Direction Of Light Propagation Research Articles

Plasmonic topological insulators for topological nanophotonics.

Photonic topological insulators are optical structures supporting robust propagation of light at their edges that are topologically protected from scattering. Here we propose the concept of plasmonic topological insulators (PTI) that not only topologically protect light at the lattice edges but also enable their confinement and guidance at the deep-subwavelength scale. The suggested PTI are composed of an evanescently coupled array of metallic nanowires that are modulated periodically along the light propagation direction. The intrinsic loss associated with the PTI is found not to deteriorate their topological protection on the edge modes. The proposed PTI may find interesting applications in nanophotonics, where the tolerance to the fabrication disorders for device applications are essential.

The 8×10 GHz Receiver Optical Subassembly Based on Silica Hybrid Integration Technology for Data Center Interconnection**Supported by the National High Technology Research and Development Program of China under Grant No 2015AA016902, the National Natural Science Foundation of China under Grant Nos 61435013 and 61405188, and the K. C. Wong Education Foundation.

An 8×10 GHz receiver optical sub-assembly (ROSA) consisting of an 8-channel arrayed waveguide grating (AWG) and an 8-channel PIN photodetector (PD) array is designed and fabricated based on silica hybrid integration technology. Multimode output waveguides in the silica AWG with 2% refractive index difference are used to obtain flat-top spectra. The output waveguide facet is polished to 45° bevel to change the light propagation direction into the mesa-type PIN PD, which simplifies the packaging process. The experimental results show that the single channel 1 dB bandwidth of AWG ranges from 2.12 nm to 3.06 nm, the ROSA responsivity ranges from 0.097 A/W to 0.158 A/W, and the 3 dB bandwidth is up to 11 GHz. It is promising to be applied in the eight-lane WDM transmission system in data center interconnection.

In situ measurement of light polarization with ellipticity-induced nonlinear magneto-optical rotation

A precise, accurate, and relatively straightforward in-situ method to measure and control the ellipticity of light resonantly interacting with an atomic vapor is described. The technique can be used to minimize vector light shifts. The method involves measurement of ellipticity-induced resonances in the magnetic-field dependence of nonlinear magneto-optical rotation of frequency-modulated light. The light propagation direction is orthogonal to the applied magnetic field $\textbf{B}$ and the major axis of the light polarization ellipse is along $\textbf{B}$. When the light modulation frequency matches the Larmor frequency, elliptically polarized light produces precessing atomic spin orientation transverse to $\textbf{B}$ via synchronous optical pumping. The precessing spin orientation causes optical rotation oscillating at the Larmor frequency by modulating the atomic vapor's circular birefringence. Based on this technique's precision, in-situ nature (which avoids systematic errors arising from optical interfaces), and independent control of the most important systematic errors, it is shown that the accuracy of light ellipticity measurements achievable with this technique can exceed that of existing methods by orders of magnitude.

LED packaging for high light quality: Status and perspectives

Light-emitting diode (LED) is regarded as the green lighting source in 21st century due to its advantages of high efficiency, energy saving, environmental protection and compact size, etc. With the popularization and application of LED in more and more lighting occasions and the improvement of people’s requirement for the quality of light source, more and more attention has been paid to the light quality of LED. The main indexes for evaluating the light quality of LED are the efficiency, angular color uniformity (ACU) and color rendering index (CRI), which are closely related to LED packaging. LED packaging is a process of assembling the chip and other packaging materials together to form the final lighting products, it plays a key role in mechanical protection, external signals and electrical connections, heat dissipation and light quality control. This paper makes a systematic exposition of the control technology of efficiency, ACU and CRI from the view of LED packaging, mainly involves three key packaging technologies: 1) thermal management. For LED products, about 60% of the input electrical power is converted to heat, therefore, the failure of the effectively heat dissipation would cause sharply increase of the temperature, which leads to the decrease of efficiency and induces reliability problems. 2) Optical design. The LED light is generate by the electron-hole recombination from the active layer of the chip, then it across the chip, phosphor layer, encapsulant layer, lens and other packaging materials. Due to scattering, refraction and absorption of the packaging materials, the light propagation direction and path would be changed, and part of the light is absorbed and converted into heat. Therefore, the optical design not only can control the LED light energy distribution, but also can reduce the light absorption in the packaging materials to improve the efficiency. 3) Phosphor coating. Compared to the multi-color LED chips packaging method, coating the monochromatic or polychromatic fluorescent powder on the LED chip to achieve white light is more popular and widely applied in the LED industrial due to its correlated color temperature (CCT) adjustability and process flexibility. For the phosphor-converted white LED, the phosphor coating plays a key role in deciding the ACU and CRI.

On the equivalence between Young's double-slit and crystal double-refraction interference experiments.

We show, both analytically and experimentally, that under common experimental conditions the interference pattern produced in a classic Young's double-slit experiment is indistinguishable from that generated by means of a doubly refracting uniaxial crystal whose optic axis makes a skew angle with the light propagation direction. The equivalence between diffraction and crystal optics interference experiments, taken for granted by Arago and Fresnel in their pioneering research on the interference of polarized light beams, is thus rigorously proven.

Rainbow trapping in a chirped three-dimensional photonic crystal

Light localization and intensity enhancement in a woodpile layer-by-layer photonic crystal, whose interlayer distance along the light propagation direction is gradually varied, has been theoretically predicted and experimentally demonstrated. The phenomenon is shown to be related to the progressive slowing down and stopping of the incident wave, as a result of the gradual variation of the local dispersion. The light localization is chromatically resolved, since every frequency component is stopped and reflected back at different positions along the crystal. It has been further discussed that the peculiar relation between the stopping position and the wave vector distribution can substantially increase the enhancement factor to more than two orders of magnitude. Compared to previously reported one- and two-dimensional photonic crystal configurations, the proposed scheme has the advantage of reducing the propagation losses by providing a three-dimensional photonic bandgap confinement in all directions. The slowing down and localization of waves inside photonic media can be exploited in optics and generally in wave dynamics, in many applications that require enhanced interaction of light and matter.

Generation of light in spatially dispersive materials

Spatial dispersion makes optical properties of materials depend on the direction of light propagation. The effect can be applied to control optical emission by sources embedded in such media. We propose a method to determine the radiation pattern of essentially any emitter located in a general spatially dispersive and optically anisotropic medium. The method is based on a decomposition of the source into waves of electric current, each creating optical plane waves whose properties are determined by the wave parameters, the refractive index, and impedance. The method is computationally fast and very accurate even in strongly spatially dispersive plasmonic metamaterials. In particular, we observe large modification of dipole emission in a bifacial and a diffraction-compensating metamaterial. The method is applicable to a large variety of nanostructured materials and, therefore, we believe that it can find numerous applications in nano-optics.

Decentered Gaussian Beam Pumped Highly Efficient Passively Q-Switched Microchip Laser for Controllable High-Order Transverse Modes

Versatile controllable high-order transverse laser modes have been generated in a decentered Gaussian beam (DGB) pumped Cr4+:YAG passively Q-switched (PQS) Nd:YAG microchip laser. The DGB has been formed by offsetting collimating lens away from the propagation direction of the pump light. Effects of the collimating lens offset distance on the beam profile and tilting angle of the DGB have been investigated experimentally and theoretically. A highly efficient and high repetition rate Laguerre–Gaussian (LG) mode PQS microchip laser is achieved when the ordinary Gaussian beam is used as the pump source. The Ince–Gaussian (IG) mode and Hermite–Gaussian (HG) mode lasers are obtained when the DGB is applied as the pump source. The IG mode laser is changed to an HG mode laser by increasing the offset distance of the collimating lens. Optical efficiencies above 41% respective to the absorbed pump power have been obtained in the DGB pumped LG and IG mode PQS Nd:YAG microchip laser. The nanosecond pulse width and peak power of over 4 kW has been achieved in DGB pumped PQS Nd:YAG microchip lasers for various high-order transverse modes. Our works on the DGB pumped PQS microchip laser for high-order transverse modes provide an effective and simple method for designing highly efficient, controllable LG, IG, and HG mode solid-state lasers.

Stimulated Raman Scattering Interferometer for Molecular-Selective Tomographic Imaging.

Development of a stimulated Raman scattering (SRS) interferometer is presented. In the apparatus, a SRS signal generated in a sample is collinearly overlapped with a reference beam, and the interference pattern between the signal and reference beams is obtained. From the interference pattern, the phase of the SRS signal can be measured. From the phase of the signal, the spatial distribution of the target molecule can be obtained with high positioning precision. In the present article, the SRS interferometer was developed and the first SRS interference signal was obtained from a polyethylene film. The phase change of the SRS interference pattern caused by scanning the sample position along the direction of the incident light propagation was measured. From the result, the sample position difference for a sub-micron was detected with uncertainties of 15%. The tomographic images of a triple-layered polymer film were successfully obtained. The SRS interferometer would become a core optical technique of a novel molecular-selective tomographic imaging method.

Proposed manipulation of spin currents in GaAs crystal using the Pockels effect

A spin current could induce linear electro-optical electric susceptibility, thus the incident light could interact with the spin current via Pockels effect. In this letter, we find by appropriately selecting the directions of the applied electric field, the propagation direction of the incident light and its polarization, there will exist spin angular momentum transfer between the incident light and the spin polarized electrons resulting in the change of the spin current. Basing on this, a method for manipulating spin current is put forward. In our method, we could adjust the magnitude of the applied electric field or the intensity of incident light to manipulate the spin currents. The numerical results show that, for the condition of the applied electric field being 1×103 Vm-1 and the intensity of incident light being 1×105 Wm-2, the ratio of the changed pure spin current versus initial pure spin current will drop to 0.1%, and the spin polarization of the generated spin-polarized current will reach to 99.9%.

High-resolution study of the yellow excitons inCu2Osubject to an electric field

We have used high resolution transmission spectroscopy to study the exciton level spectrum in Cu$_2$O subject to a longitudinal external electric field, i.e., in the geometry where the transmitted light is propagating along the field direction. Different experimental configurations given by the field orientation relative to the crystal and the light polarization have been explored. We focus on the range of small principal quantum numbers $n \leq 7$. The number of exciton states belonging to a particular principal quantum number increases with $n$, leading to an enhanced complexity of the spectra. Still, in particular for $n = 3 \ldots 5$ a spectral separation of the different lines is feasible and identification as well as assignment of the dominant state character are possible. We find a strong dependence of the spectra on the chosen light propagation direction and polarization configuration, reflecting the inadequacy of the hydrogen model for describing the excitons. With increasing the field excitonic states with different parity become mixed, leading to optical activation of states that are dark in zero field. As compared with atoms, due to the reduced Rydberg energy states with different $n$ can be brought into resonance in the accessible electric field strength range. When this occurs, we observe mostly crossing of levels within the experimental accuracy showing that the electron and hole motion remains regular. The observed features are well described by detailed calculations accounting for the spin-orbit coupling, the cubic anisotropy effects, and the symmetry-imposed optical selection rules.

Chiral quantum optics.

Advanced photonic nanostructures are currently revolutionizing the optics and photonics that underpin applications ranging from light technology to quantum-information processing. The strong light confinement in these structures can lock the local polarization of the light to its propagation direction, leading to propagation-direction-dependent emission, scattering and absorption of photons by quantum emitters. The possibility of such a propagation-direction-dependent, or chiral, light-matter interaction is not accounted for in standard quantum optics and its recent discovery brought about the research field of chiral quantum optics. The latter offers fundamentally new functionalities and applications: it enables the assembly of non-reciprocal single-photon devices that can be operated in a quantum superposition of two or more of their operational states and the realization of deterministic spin-photon interfaces. Moreover, engineered directional photonic reservoirs could lead to the development of complex quantum networks that, for example, could simulate novel classes of quantum many-body systems.

Experimental investigation on aero-optics of supersonic turbulent boundary layers at different light incident angles

The aero-optical distortion caused by the compressibility of high-speed flow field has a great influence on the development of airborne optical detection system of (hypersonic) supersonic vehicles. The turbulent boundary layer is one of the most important aspects in the aero-optical study, and has become one of the hot research points in the field of aero-optical study. The nano-particle-based planar laser scattering technique is used to measure the density distribution of the supersonic (Ma=3.0) turbulent boundary layers, and the optical path difference, which is quite crucial for the aero-optical study, is obtained by ray-tracing method. The experimental result is verified by being compared with the theoretical result computed by the aero-optical scaling method of turbulent boundary layers. Five different light incident angles (α=60°, 75°, 90°, 105°, 120°) are selected and used to examine the influences of light incident angles on the supersonic turbulent layer, and the underlying flow physics is analyzed. Research shows that the light propagation path in the supersonic turbulent boundary layer contributes to the light incident angle dependence of aero-optics. The different propagation paths lead to the difference between the light propagation distance in the flow field and the correlation results of the corresponding density fluctuation. The oblique incidence of light results in the increase of the propagation distance in the flow field, and then the aero-optics turns worse. The greater the angle between the incident direction of light and the vertical direction of the wall, the more significant the aero-optics is, the difference increases at different times, the difficulty in correcting the aero-optics is also increased. In the supersonic turbulent boundary layer, a large number of vortices with a specific orientation lead to the anisotropy of the aero-optics in the turbulent boundary layer. By calculating the spatial two-point correlation of the density fluctuations at the streamwise plane (x-y plane), the cross-correlation result of density fluctuations at any light incidence angle (α=0°-180°) can be obtained. The local coherent structure scale is nearly 0.20 mm, which is basically consistent with the aero-optical effective scale (≈ 0.18 mm) computed from the formula proposed by Mani et al. When the light is inclined downstream, the direction of light propagation is consistent with the vortex structure in the flow field, and in this direction, the correlation coefficient of density fluctuation is larger, so the aero-optics is more serious. When the light beam is tilted upstream, the correlation coefficient is smaller, so the aero-optics is weaker.

空间调制光传播方向及偏振态的多层图案化液晶聚合物薄膜研究

空间调制光传播方向及偏振态的多层图案化液晶聚合物薄膜研究

Orientational dependence of optically detected magnetic resonance signals in laser-driven atomic magnetometers

We have investigated the dependence of lock-in-demodulated M_x-magnetometer signals on the orientation of the static magnetic field mathbf {B}_{0} of interest. Magnetic resonance spectra for 2400 discrete orientations of mathbf {B}_{0} covering a 4pi solid angle have been recorded by a PC-controlled steering and data acquisition system. Off-line fits by previously derived lineshape functions allow us to extract the relevant resonance parameters (shape, amplitude, width, and phase) and to represent their dependence on the orientation of mathbf {B}_{0} with respect to the laser beam propagation direction. We have performed this study for two distinct M_x-magnetometer configurations, in which the rf-field is either parallel or perpendicular to the light propagation direction. The results confirm well the algebraic theoretical model functions. We suggest that small discrepancies are related to hitherto uninvestigated atomic alignment contributions.

Coherent control of double deflected anomalous modes in ultrathin trapezoid-shaped slit metasurface.

Coherent light-matter interaction in ultrathin metamaterials has been demonstrated to dynamically modulate intensity, polarization and propagation direction of light. The gradient metasurface with a transverse phase variation usually exhibits an anomalous refracted beam of light dictated by so-called generalized Snell’s law. However, less attention has been paid to coherent control of the metasurface with multiple anomalous refracted beams. Here we propose an ultrathin gradient metasurface with single trapezoid-shaped slot antenna as its building block that allows one normal and two deflected transmitted beams. It is numerically demonstrated that such metasurface with multiple scattering modes can be coherently controlled to modulate output intensities by changing the relative phase difference between two counterpropagating coherent beams. Each mode can be coherently switched on/off and two deflected anomalous beams can be synchronously dictated by the phase difference. The coherent control effect in the trapezoid-shaped slit metasurface will offer a promising opportunity for multichannel signals modulation, multichannel sensing and wave front shaping.

Back-focal plane Mueller matrix microscopy: Mueller conoscopy and Mueller diffractrometry

The common mode of observation in an imaging Mueller matrix polarimeter is orthoscopic observation, where the front focal plane of the objective is focused onto the camera sensor. Different points on the camera sensor correspond to different points on the sample. But for certain applications, valuable complementary information is obtained when the back focal plane of the objective is focused onto the camera sensor. This observation mode informs about the directions distribution of light so that different points on the camera sensor correspond to different directions of light propagation through the specimen. We distinguish between two types of illumination; the sample can be illuminated with a large numerical aperture, such that what is measured corresponds to usual transmission or specular reflection ellipsometry (Mueller conoscopy), or the sample may be illuminated with collimated light, and what is measured corresponds to scattering or diffraction of light (Mueller diffractometry). A polarimeter suitable for back-focal measurements is described and illustrative examples of Mueller conoscopy and Mueller diffractometry are presented.

Experimental realization of Talbot array illumination for a 2-dimensional phase grating

We present the result of a multiple phase level Talbot array illuminator (TAI) based on two dimensional phase grating. The grating was fabricated on glass by a Deep Reactive Ion Etching (DRIE) system with optimized gas combination recipe. The opening ratio of the grating is ∼0.71 with a phase step of 11π. The experimental study on Talbot array illuminations (TAI's) of this fabricated device in near-field region was carried out using a He-Ne laser. The TAI's were observed at the positions 2.81 mm, 8.59 mm, 14.28 mm, and 20.57 mm along the light propagation direction. The computer simulations of wave field distribution in near field region at different fractional Talbot distances have been generated. These simulated results are in reasonably good agreement with our experimental observed results. Our analysis on Talbot array illumination (TAI's) shows that the multiple harmonic phase modulations are transformed into intensity modulations at 1/8, 3/8, 5/8, and 7/8 fractional Talbot position. We have observed three different grating images, viz., attenuated, phase contrast, and diffraction contrast images. We also show the presence of a considerable amount of phase change at the edge of each phase sublevel that enhances the contrast of differential phase contrast imaging. Therefore, the phase contrast signal can be detected at a fractional position using single phase grating without the need of phase stepping.

Optical activity of quantum wells

We report on the observation of optical activity of quantum wells resulting in the conversion of the light polarization state controlled by the light propagation direction. The polarization conversion is detected in reflection measurements. We show that a pure $s$-polarized light incident on a quantum well is reflected as an elliptically polarized wave. The signal is drastically enhanced in the vicinity of the light-hole exciton resonance. We show that the polarization conversion is caused by the spin-orbit splitting of the light hole states and the birefringence of the studied structure. The bulk inversion asymmetry constant $\beta_{h} \approx 0.14$ eV\AA, is determined for the ground light hole subband in a 10 nm ZnSe/ZnMgSSe quantum well.

Two-dimensional photonic crystals based on anodic porous TiO2 with ideally ordered hole arrangement

Ideally ordered TiO2 hole arrays with high aspect ratios were prepared by the anodization of pretextured Ti. The obtained TiO2 acted as two-dimensional photonic crystals in which a photonic band gap is formed in all directions of light propagation in the lattice. The process allows the easy and low-cost fabrication of TiO2 photonic crystals and can be used for the preparation of functional optical devices, which require the precise control of light propagation.