arXiv subjects

C. D. Cruz

Publications and source records attributed to C. D. Cruz.

3 recordsLinked to original sources

arXiv2026

Precise measurement of the Kerr coefficient using phase-sensitive pump-probe hyperspectral imaging

Phase-sensitive pump-probe hyperspectral imaging is a precise technique for absolute two-beam measurements of the optical Kerr coefficient ($n_2$). The irradiance profile is characterized and background effects are rejected by rastering the pump beam across the probe beam to yield a complex-valued hyperspectral image of the pump-induced nonlinear response. Information about the temporal irradiance profile is carried in the spectral response. The technique is demonstrated by measuring $n_2$ of a fused silica sample near 1~$\mu$m wavelength and benchmarked against a measurement using Z-scan [Sheik-Bahae \emph{et al.}, IEEE J. Quantum Electron. \textbf{26}, 760--769 (1990)], the most widely used single-beam technique. The two measurements are consistent when the two-beam grating effect from the Raman contribution to the nonlinearity is considered. Uncertainty contributions are described in detail and the outlook is discussed for improvements in precision.

physics.optics↗

arXiv2024

Phase-sensitive pump-probe measurement of the complex nonlinear susceptibility of silicon across the direct band edge

The nonlinear response of materials, an increasingly important aspect of light-matter interaction, can be challenging to measure in highly absorbing materials. Here, we introduce an interferometric technique that enables a direct measurement of the nonlinear complex permittivity in a bulk medium from reflectivity alone. We demonstrate the utility of pump-probe supercontinuum (SC) spectral interferometry in reflection by measuring time-dependent variations in the complex dielectric function ($n$, $k$) over the visible wavelength range in bulk silicon. Transient phase shifts in the reflected SC due to a near infrared pump pulse allow us to track modifications to $k$; whereas changes in $n$ are derived from transient fluctuations in the reflected SC probe amplitude. The ultrafast response is attributed to effective two-photon absorption ($\beta$) and Kerr ($n_2$) coefficients. We observe the onset of strong two-photon absorption as the two-photon energy is tuned through the direct band edge of silicon ($E_1$ = 3.4 eV) for the first time to our knowledge. This technique allows straightforward spectroscopic measurements of the $\chi^{(3)}$ nonlinear response at the surface of absorbing materials.

physics.optics↗

arXiv2015

Shannon Mutual Information Applied to Genetic Systems

Shannon information has, in the past, been applied to quantify the genetic diversity of many natural populations. Here, we apply the Shannon concept to consecutive generations of alleles as they evolve over time. We suppose a genetic system analogous to the discrete noisy channel of Shannon, where the signal emitted by the input (mother population) is a number of alleles that will form the next generation (offspring). The alleles received at a given generation are conditional upon the previous generation. Knowledge of this conditional probability law allows us to track the evolution of the allele entropies and mutual information values from one generation to the next. We apply these laws to numerical computer simulations and to real data (Stryphnodendron adstringens). We find that, due to the genetic sampling process, in the absence of new mutations the mutual information increases between generations toward a maximum value. Lastly, after sufficient generations the system has a level of mutual information equal to the entropy (diversity) that it had at the beginning of the process (mother population). This implies no increase in genetic diversity in the absence of new mutations. Now, obviously, mutations are essential to the evolution of species. In addition, we observe that when a population shows at least a low level of genetic diversity, the highest values of mutual information between the generations occurs when the system is neither too orderly nor too disorderly. We also find that mutual information is a valid measure of allele fixation.

q-bio.PE↗