Rectifying light

By Dean Williams on July 21, 2016

An optical rectifying antenna, or ‘rectenna’, converts optical-frequency electromagnetic radiation into DC output. JPhysD authors Saumil Joshi and Garret Moddel have been investigating optical rectennas for power harvesting and detection.

In these rectennas, light is treated as electromagnetic waves that are absorbed in micron-scale antennas and rectified by ultra-high-speed diodes. Even though the light is not absorbed in the same way that semiconductor solar cells absorb photons, the quantum nature of the electromagnetic fields is evident in the devices. This brings up fascinating questions about the nature of ultra-high-frequency rectification.

Is current rectification classical or quantized?

We normally think of the rectification of electric currents as a smooth, classical process. A continuously varying voltage applied to a rectifier produces a continuously varying current, with a magnitude that is greater in the forward bias direction than in the reverse, as shown in Figure 1(a). What would quantum mechanical rectification look like, and how does it occur? Does it ever matter?

Figure 1. (a) Classical rectification occurs at low frequency and high power, such that the I(V) is sampled over what appears to be a continuous range of voltages. (b) The quantum case occurs at high frequency and low power, and the I(V) is sampled at discrete voltage steps. © IOP Publishing. All Rights Reserved.

The key to understanding this issue is the frequency of oscillation. We know from quantum mechanics that the energy of a quantum oscillator is a small factor times the product of its frequency and the Planck constant. If that energy is greater than a meaningful quantity, then the quantum nature of the rectification is in evidence. That quantity turns out to be the magnitude of the oscillating input voltage (times the electron charge). When the oscillator energy is larger than product of the input voltage time the electron charge, then we have to treat the process quantum mechanically.

In the quantum case the rectifier does not see a continuously varying input voltage. Instead, it samples the voltage at only two discrete points, corresponding to the bias voltage plus and minus the oscillator energy (divided by the electron charge). These two points are shown as red dots in Figure 1(b).

When is quantized current rectification significant?

This is significant only at extremely high frequencies. For example, at 100 THz –corresponding to 3 µm infrared light – the energy is 0.4 eV. At that frequency, rectification with voltages on the order of 1 V must be treated quantum mechanically.

One of the few situations in which this is observed is in optical rectennas, as described in our paper. An optical rectenna combines a micron-scale antenna with an ultra-fast diode to rectify the current produced by incoming optical waves. These devices can operate in quantum or classical regimes, depending on the optical frequency and input intensity. Optical rectennas are a technology where Maxwell’s electromagnetic waves and Einstein’s quantized photons meet in converting optical waves to electric power.

Is rectification at more commonly used electronic frequencies quantized?

Of course. This happens even in the low frequency, high power rectifier that is converting AC power to DC for your computer. At lower frequencies and higher voltages the diode still samples the voltage at discrete points, but those points are so closely spaced that it appears to us that nature makes no jumps (natura non facit saltus), when in fact it does!

About the authors

Garret Moddel and Saumil Joshi have been developing optical rectennas at Quantum Engineering Laboratory of University of Colorado at Boulder. The lab investigates exotic new energy conversion technologies. Garret Moddel is a professor at the University, and Saumil Joshi recently earned his PhD there and is currently a postdoctoral researcher at the University of Massachusetts Amherst.