Output Light Field Research Articles

Electrically tunable optical spatial differentiation with graphene.

In recent years, optical analog computing has experienced rapid development, among which optical differential operation has attracted great attention. Here, based on the unique optical properties of graphene, we propose an electrically tunable optical spatial differentiation by introducing a graphene layer at a quartz substrate. It is found that the output light field is sensitive to the graphene layer near the Brewster angle for small polarization output at the graphene-quartz substrate interface and can be modulated by changing the Fermi energy of graphene. In this case, the result of the optical differential operation can be dynamically regulated. Almost strict one-dimensional differential operations in different directions and almost perfect two-dimensional differential operations can be achieved. In addition, two-dimensional edge detection with different degrees of distortion in different directions can also be realized when applied to image processing. This new modulation method may provide more possibilities for tunable image edge detection and provide a potential way for developing more versatile optical simulators in the future.

Ultra-short pulse focusing algorithm for optical parametric chirp pulse amplification numerical simulation platform

The development of optical parametric chirp pulse amplification (OPCPA) numerical simulation platform involves physical models such as broadening and compression of optical pulse, parametric amplification and focusing output. In the simulation platform, the Fresnel far-field diffraction equation is usually used to simulate the characteristics of ultrashort pulse focusing. Firstly, we need to calculate the optical field distribution of different wavelength components in the ultrashort pulse, and then use the inverse Fourier transform to obtain the temporal and spatial distribution characteristics of the pulse. However, for different wavelength components, the sizes of focused field grids obtained by the far-field algorithm are not equal, and subsequent resampling is required, which will increase the amount of calculation. In addition, due to the limitation of the calculation range of the light field in the pulse broadening and compression, there is also a problem of poor resolution of the focused field. In this work, the mathematical expression that can adjust the range of the output light field and use the fast fourier algorithm is derived. The main mechanism of this algorithm is as follows. Based on the Fresnel far-field diffraction equation, the output field is sampled independently in the discrete calculation process to meet the requirements for adjustable range of the output field. After identity transformation, the output field results can be calculated by the fast Fourier algorithm. Furthermore, the sampling conditions that need to be satisfied when using the algorithm are further analyzed and discussed. It solves the problem of how to improve the resolution of light field and keep the computational grid size of each wavelength component consistent when the traditional Fresnel far field diffraction is used to simulate the focusing process, which provides the convenience for the subsequent direct time-frequency inverse transformation. The numerical simulation results reveal that the dark ring region of the ultrashort pulse focusing field shows strong spatiotemporal coupling characteristics. This algorithm has been successfully applied to the development of OPCPA numerical simulation platform, and is expected to play an important role in optimizing the design of ultrashort laser pulse device.

Multipartite quantum steering of symmetric and asymmetric structures based on four-wave mixing processes

In a quantum communication network, multipartite Einstein–Podolsky–Rosen (EPR) steering has become an important resource and has attracted much attention because of its important role in quantum secret sharing and quantum secure communication. In this paper, we investigate the multipartite EPR steering characteristics of symmetric and asymmetric structures based on four-wave mixing (FWM) processes. Multipartite quantum steering, among the four output light fields produced by two structures, is theoretically analyzed. We find that both symmetric and asymmetric structures can verify Gaussian steered monogamous relationships, but the asymmetric structure produces stronger steering and has more flexibility to manipulate monogamous relationships. The results presented here may find potential applications in secure quantum information manipulation processes.

Generation of hollow Gaussian beams by restoring structured light with meta-optics

Hollow Gaussian beams (HGBs) with annular intensity distributions but uniform phase and polarization distributions have aroused enormous interest due to their advantages in optical trapping, optical limiting, etc. Although extensive HGB generation methods have been performed previously, there are still existing limitations in polarization independence and broadband response, which hinder the application of HGBs. Here, we propose and experimentally demonstrate an HGB generation method by restoring structured light with a pair of cascaded dielectric metasurface q-plates. The metasurface q-plates are based on Pancharatnam-Berry (PB) phase and work as elements of optical spin-to-orbital angular momentum conversion. The first metasurface q-plate is used to introduce orbital angular momentum to an incident circularly polarized beam, and the output light field possesses both annular intensity distribution and spiral phase with reversed circular polarization. Since the phase retardation of the metasurface q-plates isπ, the reversed circularly polarized beam output from the first metasurface q-plate will obtain opposite spiral phase after the second metasurface q-plate. Then, the spiral phase of the output light field is counteracted, but its annular intensity distribution is retained as a new HGB. Arbitrarily polarized Gaussian beams are available for incident beams because they can be decomposed into orthogonal circularly polarized components. Furthermore, this method is applicable to a broad spectrum of visible light owing to the dispersion-free characteristic of the PB phase. This work suggests a polarization-independent and broadband HGB generation method, which may benefit HGB-based fundamental research.

Nonreciprocal transmission of vortex beam in double Laguerre-Gaussian rotational cavity system

By constructing an optorotational system composed of two linearly coupled Laguerre-Gaussian rotational cavities, we realize the nonreciprocal transmission of the vortex beam with the orbital angular momentum. Two vortex beam cavity modes driven by strong fields are coupled with a rotational mirror via the torsion, and two cavity modes interact with each other via the optical fiber. A weak probe field is incident from one side of the system for examining the optical response along one propagating direction. With the Hamiltonian of the system and the Heisenberg-Langevin equation, we can obtain the transmission of the output light field from the input-output theory. The result shows that the optical nonreciprocity of the vortex beam arises from the quantum interference between the optorotational interaction and the linear coupling interaction between two vortex beam modes, and the phase difference can be used to adjust the optical nonreciprocity. The phase difference can determine not only the occurrence of the nonreciprocity but also the direction of nonreciprocity. Moreover, the ratio of the topological charges carried by the two vortex beam fields has an influence on the transmission. Under an appropriate topological charge ratio, the unidirectional transmission of the vortex beam can be realized in such a system. It is found that whether the topological charge ratio is positive or negative, i.e. whether the vortex beam is left-hand beam or right-hand beam, does not affect the transmission; the influence of the topological charge on the transmission amplitude actually comes from the topological charge number carried by the vortex beam, due to the fact that the coupling strength between the rotating mirror mode and the cavity mode depends on the topological charge number. In addition, we also obtain the condition that the system damping rates should meet for realizing the perfect nonreciprocal propagation of the vortex beam. Finally, we can achieve the nonreciprocal group velocity of the slow light. The direction of the nonreciprocal slow light can be controlled via phase modulation. Our work provides a possible application in manipulating the vortex beam propagation. Furthermore, we extend the nonreciprocity of ordinary beams in the optomechanical system to the nonreciprocity of the vortex beam in the optorotational system. The results are expected to be applied to fabricating the ideal optical isolators for the vortex beam carrying the orbital angular momentum in optical communication.

Broadband coherent perfect absorption by cavity coupled to three-level atoms in linear and nonlinear regimes

A broadband coherent perfect absorption (CPA) scheme consisting of an optical resonator coupled with three-level atoms excited by single cavity mode is proposed and analyzed. We show the output light field from the system is completely suppressed under specific conditions when the system is excited in linear and nonlinear regimes by two identical light fields from two ends of optical cavity. An analytical broadband CPA criterion for central and sideband excitations of cavity quantum electrodynamics system is derived in linear regime. Moreover, we show the resonant excitation criterion for CPA is greatly extended in nonlinear regime. A new type of bistability behavior is found. The output field intensity and the bistability curve can be well tuned by dynamically adjusting system parameters. Our results demonstrate that the CPA is quite universal, and it should be useful in a variety of applications in optical logic and optical communication devices.

Design of Metasurface with Nanoslits on Elliptical Curves for Generation of Dual-Channel Vector Beams.

The manipulations of nanoscale multi-channel vector beams (VBs) by metasurfaces hold potential applications in various important fields. In this paper, the metasurface with two sets of nanoslits arranged on elliptic curves was proposed to generate the dual-channel focused vector beams (FVBs). Each set of nanoslits was composed of the in-phase and the out-of-phase groups of nanoslits to introduce the constructive interference and destructive interference of the output light field of the nanoslits, focusing the converted spin component and eliminating the incident spin component at the focal point. The two sets of nanoslits for the channels at the two focal points were interleaved on the same ellipses, and by setting their parameters independently, the FVBs in the two channels are generated under illumination of linearly polarized light, while their orders and polarization states of FVBs were controlled independently. The generation of the FVBs with the designed metasurfaces was demonstrated by the finite-difference time domain (FDTD) simulations and by the experimental verifications. The work in this paper is of great significance for the generation of miniaturized multi-channel VBs and for broadening the applications of metasurfaces.

Optomechanical cooling with coherent and squeezed light: The thermodynamic cost of opening the heat valve

Ground-state cooling of mechanical motion by coupling to a driven optical cavity has been demonstrated in various optomechanical systems. In our work, we provide a so far missing thermodynamic performance analysis of optomechanical sideband cooling in terms of a heat valve. As performance quantifiers, we examine not only the lowest reachable effective temperature (phonon number) but also the evacuated-heat flow as an equivalent to the cooling power of a standard refrigerator, as well as appropriate thermodynamic efficiencies, which all can be experimentally inferred from measurements of the cavity output light field. Importantly, in addition to the standard optomechanical setup fed by coherent light, we investigate two recent alternative setups for achieving ground-state cooling: replacing the coherent laser drive by squeezed light or using a cavity with a frequency-dependent (Fano) mirror. We study the dynamics of these setups within and beyond the weak-coupling limit and give concrete examples based on parameters of existing experimental systems. By applying our thermodynamic framework, we gain detailed insights into these three different optomechanical cooling setups, allowing a comprehensive understanding of the thermodynamic mechanisms at play.

Macroscopically entangled light fields

A novel method of macroscopically entangled light-pair generation is presented for a quantum laser using randomness-based deterministic phase control of coherent light in a coupled Mach–Zehnder interferometer (MZI). Unlike the particle nature-based quantum correlation in conventional quantum mechanics, the wave nature of photons is applied for collective phase control of coherent fields, resulting in a deterministically controllable nonclassical phenomenon. For the proof of principle, the entanglement between output light fields from a coupled MZI is examined using the Hong-Ou-Mandel-type anticorrelation technique, where the anticorrelation is a direct evidence of the nonclassical features in an interferometric scheme. For the generation of random phase bases between two bipartite input coherent fields, a deterministic control of opposite frequency shifts results in phase sensitive anticorrelation, which is a macroscopic quantum feature.

Super-Variable Focusing Vortex Beam Generators Based on Spiral Zone Plate Etched on Optical Fiber Facet

In this article, an all-fiber super-variable focusing vortex beam generator is proposed and demonstrated, the focal point of which can be drastically relocated by varying the wavelength of the incident light. The all-fiber generator is fabricated by etching a spiral zone plate (SZP) on the top of the composite fiber structure composed of a single mode fiber and a quarter pitch graded index fiber using focused ion beam lithography. Numerical simulation and experimental measurements have been applied in investigation of all-fiber output light field characteristics, such as the focal spot, focal length, and vortex topological charges. Results show that these parameters can be controlled by flexibly adjusting the design parameters of SZP microstructure. In particular, the position of the donut-shaped focus spot can be dynamically tuned in a wide range by varying the incident laser wavelength with the focal spot profile remains constant. The all-fiber generator can be potentially applied in particle manipulation, high capacity communication, and fiber optic endoscopic imaging, all of which require ultra-compact, multi-dimension, and flexible focusing schemes.

Seeing through multimode fibers with real-valued intensity transmission matrices

Image transmission through multimode optical fibers has been an area of immense interests driven by the demand for miniature endoscopes in biomedicine and higher speed and capacity in telecommunications. Conventionally, a complex-valued transmission matrix is obtained experimentally to link the input and output light fields of a multimode fiber for image retrieval, which complicates the experimental setup and increases the computational complexity. Here, we report a simple and high-speed method for image retrieval based on our demonstration of a pseudo-linearity between the input and output light intensity distributions of multimode fibers. We studied the impact of several key parameters to image retrieval, including image pixel count, fiber core diameter and numerical aperture. We further demonstrated with experiments and numerical simulations that a wide variety of input binary and gray scale images could be faithfully retrieved from the corresponding output speckle patterns. Thus, it promises to be useful for highly miniaturized endoscopy in biomedicine and spatial-mode-division multiplexing in telecommunications.

All-fiber focused beam generator integrated on an optical fiber tip

We report a compact all-fiber focused vortex beam generator, where a spiral zone plate (SZP) is integrated on the tip of a composite optical fiber microstructure with the femtosecond laser two-photon polymerization. The experimental characteristics of the beam produced from the focused vortex beam generators, such as the focal length, focal spot diameter, and vortex topological charge, are in excellent agreement with the results from finite-difference time-domain simulations. Adjusting the design parameters of the SZP integrated on the fiber tip, we can realize the precise manipulation of the generator’s output light field. Since the intrinsic high divergence of a vortex beam during propagation is effectively overcome, the generators may have potential applications for optical fiber optical wrench, all-fiber stimulated emission depletion microscopy, or orbital angular momentum fiber communication. Moreover, the compact generators also have a good anti-interference ability and long-term stability.

Tunable Duplex Metalens Based on Phase-Change Materials in Communication Range.

Metalenses recently have attracted attention because of their more compact size in comparison with conventional lenses; they can also achieve better optical performance with higher resolution. Duplexer is an interesting function of a metalens that can distinguish different sources and divide them into two parts for specific purposes. In this article, we design tunable duplex metalenses with phase-change material Ge2Sb2Te5 for the first time. Two types of special unit cells are designed to modulate the incident lights, and four metalenses are designed based on the two types of unit cells. Specific phase profiles are calculated for different sections of metalens in which the corresponding unit cells are settled; accordingly, the metalenses can focus the incident lights at any positions according to our design. Moreover, the metalenses become selectable via tuning the state of phase-change material, which means that the output light field can be actively controlled. The proposal of our tunable duplex metalenses will offer new opportunities for active three-dimensional imaging or optical coding.

Electrical source of pseudothermal light

We describe a simple and compact electrical version of a pseudothermal light source. The source is based on electrical white noise whose spectral properties are tailored by analog filters. This signal is used to drive a light-emitting diode. The type of second-order coherence of the output light can be either Gaussian or Lorentzian, and the intensity distribution can be either Gaussian or non-Gaussian. The output light field is similar in all viewing angles, and thus, there is no need for a small aperture or optical fiber in temporal coherence analysis.

From Focal Stack to Tensor Light-Field Display

We propose a method of using a focal stack, i.e., a set of differently focused images, as the input for a novel light field display called a "tensor display." Although this display consists of only a few light attenuating layers located in front of a backlight, it can be viewed from many directions (angles) simultaneously without the resolution of each viewing direction being sacrificed. Conventionally, a transmittance pattern is calculated for each layer from a light field, namely, a set of dense multi-view images (typically dozens) that are to be observed from different directions. However, preparing such a massive amount of images is often cumbersome for real objects. We developed a method that does not require a complete light field as the input; instead, a focal stack composed of only a few differently focused images is directly transformed into layer patterns. Our method greatly reduces the cost of acquiring data while also maintaining the quality of the output light field. We validated the method with experiments using synthetic light field datasets and a focal stack acquired by an ordinary camera.

Quantum steering in cascaded four-wave mixing processes.

Quantum steering is used to describe the "spooky action-at-a-distance" nonlocality raised in the Einstein-Podolsky-Rosen (EPR) paradox, which is important for understanding entanglement distribution and constructing quantum networks. Here, in this paper, we study an experimentally feasible scheme for generating quantum steering based on cascaded four-wave-mixing (FWM) processes in hot rubidium (Rb) vapor. Quantum steering, including bipartite steering and genuine tripartite steering among the output light fields, is theoretically analyzed. We find the corresponding gain regions in which the bipartite and tripartite steering exist. The results of bipartite steering can be used to establish a hierarchical steering model in which one beam can steer the other two beams in the whole gain region; however, the other two beams cannot steer the first beam simultaneously. Moreover, the other two beams cannot steer with each other in the whole gain region. More importantly, we investigate the gain dependence of the existence of the genuine tripartite steering and we find that the genuine tripartite steering exists in most of the whole gain region in the ideal case. Also we discuss the effect of losses on the genuine tripartite steering. Our results pave the way to experimental demonstration of quantum steering in cascaded FWM process.

Phase-sensitive cascaded four-wave-mixing processes for generating three quantum correlated beams

Theoretical studies and experimental implementations of quantum correlation are the important contents of continuous variables quantum optics and quantum information science. There are various systems for the study of quantum correlation. Here, we study an experimental scheme for generating three quantum correlated beams based on phase-sensitive cascaded four-wave-mixing (FWM) processes in rubidium vapor. Quantum correlation including intensity difference or sum squeezing, two other combinatorial squeezing, and quantum entanglement among the three output light fields are theoretically analyzed in this paper. Also, the comparison of the quantum correlations have been made between the phase-sensitive cascaded FWM processes and the phase-insensitive cascaded FWM processes. By changing the phases and intensities of the input beams, it is interesting to find that the maximum degrees of various combinatorial squeezing are equal when the two FWM processes share a common intensity gain. When the common intensity gain of the two FWM processes changes, the maximum degrees of different combinatorial squeezing will be synchronously controlled. At last we discuss the genuine tripartite entanglement and steering in our phase-sensitive cascaded scheme, and compare them with the cases of the phase-insensitive cascaded scheme.

Simulation model of super Gaussian beam pumped Q-switched solid-state laser

Computer simulation is always an important means for studying laser, while laser theory is the basis of simulation. Although the semi-classical laser theory can accurately describe the generation process of laser, its complexity leads to a need of huge resources and time for computation. However, in particular cases, the influence of some factors on the laser system can be neglected. If a simpler model is employed to describe the laser system, the time of simulation can be shortened significantly. In order to simulate the laser system more efficiently, a simulation model of Q-switched solid-state laser is proposed in this paper. In this model, the time-domain function of Q switch is introduced, which represents the modulation of Q switch loss over time. Because the cross section of the Nd:YAG rod is circularly shaped, the resonator eigenmodes are assumed to be a Laguerre-Gaussian beam for simplicity. Then, any other laser beam can be formed by superposition of the eigenmodes of the resonator. These series of resonator eigenmodes are coupled with the rate equations of laser crystals. Finally, the distribution of pump light field inside the laser crystal is approximated as super Gaussian distribution. Based on this physical model, the influence of pump power and pump light field distribution on the output beam of multimode Q-switched solid-state laser is investigated. The simulation results are in good agreement with the experimental data, which explains the validity of the proposed model. For instance, with the increase of pump power, the output power of the laser increases, but the overall slope efficiency decreases. This is because the diffraction loss m,n of the lower order mode is less than the diffraction loss of higher order mode. When the pumping power increases, the higher order mode that starts to oscillate has lower utilization efficiency of pump energy. Therefore, the overall slope efficiency of the laser is reduced. In order to analyze the mode competition in the multimode Q-switched solid-state laser more comprehensively, the processes of laser pulse generation, relaxation oscillation and continuous oscillation are calculated as one full cycle. The laws of pulse power and beam quality factor versus time are obtained. For example, the maximum instantaneous output power of the relaxation oscillation is about 30 times the steady continuous output power. This law has a certain reference value when analyzing the damage threshold of laser optical element. In the pulse generation stage, the beam quality factor is close to 1, which explains the fact that the pulse field composition is nearly the fundamental mode of the laser. In the relaxation oscillation, the value of the beam quality factor changes irregularly with time, because mode competition is in a non-equilibrium state at this time. When stable continuous oscillation occurs, the mode competition achieves dynamic equilibrium, which means that the proportion of each mode is no longer changed in the output light field.

New surface plasmon polariton waveguide based on GaN nanowires

Lasers are nowadays widely used in industry, in hospitals and in many devices that we have at home. Random laser development is challenging given its high threshold and low integration. Surface plasmon polariton (SPP) can improve random laser characteristics because of its ability to control diffraction. In this study, we establish a random laser structural model with silicon-based parcel GaN nanowires. The GaN nanowire gain and enhanced surface plasmon increase population inversion level. Our laser model is based on random particle scattering feedback mechanism, nanowire use, and surface plasmon enhancement effect, which causes stochastic laser emergence. Analysis shows that the SPP mode and nanowire waveguides coupled in the dielectric layer of low refractive index can store light energy like a capacitor under low refractive index clearance. The waveguide mode field area and limiting factors show that the modeled laser can achieve sub-wavelength constraints of the output light field. We also investigate emergent laser performance for a more limited light field capacity and lower threshold.

Local-oscillator noise coupling in balanced homodyne readout for advanced gravitational wave detectors

The second generation of interferometric gravitational wave detectors are quickly approaching their design sensitivity. For the first time these detectors will become limited by quantum back-action noise. Several back-action evasion techniques have been proposed to further increase the detector sensitivity. Since most proposals rely on a flexible readout of the full amplitude- and phase-quadrature space of the output light field, balanced homodyne detection is generally expected to replace the currently used DC readout. Up to now, little investigation has been undertaken into how balanced homodyne detection can be successfully transferred from its ubiquitous application in table-top quantum optics experiments to large-scale interferometers with suspended optics. Here we derive implementation requirements with respect to local oscillator noise couplings and highlight potential issues with the example of the Glasgow Sagnac Speed Meter experiment, as well as for a future upgrade to the Advanced LIGO detectors.