The Application Of Thin Film Absorber Coatings To Enhance The Sensitivity Of Fast Pyroelectric Detectors

The application of thin film absorber coatings to enhance the sensitivity of fast pyroelectric detectorsPeter C. LaDelfe, S. C. Stotlar, L. B. EdgettLos Alamos National Laboratory MS-G770, P.O. Box 1663, Los Alamos, NM 87544AbstractMultilayer optical absorber coatings may be applied to the faces of thermal detectors to enhance their sensitivity at wavelengths at which the detector^material exhibits low absorptivity. These coatings have extremely low mass (<100 pg/cm ) and the absorptance may be varied from 0 to near 100%. The design of these coatings is discussed, based on considerations derived from the detector application and experimental data is presented showing the effect on sensitivity.IntroductionMost pyroelectric detector materials are essentially transparent in portions of the visible and near-infrared regions of the spectrum, rendering them unsuitable for the detection of radiation at these wavelengths. This limitation can be overcome by applying an absorbing coating on either or both faces of the detector. Some workers1 * 2 have used gold black or cermets. These, however, tend to be very thick and increase the thermal time constant of the detector. In addition, the gold black is fragile to the touch.Annis and Simpson3 applied a three layer coating developed by Hilsum4 and Silberg5 to a PLZT detector. This coating consists of a dielectric layer sandwiched between two metal layers. They found that with this arrangement they were able to achieve greater than 95% absorptance.Los Alamos has been developing pyroelectric detectors for use in laser fusion and other high speed applications for the past several years.6'8 Most of these require response at 10.6 |jm where strontium barium niobate (SBN), lanthanum-doped lead zirconate (PLZT), and lithium tantalate (LiTaO~) are useful. However, other applications exist at 1.06 pm, 3-5 urn, etc. The inherent absorption in the visible and near IR of SBN and LiTaO« is shown in Figure 1. These applications necessitated the development of a means to sensitize these detector materials to radiation at wavelengths at which they are normally insensitive.In this paper, the problem of applying an absorber coating to a pyroelectric detector is discussed further. It will be shown that a two layer coating, thinner and lighter than that used by Annis and Simpson, may be used with comparable absorptance. It will also be shown that a coating may be applied to provide an absorptance level arbitrarily close to 100%, limited only by fabrication errors. The use of a pair of absorber coat­ ings, one on the entrance and the second on the exit face of the detector, shall also be discussed. The primary concern will be with detectors for very high speed applications wherein some sensitivity will be sacrificed to reduce the thermal load. The coatings for maximum sensitivity shall receive some consideration.Examples shall be drawn from the problem of coating an SBN detector for use at 1.06 urn. Finally, experimental data shall be presented showing the measured effect of coatings on detector speed and sensitivity.The coatings under consideration here consist of a single metal layer with an anti- reflection layer as shown in Figure 2. For a coating on the entrance face, the incident medium is air and the exit medium is the pyroelectric material. For a coating on the exit face, these are reversed.The materials are characterized by their complex refractive index,N. = n. - i k. J J JIn the work presented here, k. =0 for all but the metal layer. The optical performance of the coatings shall be calculated using the characteristic matrix formalism. This is described fully by Macleod.9 The reflectance, R; transmittance, T; and absorptance, A; are given by:266