TRPL Imaging

Time-Correlated Single Photon Counting (TCSPC) is used to determine the photoluminescence lifetime. In TCSPC, one measures the time between sample excitation by a pulsed laser and the arrival of the emitted photon at the detector. TCSPC requires a defined “start”, provided by the electronics steering the laser pulse or a photodiode, and a defined “stop” signal, realized by detection with single-photon sensitive detectors (e.g. Single Photon Avalanche Diodes, SPADs). The measurement of this time delay is repeated many times to account for the statistical nature of the fluorophores emission. The delay times are sorted into a histogram that plots the occurrence of emission over time after the excitation pulse.
In order to acquire a fluorescence lifetime image, the photons have to be attributed to the different pixels, which is done by storing the absolute arrival times of the photons additionally to the relative arrival time in respect to the laser pulse. Line and frame marker signals from the scanner of the confocal microscope are additionally recorded in order to sort the time stream of photons into the different pixels.

Consequently the essential components of a TRPL imaging set-up are:

• pulsed laser source (diode lasers or multi-photon excitation)
• single photon sensitive detector
• dichroic mirror (to separate fluorescence signal from excitation light)
• objective (to focus the excitation light into the sample and collect fluorescence signal)
• TCSPC unit to measure the time between excitation and fluorescence emission

Following systems can be used for TRPL imaging

• MicroTime 100 - Compact upright photoluminescence microscope >
• MicroTime 200 -  Confocal microscopic platform >
• FluoMic - Compact upright widefield photoluminescence microscope >
• FlexWave - Wavelength selection unit >

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Carrier diffusion in a GaAsP quantum well system

Carrier diffusion observed in a GaAsP quantum well system. The central spot of the image was illuminated with a 440 nm pulsed diode laser at 40 MHz and the emission was filtered with a 735/50 nm bandpass filter. Only the detection volume was scanned over the image. The false color code in the different images of the ROI indicates that the average lifetime increases for larger radii from the point of illumination. This is shown with a complete lifetime mapping in (a) and individual ROI of increasing distance from the excitation in (b), (c) and (d). A closer look at the temporal behavior of the TRPL as shown in (e) reveals that the maximum in the transient photoluminescence shifts to later times in the transient emission for increasing distance from the point of illumination. This is the result of the longer time scales for the migration of charge carriers from the center of the image into the selected region of interest before emitting a photon from recombination. While the rise times vary, the primary decay component remains constant at around 1.1 ns. Only the center of the image shows a different behaviour, where the decay is slightly shorter at roughly 1 ns since diffusion from the point of excitation acts as an additional depleting component.

Set-up:

• Olympus FluoView FV1000 with LSM Upgrade Kit

Sample courtesy of Andrea Knigge, Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Germany.

CdTe-polycrystalline wafer surface

A CdTe-polycrystalline wafer surface was scanned on a confocal microscope before and after thermal activation with a chloride compound. The respective intensity images (a) and (b) as well as the lifetime images (d) and (e) before and after treatment show a significant increase in intensity and photoluminescence lifetime after activation. The statistical distribution of the intensities (c) and lifetimes (f) over the full image are given before (blue) and after (green) activation. With only the 3 ms/pixel measurement time, a distinctive change in the average lifetime can be determined as well as significant variations of the lifetime over different regions of the CdTe structure.

Set-up:

• MicroTime 100

Sample courtesy of Hannes Hempel and Christian Kraft, Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Germany

References:

[1] Buschmann, V. et al, Journal of Applied Spectroscopy, Vol.080, p.449-457 (2013)
[2] Kraft, C. et al, Journal of Applied Physics, Vol.113, 124510 (2013)