, 0.4 and 0.2 millipascal for indium arsenide, zinc sulphide and indium antimonide — a non-monotonic dependence on the bandgap, and every value enormously above the sensitivity the CANNEX parallel-plate experiment expects to reach.Section 2.B Application, Figure 2
The proposal comes with the geometry laboratories actually use. Within the proximity-force approximation, a gold sphere of 50, 100 or 150 micrometre radius facing a biased gallium arsenide plate crosses from attraction to repulsion at 17, 21 and 30 nanometres, and the repulsive force runs from about 30 to 250 piconewtons — several orders of magnitude above the 0.1 piconewton found in the earlier sphere calculation of Chen and Fan, and inside the accuracy of ordinary atomic-force-microscope and MEMS force measurements. At short separations the force can be tuned from attractive to repulsive by changing the bias alone.Section 3, Figures 4a and 4b; Conclusion
The paper is explicit about the edge of its own validity, which is also the next thing to watch: the nonequilibrium terms grow logarithmically as the bias approaches the bandgap, and that divergence marks the semiconductor turning into a laser, so the treatment holds only while the difference between bandgap energy and applied bias stays comfortably clear of the thermal energy — 71 millielectronvolts against 26 millielectronvolts at room temperature for the largest gallium arsenide bias used here.Section 2.B Application, paragraph on the logarithmic divergence
The way in
https://doi.org/10.1364/OPTICAQ.523360How to cite it
Benjamin Spreng, Calum Shelden, Tao Gong, Jeremy N. Munday (2024) Casimir repulsion with biased semiconductors doi:10.1364/OPTICAQ.523360