Proc. SPIE. 6865, Nanoscale Imaging, Sensing, and Actuation for Biomedical Applications V
KEYWORDS: Signal to noise ratio, Imaging systems, Sensors, Skin, Optical microscopy, Image resolution, Scanning electron microscopy, Near field scanning optical microscopy, Near field, Spatial resolution
We introduce a new aperture-type near-field scanning optical microscopy (NSOM) system, which rely on large area
(e.g., > 200 x 200 nm) aperture geometries that have sharp corners. The spatial resolution of this new near-field
imaging modality is not determined by the size of the aperture, but rather by the sharpness of the corners of the large
aperture. This approach significantly improves the light throughput of the near-field probe and therefore increases
the optical signal-to-noise ratio (SNR). Here we discuss the basics of this new near-field microscopy approach and
illustrate both theoretically and experimentally that an array of detectors can be utilized to further improve the SNR
of the near-field image.
Necrotic-core fibroatheromas (NCFA) with thin, mechanically weak fibrous caps overlying lipid cores comprise the majority of plaques that rupture and cause acute myocardial infarction. Laser speckle imaging (LSI) has been recently demonstrated to enable atherosclerotic plaque characterization with high accuracy. We investigate spatio-temporal analysis of LSI data, in conjunction with diffusion theory and Monte Carlo modeling of light transport, to estimate fibrous cap thickness in NCFAs. Time-varying laser speckle images of 20 NCFAs are selected for analysis. Spatio-temporal intensity fluctuations are analyzed by exponential fitting of the windowed normalized cross-correlation of sequential laser speckle patterns to obtain the speckle decorrelation time constant, (), as a function of distance from the source entry location. The distance, , at which () dropped to 65% of its maximum value is recorded. Diffusion theory and Monte Carlo models are utilized to estimate the maximum photon penetration depth, zmax(), for a distance equal to , measured from LSI. Measurements of zmax() correlate well with histological measurements of fibrous cap thickness (R=0.78,p<0.0001), and paired t-tests show no significant difference between the groups (p=0.4). These results demonstrate that spatio-temporal LSI may allow the estimation of fibrous cap thickness in NCFAs, which is an important predictor of plaque stability.
Semiconductor optical amplifiers (SOAs) based on nanostructure gain media such as quantum dots (QD) and quantum dashes (QDASH) have several basic characteristics which offer significant performance improvements over commonly used quantum well (QW) or bulk amplifiers. Among these are broadband optical gain bandwidth (which is two to three times broader than that of QW/bulk gain media), fast gain dynamics, large saturation powers, and low α parameter and population inversion factor. Originally, these properties have been demonstrated for QD/QDASH SOAs operating at 1000 nm and 1300 nm. However, it is imperative that QD/QDASH SOAs operating at 1550 nm be materialized in order for them to have the expected impact on fiber-optic communication. Operation at 1550 nm has been achieved using InAs / InP QD and QDASH laser structures. In this paper the unique gain and noise properties of InAs / InP QDASH SOAs operating at 1550 nm will be presented. Specifically, cross-gain-modulation, four-wave-mixing and chirp measurements which explore the complex spectral cross relaxation dynamics of these SOAs will be described and highlighted in the context of simultaneous, distortionless, high bit-rate multiwavelength data amplification, as well as wideband / high-speed optical signal processing applications. Also, an experimental study of the gain and noise in saturated QDASH SOAs will be described together with a theoretical analysis comprising both coherent and incoherent gain phenomena. The impact of the partially inhomogeneously broadened gain spectrum, fast population pulsation dynamics, α parameter and wetting layer density of states on the noise characteristics will be discussed.
Semiconductor lasers and amplifiers were developed based on self-assembled quantum-dot gain material. This paper gives an overview about the recent work on GaAs- and InP-based quantum-dot devices mainly dedicated for telecom applications. The major advantage of quantum-dot like gain material, i.e. the possibility to tailor the spectral and spatial gain properties of an amplifying material, was used to optimize different device aspects, like low threshold current, broad band amplification or low temperature sensitivity. High performance GaAs-based continuous wave (cw) operating quantum-dot lasers could be fabricated with threshold currents of about 2 mA (L = 400 μm). Single mode emitting devices with emission wavelengths > 1.3 μm were realized by laterally coupled feedback gratings with threshold currents below 5 mA, output powers > 5 mW and cw operation temperatures up to 85 °C. Modulation frequencies of up to 7.5 GHz were obtained for standard device structures. For long wavelength telecom applications quantum-dot like material with dash geometry was developed on InP substrates with basic properties in the transition region between quantum-dot and -wire systems. A very large tuning range of the emission wavelength between 1.2 and 2.0 μm (room temperature) was obtained which allow the realization of material with ultra-wide gain bandwidth. Quantum-dash laser structures reaches threshold current densities < 1 kA/cm2. Ridge waveguide lasers with a cavity length of 1.9 mm show cw threshold currents of about 100 mA and maximum output powers > 40 mW per facet. With 300 μm long facet coated devices cw threshold currents of 23 mA and maximum operation temperatures in pulsed mode of 130 °C were achieved. Semiconductor optical amplifiers were fabricated by using broad band quantum-dash material. For a 1.9 mm long device, up to 22 dB gain was obtained with a three times larger spectral range than in comparable quantum well devices. High speed nearly pattern free signal amplification up to 10 GBit/s could be demonstrated and wavelength conversion experiments were performed.