## What is an all-optical switch?

An all optical switch is a device that allows one optical signal to control another optical signal, i.e. control of light by light.

The above definition of an all-optical switch is rather general, encompassing many possible devices. Here we will illustrate three types of all-optical switchs.

 Schematic of an all-optical switch that changes the direction of output light as in our experiment.© Andrew Dawes and Lucas Illing, 2005

As a first example, we illustrate the behavior of our switch, shown to the right. In this device, a single beam of light (blue) is emitted from a nonlinear material and exits in a certain direction. This is one of the two states of the switch; let's call it the "off" state. The switch is turned to the "on" state, when a second light beam, the switching beam (red), is injected into the nonlinear optical material. As a result, the output beam (blue) is emitted in a different direction.

 Schematic of an all-optical switch that redirects light. © Andrew Dawes and Lucas Illing, 2005

On the left, we show a second example of an all-optical switch. Here, a single beam of light (blue) passes through a nonlinear material and exits in a certain direction. This is the ``on'' state of the switch. The ``off'' state of the switch is achieved when a weak switching beam (red) is injected into the nonlinear optical material changes the direction of the output beam (blue).

 Schematic of an absorptive all-optical switch. © Andrew Dawes and Lucas Illing, 2005

On the right, we illustrate the behavior of a third example of an all-optical switch. In this device, a single beam of light (blue) passes through a nonlinear material and exits in a certain direction. This is one of the two states of the switch; let's call it the "on" state. The switch is turned to the "off" state, when a second light beam, the switching beam (red), is injected into the nonlinear optical material. In this case, the light of both the first beam (blue) and the switching beam (red) is absorbed by the material and there is no output light at all.

## Why is an all-optical switch useful?

Electrical switches have been used as long as electricity has been used. The most basic electrical switch is simply a pair of wires that can be separated or put in contact; ``off'' or ``on'' respectively. In an all-electrical switch, electrical signals are used to open or close the switch. An all-optical switch performs the same function but instead of electrical signals, it controls optical signals: light.

Without any doubt, all-electrical switches are extremely useful. It is great to be able to turn appliances "on" and "off" electronically and to use switches to direct electronic signal-streams around a network. Arguably, even more important is the use of all-electrical switches like the transistor as a building block for digital logic circuits. The idea is that the two states of a switch ("on" and "off") can be used as a physical representation of the binary integers or logic levels (0 and 1) and that logic rules used for computation can be implemented all-electronically because the state of the switch is controlled by another electrical signal.

All-optical switches can in principle fulfill the same functions as all-electronic switches, e.g. direct signal-streams around optical networks or serve as building blocks for optical computers. Would this be useful?

An area for which all-optical switches are very important is communications, because nowadays most long-distance telephone and Internet communication is carried on optical fibers. These thin strands of glass let large amounts of information travel long distances at nearly the speed of light. Although you are currently using many optical fibers by reading this web page, the data your computer receives isn't an optical signal during the whole trip. At any point where data signals change fibers to get to their destination (like your car changing highways), the signal has to be turned from light into electricity so the destination address can be read and your data can be pointed in the right direction. The process of converting signals from light to electricity and back uses extra power (and generates extra heat) that can be expensive if the conversion has to happen quickly or many times in a row. The efficiency of optical communications can be increased if devices (like our all-optical switch) are designed to guide signals while they are in optical form.

All optical computing is still very much a technology of the future. It has some main advantages as compared to electronic computer, such as small size/high density, high speed, and low heating of junctions and substrate. Thus, all-optical computers might be economically viable. However, we are more interested in the possibility of using an improved version of our all-optical switch for quantum computation and quantum communication. An all optical switch useful for quantum computation and communication should be able to sense or manipulate single photons. Therefore, it is one of our goals to modify our switch such that a single switching photon can toggle between the "on" an "off" state.

## Why doesn't a good all-optical switch exist yet?

In vacuum, or in air, light beams simply pass through one another without interacting. Therefore, in vacuum, it is not possible to change the direction of one beam of light with another. On the other hand, in a nonlinear material, a light beam of sufficient strength changes the optical properties of the material which in turn affects any beams of light also propagating through the material. Therefore one beam applied to the material can control the interaction between the material and another beam. As a result, one beam can cause a second beam to change direction.

The problem is that for this type of light-by-light control to occur, the light and the material must interact readily. Typically, materials only respond in the desired way in the presence of strong beams of light. This means high power beams are usually needed to observe even tiny light-by-light interactions. Such high power requirements limit the development of practical all-optical devices because many stages of amplification would be needed increasing the cost. Thus the difficulty is in finding systems with the right light-matter interactions necessary to make practical all-optical switches.

To build a useful all-optical switch, one must improve how readily the material responds to light. Several techniques exist for increasing the strength of the light-matter interaction. Our experiment involves using light whose frequency matches the frequency of light emitted by an atom when an electron relaxes from an excited state to the ground state. This technique is called "resonant enhancement" of the light-matter interaction, which simply means that by shining the same frequency of light on the atom that it naturally emits, the light and atom interact more readily than if a different frequency of light was used.

A nonlinear material made of one specific type of atom can have very strong light-matter interactions because of resonant enhancement. For example, we use a material consisting of only rubidium atoms which allows the strong light-by-light interactions needed for all-optical switching. On the other hand, many modern telecommunication devices are made from semiconductor materials where resonant enhancement is not as effective. Thus, more research is needed to find ways to apply the principles that allow all-optical switching in our device and existing technology.