Deconvolution method for two-dimensional spatial-response mapping of lithographic infrared antennas.

The spatial impulse response of antenna-coupled infrared detectors with dimensions comparable with the wavelength is obtained from a two-dimensional scan of a tightly focused CO2-laser beam. The method uses an experimental setup with submicrometer resolution and an iterative deconvolution algorithm. The measured spatial response is compared with numerically computed near-field distributions of a dipole antenna, with good agreement. © 1999 Optical Society of America OCIS codes: 040.0040, 040.3060, 040.5570, 100.1830, 230.5440. Novel submicrometer-sized thin-film detectors have been demonstrated in the infrared ~IR! at wavelengths near 10 mm. Their sensing mechanism is based on the operation of metal‐ oxide‐metal ~MOM! diodes 1‐3 or microbolometers. 4 The power collection capability of these ultrasmall detectors is enhanced by the use of integrated antennas. The measured response is closely related to the shape of the antenna ~e.g., dipole, bow tie, or spiral! and its physical dimensions. These detectors are the smallest currently available in the IR region. Characterization of their two-dimensional spatial response is therefore a challenging task. In this paper, we demonstrate a method to map the spatial response of these devices. The signal obtained from a detector is proportional to the irradiance distribution integrated over the collecting area of the device. For classical macroscopic IR sensors the photosensitive region is well defined and is usually described as an effective area within which a constant spatial response is assumed; the outside region is zero. To characterize such devices, one typically scans a probe beam across the photosensitive region, measuring the output of the detector as a function of the position of the probe beam. The measured output signal is, in general, the convolution of the sensor’s spatial response and the beam profile. If the dimensions of the photosensitive region are large compared with the beam, the detector’s spatial response can be approximated directly as the measured output signal. The subwavelength IR detectors used in our study are smaller than the waist dimensions of laser beams, even when low fy# focusing optics are used. Consequently, our problem involves the mapping of a structure smaller than the dimension of the probe beam used for the measurement. This problem has been addressed and solved in other areas of optics. The restoration of images produced by aberrated systems is possible when the transfer function of the optical system is well known. An interesting application of this principle was developed for the Hubble Space Telescope. 5 Its point-spread function ~PSF! was calculated with high accuracy, and blurred images were enhanced by use of the known PSF in a deconvolution procedure. The algorithms developed for this case have been adapted to retrieve the spatial response of our small IR detectors. Section 2 describes the theoretical foundations of the deconvolution method and shows the application of the iterative algorithm, along with the characterization method of the illuminating beam and the parameterization of the spatial response. To illustrate the deconvolution method, we present in Section 3 the determination of the thermal and the antenna spatial response of MOM antenna-coupled detectors. The practical problems of characterizing the beam before the deconvolution algorithm are discussed. We also analyze several methods to provide dimen