Summer project Xu Lixiang PB

Part 1—construct a pulse picking with AOM Problem: dividing pulse laser’s frequency by ½, ¼,1/8 to extend the range of testing lifetime; Solution: Using AOM to achieve pulse picking; Result: Partly successful;

Details: (1) When we measure the life time of one kind of quantum dot, it is much longer than the periodic time of pulse laser, just like figure 1. (2)To conquer this problem, we need to shorten the periodic time of pulse laser, while the repeating rate of pulses depends on the structure of laser’s cavity, only available to adjust for a little. (3)Then, AOM pulse picking is a better choice, which can pick laser pulses under the control of RF input. The principle is to apply a short RF pulse to the acousto-optic modulator so as to deflect the wanted pulse into a slightly modified direction. The deflected pulse(first order) can then pass an aperture, where the others are blocked. So when RF input signal is zero, there is no pulse output in the first order. Then we can use a square RF input to achieve any kind of pulse picker.( figure 2) Figure 2: simple AOM pulse picker

(4)Another problem is sync: the signal of laser’s controller couldn’t sync with external RF signal, so when we use the external RF signal as time trigger and AOM pulse picker input, the lifetime figure is not exponential decay but an approximately flat uniform distribution. A better choice is to use electronic circuits to achieve sync. Now wait for our electronic shop to complete this electronic circuit.

Part 2—Enhancement of PL of Plasmon Problem: How to improve the accuracy and reliability of the result, when measuring such kind of enhancement on inhomogeneous samples. Solution: dot-to-dot measurement on marked substrate. Result: Very successful and reliable. Details: (1)For the reason of sample’s inhomogeneity, it’s hard to get a accurate result of the enhancement of PL, without and with nanostructure metal. (2)One way is to calculate the average enhancement of whole area, in this case we need very uniform sample, which is not easy to fabricate.

(3)For non-uniform sample, we make a simple but useful trick: replace the ordinary substrate with marked one, dilute samples to let less than 10 dots in one marked square, measure the PL intensity of one dot and mark its position(by taking photos), then after handling the sample with nanostructure metals, find out that dot and measure its PL intensity.

Part 3—Simulation of Plasmon Problem: Design a structure to trap spp(surface plasmon polaritons). Solution: design a structure like photonic crystal with defect and simulate it on comsol. Result: not finished yet. Details: (1)First step: find the plasmon mode in such kind of structure, but failed. So now I’m trying to find the mode in a 2D structure. (2)Use this mode as source to excite the 3D structure (just like this figure) and study its propagation and trap.