We present system for long-term continuous PPG monitoring, and physical model for PPG analysis. The system is based on ideology of light scattering modulated by the process of RBC aggregation. OXIRATE’s system works in reflection geometry. The sensor is tiny, completely mobile phone compatible, it can be placed nearly everywhere on the body surface. These technical features allow all-night comfortable PPG monitoring that was performed and analyzed. We can define various sleep stages on the basis of different reproducible time-behavior of PPG signal. Our system of PPG monitoring was used also for reflection pulse oximetry and for extreme PPG studies, such as diving.
In spite of rather impressive achievements in intersubband quantum cascade lasers their current parameters are still far
from the needs of practical implementation. We compare theoretical prospects of THz gain for two cases: intersubband
GaAs-based quantum cascade lasers and interband laser based on coupled quantum wells InAs-GaSb. Our
methodology of such a comparison is reduced to following: The most typical design of GaAs-based QCL is compared
with an InAs-GaSb coupled quantum well laser operating in the same frequency range. The detailed density matrix
based calculation shows that the maximal possible gain for CQWL can be three orders of magnitude higher. We
present details on LO-phonon emission rates in typical QCL structures. This calculation supports the statement that
low depopulation selectivity might be an essential feature of QCL in few THz spectral range.
We present a theoretical overview of key physical limitations for application-oriented nanostructure design. We focus
on such promising applications as: nanodot-assisted optical imaging, and photo-thermal therapy with the help of
nanostructures. For these applications we consider the following nanostructures: metal-coated nanoshells and metal
nanoparticles. The actual design of relevant nanoobjects for particular applications must include consideration of such
phenomena as: plasmon resonance, light scattering, light absorption. These phenomena are considered for model
systems of various designs for different parameters of radiation. Our model estimations are compared with
experimental results when such results are available. The conclusions are formulated as a paradigm "desired vs.
feasible".
We present here a bird eye view on the basic physical factors limiting the efficiency of nanostructure-based THz lasing. The origin of these limitations is the contradiction between the requirements of THz gaps, high radiative matrix element value, and selective depopulation. Various ways were suggested to go out of these limitations. They include: sophisticated nanostructure layout, the usage of QCLs, and switch to bipolar THz lasing. We present the results of detailed density matrix based calculations comparing these approaches. In theory InAs-GaSb bipolar THz lasers are the most promising.
Our analysis of spectral behavior of time-variant optical characteristics caused by RBC aggregation is applied to issues
of non-invasive blood monitoring. Modulations of blood flow cause the change in geometry of RBC aggregates and corresponding variance of light scattering. This changes cause the variation of optical transmission, reflection, and polarization of outcoming light. The last can be translated back in absorption coefficients of various blood constituents, refractive index mismatch, etc. For instance, in case of long occlusion simultaneous measurements of both the azimuthal angle and the ellipticity of outcoming light can provide sufficient data to determine the blood glucose.
We present here a bird-eye view of time-dependent optical transmission of blood in red-near infrared spectral range.
This issue is of the key importance both for fundamental understanding and for various applications connected with
non-invasive optical blood analysis. A number of experiments measuring kinetics of blood transmission in the case of
natural heart pulsations and of artificial kinetics following over-systolic occlusion is reviewed. The comprehensive
theoretical approach has to consider scattering-associated mechanism rather than the widely accepted absorption-associated
one. Light scattering occurs on RBC aggregates. The size of aggregates and their shape change in time due
to blood flow variations. It results in the corresponding changes of optical transmission.
We argue that nanostructure based THz lasers of standard design have a principal limitations of gain value. These limitations rise from the obvious necessity to engineer both THz gap and population inversion simultaneously. Typical approach to the gap engineering inherited from midIR lasers utilizes intersubband transitions. However, contrary to midIR range, for THz lasing selective depopulation is problematic. The problem is that the selectivity of
both depopulation mechanisms, LO phonon emission and electron - electron scattering, in THz region is substantially weaker than in midIR region. We suggest to use InAs/GaSb coupled quantum wells as a way to overcome this fundamental limitation. This is the only heterostructure where THz lasing can be based not on intersubband but on interband transitions. A proper design of this structure leads to a hybridization gap coming from anti-crossing of the GaSb valence band and InAs conduction band naturally appearing in the THz range. Two more advantages of this design are (i) a large value of the interband dipole matrix element and (ii) W-shaped spectrum leading to a singular density of states. These advantages lead to a gain much higher than for intersubband THz lasing.
We suggest a new structure for THz generation based on coupled quantum wells of InAs-GaSb. This structure uniquely combines the advantages of both the p-n junction laser and the cascade laser. Actual generation results from optical transitions between the e3 and e2 levels in InAlAs quantum well, and resonant with them effective band gap between the conduction band of InAs and the valence band of GaSb quantum well, e1-hh1. The separation between e1 and e2 equals to LO phonon energy that provides population inversion between e3 and e2. We consider two ways of structure design that differ by the carrier dispersion: W-shaped dispersion of the carriers in ground states and regular V-shaped dispersion. All these structures bring in the advantages of the system with equidistant levels, i.e., good temperature characteristics and high probability of radiative transition leading to low threshold current compared to alternative designs. We present a comparative analysis of various mechanisms of carrier relaxation (LO phonons and electron-electron scattering) and point out an optimal band structure favoring high efficiency of THz emitter. Corresponding band structure calculations supply one with the range of quantum well parameters providing all the features presented above.
We prove experimentally that RBC aggregation is among the major factors affecting time evolution of light transmission in both the normal situation of pulsatile blood flow and the situation of over-systolic vessel occlusion. Optical transmissions of tissue in vivo have been measured in red/near-infrared region. Sudden blood flow cessation causes the light transmission rising. For certain wavelengths range this growth becomes non-monotonic. The correspondence between in vivo measurements and the theoretical simulations is reached if we attribute the transmission growth to the change of average size of scatterers. The most important blood parameters such as hemoglobin, glucose, oxygen saturation, etc., influence the transmission growth following over-systolic occlusion and, therefore, may be extracted from the detailed analysis of the time evolution of optical transmission. It forms a basis for new kind of non-invasive measurements, i.e., occlusion spectroscopy. The results of in vivo clinical trials are presented for glucose and hemoglobin.
We develop theoretical models of light transmission through whole blood considering RBC aggregation. RBC aggregates are considered to be the main centers of scattering in red/near- infrared spectral region. In pulsatile blood flow the periodic changes of aggregate geometry cause oscillations of light scattering. Thus scattering-assisted mechanism has to be taken into account in pulse oximeter calibration. In case of over-systolic vessel occlusion the size of aggregates grows, and the light transmission rises. Light diffraction on a single scatterer makes the transmission growth non- monotonic for certain spectral range. For the most typical set of aggregate parameters this range corresponds to wavelengths below 760 nm, and this prediction fits well both in vivo and in vitro experimental results. This spectral range depends on the refraction index mismatch and the geometry of aggregates. Both of them may be affected by the chemistry of blood. For instance, changes of glucose and hemoglobin have different effect on light transmission time response. Consequently, their content may be determined from time evolution of optical transmission.
Optical transmission of tissue in-vivo and model red blood cells (RBS) suspensions in vitro have been measured in red and near infrared region targeting better understanding of the nature of in vivo pulsatile signals. It is shown experimentally (both in vitro and in vivo) that the pulsatile signal may result not just from volumetric changes, but also from light scattering fluctuations. Theoretical predictions on time evolution of optical transmission for the case of very long over-systolic occlusion also have been proved experimentally for both in vivo measurements and in vitro model sets. The interconnection of the shape of optical signal and geometry of RBC aggregates is confirmed.
We consider a number of diffusive and transport models of light transmission through whole blood, targeting better understanding of nature of optical transmission pulsations for blood flow modulated by heartbeats. We claim the existence of scattering- associated mechanism rather than the absorption-associated one. Single erythrocytes and their aggregates are considered to be the main centers of scattering in the red- near infrared spectral region. The shape and size of aggregates change in time due to blood flow changes. The corresponding changes of optical transmission are simulated.
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