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3.1.2: Streak Camera

Streak camera is another method of measuring time-resolved fluorescence. Its principle slightly resembles the operation of old-fashioned analog oscilloscope, and the operation scheme is depiced in fig. 14. It consists of three stages.

 

Fig. 1) Operation of a streak camera. Fluorescence photons knock out electrons from photocatode. The electrons are accelerated in an electric field and enter the space between deflection electrodes, to which a fast-varying  sweeping voltage is connected. Sweeping voltage converts the time of arrival of the electrons into position in space. The start of the sweep is synchronized with an excitation laser pulse. Deflected electrons hit the phosphor screen, the image on which is registered by a CCD camera.

 

 

  • Fluorescence photons from the sample knock out electrons from photocatode.
  • Electrons are accelerated in an electric field, after which they enter the space with fast-varying sweeping electric field. In this manner, the arrival time of the electrons (or, ultimately, photons) is converted into position of electron beam in space.
  • Deflected electrons hit the phosphor screen, producing the image, which is recorded by the CCD camera. From the position of the electron flashes on the screen, their arrival time can be estimated.

Thus, in streak camera, photons are converted into electrons, whose arrival time is converted into spatial coordinate by the sweeping voltage (electrons that arrive later find a higher voltage on the deflection plates and are displaced more). Subsequently, the electron distribution is converted back into the distribution of light (image) on the phosphor screen. To increase sensitivity, the electron image is usually amplified by a microchannel plate PMT, placed before the phosphor. The image is detected by a sensitive CCD camera.

The main advantage of a streak camera compared to the other fluorescence techniques is the fact that deflection plates use only one spatial direction (vertical in fig. 13). The remaining spatial coordinate can therefore be used for spreading the spectrum by a spectrograph. Thus, the entire two dimensional (spectro-temporal) data is collected in a single experiment. Additionally, streak camera is faster compared to TCSPC, because it is based on a simple vacuum tube. Typical time resolution of a syncroscan[1] screak camera can be down to 1 ps. Given the fact that this is the resolution of the entire time-resolved fluorescence spectrum (eq.), the speed of data collection with streak camera exceeds TCSPC by a large margin (and, of course is more expensive by more or less the same margin).

 


[1] Synchroscan is a type streak camera that allows accumulating data from multiple laser shots on the CCD. This type of camera is many times more sensitive than single-shot streak cameras; however, the jitter in the synchronization circuitry slightly ‘smears out’ the time response.