Quantum Grid IR Spectrometer Enables Multicolor Detection

Anne L. Fischer

Two- and four-color infrared imaging employing multiple detectors is used for applications such as target discrimination, land mine detection and geological surveying. Now scientists from the US Army Research Laboratory in Adelphi, Md., and from Polytechnic University in Brooklyn, N.Y., have designed a multicolor infrared spectrometer that uses a linear array of quantum grid infrared photodetectors based on a single material.

The top view of an array of quantum grid infrared photodetectors shows the grid lines of an individual photodetector.

In current two- and four-color IR imaging, materials such as PtSi, InSb and HgCdTe are used to detect specific ranges of wavelengths, explained the Army laboratory's Kwong-Kit Choi. It would be desirable to simplify such imagers by using one type of detector, but there is no easy way to control the absorption spectra of these intrinsic infrared materials, which are governed by their bandgaps.

By using a property of quantum well materials, the researchers can achieve unlimited multicolor detection with a single detector material. Because quantum well structures respond only to the electric field component of an electromagnetic field vertical to the material layers, they do not absorb normal incident light. A detector geometry that can change the direction of the electric field under normal incidence thus can be used to select a specific detection wavelength from a broadband material. An array of such detectors with different geometries can detect different ranges of wavelengths, forming a spectrometer.

In their spectrometer, the researchers adopted a quantum grid infrared photodetector structure for wavelength selection that comprises an array of grid lines of the detector material, with the width determining the absorption wavelength. The IR material is a binary superlattice structure for 8- to 14-µm broadband absorption that employes a coupled dissimilar quantum well pair as its basis.

In contrast to other light-coupling schemes that they have studied, this one uses multipole scattering from metal strips on top of the grid. Multicolor infrared detection is achieved by integrating these quantum grid photodetectors in unit cells of a two-dimensional array. This technique may prove useful in chemical detection and material analysis because it not only will detect an object, but also will analyze its emission spectrum, Choi said.

The team is working to reduce the size of the quantum grid infrared photodetectors and to develop geometries so that the spectrometer units can be more closely packed to enable imaging. The present size of a photodetector is 1000 × 400 µm, making the approach suitable for the construction of a spectrometer, but not of an imager.

One of the patterns the group is investigating takes the shape of a daisy, with grid lines arranged radially from the central bonding pad. With this "daisy quantum well infrared photodetector" geometry, detector pixels as small as 18 × 18 µm may be realized.