| Fig. 1. SEM images of n-aSiC samples after anodization in electrolytes with different HF concentrations
S
Fig. 2. PL spectra of n- aSiC samples after anodization in different HF-concentration electrolytes excited by 325 nm light
EM images of the anodized n-aSiC samples are shown in fig. 1. For comparison, in this figure we have put also the image of a n-aSiC sample which has been etched in 1% HF etchant in 15 minutes (1f image). First, from this figure, we can say that the sample which has been etched in the 1% HF etchant had a porous layer with the separate holes deep-rooted in the n-aSiC film (presented by the darker points in the 1f image). However, samples, which have been etched in dilute HF etchants with HF concentration varied from 0.05 to 0.4%, have a porous layer with totally different morphology. In our opinion, this layer is not etched down but grown up on the surface. It can be seen clearly from Fig. 1 that in the 0.05 to 0.4% HF concentration region, when the HF concentration in the electrolyte increased, the grown-up layer in the form of the white clusters broadened wider and wider. And when the HF concentration in the electrolyte has reached 0.4%, these white clusters will cover almost entirely the sample surface. Beginning from 0.5% HF electrolyte, the pore morphology has turned to the form of the separate holes which have been etched down from the surface.
The results of PL measurements on the porous n-aSiC layers using the 325 nm light excitation from a He-Cd laser are shown in Figure 2. From this figure it can be seen that the PL intensity of the etched sample depends strongly on the HF concentration of the etching solution. In particular, the more dilute HF etchant will give stronger PL intensity.
In Figure 2 we can see also that the PL intensity of the sample which has been etched in 1% HF etchant is hundreds times weaker than that of samples have been etched in diluted HF etchant. In fact, when carrying out PL measurements, by the naked eyes we could see the blue light emitting from some samples which have been etched in diluted HF solutions. Generally, samples etched in the dilute HF etchants exhibit PL peak located in the region from 500 nm to 540 nm. We would like to note also that with the time, the PL from the porous n-aSiC samples did not degraded but increased. In our opinion, this fact shows that, in contrary with the PL from PSi, the oxidation of the PSiC samples in air will increase the PL intensities.
CONCLUSIONS
Anodic etching of n-aSiC thin films in the aqueous HF electrolytes with HF concentrations less than 1% have shown that: i) the surface morphology of the porous layer obtained is completely different from those of the samples etched in electrolytes with the HF concentration greater than 1%; ii) the PL intensity of the porous layer obtained is hundreds times higher than that of the samples etched in electrolytes with the HF concentration greater than 1%.
ACKNOWLEDGMENTS
The authors would like to thank MSc. Do Hung Manh and MSc. Nguyen Thi Thanh Ngan for the SEM and photoluminescence measurements. This work was supported by the National Foundation for Science and Technology Development.
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Determination of some optical parameters characterizing homogenised fresh milk
with double integrating sphere technique
Nguyen Tuan Anh1*, Dang Xuan Cua1, Phung Quoc Bao2,
Bach Gia Duong3, Vu Anh Phi2
1 National Center for Technological Progress (NACENTECH), Vietnam
2 Faculty of Physics, Ha Noi University of Science, Vietnam
3 Hanoi University of Technology, Vietnam
Abstract
Some optical parameters of homogenised fresh milk, such as absorption coefficient µa, scattering coefficient µs, anisotropy g, at 820nm have been determined by using double integrating sphere technique. The milk concentration range under consideration is from 0 to 5% vol. The obtained results show that for concentrations higher than 5%, µa, µs, and g reach their stable values, that is 4.5 0.2mm-1, 55 2mm-1, and 0.975 ± 0.005, respectively. Moreover, there exits a milk concentration range, from 0.8 to 2% vol., where both µa and µs vary linearly, while g tends to its maximum value.
Key words: optical parameters homogenised fresh milk, double integrating sphere technique
INTRODUCTION
The basic quantity of interest in the state of the art of light propagation in photomedicine and photobiology is the radiant energy fluorescence rate which in turn can be considered as function of three optical parameters, namely, absorption coefficient µa, scattering coefficient µs, anisotropy g [1] and can be described by the Radiative Transfer Equation:
 (1)
where I(r,s) is the incoming light intensity per solid angle unit at position r via s direction; d’ is a solid angle element and p(s,s’) is scattering phase function from direction s’ to direction s.
The most widely used scattering phase function is Henyey-Greenstein function depending on anisotropy g and angle between s and s’ in the form:
 (2)
In this paper, the parameters µa, µs and g of homogenised fresh milk - a readily available and inexpensive phantom medium [2] - have been determined by using double integrating sphere technique. When illuminating the homogenised fresh milk by a light beam, the beam is scattered by protein particle with 0.15µm in diameter and fat globule with diameter in the range [0.5µm, 2µm] (Fig. 1).
Figure 1. Homogenised fresh milk used as phantom medium
The milk concentration range under consideration is from 0 to 5% vol. For concentrations higher than 5%, µa, µs, and g reach their stable values, that is 4.5 0.2mm-1, 55 2mm-1, and 0.975 ± 0.005, respectively. Moreover, there exits a milk concentration range, from 0.8 to 2% vol., where both µa and µs vary linearly, while g tends to its maximum value.
EXPERIMENTAL SETUP
From equations (1) and (2), one can see that optical characteristics of turbid media can be determined only by three optical parameters: absorption and scattering coefficients µa , µs and scattering phase function p(s,s’) or anisotropy factor g. According to the Kubelka-Munk model, these parameters can be evaluated by the following formulae [3]:
 (3)
where Rcd, Tcd, are backward and forward scattering components, respectively and Tc is collimated component. On its part, these components can be measured by a double integrating sphere based equipment.
During the measurement, a series of samples with increasing concentrations have been prepared by mixing fresh milk with distilled water. The used homogenised fresh milk has fat concentration of 4%. A cuvette containing the as-prepared solutions is placed between two identical integrating spheres of 5cm in diameter (Fig. 2).
Figure 2. Determination of optical parameters with double integrating technique
The laser working at 820nm, has an output power of 10mW. The photodiodes has responsitivity of 0.65A/W at wavelength of 820nm. The collimated laser beam oriented through the sample is measured by detector PD1. All light backward-scattered from the sample is collected by sphere 1 and detected by detector PD2. The forward-scattered light is collected by sphere 2 and detected by detector PD3.
RESULTS AND DISCUSSION
The integrated backward scattering component Rcd measured by detector PD2, the integrated forward scattering component Tcd measured by detector PD3, and the collimated component Tc measured by detector PD1 for different concentrations are displayed in Fig.3 and Fig.4, respectively.
Figure 3. Concentration-dependence of Tcd and Rcd
Increasing with the solution concentration, Rcd reaches its maximum value of 0.65 at concentration of about 1.2% vol. and decreases gradually to a stable value of 0.25 for high concentrations. Meanwhile, the integrated forward scattering component Tcd increases to a saturated value of 0.25 at concentrations higher than 5%.
One can see that Rcd, Tcd tend to the same limit at the concentrations higher than 5%.
Figure 4. Concentration-dependence of Tc
Tc follows the Beer law and exponentially dependences on the solution concentration. It decreases quickly in the high concentration range and practically tends to zero.
The absorption coefficient µa, scattering coefficient µs, and anisotropy g, at 820nm, have been calculated from corresponding as-measured data with the formulae (3) and graphically represented in Fig. 5 and Fig. 6, respectively.
Figure 5. Concentration-dependence of µa and µs
By increasing solution concentrations, both absorption and scattering coefficients increase gradually and then reach their own saturated values for concentrations higher than 5% vol.. The saturated values of µa, µs are estimated 4.5 0.2mm-1 and 55 2mm-1, respectively. It is interesting that there exits a concentration range, from 0.8 to 2.0% vol., where µa, µs vary linearly.
Figure 6. Concentration-dependence of g
As for the anisotropy factor g, it decreases from 0.99 at low concentrations to a stable value of 0.975 ± 0.005 for concentrations higher than 5% vol.
IV. Conclusion
The optical parameters of homogenized fresh milk in the concentration range from 0 to 5% vol. have been investigated in detail. The obtained results at high concentrations are in good agreement with those in related papers [4].
The dependence of µa, µs and g on the concentration of a turbid medium leads to the idea that using these parameters as quality factors, especially for high scattering media. In recent years, this new approach has been widely mentioned, particularly in medical applications (e.g. methods of optical tomography and optical biopsy).
V. ACKNOWLEDGEMENTS
The authors are grateful to the Vietnamese Ministry of Science and Technology (MOST) for financial supports.
References
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