I want to measure the PL spectra for devices of different sizes. How could I change the size and shape of excitation laser spot, for example a line spot of different lengths or a circle spot of different radius, without changing the excitation power density?
What kind of equipment do I need ?
You might be able to use a spatial light modulator (SLM) to make a line. For a circular spot this is much easier. You can change the focused beam diameter by changing the diameter of the collimated input excitation beam. The larger in input mean, the smaller the focused spot. A very crude (but effective) way to do this is to put an iris in the excitation beam path. If you close down on the iris, you will clip the beam in the near field, which will broaden the beam spot on your sample. The downside with clipping the beam is that your input beam is no longer gaussian. To overcome this, you could instead used a variable beam expander (from thorlabs for example), which can be used like variable telescope to change the beam diameter. You will need to adjust the power if you want to keep the intensity uniform.
For Circular, you can just use different focal length lens. for example, If you are using 100 mm focal length lens and the sample is at the focal point. Now you can use the 150 mm lens to get bigger spot size and 200 mm lens to get even bigger spot size.
In case if you are interested in PL for small size structures (smaller than the optical diffraction limit, such as qunatum devices, quantum dots, nano-structured materials, etc.), then you use Near Field Scanning Microscopy (NSOM) approcah in your PL experiment using an optical mask. This will allow yo uto overcome the optical diffration limits associated with optical systems, and the limitation with the laser wavelength. For example see: http://www.ncbi.nlm.nih.gov/pubmed/25359798 or http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1015926
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