These results demonstrated that intrinsically SL photodetectors do not exhibit 1/ f noise. Recently, direct measurements of the noise spectra of high-performance SL heterodiodes based on a variant CBIRD design ( Hill et al., 2009a) were performed at different operational conditions to understand the effects of dark current and of the surface current on detector noise ( Soibel et al., 2010). In the initial works, the noise spectra of the illuminated SL detector were studied and 1/ f noise was not observed, yet the detector noise in these experiments was dominated by photon shot noise ( Mohseni and Razeghi, 2001 Plis et al., 2006). The 1/ f noise plays a significant role in MCT detectors, so significant research efforts were dedicated to understand the origin of the noise in these devices.

The detector excess noise, which is very often 1/ f noise, can significantly degrade the detector performance and has to be minimized or preferably eliminated. The detector “fundamental” noise is given by S i = 2 e ( I + 2 I 0 ) = 2 e I + 4 k B T ∕ R 0, where I and I 0 = k B T ∕ R 0 are the diode current and the diode saturation current, respectively, T is the temperature, k B is Boltzmann's constant, and R 0 is the differential resistance at zero bias ( van der Ziel, 1970). The photon shot noise is given by S ph = 2 η ΦA, where η is the detector external quantum efficiency and Φ is the radiation flux density on the detector of area A. The noise power spectrum of a photovoltaic detector, S pv, is given by S pv ( f ) = S ph + S i + S e( f), where S ph, S i, and S e ( f) are the photon shot noise, the detector “fundamental” (shot and thermal), and detector excess noise, respectively. NEI gives the optical intensity that produces the electrical signal equivalent to the noise signal of the detector, so the lower noise of the detector, the lower optical flux it can detect. The detector performance is limited by the noise equivalent intensity (NEI) value that defines the minimal optical power the detector (or FPA) is capable of resolving for given optics and integration time. Gunapala, in Semiconductors and Semimetals, 2011 5.3 Noise Measurement Chemical treatments to the hair have also been investigated for cosmetic applications.ĭavid Z-Y. The 3 μm resolution (approximately) allows an excellent definition of the various internal structures of the hair, namely, the medulla, the cortex, and the cuticle. There, the signal-to-noise ratio has allowed the measurement of the spatial distribution of NH, CH, and CO groups through the spatially resolved absorption of NH stretch, the difference in the asymmetrical stretch modes, and the CO stretching, respectively. In the mid-infrared, the diffraction limit implies a spatial resolution equal to the 3–15 μm wavelength, a resolution suitable for studying the various functional groups of biological materials.Ī setup developed in Brookhaven involves a commercial Spectra Tech Irms scanning infrared microspectrometer combined with an interferometer fed with the collimated beam of SR ( Nanba, 1989), which has been used, for instance, in the investigation of the 100 μm cross section of human hair ( Bosch, 1998).

To circumvent this limitation, the use of SR as the probe source allows the apparatus to reach the diffraction limit, that is, a spatial resolution equal to the wavelength. However, the signal level clearly decreases when the microscope optics impose apertures of the order of the wavelength. Combining a Fourier transform interferometer with a microscope optical system provides a spectroscopic image of the chemical content of the sample. The infrared spectrum, and more specifically the mid-infrared region where most of the intermolecular modes of a molecule lie, forms a unique fingerprint of that compound. However, for nonhomogeneous materials of which rocks, tissues, organs, and cells are examples, spectroscopic imaging can give far more information about a sample than their analogous global measurement. Haque, in Comprehensive Biomedical Physics, 2014 8.04.7.3 Infrared Spectral Imaging and Microscopyįor homogeneous samples, the single-spectrum technique is well adapted as it will generally allow measurements with optimized signal-to- noise spectra.

Dark noise bands