Identification of potential threats in low-light conditions through imaging is commonly achieved through closed-circuit
television (CCTV) and surveillance cameras by combining the extended near infrared (NIR) response (800-10000nm
wavelengths) of the imaging sensor with NIR LED or laser illuminators. Consequently, camera systems typically used
for purposes of long-range observation often require high-power lasers in order to generate sufficient photons on targets
to acquire detailed images at night. While these systems may adequately identify targets at long-range, the NIR
illumination needed to achieve such functionality can easily be detected and therefore may not be suitable for covert
applications. In order to reduce dependency on supplemental illumination in low-light conditions, the frame rate of the
imaging sensors may be reduced to increase the photon integration time and thus improve the signal to noise ratio of the
image. However, this may hinder the camera’s ability to image moving objects with high fidelity. In order to address
these particular drawbacks, PHOTONIS has developed a CMOS imaging sensor (CIS) with a pixel architecture and
geometry designed specifically to overcome these issues in low-light level imaging. By combining this CIS with field
programmable gate array (FPGA)-based image processing electronics, PHOTONIS has achieved low-read noise imaging
with enhanced signal-to-noise ratio at quarter moon illumination, all at standard video frame rates. The performance of
this CIS is discussed herein and compared to other commercially available CMOS and CCD for long-range observation
The material constitution of modern photocathodes (i.e. third generation) has remained a constant for almost two decades. The active GaAs layer is grown by metal organic chemical vapor deposition (MOCVD) and processed to create a negative electron affinity (NEA) surface for photoemission. Thus, these types of cathodes are limited in their spectral response by the band gap energy of the GaAs. There is interest in extending this range past 1000nm while preserving a high quantum efficiency (QE). This would allow taking advantage of an increased luminescence of night sky in the infrared. MOCVD grown InGaAs photocathodes have a photoresponse (PR) in the near infrared. Still, a major drawback to date has been its low QE. We believe that the use of molecular beam epitaxy (MBE) to grow this alloy will permit the fabrication of a higher quality device beyond today's standards, with improved equivalent background illumination and higher QE over a 700nm to 1100nm spectral range. To demonstrate this concept two reflection mode InGaAs photocathode were grown. These cathodes were NEA activated with <i>Cs</i>:<i>O in situ </i>in the MBE reactor after their growth and their PR recorded. Following the activation, optical characterization techniques (i.e. photoluminescence, Raman spectroscopy) were employed to probe electron and phonon energy to relate fundamental material parameters to the observed PR. The collected information is being used to correct and enhance growth characteristics to increase spectral response and QE.
Modern image tube intensifier photocathodes rely on a GaAs active layer, which has traditionally been grown using metallorganic chemical vapor deposition (MOCVD) due to its high throughput and lower cost of operation. Molecular beam epitaxy (MBE) processes have not been thoroughly investigated in that context. The latter technique demonstrates greater structural interface control as well as an improved growth quality for a multitude of applications. Still, at this point it is uncertain, considering actual fabrication techniques for image intensifiers, that the higher growth quality will result in an improvement of devices. Studies are being carried out to compare fundamental optical parameters between GaAs photocathodes grown by both MOCVD and MBE following the same growth and fabrication guidelines. These experiments involve using photoluminescence and Raman spectroscopy to obtain electron and phonon energy information on the materials. An atomic force microscope (AFM) is employed to compare the surface roughness of both methods. In addition, the white light responses of the photocathodes are also evaluated during the creation of a negative electron affinity (NEA) surface to observe any differences between the two growth techniques.