Photo-detectors, particularly those operating in the infrared are subject to thermally generated dark currents possessing shot noise that limits the achievable signal to noise performance. The dark current can be reduced by cooling the detector. This requires considerably more power beyond what is necessary to operate the detector. Since dark current is proportional to the area of the photo-detector, optical techniques that reduce this area can improve performance.
Broadband heterodyne detection shows promise for room temperature operation of infrared detectors and spectrometers, allowing significant reductions in system power and weight requirements.
LLNL researchers have developed a broadband heterodyne detection system that incorporates several significant improvements that move the state of the art toward quantum noise limited performance. The design comprises of an optical element that increases the intensity of the incoming light on the detector by a factor exceeding 50x. It is based on the properties of surface plasmons in interaction with optical diffraction structures or gratings. LLNL’s approach does not rely on a resonance phenomenon making it suitable for applications such as infrared cameras as well as spectrometers.
One improvement is a new detector architecture. The infrared signal of interest is mixed with light from a broadband light source to create a lower-frequency—e.g., RF—signal that is more readily detected than the infrared signal itself. To achieve the required speed and sensitivity for room temperature heterodyne detection, the RF detector element is decoupled from the photodetector.
A second improvement is the use of an optical upconversion scheme, previously demonstrated for mm-wave detection, to enable sensitive room-temperature detection of heterodyne RF signals. In this approach, RF energy in a waveguide is coupled to a collinear laser beam. Modulations in the laser intensity can then be measured and correlated with the heterodyne signal.
A third improvement is the use of a field-emitting carbon nanotube antenna. Carbon nanotubes have been demonstrated to be useful as sensitive detectors of RF radiation when operating in field emission mode to an adjacent electrode. A field-emitting carbon nanotube oscillates significantly in the presence of an RF field, modulating the current. In the case of the infrared photodetector, the modulation of this current can then be correlated with the presence of an RF heterodyne signal at the photodetector. This enables room-temperature detection of the RF signal with extremely small (~nW) power requirements.
A fourth improvement is the incorporation of a plasmonic superlens that increases the intensity of the light impinging on the detector by a factor of >50, considerably more than any of the prior art techniques.
Improved detector for infrared hyperspectral imaging systems, particularly for surveillance applications is the goal to enable ground-based persistent surveillance and micro-power space surveillance with infrared hyperspectral imaging capability. The major markets for infrared hyperspectral imaging systems are homeland security, law enforcement and military. Specific applications include:
- Ground-based persistent surveillance
- Micro-power space surveillance
- Atmospheric remote sensing
- Astronomy and planetary remote sensing
- Field applications in chemical agent and bioagent detection
- Agriculture monitoring
- QA/QC in industrial processes