Table of contents

We summarize the development of high harmonic generation (HHG) with linearly polarized Laguerre–Gaussian (LG) beams and their superpositions to explain the non-perturbative aspects of HHG. Furthermore, we show that circularly polarized extreme ultraviolet vortices with well-defined orbital angular momentum (OAM) can be generated by HHG with bicircular LG beams. We introduce photon diagrams in order to explain how to calculate the OAM and the polarization of the generated harmonics by means of simultaneous conservation of spin angular momentum and OAM. Moreover, we show how the intensity ratio of the driving fields in HHG with bicircular LG beams further enhances the generation of circularly polarized twisted attosecond pulse trains.

It is shown that the helicity amplitudes can be used to describe and analyze the properties of the electromagnetic field in classical and in quantum theory. On the one hand they embody the relativistic content of electromagnetic theory. On the other hand they give a concise description of such experimentally important notions as polarization, the Stokes parameters and the Poincaré sphere.

We consider the energy and helicity densities of circularly polarised light within a lossless chiral medium, characterised by the chirality parameter β. A form for the helicity density is introduced, valid to first order in β, that produces a helicity of ±ℏ per photon for right and left circular polarisation, respectively. This is in contrast to the result obtained if we use the form of the helicity density employed for linear media. We examine the helicity continuity equation, and show that this modified form of the helicity density is required for consistency with the dual symmetry condition of a chiral medium with a constant value of /μ. Extending the results to arbitrary order in β establishes an exact relationship between the energy and helicity densities in a chiral medium.

In this paper, a novel hybrid graphene-dielectric slot waveguide (HGDSW) is theoretically proposed. The characteristics of the fundamental hybrid modes (TM polarization) including effective refractive index (n_eff), normalized mode area (A_eff/A₀), propagation length (L_p) and figure of merit are numerically investigated. The results reveal the hybrid mode guided in the proposed HGDSW can be tuned by the key geometric structure parameters and the chemical potential of graphene. Besides, the HGDSW can support low loss and effective deep-subwavelength light transmission in THz wavelength, in conjunction with enhanced overall high performance. Moreover, ultra-low crosstalk between two parallel HGDSWs can be achieved. The HGDSWs reveal the potential capabilities in the ultra-compact photonic integration field.

A phase-compensation method is presented to fully optimize multilayer reflectance bandwidth, spectral phase and group delay dispersion. For a given multilayer (A), a set of different phase-compensated dielectric mirrors (B) is achieved as a function of a single parameter: the so-called reference wavelength λ_r. With a correct selection of the parameter λ_r, a set of different dielectric multilayers can be obtained with fixed broadband reflectance regions and smooth spectral phases (independently of the layers' thickness distribution) so that ultrashort pulses can be entirely reflected in such dielectric mirrors with a negligible amount of absorption and distortion. Hence, with an adequate numerical analysis via our phase-compensation method, experimental designs can be easily performed to obtain ad hoc dielectric mirrors for ultrashort pulse management.

A quantitative spectrometer-based photothermal optical coherence tomography (PT-OCT) system is employed to investigate and image the photothermal trapping of gold nano-rods (GNRs) in clear and biological media. The PT-OCT system is calibrated through dynamic phase measurements of piezo motion with known driving parameters. We measure and compare the displacement sensitivities of the PT-OCT system at different camera exposure time settings in two configurations: with a distinct reference path; and with a common path. The displacement sensitivity of the system in the common path configuration is improved from 1.5 to 0.17 nm by performing Fourier analysis on the output phase. The minimum Ti:Sa power capable of inducing a detectable photothermal response of GNRs is measured to be 0.5 mW. This value agrees with the latest reported minimum Ti:Sa power for photothermal trapping GNRs. The PT-OCT system is used to generate en-face images of photothermal trapped GNRs in the water solution and in the biological tissue. By displaying the difference between successive en-face phase images, spatial distribution patterns of the aggregated GNRs, resulted from the photothermal trapping, are clearly outlined with great contrast. The photothermal trapping of GNRs in tissue shows a greater complexity than in the clear media. The limitation of the PT-OCT technology is discussed. The study proves the potential of PT-OCT for imaging the photothermal trapping of GNRs.

We propose a simple method of linearizing spectral interference fringes of spectral domain optical coherence tomography based on B-scan Doppler frequency shift (DFS), which can be obtained by offsetting the laser beam from the pivot of a scanning mirror. We show that DFS is proportional to wavenumber. A DFS based calibration curve can be then extracted from either a single mirror image or multiple tissue images. By examining the convergence of the nonlinear coefficients of the DFS curve fitted with a polynomial equation, tissue images themselves can be used to linearize the wavenumber without requiring mirror images or additional hardware. The method is verified with both ex vivo and in vivo tissue images. Using B-scan Doppler shift, we also demonstrate significantly improved imaging quality by combining complex conjugate ambiguity removal, speckle reduction, and spectral calibration with tissue images.

We numerically demonstrated the generation of terahertz-mid-infrared (THz-MIR) anisotropic vortex beams from few-cycle vortex-laser-induced air plasma based on both the intra-pulse four-wave mixing and the photocurrent models. It is found that for the low-frequency THz components, their phase variance on the azimuthal angle follows a simple stepwise increase with a step of π, indicating the unsuccessful inheritance of the orbital angular momentum (OAM) from the few-cycle vortex driving laser. However, for the high-frequency MIR components, they are anisotropic vortex beams with a nonlinear variance in the phase profile showing angular acceleration, which indicates they carry OAM. Also, their intensity distributions vary from two-petal structure slowly to ring-shaped structure as the frequency increases. Physically, the behaviors of their phase and intensity distributions can be explained well by the interference theory of the two models. Interestingly, according to the fitting formula of the phase distributions, the calculated topological charges (TCs) of all components in the THz-MIR frequencies range are equal to that of the driving laser. Moreover, the phase nonlinearity and the intensity distributions strongly depend on the corresponding THz-MIR frequencies. The results from both schemes are the decoupling of the OAM, the TC, the geometry and the power distribution in anisotropic vortex beams.

We experimentally demonstrate a linear-cavity all-fiber passively Q-switching cylindrical vector beam (CVB) laser operating in high-order mode. This CVB fiber laser operates without any mode converter which always leads to high insertion loss, and it can realize high efficiency. In this fiber laser, the stable Q-switching pulse is achieved with a slope efficiency of 39%. By properly adjusting the polarization controllers, radially polarized and azimuthally polarized beams can be obtained. Our work proves the feasibility of achieving the stable Q-switching CVB pulse with high-order mode directly oscillating, and it may have an enormous potential for enhancing the efficiency.

Existing technologies and methods for measuring transmitted wavefronts typically operate at only a few specific wavelengths. In this paper, we propose a new method for estimating the wavefront distortion of an optical transmission system in a broad bandwidth. We establish the relationship between the transmitted wavefront and wavelength, using Zernike fringe coefficients to represent the wavefront. From simulations of several different types of optical systems, we found that two formulas can be used to express Zernike-wavelength curves: the Conrady dispersion formula and a new formula that we have named the apochromatic characteristic formula. To reduce the influence of measurement errors on predictions, fitting method is biased in favor of simulation rather than experimental data. We illustrate the validity of this technique by reconstructing a wavefront transmitted at 671 nm using Zernike polynomials, demonstrating that the predicted wavefront is very similar to the wavefront measured experimentally. Using the Conrady formula, we illustrate that the wavefront of a monochromatic system can be predicted for any wavelength in a broad range, using data from three standard wavelengths. As Seidel coefficients correspond to Zernike coefficients, the relationship between optical aberration and wavelength detailed here can also be applied to areas, such as optical design, optical computing and adaptive optics.

A possibility for remote measurement of laser beam and atmospheric turbulence characteristics using the target-in-the-loop atmospheric sensing (TILAS) technique is experimentally demonstrated over a 7 km atmospheric propagation path. The results show that the TILAS approach can be applied for remote sensing of the target-plane intensity scintillations and path-integrated refractive index structure parameter ${{\boldsymbol{C}}}_{{\boldsymbol{n}}}^{2}$ at diverse atmospheric turbulence conditions.

This communication reports an investigation undertaken towards setting collimation of an optical beam using a self-imaging technique and histogram error (HE) based approach. The beam under test illuminates an amplitude type Ronchi grating. After the grating, a beam splitter is placed such that the grating's self-images are formed in two perpendicular directions, at the different Talbot planes. The images are then recorded using two identical CCD cameras. Towards implementing a HE based algorithm, first, element-by-element subtraction of the normalized histogram of both self-images is computed. Next, the sum of the elements of the resultant image matrix is determined. Finally, the square of the sum yields the HE. HE provides an estimate of the collimation errors in the beam. For an incident collimated beam, the self-images recorded at different Talbot planes have identical unit magnification with respect to the grating; however, when the beam diverges or converges, the size and fringe width of self-images are differentially magnified or demagnified. Hence, when the beam is collimated, the HE is minimum. For the decollimated beam, the value of HE is higher, and increases as the decollimation errors increase. Using the proposed method, we could set the collimation position to a resolution of 1 μm, which relates to ±0.22 μ radians in terms of collimation angle (for a lens of focal length 300 mm and diameter 40 mm). Experimental results conclusively establish the viability of the technique. Good accuracy and precision in the measurement have been achieved.

A bend-resistant side-leakage photonic crystal fiber with large-mode-area (LMA) is proposed. Simulation results demonstrate that the robust single-mode operation is obtained thanks to an extremely high loss ratio (the minimum loss of high order modes to the loss of fundamental mode (FM)) at the wavelength of 2 μm while the bend radius is as small as 10 cm. Meanwhile, the FM suffers a low bend loss of 0.006 dB m⁻¹ with an effective mode area exceeding 2000 μm². The bend-resistant performance can be maintained over a broad range of wavelengths from 1.75 to 2.5 μm. Except for a wide waveband, a large tolerance for bend radius is also obtained. The fiber is able to operate under single-mode operation within a range of bend radius from 9 to 24 cm. LMA and single-mode operation under a small bend radius make this fiber have great potential in developing compact high power fiber lasers.

High accurate phase analysis of waveforms as fringe patterns is essential for a wide range of scientific and engineering disciplines. However, precise phase analysis under extremely low signal-to-noise conditions is a challenging task for conventional phase-shifting methods. Here, a novel accurate phase recovering technique, called the spatiotemporal phase-shifting method (ST-PSM), is developed to measure the phase information robustly by utilizing two-dimensional intensity data in spatial- and temporal-domains simultaneously. Our simulation results indicated that ST-PSM had strong tolerance to random noise, and a self-neutralizing function to eliminate the periodical phase error due to the nonlinearity of detector, intensity saturation, vibration or phase-shifting error. The effectiveness was demonstrated experimentally from a non-contact shape measurement in fringe projection profilometry under extreme underexposure and overexposure recording conditions. Furthermore, by incorporating modern GPU parallel computing technology, a 4-step phase-shifted fringe pattern with 8 K image size can be calculated within one second. Due to its robustness and high accuracy with a fast calculation, therefore, we believe this technique has a significant impact on a variety of research and scientific fields.