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Photoreflectance spectroscopy of the mesoporous silicon structures.

Lev P. Avakyants, Lomonosov Moscow State University, Russia
Sergey E. Svyakhovskiy, Lomonosov Moscow State University, Russia
Artem E. Aslanyan, Lomonosov Moscow State University, Russia
Anatoliy V. Chervyakov, Lomonosov Moscow State University, Russia


The photoreflectance (PR) spectroscopy is an efficient, non-destructive technique to investigate semiconductor materials [1]. The modulation technique allows to measure variations of the reflectance of the sample as small as 10^{-7} [2] which makes photoreflectance the far-reaching technique to measure optical nonlinearities.

In this work, we suggest the photoreflectance spectroscopy as a method to study the optical nonlinearity of one-dimensional photonic crystal. The measurement of reflectance and photoreflectance spectra was performed for porous silicon photonic structure and supported by the calculation of optical response by the transfer-matrix method. We measured the dependence of the photoreflectance signal on the modulating radiation power and obtained the nonlinear refraction coefficient of porous silicon. It is shown that optical nonlinearity is caused by thermal effects. The thermal coefficient of the refractive index was measured.

The mesoporous photonic structure was produced by electrochemical etching of silicon [3]. The theoretical calculation of spectra of reflectance, photoreflectance, and density of optical states were performed for all samples by the transfer matrix method. The refractive indices of porous silicon layers were calculated by the Bruggeman effective media approximation.

The photoreflectance spectroscopy was performed by measurement of modulated reflectance signal using the lock-in technique. The probe beam was incoherent (70 W tungsten lamp) measured by a double grating monochromator and Si photodiode in the spectral range of 550–1000 nm. The modulation beam (pump) was a DPSS laser with a wavelength of 532 nm and a power 1-100 mW, chopped at 270 Hz with duty cycle 1/2.

Photoreflectance spectra contain sharp features at energies near the critical points of the band structure of the sample. The maxima of the PR spectrum coincide with the maxima of the density of the spectrum of the photonic state. Increasing of DoS means the decrease of group velocity causing the light localization and enhancement of the interaction of light and material and leads to the enhancement of photoreflectance.

The photoreflectance spectra were fitted with the theoretical model. The variation of the refractive index was retrieved from the model and the lowest pump power (1mW) was as small as 10^-6. These results enable us to measure small variations of the refractive index of porous silicon structures which can be used in biosensing applications.

[1] J. Misiewicz, P. Sitarek, G. Sek, and R. Kudrawiec, Photoreflectance and contactless electroreflectance measurements of semiconductor structures by using bright and dark configurations, Materials Science 21, 263, 2003.
[2] A. Rosencwaig, J. Opsal, W. L. Smith, and D. L. Willenborg, Detection of thermal waves through optical reflectance, Applied Physics Letters 46, 1013, 1985.
[3] S.E. Svyakhovskiy, A.I. Maydykovsky, and T.V. Murzina. Mesoporous silicon photonic structures with thousands of periods. Journal of Applied Physics, 112(1):013106, 2012.


Sergey Svyakhovskiy
Lomonosov Moscow State University


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