Lifu Zhang

Folate Receptor-Targeted Aggregation-Enhanced Near-IR Emitting Silica Nanoprobe for One-Photon in Vivo and Two-Photon ex Vivo Fluorescence Bioimaging

A two-photon absorbing (2PA) and aggregation-enhanced near-infrared (NIR) emitting pyran derivative, encapsulated in and stabilized by silica nanoparticles (SiNPs), is reported as a nanoprobe for two-photon fluorescence microscopy (2PFM) bioimaging that overcomes the fluorescence quenching associated with high chromophore loading. The new SiNP probe exhibited aggregate-enhanced emission producing nearly twice as strong a signal as the unaggregated dye, a 3-fold increase in two-photon absorption relative to the DFP in solution, and approximately 4-fold increase in photostability. The surface of the nanoparticles was functionalized with a folic acid (FA) derivative for folate-mediated delivery of the nanoprobe for 2PFM bioimaging. Surface modification of SiNPs with the FA derivative was supported by zeta potential variation and (1)H NMR spectral characterization of the SiNPs as a function of surface modification. In vitro studies using HeLa cells expressing a folate receptor (FR) indicated specific cellular uptake of the functionalized nanoparticles. The nanoprobe was demonstrated for FR-targeted one-photon in vivo imaging of HeLa tumor xenograft in mice upon intravenous injection of the probe. The FR-targeting nanoprobe not only exhibited highly selective tumor targeting but also readily extravasated from tumor vessels, penetrated into the tumor parenchyma, and was internalized by the tumor cells. Two-photon fluorescence microscopy bioimaging provided three-dimensional (3D) cellular-level resolution imaging up to 350 μm deep in the HeLa tumor.

Effect of initial frequency chirp on Airy pulse propagation in an optical fiber

We study both analytically and numerically the propagation dynamics of an initially chirped Airy pulse in an optical fiber. It is found that the linear propagation of an initially chirped Airy pulse depends considerably on whether the second-order dispersion parameter β(2) and chirp C have the same or opposite signs. For β(2)C<0, the chirped Airy pulse first undergoes an initial compression phase, then reaches a breakup area as depending on the values of C, and then experiences a lossy inversion transformation such that it continues to propagate with an opposite acceleration. The chirped Airy pulse is always dispersed during propagation in the case of β(2)C>0. The impact of truncation coefficient and Kerr nonlinearity on the chirped Airy pulse propagation is also disclosed separately.

Observation of central wavelength dynamics in erbium-doped fiber ring laser.

We report on the observation of central wavelength dynamics in an erbium-doped fiber ring laser by using the nonlinear polarization rotating technique. The evolution of central wavelength with the laser operation state was observed experimentally. Numerical simulations confirmed the experimental observation and further demonstrated that the dynamics of wavelength evolution is due to the combined effects of fiber birefringence, fiber nonlinearity, and cavity filter.

Manipulation of Raman-induced frequency shift by use of asymmetric self-accelerating Airy pulse.

We investigate the evolution of asymmetric self-accelerating finite energy Airy pulses (FEAP) in optical fibers with emphasis on the role of Raman scattering. We show that the Raman-induced frequency shift (RIFS) of soliton initiated by an asymmetric self-accelerating FEAP depends not only on the launched peak power but also on the truncation coefficient imposed on the asymmetric self-accelerating FEAP. We find that the RIFS of asymmetric self-accelerating FEAP increases with a decrease in the truncation coefficient, while the peak power and spectrum width of the outermost red shift of the shedding soliton spectrum are almost unchanged. The time and frequency shifts of the shedding soliton are found to be sensitive to the truncation coefficient when the truncation coefficient is in the range of 0 to 0.1. These excellent features would lead to the realization of a RIFS-based tunable light source by launching self-accelerating FEAP with different truncation coefficient into an optical fiber.

Multiple hot images from an obscuration in an intense laser beam through cascaded Kerr medium disks

We present a theoretical investigation on the formation of hot images in an intense laser beam through cascaded Kerr medium disks, to disclose the distribution and intensity of hot images in high-power disk amplifiers. It is shown that multiple hot images from an obscuration may be formed, instead of one hot image as reported previously in the literature. This gives a clear explanation for the curious damage pattern of hot images, namely, damage sites appearing on alternating optics in periodic trains. Further analysis demonstrates that the distribution and intensity of hot images depend closely on the number of Kerr medium disks, the distance from the obscuration to the front of the first disk downstream, the space between two neighboring disks, and the thickness and B integral of each disk. Moreover, we take two cascaded Kerr medium disks for example to detail multiple hot images from an obscuration and confirm the theoretical results by numerical simulations.

Dynamic propagation of finite-energy Airy pulses in the presence of higher-order effects

The dynamic propagation of the finite-energy Airy pulse (FEAP) is studied numerically in the presence of higher-order effects, including Raman scattering, self-steepening (SS), and third-order dispersion (TOD). It is shown that the Raman-induced frequency shift (RIFS) of a FEAP can be tailored by varying the truncation coefficient of the FEAP. In particular, the combined effects of Raman scattering and SS (or TOD) on the nonlinear propagation of a FEAP are identified. It is found that both TOD and SS effects tend to slow down the RIFS during the nonlinear propagation of a FEAP. In addition, the simultaneous contributions of Raman, TOD, and SS effects to the nonlinear propagation of FEAP are further discussed. Compared to the conventional symmetric pulses, such as Sech pulses, the FEAP can generate a broadening spectrum that is extended toward the blue side, in addition to the conventional red-shifted components. These results demonstrate that the FEAP can be used for supercontinuum generation and broadband sources.

Modulation instability of finite energy Airy pulse in optical fiber

We have investigated and analyzed the modulation instability (MI) of finite energy Airy pulse (FEAP) in an optical fiber in order to reveal the impact of truncation coefficient on the nonlinear propagation dynamics of FEAP with or without amplitude perturbation. We have also characterized the difference between the propagation process of smooth FEAP and that of modulated FEAP. It is shown that, for a smooth FEAP, the side lobes prior to the main lobe first undergo compression and then break up into multiple sub-pulses during propagation in the case of small truncation coefficient; while the opposite occurs in the case of large truncation coefficient. For a FEAP with amplitude modulation, the breakup of the main lobe induced by MI precedes that of side lobes for arbitrary values of truncation coefficients; but the evolution of secondary lobes is made by a transition from splitting process to a simple compression process with increasing truncation coefficient. The propagation dynamics of secondary lobes with number symbol larger 2, marked the secondary lobes starting number 1 from near to far according the distance between itself and the main lobe, is insensitive to the truncation coefficients variation in both cases. Finally, the MI gain spectra of FEAP with different truncation coefficients are obtained by numerically solving the nonlinear Schrödinger equation and the results have been compared with the theoretical predictions.

Propagation dynamics of super-Gaussian beams in fractional Schrödinger equation: from linear to nonlinear regimes.

We have investigated the propagation dynamics of super-Gaussian optical beams in fractional Schrödinger equation. We have identified the difference between the propagation dynamics of super-Gaussian beams and that of Gaussian beams. We show that, the linear propagation dynamics of the super-Gaussian beams with order m > 1 undergo an initial compression phase before they split into two sub-beams. The sub-beams with saddle shape separate each other and their interval increases linearly with propagation distance. In the nonlinear regime, the super-Gaussian beams evolve to become a single soliton, breathing soliton or soliton pair depending on the order of super-Gaussian beams, nonlinearity, as well as the Lévy index. In two dimensions, the linear evolution of super-Gaussian beams is similar to that for one dimension case, but the initial compression of the input super-Gaussian beams and the diffraction of the splitting beams are much stronger than that for one dimension case. While the nonlinear propagation of the super-Gaussian beams becomes much more unstable compared with that for the case of one dimension. Our results show the nonlinear effects can be tuned by varying the Lévy index in the fractional Schrödinger equation for a fixed input power.

Obscuration size dependence of hot image in laser beam through a Kerr medium slab with gain and loss

We present a study of the formation of a hot image in an intense laser beam through a slab of Kerr medium with gain and loss, beyond the thin-medium approximation, to especially disclose the dependence of the hot image on the size of obscuration. Based on the angular spectrum description of light propagation and the mean-field approximation we obtain the expression for intensity of the hot image, which clearly shows the dependence of intensity of the hot image on the size of obscuration. It is shown that, as the size of obscuration increases, the intensity of the corresponding hot image first increases gradually, after reaching a maximum value, it decreases rapidly to a minimum value, meaning that there exists an optimum size of obscuration, which leads to the most intense hot image. Further analysis demonstrates that the optimum size of obscuration is approximately determined by the effective fastest growing spatial frequency for a given case. For the output light beam of a given intensity, with the gain coefficient of the Kerr medium slab increasing, or the loss coefficient decreasing, the optimum size of obscuration becomes bigger, while the hot image from the obscuration of a given size becomes weaker, suggesting that high gain and low loss can efficiently suppress the hot image from obscurations. The theoretical predictions are confirmed by numerical simulations.

Modulation instability in the oppositely directed coupler with a quadratic nonlinearity

We investigate modulation instability (MI) in a nonlinear oppositely directed coupler with a quadratic nonlinearity, where one channel is made from a positive-index material (PIM) and another channel is fabricated from a negative-index material (NIM), trying to identify the different MI properties from those in a conventional parametric gap system with grating. Both the analytic continuous wave (CW) solutions and dispersion relation are obtained. By using standard linear instability we in detail discuss how the ratio of the backward to forward propagating wave’s power, the phase mismatch, and the coupled coefficient influence the dynamical behavior of MI. Large stable regions are found if the fundamental harmonics (FH) falls in the normal dispersion when compared to the case in the conventional Bragg grating with a quadratic nonlinearity where the CW solutions are unstable in most cases. In addition, we also observe the large stable regimes of the CW solutions even when the coupled strength for the second harmonics (SHs) is weaker than that of the FH. These findings suggest that the oppositely directed coupler with a NIM channel provides more ways to manipulate the MI and soliton.