Optical trapping microrheology in cultured human cells

We present the microrheological study of the two close human epithelial cell lines: non-cancerous HCV29 and cancerous T24. The optical tweezers tracking was applied to extract the several seconds long trajectories of endogenous lipid granules at time step of 1μs. They were analyzed using a recently proposed equation for mean square displacement (MSD) in the case of subdiffusion influenced by an optical trap. This equation leads to an explicit form for viscoelastic moduli. The moduli of the two cell lines were found to be the same within the experimental accuracy for frequencies 102 - 105 Hz. For both cell lines subdiffusion was observed with the exponent close to 3/4, the value predicted by the theory of semiflexible polymers. For times longer than 0.1s the MSD of cancerous cells exceeds the MSD of non-cancerous cells for all values of the trapping force. Such behavior can be interpreted as a signature of the active processes and prevents the extraction of the low-frequency viscoelastic moduli for the living cells by passive microrheolog

Model-Based Estimation of Three-Dimensional Stiffness Parameters in Photonic-Force Microscopy

We propose a system to characterize the 3-D diffusion properties of the probing bead trapped by a photonic-force microscope. We follow a model-based approach, where the model of the dynamics of the bead is given by the Langevin equation. Our procedure combines software and analog hardware to measure the corresponding stiffness matrix. We are able to estimate all its elements in real time, including off-diagonal terms. To achieve our goal, we have built a simple analog computer that performs a continuous preprocessing of the data, which can be subsequently digitized at a much lower rate than is otherwise required. We also provide an effective numerical algorithm for compensating the correlation bias introduced by a quadrant photodiode detector in the microscope. We validate our approach using simulated data and show that our bias-compensation scheme effectively improves the accuracy of the system. Moreover, we perform experiments with the real system and demonstrate real-time capabilities. Finally, we suggest a simple adjunction that would allow one to determine the mass matrix as well

Coherent interaction of laser pulses in a resonant optically dense extended medium under the regime of strong field-matter coupling

Nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. A special attention is paid to the case, where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser-fields can be coherently absorbed and reemitted by the medium. Thus, the field of medium reaction plays a key role in the interaction processes, which leads to the collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in the fast excitation interchanges between the field and a medium in the form of the optical ringing, which is analogous to polariton beating in the solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave-packets of different group velocities, is shown to significantly affect propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to the resonance in the broadband probe-spectrum, exceed the absorption-line width, since, under the strong-coupling regime, the frequency of the optical ringing exceeds the rate of incoherent relaxation. Contrary to the stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of the collective oscillations and cannot be obtained in the framework of the single-atom model.Comment: 10 pages, 8 figures, to be published in Phys. Rev.

High Resolution Absorption Spectroscopy of the 3ν1 and 3ν1+ν3 Bands of Propyne

info:eu-repo/semantics/publishe

Optical trapping microrheology in cultured human cells

We present the microrheological study of the two close human epithelial cell lines: non-cancerous HCV29 and cancerous T24. The optical tweezers tracking was applied to extract the several seconds long trajectories of endogenous lipid granules at time step of 1μs. They were analyzed using a recently proposed equation for mean square displacement (MSD) in the case of subdiffusion influenced by an optical trap. This equation leads to an explicit form for viscoelastic moduli. The moduli of the two cell lines were found to be the same within the experimental accuracy for frequencies 102 – 105 Hz. For both cell lines subdiffusion was observed with the exponent close to 3/4, the value predicted by the theory of semiflexible polymers. For times longer than 0.1s the MSD of cancerous cells exceeds the MSD of non-cancerous cells for all values of the trapping force. Such behavior can be interpreted as a signature of the active processes and prevents the extraction of the low-frequency viscoelastic moduli for the living cells by passive microrheology

The overtone spectrum of carbonyl sulfide in the region of the nu(1)+4 nu(3) and 5 nu(3) bands by ICLAS-VECSEL

The high-resolution overtone spectrum of OCS has been recorded in the region of the v(1) + 4v(3) and 5v(3) bands by intracavity laser absorption spectroscopy based on an optically pumped vertical external cavity surface emitting laser (VECSEL). The extremely weak v(1) - 4v(3) band at 8954.87cm(-1) was found to be isolated. The 5v(3) band at 10080.91 cm(-1) is accompanied by two weaker bands at 9933.53 and 10114.02 cm(-1) assigned to the 12(0)4-00(0)0 and 04'4-00'0 bands, respectively. In addition, the 01(1)5-01(1)0 hot band was detected together with the extremely weak band heads of the R branch of the 02(0,2)5-02(0,2)0 hot bands. Finally, the 5v(3) band of the (OCS)-O-16-C-12-S-34 minor isotopomer, present in natural abundance in the sample, was also observed and rotationally analyzed. Effective state parameters could be retrieved by standard band-by-band rotational fitting of the line positions, leading to a typical rms of 0.006cm(-1). The observed line positions were compared to the predictions of the global model described by Rhaibi et al. [J. Mol. Spectrosc. 191 (1998) 32-44]. In general, the agreement is excellent, close to the experimental uncertainty (0.01 cm(-1)) thus confirming the high predictive ability of this effective Hamiltonian model. Weak but significant deviations up to 0.1cm(-1) were, however, identified for two rotational levels of the highly excited 2,16(0),0 dark state, observed through a local interaction with the 0005 state. In the case of the (OCS)-O-16-C-12-S-34 isotopomer, the predicted line wavenumbers of the 5v(3) band were globally overestimated by about 0.04cm(-1). The new data have been included in the corresponding global model, leading to almost unchanged values of the molecular parameters and a statistical agreement with the experiment. (C) 2003 Elsevier Science (USA). All rights reserved