arXiv subjects

Zhi-Han Zhu

Publications and source records attributed to Zhi-Han Zhu.

31 records · Page 2Linked to original sources

arXiv2020

Parametric upconversion of Ince-Gaussian modes

Ince-Gaussian (IG) mode, a recently discovered type of structured Gaussian beam, corresponds to eigenfunctions of the paraxial wave equation in elliptical coordinates. This propagation-invariant mode is of significance in various domains, and in particular, its nonlinear transformation; however, there have been few relevant studies to date. In this work, we report the parametric upconversion of IG modes and associated full-field selection rule for the first time. We demonstrate that IG signals can be perfectly upconverted by a flattop-beam pump; in contrast, significant mode distortion occurred when using the most common Gaussian pump. Particular attention was given to the origin of the distortion, i.e., radial-mode degeneration induced by the sum-frequency generation excited by Gaussian pump. This proof-of-principle demonstration has great significance in relevant areas, such as high-dimension quantum frequency interfacing and upconversion imaging.

physics.optics↗

arXiv2020

Classically-entangled Ince-Gaussian modes

Complex vector light modes, classically-entangled in their spatial and polarisation degrees of freedom (DoF), havebecome ubiquitous in a vast diversity of research fields. Crucially, while polarisation is limited to a bi-dimensionalspace, the spatial mode is unbounded, it can be specified by any of the sets of solutions the wave equation can supportin the different coordinate systems. Here we report on a class of vector beams with elliptical symmetry where thespatial DoF is encoded in the Ince-Gaussian modes of the cylindrical elliptical coordinates. We outline their geometricrepresentation on the Higher-Order Poincaré Sphere, demonstrate their experimental generation and analyse the qualityof the generated modes via Stokes polarimetry. We anticipate that such vector modes will be of great relevance inapplications, such as, optical manipulations, laser material processing and optical communications amongst others.

physics.optics↗

arXiv2020

Measuring the non-separability of vector modes with digital micromirror devices

The non-separability between the spatial and polarisation Degrees of Freedom (DoFs) of complex vector light fields has drawn significant attention in recent time. Key to this are its remarkable similarities with quantum entanglement, with quantum-like effects observed at the classical level. Crucially, this parallelism enables the use of quantum tools to quantify the coupling between the spatial and polarisation DoFs, usually implemented with polarisation-dependent spatial light modulators, which requires the splitting of the vector mode into two orthogonal polarisation components. Here we put forward a novel approach that relies on the use of Digital Micromirror Devices (DMDs) for fast, cheap and robust measurement, while the polarisation-independent nature of DMDs enables a reduction in the number of required measurements by 25\%. We tested our approach experimentally on cylindrical vector modes with arbitrary degrees of non-separability, of great relevance in a wide variety of applications. Our technique provides a reliable way to measure in real time the purity of vector modes, paving the way to novel applications where the degree of non-separability can be used as an optical sensor.

physics.optics↗

arXiv2019

Vectorial nonlinear optics: type-II second-harmonic generation driven by spin-orbit coupled fields

Vectorial nonlinear optics refers to the investigation of optical processes whose nonlinear polarization (NP) undergoes spin-orbit coupling (SOC) interactions where, in general, the driving light field or the new field generated by the interaction containing the SOC property. To contribute to fundamental knowledge in this domain, we examine the type-II second harmonic generation (SHG) induced by vectorial laser modes. First, we provide a general theory to analyze the vectorial SHG process. Second, by using two typical vector modes as examples, we show how the SOC of the pump field dictates nonlinear interaction. Finally, we corroborate our theoretical predictions through experiments to confirm the crucial role of the SOC in nonlinear interactions. These results enhance our fundamental understanding of SOC-mediated nonlinear optics and lay the foundation for further fundamental studies as well as possible applications.

physics.optics↗

arXiv2019

Single-shot Stokes polarimetry enabled by a digital micromirror device

Stokes polarimetry (SP) is a powerful technique that enables spatial reconstruction of the state of polarization (SoP) of a light beam using only intensity measurements. A given SoP is reconstructed from a set of four Stokes parameters, which are computed through four intensity measurements. Since all intensities must be performed on the same beam, it is common to record each intensity individually, one after the other, limiting its performance to light beams with static SoP. Here, we put forward a novel technique to extend SP to a broader set of light beams with dynamic SoP. This technique relies on the superposition principle, which enables the splitting of the input beam into identical copies, allowing the simultaneous measurement of all intensities. For this, the input beam is passed through a multiplexed digital hologram displayed on a polarization-insensitive Digital Micromirror Device (DMD) that grants independent and rapid (20 kHz) manipulation of each beam. We are able to reliably reconstruct the SoP with high fidelity and at speeds of up to 27 Hz, paving the way for real-time polarimetry of structured light

physics.optics↗

arXiv2019

Full characterization of spin-orbit coupled photons via spatial-Stokes measurement

Characterization and analysis of spin-orbit coupled (SOC) states, as a measurement problem, play a vital role in research on the modern optics and photonics based on structured light. Here, we demonstrate determination of photonic SOC states via spatial-Stokes measurement, in which two spatial complex probability amplitudes of spin-dependent spatial modes within SOC states and their relative (intramode) phase can be measured directly. Compared with the standard quantum-state tomography, by avoiding wavefront-flattening operations, the apparatus can completely record photons' realistic SOC structure, leading to a more accurate and precise determination of wavefunction. This simple and general approach for SOC state determination can provide a powerful toolkit for in-situ measuring photonic SOC state, characterizing the quality of SOC light source and associated geometric-phase devices.

physics.optics↗

arXiv2019

Does the structure of light influence the speckle size?

It is well known that when a laser is reflected from a rough surface or transmitted through a diffusive medium, a speckle pattern will be formed at a given observation plane. Speckle is commonly produced by laser beams with a homogeneous intensity, for which, well-known relations have been derived, relating the speckle size to the area of illumination. Here we investigate the speckle generated by higher-order Laguerre-Gaussian (LG) modes, characterized by a non-uniform intensity distribution of concentric rings.We show that the ring-structure of the LG modes does not play any role in the speckle size, which happens to be the same as that obtained for a homogeneous intensity distribution. This allow us to provide with a simple expression that relates the speckle size to the spot size of the LG modes. Our findings will be of great relevance in many speckle-based applications.

physics.optics↗

arXiv2019

Generating single longitudinal mode entangled photons in telecom band via a submillimeter monolithic cavity

A high-quality, compact, and narrow-bandwidth entangled photon source (EPS) is indispensable for realization of many quantum communication protocols. Usually, a free space cavity containing a nonlinear crystal is used to generate a narrow bandwidth EPS through spontaneous parametric down-conversion (SPDC). One major drawback is that this occupies a large space and requires complex optical and electrical control systems. Here we present a simple and compact method to generate a single-longitudinal-mode time-energy EPS via type II SPDC in a submillimeter Fabry-Perot cavity. We characterize the quality of the EPS by measuring the coincidence to accidental coincidence ratio, the two-photon time cross-correlation function and the two-photon interference fringes. All measured results clearly demonstrate that the developed source is of high quality when compared with EPSs generated using other configurations. We believe this source is very promising for applications in the quantum communication field.

quant-ph↗

arXiv2019

In-situ detecting cooperative-target's speed and rotation inertia using structured light

Laser remote sensing represents a powerful tool that enables the accurate measurement of the speed of moving targets. Crucially, most sensing techniques are 2-Dimensional and only enable direct determination of the speed along the line of sight. A disadvantage that is very often overcome using two-dimensional techniques that in many cases are hard to implement and expensive. Here we put forward a novel 3-Dimensional technique that enables the direct and simultaneous measurement of both, the speed and the spin rate of cooperative targets. This technique is based on the use of complex vector light beams, whose polarization and spatial degree of freedom are coupled in a non-separable way. We present experimental evidence of our technique by performing a laboratory proof-of-principle experiment.

physics.optics↗

arXiv2018

Dynamic tomography of the spin-orbit coupling in nonlinear optics

Spin-orbit coupled (SOC) light fields with spatially inhomogeneous polarization have attracted increasing research interest within the optical community. In particular, owing to their spin-dependent phase and spatial structures, many nonlinear optical phenomena which we have been familiar with up to now are being re-examined, hence a revival of research in nonlinear optics. To fully investigate this topic, knowledge on how the topological structure of the light field evolves is necessary, but, as yet, there are few studies that address the structural evolution of the light field. Here, for the first time, we present a universal approach for theoretical tomographic treatment of the structural evolution of SOC light in nonlinear optics processes. Based on a Gedanken vector second harmonic generation, a fine-grained slice of evolving SOC light in a nonlinear interaction and the following diffraction propagation are studied theoretically and verified experimentally, and which at the same time reveal several interesting phenomena. The approach provides a useful tool for enhancing our capability to obtain a more nuanced understanding of vector nonlinear optics, and sets a foundation for further broad-based studies in nonlinear systems.

physics.optics↗

arXiv2017

Revealing photons' past via quantum twisted double-slit experiment

Are quantum states real? This most fundamental question in quantum mechanics has not yet been satisfactorily resolved, although its realistic interpretation seems to have been rejected by various delayed-choice experiments. Here, to address this long-standing issue, we present a quantum twisted double-slit experiment. By exploiting the subluminal feature of twisted photons, the real nature of a photon during its time in flight is revealed for the first time. We found that photons' arrival times were inconsistent with the states obtained in measurements but agreed with the states during propagation. Our results demonstrate that wavefunctions describe the realistic existence and evolution of quantum entities rather than a pure mathematical abstraction providing a probability list of measurement outcomes. This finding clarifies the long-held misunderstanding of the role of wavefunctions and their collapse in the evolution of quantum entities.

quant-ph↗

arXiv2017

Generating strong cylindrical vector pulses via stimulated Brillouin amplification

Light with transverse polarization structure, such as radial and azimuthal polarization, enables and revives lots of applications based on light-matter interaction due to their unique focal properties. To date, studies referring to this topic mainly concentrate in weak-light domain, yet it should have gained more attention in strong-light domain. A main factor contributing to the current situation is that the generation devices cannot afford high power. Here, in this proof-of-principle work, we demonstrate the generation of strong single-longitudinal-mode (SLM) cylindrical vector (CV) short pulses via stimulated Brillouin amplification, specifically, the energy is transferred from a 700 ps pump to a 300 ps Stokes pulse via parametrically generated coherent phonons. After amplification, a 100 mJ-level SLM CV pulse light with 300 ps duration is obtained. Meanwhile, the phase and polarization structures are high fidelity maintained. This result provides a practicable way to generate strong CV light, and by extending this mechanism into multi-pump beam combing system, even an ultra-high intensity CV light can be expected.

physics.optics↗

arXiv2016

Generation and reverse transformation of twisted light by spatial light modulator

A spatial light modulator (SLM) is one of the most useful and convenient device to generate structural light beams such as twisted light and complexed images used in modern optical science. The unbounded dimension of twisted light makes it promising in harnessing information carrying ability of a single photon, which greatly enhances the channel capacity in optical communications. We perform a detail theoretical study of the birth, evolution and reverse transformation of twisted light generated from a phase-only SLM based on diffraction theory, analytical expressions are obtained to show the special evolution behaviors of the light beam with the propagation distance. Beam intensity distributions calculated theoretically are in well agreement with experimental observations. Our findings clearly reveal how twisted light gradually emerges from a Gaussian spatial shape to a ring shape, and also the ring shape will gradually evolve to a quasi-Gaussian shape conversely, in the reverse transformation. These results will provide guidelines for using SLM in many optical experiments and OAM-mode multiplexing in optical communications.

physics.optics↗