A light emission microscopy (LEM) setup is constructed in the scope of SENSE at the MPI in Munich, specifically to investigate optical cross-talk in SiPMs. LEM allows for the direct measurement of optical cross-talk from the emission of light from the device.
Optical cross-talk reduces the quality of the signal, since more cells in the device trigger than expected, and is therefore an important parameter for understanding the photon detection efficiency. Photons emitted during avalanche propagation that are resorbed in neighbouring cells or even in the inactive region of the same cell, causing additional current pulses, either through direct propagation through the silicon, or indirect propagation (e.g. via reflects from the surfaces).
For the measurement of the optical cross-talk a pulsed laser is shot on an individual cell to start an avalanche process. Then the light occuring in the neighbouring cells is detected. This allows the direct measurement of optical cross-talk which will be of high interested for future high-energy and astroparticle experiments.
Using the Light Emission Microscopy (LEM) the measurement and visualization of photon emission in all kind of soild state devices is possible. Below you find a schematic view and a picture of the setup.
Diagram of the Light Emission Microscopy setup.
Photo of the Light Emission Microscopy setup.
An objective lens serves two purposes in the setup; first to focus the laser down to a small spot size (<10 μm), which is much smaller than the typical SPAD size (~100 μm), and second to resolve the photon emission from the device onto the CCD. The LEM setup is installed inside a dark box with a feed through opening for the power cables, fibers, and CCD camera readout. A long-pass filter blocks out stray laser light from reaching the CCD. Background images are recorded and subtracted to produce the final image of the light emission from the SiPM.
Example of experimental results
Below the background-subtracted image is shown. The light emission from the fired cell as well as the light emission from the neighbouring cells is visible. The light in these neighbouring cells is caused from optical cross-talk.
3-dimensional image of light emission observed from a 3x3 mm² Hamamatsu LCT4 SiPM and recorded with an Andor Clara CCD camera [1039px x 1040px]. Only the cell in which the laser was fired and neighboring cells are shown.
Derek Strom & Razmik Mirzoyan
Max Planck Institute for Physics
Phone nr : +49 89 323 54 422