[en] We revisit the problem of preparing a mechanical oscillator in the vicinity of its quantum-mechanical ground state by means of feedback cooling based on continuous optical detection of the oscillator position. In the parameter regime relevant to ground-state cooling, the optical back-action and imprecision noise set the bottleneck of achievable cooling and must be carefully balanced. This can be achieved by adapting the phase of the local oscillator in the homodyne detection realizing a so-called variational measurement. The trade-off between accurate position measurement and minimal disturbance can be understood in terms of Heisenberg’s microscope and becomes particularly relevant when the measurement and feedback processes happen to be fast within the quantum coherence time of the system to be cooled. This corresponds to the regime of large quantum cooperativity $C_{\mathrm{q}}\gtrsim <!--\; ???\; -->1$, which was achieved in recent experiments on feedback cooling. Our method provides a simple path to further pushing the limits of current state-of-the-art experiments in quantum optomechanics. (paper)

[en] We demonstrate that surface plasmon resonances excited by photon tunneling through an adjacent dielectric medium enhance the photocurrent detected by a graphene photodetector. The device is created by overlaying a graphene sheet over an etched gap in a gold film deposited on glass. The detected photocurrents are compared for five different excitation wavelengths, ranging from ${\mathrm{\lambda <!--\; ??\; -->}}_0=570\mathrm{n}\mathrm{m}$ to ${\mathrm{\lambda <!--\; ??\; -->}}_0=730\mathrm{n}\mathrm{m}.$ Although the device is not optimized, the photocurrent excited with incident p-polarized light (which excites resonant surface plasmons) is significantly amplified in comparison with that for s-polarized light (without surface plasmon resonances). We observe that the photocurrent is greater for shorter wavelengths (for both s- and $p$-polarizations) with increased photothermal current. Position-dependent Raman spectroscopic analysis of the optically-excited graphene photodetector indicates the presence of charge carriers in the graphene near the metallic edge. In addition, we show that the polarity of the photocurrent reverses across the gap as the incident light spot moves across the gap. Graphene-based photodetectors offer a simple architecture which can be fabricated on dielectric waveguides to exploit the plasmonic photocurrent enhancement of the evanescent field. Applications for these devices include photodetection, optical sensing and direct plasmonic detection. (paper)

[en] We propose a scheme of loss-resilient entanglement swapping between two distant parties in lossy optical fibre. In this scheme, Alice and Bob each begin with a pair of entangled non-classical states; these ‘hybrid states’ of light are entangled discrete variable (Fock state) and continuous variable (CVs) (coherent state) pairs. The CV halves of each of these pairs are sent through lossy optical fibre to a middle location, where these states are then mixed (using a 50:50 beam-splitter) and measured. The detection scheme we use is to measure one of these modes via vacuum detection, and to measure the other mode using balanced homodyne detection. In this work we show that the $|{\mathrm{\Phi <!--\; ??\; -->}}^{+}⟩<!--\; ???\; -->=(|00⟩<!--\; ???\; -->+|11⟩<!--\; ???\; -->)/\sqrt{2}$ Bell state can theoretically be produced following this scheme with high fidelity and entanglement, even when allowing for a small amount of loss. It can be shown that there is an optimal amplitude value (α) of the coherent state, when allowing for such loss. We also investigate the realistic circumstance when the loss is not balanced in the propagating modes. We demonstrate that a small amount of loss mismatch does not destroy the overall entanglement, thus demonstrating the physical practicality of this protocol. (paper)

[en] In this work, we comprehensively investigate a Dammann grating (DG) that can generate a 5 × 5 diffraction spot array with an extending angle of ${18}^{\circ <!--\; ???\; -->}\times <!--\; ??\; -->{18}^{\circ <!--\; ???\; -->}$ around the fiber communication wavelength of $1550\mathrm{n}\mathrm{m}$. The DG is a simple metasurface structure composed of a silicon cuboid nanorod array on a silica substrate, and only two different sizes of nanorods with square cross-sections and uniform spatial orientations are used. These simple units and this configuration are favorable in practice, and the C4 symmetry cross section of the nanorods ensures the polarization-independent operation of the DG. The phase modulation of the nanorods is achieved by the guiding mode propagating in them rather than electric or magnetic Mie-type resonance, which makes the design of the cuboid nanorods easy and robust. More importantly, the two-dimensional nanorod array is generated from a one-dimensional array, which further decreases the design and fabrication complexity. (paper)

[en] The Stratton–Chu integral representation of electromagnetic fields is used to study the spatio-temporal properties of large bandwidth laser pulses focused by high numerical aperture mirrors. We review the formal aspects of the derivation of diffraction integrals from the Stratton–Chu representation and discuss the use of the Hadamard finite part in the derivation of the physical optics approximation. By analyzing the formulation we show that, for the specific case of a parabolic mirror, the integrands involved in the description of the reflected field near the focal spot do not possess the strong oscillations characteristic of diffraction integrals. Consequently, the integrals can be evaluated with simple and efficient quadrature methods rather than with specialized, more costly approaches. We report on the development of an efficiently parallelized algorithm that evaluates the Stratton–Chu diffraction integrals for incident fields of arbitrary temporal and spatial dependence. This method has the advantage that its input is the unfocused field coming from the laser chain, which is experimentally known with high accuracy. We use our method to show that the reflection of a linearly polarized Gaussian beam of femtosecond duration off a high numerical aperture parabolic mirror induces ellipticity in the dominant field components and generates strong longitudinal components. We also estimate that future high-power laser facilities may reach intensities of ${10}^{24}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->2}$. (paper)

[en] Asymmetric transmission of circularly polarized waves is a well-established property of lossy, anisotropic, two-dimensionally chiral patterns. Here we show that asymmetric transmission can be observed for oblique incidence onto any lossy periodically structured plane. Our results greatly expand the range of natural and artificial materials in which directionally asymmetric transmission can be expected, making it a cornerstone electromagnetic effect rather than a curiosity of planar chiral metamaterials. Prime candidates for asymmetric transmission at oblique incidence are rectangular arrays of plasmonic spheres or semiconductor quantum dots, lossy double-periodic gratings and planar metamaterial structures

[en] Fluorescence imaging is an attractive imaging modality for understanding biological mechanisms. Among such techniques, continuous wave fluorescence-enhanced diffuse optical tomography (CW-fDOT) is interesting since it is adapted to CCD cameras which allow non-contact measurements on a large number of detection elements for low cost. Here, by extending the derivations previously applied to continuous wave diffuse optical tomography (CW-DOT), we demonstrate that multiple optical configurations may lead to the same sets of optical measurements. We first explain in a ideal case and then in a 'real-world' example when such situations happen. Therefore, one cannot absolutely quantify the fluorescence amount from CW-fDOT measurements without adding priors

[en] Coherent beam combining of high-order-mode fiber lasers is an attractive choice for high-power lasers as high-order-mode fiber lasers have a higher power handling capability than fundamental mode lasers. As active phasing provides a robust way for in-phase mode coherent beam combining, coherent beam combining of high-order-mode fibers with active phase control is proposed and a proof-of-concept experiment is performed. In the experiment, LP₁₁ mode lasers are generated by off-center launching of single-mode fiber laser beams into few-mode fibers and active phasing is realized using a stochastic parallel gradient descent algorithm. The results show that the long-exposure fringe contrast is increased from zero to 40% when the system evolves from open-loop to closed-loop. A scalable architecture for coherent beam combining of high-order-mode amplifiers with active phase control in a master oscillator power amplifier configuration (MOPA) is also proposed

[en] The electric field of an ultrashort laser pulse often fails to separate into a product of purely temporal and purely spatial factors. These so-called spatio-temporal couplings constitute a broad range of physical effects, which often become important in applications. In this review, we compile some recent experimental and theoretical work on the understanding, avoidance and applications of these effects. We first present a discussion of the characteristics of pulses containing spatio-temporal couplings, including their sources, a mathematical description and the interdependence of different couplings. We then review different experimental methods for their characterization. Finally, we describe different applications of spatio-temporal couplings and suggest further schemes for their exploitation and avoidance. (review article)

[en] From the mathematical viewpoint, a congruence of rays may be modelled by some mapping. In this frame, the caustic corresponds to the points where the rank of the mapping decreases by at least one unit. The umbilical singularities are the special points where the rank decreases by two units and they constitute, by definition, the Thom singular set Σ². We propose to exploit this mathematical property to calculate directly the umbilical singularities, without any calculation of the caustic surface. The method is applied in two explicit examples. In the first example we consider a wavefront and we show that the equations of the set Σ² lead to the classical result on the equality of the principal curvatures on the wavefront. In the second example we use the Σ² method for the localization of umbilics produced by a paraboloidal reflector. For comparison, we present also the same calculation by starting from the equation of the caustic