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Sunday, April 10, 2011

From quantum Zeno effect to all-optical switch, part I.

Needless to say, scientists have been puzzled and fascinated by the quantum nature of the physical law for more than a century. The history of science is all over it and evolves with it. Having this in mind, it is very reasonable to see that the Science magazine has named the discovery of the quantum machine as the most significant scientific advance of 2010. It is the first quantum mechanical resonator that can actually be seen by bare eyes and deserves another detailed blog by itself.

How about in the optical world? Have we successfully implemented or utilized the quantum nature of materials for cool applications? The exciting answer is YES, and we will be looking at some of them in this short blog:

Let’s start from one of the most bizarre behavior that quantum mechanics can do – Quantum Zeno Effect. It states that if your observation of an event is frequently enough, its decay to the natural state of equilibrium will be affected significantly, either being slowed down, frozen, or accelerated. In fact, scientists call it anti-Quantum Zeno Effect, if the process is being accelerated (by the way, you will be able to hear the talk from its explorer -- Gershon Kurizki in CLEO 2011: QELS Fundamental Science).

The name “Quantum Zeno effect” adopts a broader meaning when it enters the optical world. We now use this term to describe manipulating the evolutions of the populations of different quantum states (or photons with different colors) by external perturbation.

If you feel the aforementioned is hard to digest, I promise the following will be not. We will be looking at some real examples and these are aiming for a high goal -- all-optical switches. If you wonder why all-optical switch is important, just think about how hot your CPU can get most of the time and how fast light can travel compared with electrons.

Take a look at figure 1. Two optical fibers connected by a microdisk made of GaAs. As shown in the figure, the signal light is shown in green. The disk couples the signal weakly. It comes in from the lower left (upper right) side, coupled to the disk, to the second fiber, and leaks to the upper left (lower right). Now if we carefully put in another pump light, marked as blue, it would perturb the property of the disk such that the ability to couple the signal light will be ceased completely. As a result, no signal light can be coupled to another fiber through the disk. In other words, you can detect signal from the upper left by putting it from the lower left with the pump-off. With pump-on, you detect no signal into the second fiber. This is a switch controlled by pump light.

Figure 1. one model of all-optical switch utilizing a microdisc. Courtesy of Y. Huang and and P. Kumar in Optics Letters Vol. 35 2376 (2010).  
In terms of quantum mechanics, the pump light opens a new channel of interaction inside the microdisk. It affects the existence of signal photons in the disk by draining them into another wavelength of light (shown in red in the figure). So the populations of photons with different colors are rebalanced. Virtually no signal photons are found in the disk when pump is on, and as a result, no leakage of it on the second fiber can be found.

Yu-Ping Huang, Joseph B. Altepeter, and Prem Kumar also present another similar methodology utilizing second harmonic generation (SHG) principle, as shown in figure 2. When there is no pump in the waveguide (WG-I), SHF process dominates. You put in signal with frequency ws; you get 2ws in the output. If the pump light is in, then every moment you have 2ws in the waveguide, it will interact with the pump and be drained to another frequency. So light with 2ws never builds up, and intensity of ws is not affected too much. The net result is that you still have the ws as the output. In summary, you have 2ws (ws) as output with pump-off (on). This is indeed another neat way of doing optical switch.

Figure 2. an all-optical switch based on SHG principle. Courtesy of Y. Huang, J. Altepeter, and P. Kumar.

Using two-photon absorption for the optical switch is yet another exotic way. Considering the design in figure 3, a toroidal resonator couples two fibers. The input light E1A (E1B) on fiber 1 (2) has frequency wA (wB). The resonator has strong two-photon absorption of wA + wB but nearly no absorption for wA, wB, 2wA, or 2wB. It is found out that, with the existence of strong E1B, you cannot find any E1A in the second fiber. In other words, by inputting E1B or not, we can control the existence of E1A on the fiber 2. The principle behind it is very similar to what we just discussed. With strong E1B in the resonator, any light of E1A will be destroyed through two-photon absorption process, so only strong E1B remains in the resonator. With the non-existence of E1A in the resonator, none of it can be coupled to the fiber 2. In addition, we can also use the strength of E1A to control the existence E1B in the first fiber! So a multi-functional optical switch emerges.

Figure 3. An all-optical switch based on two-photon absorption resonator. iR and T are coupling coefficient and transmission coefficient of the system. Courtesy of B. Jacobs and J. Franson in Physical Review A 79 063830 (2009).

Since two-photon absorption plays a core role of optical switches, researchers like Seth R. Marder and Joseph W. Perry are trying very hard to synthesize new organic compounds with desired two-photon absorption. You can learn about it in the morning section of QELS Fundamental Science.

We will look into more different kinds of all optical switches in the next blog.

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

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