From quantum Zeno effect to all optical switch, part II.

In the last blog, we took a trip starting from quantum Zeno effect and reached to one of its applications -- all-optical switch -- at a quick pace. This time, we will look into more phenomena that researchers use in order to achieve this all-optical switch future.

We discussed about photonic crystals (PCs) and their versatility in a recent blog. We learned that by changing the patterns of the PCs, it is able to select which color of light that can travel within it or be rejected. While the patterns play the crucial role in PCs, we have to realize that it is the modulation of the refractive index produced by the patterns that give PCs their unique physical properties. With this being said, it is not difficult to understand that if the refractive index of the material that PCs are made of can be changed, we are able to affect (or tune) PCs’ properties. This is exactly what researchers are trying to do recently:

Considering the silicon PC shown in figure 1a, there are two colors of light allowed to propagate in it (mode c and mode s). Now, it is known that putting some free electrons in the conduction band of Si would change its refractive index. To use this feature, researchers shine this PC with some light (pump) such that a few electrons in the Si can be kicked to the conduction band. Changing the refractive index shifts the center frequencies of mode c and mode s directly. In addition, since PC is so sensitive to its refractive index, just a few hundred fJ of energy is required to tune the transmittance property of the PC. The all-optical switch is then realized by the following: Let’s input two colors of light into the PC -- one is very close to mode s and one is right at mode s (figure 1b). Without the additional pumping light, mode s is transmitted. With the pump, mode s is suppressed and the other color now is able to transmit since the transmittance property is shifted. So by pump-on/pump on, we will have different colors of light coming out -- an all-optical switch, as we expect.

Figure 1. an all-optical switch based on a silicon PC. (a) The structure and the transmittance curve of this specific PC. (b) with/without pump, the transmittance of the PC is shifted. Here we use mode s as an example. Courtesy of T. Tanabe, M. Notomi, S. Mitsugi, A. Shinya, and E. Kuramochi on APL 87 151112 (2005).

Another example is a PC made of polystyrene. The pump beam can also control the transmittance of it. The structure and the transmittance curve are shown in figure 2. By pumping this PC with femtosecond laser pulses of a few nJ, it is found that the transmittance can be changed by more than 60%. And this feature definitely makes it a strong candidate for all-optical switch application.

Figure 2. (a) A SEM image of a PC made of polystyrene. (b) The transmittance curve of this PC without being pumped by optical pulses. Courtesy of Y. Liu, F. Qin, Z. Wei, Q. Meng, D. Zhang, and Z. Li on APL 95 131116 (2009).

Let’s change the gear and look at something that will also be presented in CLEO 2011. A phenomenon called inverse Raman scattering (IRS) is utilized for all-optical switch application. We are all very familiar with Raman scattering, in which a material is pumped with a strong light, and you can detect some other colors of the light in the output due to inelastic scattering of the pump light in the material. If now we input two frequencies of light -- one is pump, the other has bluer color such that the frequency difference between these two are equal to the energy loss of the inelastic scattering, IRS would drain the energy from the high frequency light to the pump. So by putting pump or not in to the material, we can actually decide we want to drain the energy from the high frequency light out or not. This is actually realized by using a optical fiber or a silicon ring resonator. Excitingly, these will be presented during the conference. So, do not forget to check it out if you are interested.

There are more to say on this topic, such as using “four wave mixing on a silicon photonic chip” or “quantum dots coupled with a PC” to achieve the all-optical switch goal. The pool of exploration is open, just get ready and jump in!

DISCLAIMER

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

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.

DISCLAIMER

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