Page 73 - AWSAR 2.0

like a mirror and deflected a light beam pointed at it. The heat given out to the solvent also created a gradient of temperature, that is, the parts of the solvent nearer to the focus of the light beam were at a higher temperature, while the parts away from it were at a lower temperature. This gradient of temperature was responsible for holding the bubble in a fixed position, which was critical for it to function as a stable switch.
Initially, when we measured the power of the light transmitted through the solution, we saw cyclic changes in the light power the power alternately increased and decreased. This system, therefore, behaved more like an oscillator, which toggled between on and off states instead of a stable switch. We also had no control over these states. On looking closely at this system using a microscope, we found that after its formation, the bubble slowly grew in size. On reaching a certain size, it escaped the focus of the light
beam. Immediately after this,
another bubble formed and this
cycle continued.
Therefore, the next
challenge for us was to stabilize
the bubble so that we could
control the on and off states
of our switch. To solve this
problem, we replaced the low
boiling point solvent hexane
with a higher–boiling point
solvent toluene. This helped
in making the growth of the
bubble a very slow process and hence led to a stable bubble. We could stabilize the bubble for many hours in toluene. Now, we could use this bubble to control the direction of our signal light.
Another challenge that we faced was that the signal light must not interact with the medium. Otherwise, it might disturb the stability
of the bubble. The solution to this problem was to use a signal light of energy that would not get absorbed by the quantum dots. We chose red light as our signal light. Red light, being lower in energy than the absorption threshold of our quantum dots, was incapable of forming the bubble. If we switched off the blue light, the red light would simply pass through the quantum dots without doing much. Thus, blue light was essential for the formation of a bubble and hence controlling the switch. Therefore, we achieved the control of a red light beam by a blue light beam via a tiny bubble.
It turned out that the powers required for the bubble formation were of the order of milliwatts (close to 50 mW). These powers were about a thousand times less than the powers typically needed for such systems. Thus, we were able to demonstrate a simple optical transistor capable of working at very low powers.
After confirming that our system functioned as a low power optical switch, we went one step further and decided to check whether we could carry out basic computations using it. To perform operations such as addition, subtraction, etc., a computer needed to work with a combination of transistors that formed logic circuits or logic gates. Using our optical transistors, we could operate such logic circuits at very low
powers.
Thus, by using a light-generated bubble
in a quantum dot solution, we made a low power optical transistor and also showed its application in designing logic circuits. Although it needed further optimization in terms of efficiency and speed, it was a small step toward low-power all-optical computation.
Dr Nihit Saigal
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The problem that we defined for our research was: “How can we control a light beam by another low-power light beam using quantum dots as media?” During our journey through several experiments in the laboratory, we came up with a unique solution to this problem.