Towards nonreciprocal pulse dynamics in a time-varying medium

Due to the limitations of measurement equipment and the influence of factors such as the environment and target, measurement errors may occur during the data acquisition process of airborne LiDAR bathymetry (ALB). The refractive index of water is defined as the propagation ratio of the speed of light waves in a vacuum to that in water; this ratio influences not only the propagation speed of the laser pulse in water but also the propagation direction of the laser pulse entering water. Therefore, the influence of refractive index changes in water on the ALB errors needs to be analyzed. To this end, the principle of ALB is first briefly introduced. Then, the calculation method for the refractive index of water is described with Snell’s law and an empirical formula. Finally, the influence of refractive index changes on ALB errors is analyzed using the derived formula at the water–air interface and in the water column. The experimental results showed that in a constant elevation of 50 m for a bathymetric floor, the refractive index changes in water caused by temperature, salinity, and depth are less than 0.001. The maximum bathymetric error and maximum planimetric error caused by the refractive index changes at the water–air interface are 0.036 m and 0.015 m, respectively. The ALB errors caused by refractive index changes in the water column are relatively low, and the water column does not need to be layered to calculate the ALB errors. The influence of refractive index changes in water on the ALB error is minimal, accounting for only a small proportion of all bathymetric errors. Thus, it is necessary to determine whether the effect of the ALB error due to refractive index changes in water needs to be corrected based on the accuracy requirements of the data acquisition. This study and analysis can provide a reference basis for correcting ALB errors.

Understanding and controlling laser-induced refractive index modifications in bulk chalcogenide glasses is important for a range of photonics applications targeting the mid-infrared spectral region. We focus here on material engineering aspects and pulse spatio-temporal design characteristics that are able to induce and maintain positive refractive index changes in laser-irradiated Sulfur-based chalcogenide glass, mandatory for 3D photonic design. Specifically we study the photoinscription process of a Ge-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated by focused ultrafast near-infrared laser pulses where Ge doping plays a determinant role in generating high-contrast positive index changes. By means of aposteriori and real-time in situ observations we show that positive refractive index changes (type I) are the result of a restructuring of the glass matrix and a photo-induced contraction process initiated by two-photon electronic excitation leading to bond softening, molecular mobility, structural changes and rearrangements. Oppositely, negative refractive index changes (type II) could be associated with two different processes: photo-expansion at higher intensities and hydrodynamic evolution initiated by plasma generation and laser heating, with thermomechanical relaxation and stress unload. Alongside the role of Ge in setting various degrees of the matrix connectivity, the structural arrangement developed under different thermal history schemes for glass preparation is equally important as it defines to which extent further structural flexibility is possible. Thus we indicate the role of glass matrix metastability in generated high-contrast refractive index changes and we show that a higher degree of relaxation is an impediment for contrasted positive index changes, while these are developing in unrelaxed glasses, where several degrees of structural flexibility exist. Alongside dynamic time-resolved imaging experiments probing the development and relaxation of excitation, we also show, via static Raman analysis of the modified regions, that significant structural changes are induced by laser irradiation and we discuss the potential processes involved.

This study explores how hydrostatic pressure, temperature, incident light intensity, and structural parameters modulate optical absorption coefficients and refractive index changes in GaAs/Al0.3Ga0.7As spherical quantum dots under the screened cosine Kratzer potential. We use the Nikiforov-Uvarov method to solve the Schrödinger equation to determine the relevant wave functions and energy levels. And using iterative method and density matrix theory, we derived analytical expressions for the optical absorption coefficients and refractive index changes. The results of the numerical analyses demonstrate that both the structural parameters of the quantum dots and the external environmental factors significantly influence the optical absorption coefficients and refractive index changes, with varying degrees of impact. It is particularly noteworthy that these factors are extremely important for modulating the peak amplitude and resonance peak position of the optical absorption coefficients and refractive index changes, providing important theoretical basis for us to deeply understand and modulate the nonlinear optical properties of quantum dots.

Intersubband optical absorption coefficients and refractive index changes in modulation-doped asymmetric double quantum well

An optical waveguide sensor was fabricated by forming a multilayer film made by porous anodic alumina (PAA) and Al layers on a glass substrate. The fabricated sensor system was based on the monitoring of a waveguide coupling mode, which is sensitive to the change in the refractive index of the PAA layer caused by collection of target molecules into the pores of the PAA layer. The PAA/Al multilayer film was formed by partial anodization of an Al film deposited on the glass substrate, and the waveguide coupling mode was examined by measuring angular spectra (reflectivity dependence on the incident angle of monitoring light; green He-Ne laser, 534.5 nm). A deep and sharp waveguide coupling dip was obtained for the PAA/Al multilayer system where the thicknesses of the PAA and Al layers were 200 and 17 nm, respectively. The optical sensor response of the PAA/Al multilayer system was compared to the responses of a surface plasmon resonance (SPR) sensor made by a Au thin film on a SF10 glass substrate. It was inferred that the optical waveguide sensor made by the PAA/Al multilayer could detect a smaller change in the refractive index of a solution, and it provided higher resolution than the SPR sensor. The sensor response for a change in the complex refractive index of the PAA layer was examined next, and it was found that the optical waveguide sensor was sensitive to the change in the imaginary part of the complex refractive index rather than the change in the real part. This result indicated that the sensitivity of the optical waveguide sensor could be improved by using the light absorption of a target compound.

Linear and nonlinear optical absorption coefficients and refractive index changes in asymmetrical Gaussian potential quantum wells with applied electric field

Cytoplasmic viscosity is a crucial parameter in determining rates of diffusion-limited reactions. Changes in viscosity are associated with several diseases, whilst nuclear viscosity determines gene integrity, regulation and expression. Yet how drugs including DNA-damaging agents affect viscosity is unknown. We demonstrate the use of a platinum complex, Pt[L]Cl, that localizes efficiently mostly in the nucleus as a probe for nuclear viscosity. The phosphorescence lifetime of Pt[L]Cl is sensitive to viscosity and provides an excellent tool to investigate the impact of DNA damage. We show using Fluorescence Lifetime Imaging (FLIM) that the lifetime of both green and red fluorescent proteins (FP) are also sensitive to changes in cellular viscosity and refractive index. However, Pt[L]Cl proved to be a more sensitive viscosity probe, by virtue of microsecond phosphorescence lifetime versus nanosecond fluorescence lifetime of FP, hence greater sensitivity to bimolecular reactions. DNA damage was inflicted by either a two-photon excitation, one-photon excitation microbeam and X-rays. DNA damage of live cells causes significant increase in the lifetime of either Pt[L]Cl (HeLa cells, 12.5–14.1 µs) or intracellularly expressed mCherry (HEK293 cells, 1.54–1.67 ns), but a decrease in fluorescence lifetime of GFP from 2.65 to 2.29 ns (in V15B cells). These values represent a viscosity change from 8.59 to 20.56 cP as well as significant changes in the refractive index (RI), according to independent calibration. Interestingly DNA damage localized to a submicron region following a laser microbeam induction showed a whole cell viscosity change, with those in the nucleus being greater than the cytoplasm. We also found evidence of a by-stander effect, whereby adjacent un-irradiated cells also showed nuclear viscosity change. Finally, an increase in viscosity following DNA damage was also observed in bacterial cells with an over-expressed mNeonGreen FP, evidenced by the change in its lifetime from 2.8 to 2.4 ns.

Self phase modulation is a nonlinear effect that is observed when a laser beam is focused on to a high-absorbing thermal medium. A regular tea sample in a plastic cuvette is used as the nonlinear absorbing sample. The change in the refractive index of the medium occurs due to the heat generated by the focused pump beam, which in turn changes the refractive index. In this paper, self phase modulation is investigated in different ways. An Ar-Ion laser of 514 nm is used as the pump beam and a 632 nm He-Ne laser is used as the probe beam. The probe beam is introduced from the opposite side of the pump beam. Ring patterns are observed from the each side of the sample. Regular far field ring patterns are observed from the pump beam, and two sets of rings are observed with the probe beam. The behaviors of these inner and outer rings are monitored for different pump powers. The steady state heat equation is solved to obtain an exact solution for the radial heat distribution and far field ring patterns are simulated using the Fresnel–Kirchhoff diffraction integral. Ring patterns are theoretically explained using simulations results, and compared with experimental observations. Finally, an interferometric setup using the low power He-Ne laser is also used to determine the induced change in refractive index. Results are compared with those obtained directly from self-phase modulation and from the probe beam method.

Using a new simulation method, the influence of refractive index changes during the bleaching of the photoresist on process parameters is investigated. For standard applications using thin resists, refractive index changes above 0.1 result in considerably modified dose latitudes, swing curves and iso- dense bias. In special applications with thick resists, the same effect occurs for much smaller refractive index changes. Optimized refractive index changes can be used for the fabrication of structures with high aspect ratios.

A method employing both multiple-beam and two-beam interferometry for the measurement of extremely small changes in thickness and refractive index of glass plates is described. The glass plate which is to be measured is scanned through a narrow laser beam in one arm of a Twyman-Green interferometer, the output of which constitutes one arm of a two-beam optical measuring set. By measuring the changes in intensity of the light emerging from the interferometer (once with the interferometer mirrors in use and once with the mirrors blocked), two sets of data are obtained which permit the separate determination of the variations of thickness and refractive index. Since the optical measuring set can easily detect changes in light intensity of one part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , changes in thickness of the order of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda/1000</tex> and changes in refractive index of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> can be measured. By scanning over the area of the sample, a contour diagram depicting both thickness variations and refractive index variations can be constructed.

Palm oil is a major contributor to the economy, especially in regions like Riau, Indonesia, where it accounts for a significant portion of the Gross Domestic Product (GDP) and national crude palm oil (CPO) production. However, the rising demand for palm oil has led to unstable prices and health concerns, particularly when the oil is reused multiple times. Repeated use of palm oil can increase the risk of health issues such as coronary heart disease and cancer due to the presence of saturated fats and carcinogenic compounds. To address these concerns, the study explores methods to assess palm oil quality, specifically using a fiber optic sensor-based evanescent absorption technique. This method detects changes in the oil by measuring the concentration of substances on the sensor surface without damaging the sample. It operates with high sensitivity, detecting even small changes in the refractive index. Machine learning, particularly Convolutional Neural Networks (CNNs), is used to automate analyzing images and detecting patterns related to palm oil quality. The study being summarized focuses on detecting the quality of palm oil from five different brands of cooking oil in Indonesia. The detection device uses the evanescent absorption method, measuring the voltage of the optical signal converted by a photodiode. The study found that the ratio of initial to measured voltage indicates changes in turbidity and refractive index, which increase as the oil is reused. Among the brands tested, Brand B showed the highest ratio after three uses and a high sensitivity, which is 0.456, indicating it becomes turbid quickly. Brand C has a response time of 67–115 seconds, suggesting it could be reused more times without significant degradation

The birefringence of a red blood cell (RBC) is quantitatively monitored as it becomes infected by a malarial parasite. Large changes occur in the cell's refractive index at different stages of malarial infection. The observed rotation of an optically trapped, malaria-infected RBC is not a simple function of shape distortion: the malarial parasite is found to itself exercise a profound influence on the rotational dynamics by inducing stage-specific birefringence. Our measurements shed new light on the competition between shape- and form-birefringence in RBCs. We demonstrate the possibility of using birefringence to establish very early stages of infected parasites and of assessing various factors that contribute to birefringence in normal and infected cells. Our results have implications for the development and use of noninvasive techniques of quantifying changes in cell properties induced by malaria disease pathology.

Here, we demonstrate the preparation of a hybrid nanoplasmonic probe array, a nanoforest, which consists of gold nanostructures coated on top of silica nanospheres and evaporated gold film structure onto bottom through masks. Optically tunable plasmonic surfaces could be achieved by varying the nanoforest fabrication conditions, which acted as a selective nanoantenna. Plasmon coupling effects were effectively controlled by adjusting the vertical distance (nanogap) between the gold nanostructures on the silica bead and the bottom. Such control is critical to highly sensitive molecular detection applications. As a proof of concept, nanoforests were used as colorimetric sensors of refractive index changes. The sensors were highly sensitive and displayed distinct colors for small refractive index (RI) changes. The flexible and sensitive near/far-field optical properties render these nanoforests excellent plasmonic substrates for quantitative, sensitive and label-free sensing with nanoscale spatial resolution.

Using the density matrix approach and iterative method, we have theoretically investigated the optical properties of a disk-like quantum dot (QD) with hard-wall confining potential subjected to an external static magnetic field in the presence of a Rashba spin–orbit interaction. The results of numerical calculations for the typical InAs QD show that the linear, nonlinear and total absorption coefficients (ACs) and refractive index (RI) changes are sensitive to the parameters of the magnetic field and dot size. Moreover, the resonance peaks of ACs and RI changes redshift when increasing the Rashba spin–orbit interaction strength. The role of this interaction as a control parameter on the ACs and RI changes has been demonstrated in detail.

The linear and nonlinear optical properties of an off-center hydrogenic donor impurity in nanowire superlattices: Comparison between arrays of spherical and cylindrical quantum dots