Photon Penetration Depth Research Articles

使用敏感度补偿方法提高近红外断层成像的深度分辨率

The sensitivity of diffuse optical tomography (DOT) imaging exponentially decreases with the increase of photon penetration depth, which leads to a poor depth resolution for DOT. In this letter, an exponential adjustment method (EAM) based on maximum singular value of layered sensitivity is proposed. Optimal depth resolution can be achieved by compensating the reduced sensitivity in the deep medium. Simulations are performed using a semi-infinite model and the simulation results show that the EAM method can substantially improve the depth resolution of deeply embedded objects in the medium. Consequently, the image quality and the reconstruction accuracy for these objects have been largely improved.

Performance analysis of near-field thermophotovoltaic devices considering absorption distribution

This paper elucidates the energy transfer and conversion processes in near-field thermophotovoltaic (TPV) systems, considering local radiation absorption and photocurrent generation in the TPV cell. Radiation heat transfer in a multilayered structure is modeled using the fluctuation–dissipation theorem, and the electric current generation is evaluated based on the photogeneration and recombination of electron–hole pairs in different regions of the TPV cell. The effects of near-field radiation on the photon penetration depth, photocurrent generation, and quantum efficiency are examined in the spectral region of interest. The detailed analysis performed in the present work demonstrates that, while the near-field operation can enhance the power throughput, the conversion efficiency is not much improved and may even be reduced. Subsequently, a modified design of near-field TPV systems is proposed to improve the efficiency.

Size-Dependent Angular Distributions of Low-Energy Photoelectrons Emitted from NaCl Nanoparticles

Angle-resolved threshold photoelectron spectra are recorded for size-selected beams of sodium chloride nanoparticles (radius = 25−250 nm). The photoelectron angular distributions exhibit a size-dependent asymmetry that is inversely proportional to particle radius. A model of the internal electric field amplitude and the photoelectron escape probability inside the particles reveals that the asymmetry in photoemission arises because the finite dimensions of the particle are comparable to both the photon penetration depth (14−22 nm) and electron escape length (10 nm).

Penetration depth of low-coherence enhanced backscattered light in subdiffusion regime

The mechanisms of photon propagation in random media in the diffusive multiple scattering regime have been previously studied using diffusion approximation. However, similar understanding in the low-order (subdiffusion) scattering regime is not complete due to difficulties in tracking photons that undergo very few scatterings events. Recent developments in low-coherence enhanced backscattering (LEBS) overcome these difficulties and enable probing photons that travel very short distances and undergo only a few scattering events. In LEBS, enhanced backscattering is observed under illumination with spatial coherence length L{sc} less than the scattering mean free path l{s}. In order to understand the mechanisms of photon propagation in LEBS in the subdiffusion regime, it is imperative to develop analytical and numerical models that describe the statistical properties of photon trajectories. Here we derive the probability distribution of penetration depth of LEBS photons and report Monte Carlo numerical simulations to support our analytical results. Our results demonstrate that, surprisingly, the transport of photons that undergo low-order scattering events has only weak dependence on the optical properties of the medium (l{s} and anisotropy factor g) and strong dependence on the spatial coherence length of illumination, L{sc} relative to those in the diffusion regime. More importantly, these low-order scattering photons typically penetrate less than l{s} into the medium due to the low spatial coherence length of illumination and their penetration depth is proportional to the one-third power of the coherence volume (i.e., [l{s}piL{s}{2}]1/3) .

Magnetic depth profiles from resonant soft x-ray scattering: Application to Dy thin films

We employ the strong variation of the photon penetration depth across an electronic resonance in soft x-ray scattering for a depth resolved study of an antiferromagnetic (AFM) thin film. We directly observed the development of the helical AFM structure in thin Dy(001) films on W(110) across the temperature-induced phase transition from ferromagnetic to helical AFM order.

An amorphous SIC/SI image photodetector with voltage-selectable spectral response

The sensitive devices are multilayer stacked structures p–i–n/p–i–n based on a-SiC : H and a-Si : H between two transparent conductive contacts. The thickness and the absorption coefficient of the front p–i–n cell is optimized for blue collection and red transmittance and the thickness of the back one adjusted to achieve full absorption in the green and high collection in the red spectral ranges. Color discrimination is achieved through the modulation of one, two or both cell depletion regions by an applied external voltage. The devices are characterized through the analysis of the photocurrent and spectral response under different steady state optical bias and applied voltages. In order to achieve full color discrimination and to evaluate the sensors responsivity to different light wavelengths, the photocurrent generated by a modulated red light is measured under different optical/electric bias. The sensor element is illuminated through the back diode with red modulated light and the optical bias applied onto the front diode. From the experimental results it is observed that when using a red modulated light the thin a-SiC : H front absorber (200 nm) maximizes the conversion efficiency for blue front optical bias and the thickest back absorber layer (1000 nm) minimizes the conversion efficiency in the red range. Results show that the conversion efficiency to a red modulated light under blue front optical bias is maximized when a 200 nm a-SiC : H front absorber is used, and minimized in the red range if the absorber layer of the back diode is around 1000 nm thick. In those devices the green photons absorption occurs mainly across the front diode, the n–p defectous interface and at the front side of the back diode. Under reverse bias and blue irradiation the collection is high since the back diode becomes fully depleted due to its self-biasing process. Under red illumination the a-Si:H back absorber acts as a load due to the high light penetration depth of the red photons and the low collection is determined by the dark characteristics of the front diode. In the green spectral range the reverse bias increases the potential drop across the back diode and the collection increases linearly. The effect of the applied voltage on the color selectivity and spectral sensitivity is discussed and supported by a physical model based on a numerical simulation.

Measurement of fibrous cap thickness in atherosclerotic plaques by spatiotemporal analysis of laser speckle images

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, tau(rho), as a function of distance rho from the source entry location. The distance, rho', at which tau(rho) 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(rho'), for a distance equal to rho', measured from LSI. Measurements of zmax(rho') 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.

Why the quest of new rare earth doped phosphors deserves to go on

The scientific research on phosphors has a long history starting more than 100 years ago. But recently the appearance of new kinds of displays and lighting devices (plasma display, fluorescent lamp without mercury...) induced an increase of the research of new phosphors with better luminous efficiency than those available up to now. It has been shown that the behaviour of classical phosphors in a plasma display panel is quite different than in a cathode ray tube and that the vacuum ultraviolet (VUV) excitation process has to be studied with care in order to improve the phosphors efficiency. It is well established now that a good phosphor for electronic or ultraviolet excitation, is not necessarily a good choice for excitation in VUV. This is probably due to the fact that the excitation process is very different in that case. We will illustrate this difference on the well-known LaPO 4 :Ce 3 + , Tb 3 + phosphor. The penetration depth of the VUV photons is extremely small inducing a large contribution of the surface of the phosphor. We have shown that, for most phosphors, only a few tens of nanometers of the phosphor grain are really useful and we propose a way to realize phosphors powders using less than 20% of doped materials. However the traps of the material play a crucial role in the fluorescence properties due to the fact that autoionization process is likely when dopant ions absorb high energy photons. Fast aging process is one of the main drawbacks of VUV excitation. We demonstrate this effect on BaMgAl 1 0 O 1 7 :Eu 2 + , the blue emitting phosphor widely used up to now in Plasma Displays Panels and fluorescent lamps. Low energetic efficiency is another drawback of VUV excitation that can be solved only in the framework of fundamental studies. Quantum cutting emission may be a solution and calls new research to find good phosphors characterized by a high quantum efficiency, a high fluorescence efficiency and an adapted colorimetry.

Multicanonical Monte-Carlo simulations of light propagation in biological media

Monte-Carlo simulation is an important tool in the field of biomedical optics, but suffers from significant computational expense. In this paper, we present the multicanonical Monte-Carlo (MMC) method for improving the efficiency of classical Monte Carlo simulations of light propagation in biological media. The MMC is an adaptive importance sampling technique that iteratively equilibrates at the optimal importance distribution with little (if any) a priori knowledge of how to choose and bias the importance proposal distribution. We illustrate the efficiency of this method by evaluating the probability density function (pdf) for the radial distance of photons exiting from a semi-infinite homogeneous tissue as well as the pdf for the maximum penetration depth of photons propagating in an inhomogeneous tissue. The results agree very well with diffusion theory as well as classical Monte-Carlo simulations. A six to sevenfold improvement in computational time is achieved by the MMC algorithm in calculating pdf values as low as 10(-8). This result suggests that the MMC method can be useful in efficiently studying numerous applications of light propagation in complex biological media where the remitted photon yield is low.

DUV photons to probe the photoconductive properties of diamond films in the surface proximity

We suggest the use of deep UV photons as a probe to investigate the photoelectrical properties of different depths in the diamond films. The penetration depth of UV photons decreases rapidly towards shorter wavelengths in the region of and just below the band gap energy, achieving ∼2 nm at wavelengths approximately 100 nm. Therefore, they are suitable for probing the bulk properties and the surface proximity of diamond layers. We have irradiated different CVD diamond samples with UV laser pulses at 193, 213 and 266 nm as well as continuous radiation emitted by a UV source in the 140–250 nm wavelength range. We have analysed the photoconductivity at different depths as a function of material thickness, electric field value and polarity, radiation intensity, time response to laser pulses and to illumination transients. We have observed different behaviours above and below the band gap energy that could be due to polarization effects and a photo-response quenching above the band gap that is enhanced by reversing the electric field.

Noninvasive determination of tissue optical properties based on radiative transfer theory

We present a feasibility study of a new method for determining the tissue optical properties, including the absorption and scattering coefficients and the scattering asymmetry factor. A state-of-the-art radiative transfer model for the coupled air/tissue system, based on rigorous radiative transfer theory, is used in our forward modeling simulations. The concept of the effective photon penetration depth is introduced and used to help determine the depth below, which information about the tissue will not be available through noninvasive imaging of a biological tissue using reflected diffuse light. Simulation results show that for accurate determination of tissue optical properties, one can use radiative transfer theory in conjunction with measurements of reflected radiances as well as other existing techniques.

Rare-earth doped phosphors: oldies or goldies?

The scientific research on phosphors has a long history starting more than 100 years ago. But recently the appearance of new kinds of displays and lighting devices (plasma display, fluorescent lamp without mercury, etc.) induced an increase of the research of new phosphors with better luminous efficiency than those available up to now. It has been shown that the behavior of “classical” phosphors in a plasma display panel is quite different than in a cathode ray tube and that the vacuum ultraviolet (VUV) excitation process has to be studied with care in order to improve the phosphors efficiency. That is particularly true in PDPs. It is well established now that a good phosphor for electronic or ultraviolet excitation is not necessarily a good choice for excitation in VUV. This is probably due to the fact that the excitation process is very different in that case and also because the penetration depth of the VUV photons is extremely small inducing a large contribution of the surface of the phosphor. We will illustrate this with some examples. Methods to accelerate luminous intensity decrease under VUV excitation will be described. Low efficiency, fast aging process are both drawbacks that can be solved only in the framework of fundamental studies. Quantum cutting emission may be a solution for the first one but no satisfactory process was proposed for the moment to solve the second.

Photocatalytic degradation of nitrobenzene using titanium dioxide and concentrated solar radiation: chemical effects and scaleup

Photocatalytic degradation of nitrobenzene (NB) using titanium dioxide (Degussa P-25) as photocatalyst and concentrated solar radiation has been studied. The effects of various factors, such as the presence of anions common in the industrial wastewater, the initial pH and the addition of FeSO 4, were investigated. The intermediates detected were o, p,m-nitrophenols and dihhydroxy derivatives indicating that degradation proceeds via OH radicals. Degradation of NB was studied in three reactors of different diameters to ascertain the effect of photon penetration depth on the degradation.

Preliminary studies on the photon path in breast tissue model by NIR-TRS.

A typical tumor releases angiogenesis factor that induces the capillary growth around the tumor and, therefore, a greater amount of blood is present around the tumor. A tumor usually, therefore, becomes a local absorber due to its higher hemoglobin concentration. The ultimate goal of this project is to localize the position of a tumor in a breast tissue at an early stage using near infrared spectroscopy. Computer simulations were performed to obtain TRS spectra at various locations in a system with optical properties of human breast. The time domain output pulses, then, were transferred to the frequency domain and the data were analyzed at various modulated frequencies. The changes in the photon path with respect to the frequency were systematically studied for absorber localization in three-dimensions. Two different source-detector configurations, transmittance and reflectance were studied to explore the best TRS spectra acquisition procedure for tumor localization. Our previous study results have shown that in reflectance measurements the photon penetration depth is dependent on both the source-detector (S-D) separation distance and the modulation frequency. More specifically, the photon penetration depth decreases at smaller source and detector separations and higher frequencies. In this study, the effects of these two parameters on the photon path were studied in both transmittance and reflectance. This study results may be used effectively in determining the position.

UV curing of optical fibre coatings using excimer lamps

In this paper, UV curing of optical fibre coatings using excimer lamp radiation has been demonstrated for the first time. These layers were prepared on silicon and quartz substrates by conventional spin coating and subsequently exposed to 172, 222 nm or 308 nm light from different excimer lamp sources. Ellipsometry, Fourier transform infrared (FTIR) spectroscopy and ultraviolet spectrophotometry have been used to characterise any modifications to the coatings and to generally monitor the evolution of the curing process. The absorption coefficient and photon penetration depth, as well as the intensity profiles in the coating, of each of the different wavelengths have been measured and calculated. The FTIR spectra revealed that the coatings were cured in less than 1 s under the 308 nm excimer lamp while the 222 nm excimer lamp was found to be extremely inefficient for the curing of the optical fibre coatings. After curing with the 172 nm excimer lamp, the surface of the coatings became cracked, irregular and milky in appearance. Such features are not observed when irradiating with 222 and 308 nm. The different UV curing effects of these three wavelengths will be discussed.

Snow grain size retrieved from near‐infrared radiances at multiple wavelengths

A comprehensive forward radiative transfer model is used to construct a snow grain size retrieval algorithm that relies on the use of NIR radiances. Data collected by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) at the wavelengths 0.86, 1.05, 1.24 and 1.73 µm are used to retrieve snow grain size. Based on a single‐layer (homogeneous) snow model, the retrieved snow grain size appears to depend on wavelength. The photon penetration depth defined as the e‐folding flux attenuation depth has be computed for different snow grain sizes and different wavelengths. It reveals that this apparent wavelength dependence occurs because (i) the snow grain size generally increases with depth, and (ii) the photon penetration depth decreases with increasing wavelength. The results show that the wavelength dependence of the photon penetration depth can be used to retrieve the depth dependence of the snow grain size.

Development of stereotactically guided laser interstitial thermotherapy of breast cancer: in situ measurement and analysis of the temperature field in ex vivo and in vivo adipose tissue.

The size (0.5-1.0 cm) of early nonpalpable breast tumors currently detected by mammography and confirmed by stereotactic core biopsy is of the order of the penetration depth of near infrared photons in breast tissue. In principle, stereotactically biopsied tumors, therefore, could be safely and efficiently treated with laser thermotherapy. The aim of the current study is to confirm the controlled heating produced by clinically relevant power levels delivered with an interstitial laser fiber optic probe adapted for use with stereotactic mammography and biopsy procedures. Temperature increases and the resultant thermal field produced by the irradiation of ex vivo (porcine and human) and in vivo (porcine) tissue models appropriate to the treatment of human breast tissue by using cw Nd:YAG laser radiation delivered with a interstitial fiber optic probe with a quartz diffusing tip, were recorded with an array of fifteen 23-gauge needle thermocouple probes connected to a laboratory computer-based data acquisition system. By using a stepwise decreasing power cycle to avoid tissue charring, acceptably symmetric thermal fields of repeatable volumetric dimensions were obtained. Reproducible thermal gradients and predictable tissue necrosis without carbonization could be induced in a 3-cm-diameter region around the fiber probe during a single treatment lasting only 3 minutes. The time-dependences of the temperature rise of the thermocouples surrounding the LITT probe were quantitatively modeled with simple linear functions during the applied laser heating cycles. Analysis of our experimental results show that reproducible, symmetric and predictable volumetric temperature increases in time can be reliably produced by interstitial laser thermotherapy.

Development of stereotactically guided laser interstitial thermotherapy of breast cancer: In situ measurement and analysis of the temperature field in ex vivo and in vivo adipose tissue

Background and Objective The size (0.5–1.0 cm) of early nonpalpable breast tumors currently detected by mammography and confirmed by stereotactic core biopsy is of the order of the penetration depth of near infrared photons in breast tissue. In principle, stereotactically biopsied tumors, therefore, could be safely and efficiently treated with laser thermotherapy. The aim of the current study is to confirm the controlled heating produced by clinically relevant power levels delivered with an interstitial laser fiber optic probe adapted for use with stereotactic mammography and biopsy procedures. Study Design/Materials and Methods Temperature increases and the resultant thermal field produced by the irradiation of ex vivo (porcine and human) and in vivo (porcine) tissue models appropriate to the treatment of human breast tissue by using cw Nd:YAG laser radiation delivered with a interstitial fiber optic probe with a quartz diffusing tip, were recorded with an array of fifteen 23-gauge needle thermocouple probes connected to a laboratory computer-based data acquisition system. Results By using a stepwise decreasing power cycle to avoid tissue charring, acceptably symmetric thermal fields of repeatable volumetric dimensions were obtained. Reproducible thermal gradients and predictable tissue necrosis without carbonization could be induced in a 3-cm-diameter region around the fiber probe during a single treatment lasting only 3 minutes. The time-dependences of the temperature rise of the thermocouples surrounding the LITT probe were quantitatively modeled with simple linear functions during the applied laser heating cycles. Conclusion Analysis of our experimental results show that reproducible, symmetric and predictable volumetric temperature increases in time can be reliably produced by interstitial laser thermotherapy. Lasers Surg. Med. 26:67–75, 2000. © 2000 Wiley-Liss, Inc.

A numerical study of the statistics of penetration depth of photons re-emitted from irradiated media

Abstract An important question in the utilization of optical methods to estimate optical parameters non-invasively in human tissue is that of describing properties of the trajectories of laser-injected photons. It is relatively easy to calculate an expression for the average depth experienced by these photons starting from either a random-walk or a diffusion model. It is much harder to characterize more detailed descriptive parameters. Here we make use of a novel variation of the method of exact enumeration to provide essentially numerically exact results for the random-walk model of a continuous-wave experiment.

A numerical study of the statistics of penetration depth of photons re-emitted from irradiated media

An important question in the utilization of optical methods to estimate optical parameters non-invasively in human tissue is that of describing properties of the trajectories of laser-injected photons. It is relatively easy to calculate an expression for the average depth experienced by these photons starting from either a random-walk or a diffusion model. It is much harder to characterize more detailed descriptive parameters. Here we make use of a novel variation of the method of exact enumeration to provide essentially numerically exact results for the random-walk model of a continuous-wave experiment.