Tech Briefs

A photon imaging counter is sensitive at very low light levels.

Photon counting detectors with high spatiotemporal resolution are key tools of imaging techniques where picosecond timing and micrometer position imaging are required, such as time correlated single photon counting (TCSPC) or fluorescence lifetime imaging microscopy (FLIM) where fast feedback is critical. A newly developed an imaging photon counter provides a wide field of view (18 mm active area) imaging system that is able to detect individual photons within resolution of 10 ps in time and 40 m FWHM in position at a 16 MHz image resolution.

Highly Sensitive Detection

Developed by Photonis, the imaging photon counter is an imaging system that combines two highly sensitive technologies, microchannel plates (MCPs) and high quantum efficiency (Hi-QE) photocathodes to achieve its unique combination of high count rates and high resolution with a full 18-mm field of view. The imaging photon counter provides extreme low-light images without sub-zero cooling and connects to any microscope or imaging device via a standard C-mount.

Fig. 1 - Comparative QE of available photocathodes, including Hi-QE versions.

Photons are converted into electrons and the electron signal is amplified by a proprietary low-light dual MCP-based detector with a custom 22-mm cross-strip (XS) anode. Events detected in the anode strips are digitized by the electronics unit that achieve global count rates up to 5 MHz with local count rates (100 μm spot) up to 10 kHz. The X and Y positions of each photon are calculated by the charge distribution in the bidi-mensional anode. The spatial resolution in both directions is <40 μm.

The MCPs inside the detector are stacked into a chevron configuration, equipped with 6 μm pore and 80:1 length to diameter channel ratio, allowing electron gains between 104 and 107. Additionally, these MCPs can be coated to be more sensitive to a specified wavelength.

The MCP-based detector is also equipped with a Hi-QE photocathode that offers up to 50 percent improvement of measured QE when compared with standard S20 or S25 photocathodes while lowering dark counts as much as 10X. When tested in a dual-MCP detector, the pulse height distribution is nearly symmetrical. The Hi-QE option supports a unique combination of fast response times, low dark counts, and increased spectral sensitivity that support high count rates in light-starved environments.

Hi-QE photocathodes improve quantum efficiency by narrowing the response range to the specific desired spectral range of the application. Hi-QE photo-cathodes are available in UV, blue, green or red response ranges that can provide sensitivity from 155 to 900 nm, with greater than a 50 percent improvement from 800 to 900 nm when compared with standard S25 photocathodes. The Dark count rate of the Hi-QE S20 photocathodes is extremely low at 20-30 cps/cm2, compared with conventional S20 or S25 photocathodes, which have rates 1,000–2,000 cps/cm2 (see Figure 1).

Applications

Imaging photon camera shown with optional lens.

The imaging photon counter is ideal for the most demanding low-light and high-speed applications, such as super-resolution molecule imaging technologies, high-speed live cell imaging, or fast astronomy events. It is designed to be extremely sensitive at very low light levels while offering exceptionally high count rates independent of timing and spatial resolutions.

Unlike digital sensor-based cameras, the imaging photon counter has zero read-out noise, allowing fast TCSPC techniques even in the sparsest light conditions. In FLIM applications, the position of individual molecules, their fluorescence time, and diffusion or dynamics can be determined at resolution levels unmatched by any commercially available imaging system.

This article was written by Raquel Ortega, PhD, Application Engineer at Photonis. Dr. Ortega would like to thank O. Siegmund, A. Tremsin, J. Vallerga and J. Hull for their previous work on Cross Strip Imaging Anodes or Microchannel Plates; D. Orlov, J. DeFazio, S. Duarte-Pinto and E. Kernen at Photonis for their work on High Quantum Efficiency Photocathodes in Photon Counting Detectors, and National Institute of Standards and Technology staff for feedback. For more information, visit here.