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Saturday, April 16, 2011

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!

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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