Showing papers by "Qiwen Zhan published in 2016"

Actively mode-locked all fiber laser with cylindrical vector beam output

Abstract: We demonstrated an all fiber actively mode-locked laser that emits a cylindrical vector beam. An intra-cavity few-mode fiber Bragg grating inscribed in a short section of four-mode fiber is employed to provide mode selection and spectrum filtering functions. Mode coupling is achieved by offset splicing between the single-mode fiber and the four-mode fiber in the laser cavity. A LiNbO3 Mach-Zehnder modulator is used to achieve active mode-locking in the laser. The laser operates at 1547 nm with 30 dB spectrum width of 0.2 nm. The mode-locked pulses have a duration of 2 ns and repetition of 12.06 MHz. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity.

Generation and Conversion of Higher Order Optical Vortices in Optical Fiber With Helical Fiber Bragg Gratings

Abstract: Helical fiber Bragg grating (H-FBG) induced by UV-side exposure exhibits wavelength-dependent coupling between different modes. In this paper, we propose an effective method to generate high-order optical vortices (OVs) using H-FBG written in a multimode fiber. We simulate the conversion of OVs with different topological charges: 0 → ± 1, ±1 → 0, 0 → ± 2, 0 → ± 3 with conversion efficiency as high as 97%. We prove that higher order topological charge can be obtained simply by changing the parameters of the fiber to increase the number of modes in the fiber. We also analyze the conversion of H-FBG under various conditions of the induced asymmetric index profile and the length of H-FBG.

Bi 2 Te 3 based passively Q-switched fiber laser with cylindrical vector beam emission.

Abstract: We report a passively Q-switched fiber laser with cylindrical vector beam output modes using a few-mode fiber Bragg grating as a polarization-selective output coupler and Bi2Te3 as the saturable absorber. Both radially and azimuthally polarized beams can be readily generated, and the output polarization can be switchable by tuning the polarization controllers inside the laser cavity. The repetition rate of the Q-switched laser can be tuned from 31.54 to 49.40 kHz when the pump power increases from 103.5 to 139.5 mW. The fiber laser operates at a single wavelength of 1557.5 nm with a 3 dB linewidth of <0.04 nm.

Generation of arbitrary vector fields based on a pair of orthogonal elliptically polarized base vectors

Abstract: We present an arbitrary vector field with hybrid polarization based on the combination of a pair of orthogonal elliptically polarized base vectors on the Poincare sphere. It is shown that the created vector field is only dependent on the latitude angle 2χ but is independent on the longitude angle 2ψ on the Poincare sphere. By adjusting the latitude angle 2χ, which is related to two identical waveplates in a common path interferometric arrangement, one could obtain arbitrary type of vector fields. Experimentally, we demonstrate the generation of such kind of vector fields and confirm the distribution of state of polarization by the measurement of Stokes parameters. Besides, we investigate the tight focusing properties of these vector fields. It is found that the additional degree of freedom 2χ provided by arbitrary vector field with hybrid polarization allows one to control the spatial structure of polarization and to engineer the focusing field.

Self-starting passively mode-locked all fiber laser based on carbon nanotubes with radially polarized emission

Abstract: We demonstrate an all fiber passively mode-locked laser emitting a radially polarized beam by using a few-mode fiber Bragg grating to achieve mode selection and spectrum filtering. An offset splicing of single-mode fiber with four-mode fiber is utilized as a mode coupler in the laser cavity. Carbon nanotubes are introduced into the laser cavity as the saturable absorber to achieve self-start mode locking. The laser operates at 1547.5 nm with a narrow spectrum width of 0.3 nm at 30 dB. The emitted mode-locked pulses have a duration of 22.73 ps and repetition of 10.61 MHz. A radially polarized beam has been obtained with high mode purity by adjusting the polarization in the laser cavity.

Reflection type metasurface designed for high efficiency vectorial field generation.

Abstract: We propose a reflection type metal-insulator-metal (MIM) metasurface composed of hybrid nano-antennas for comprehensive spatial engineering of the properties of optical fields. The capability of such structure is illustrated in the design of a device that can be used to produce a radially polarized vectorial beam for optical needle field generation. This device consists of uniformly segmented sectors of high efficiency MIM metasurface. With each of the segment sector functioning as a local quarter-wave-plate (QWP), the device is designed to convert circularly polarized incidence into local linear polarization to create an overall radial polarization with corresponding binary phases and extremely high dynamic range amplitude modulation. The capability of such devices enables the generation of nearly arbitrarily complex optical fields that may find broad applications that transcend disciplinary boundaries.

Varying polarization and spin angular momentum flux of radially polarized beams by anisotropic Kerr media.

Abstract: Light fields with structured polarization distribution interacting with structured media will result in many novel optical effects in both the linear and nonlinear regimes. In this work, we report a theoretical investigation of both vectorial self-diffraction behaviors and polarization evolution characteristics of a radially polarized beam induced by anisotropic Kerr nonlinearity. By taking the polarization–orientation dependence of the third-order refractive nonlinearity, we study the far-field vectorial self-diffraction patterns of the radially polarized beam using the vectorial Rayleigh–Sommerfeld formulas. Numerical results reveal that the self-diffraction patterns with a four-fold rotational symmetry exhibit hybrid states of polarization. Moreover, the interaction of radially polarized beams with the anisotropic nonlinear Kerr media leads to the redistribution of the spin angular momentum (SAM) flux in the far-field plane. The presented work opens up new avenues for varying polarization and SAM through anisotropic optical nonlinearity.

Detection of orbital angular momentum using a photonic integrated circuit.

Abstract: Orbital angular momentum (OAM) state of photons offer an attractive additional degree of freedom that has found a variety of applications. Measurement of OAM state, which is a critical task of these applications, demands photonic integrated devices for improved fidelity, miniaturization, and reconfiguration. Here we report the design of a silicon-integrated OAM receiver that is capable of detecting distinct and variable OAM states. Furthermore, the reconfiguration capability of the detector is achieved by applying voltage to the GeSe film to form gratings with alternate states. The resonant wavelength for arbitrary OAM state is demonstrated to be tunable in a quasi-linear manner through adjusting the duty cycle of the gratings. This work provides a viable approach for the realization of a compact integrated OAM detection device with enhanced functionality that may find important applications in optical communications and information processing with OAM states.

Manipulation metallic nanoparticle at resonant wavelength using engineered azimuthally polarized optical field.

Abstract: In this work, we proposed a novel strategy to manipulate the behavior of the metallic nanoparticle under the resonant condition by using engineered azimuthally polarized optical field. Through optimizing the spatial phase distribution of the illumination, the optical force can be tailored to support stable optical trapping while avoiding trap destabilization caused by optical overheating effect simultaneously. Besides, the resonant particle can be stably trapped at predefined location in 3 dimensional space, or revolves around the beam axis with characteristics that can be holistically controlled in terms of both trajectory and rotation direction. The technique demonstrated in this work may open up new avenues for optical manipulation.

Integrated Broadband Bowtie Antenna on Transparent Silica Substrate

Abstract: The bowtie antenna is a topic of growing interest in recent years. In this paper, we design, fabricate, and characterize a modified gold bowtie antenna integrated on a transparent silica substrate. The bowtie antenna is designed with broad RF bandwidth to cover the X-band in the electromagnetic spectrum. We numerically investigate the antenna characteristics, specifically its resonant frequency and enhancement factor. Our designed bowtie antenna provides a strong broadband electric field enhancement in its feed gap. Taking advantage of the low-k silica substrate, high enhancement factor can be achieved without the unwanted reflection and scattering from the backside silicon handle which is the issue of using an SOI substrate. We simulate the dependence of resonance frequency on bowtie geometry, and verify the simulation results through experimental investigation, by fabricating different sets of bowtie antennas on silica substrates and then measuring their resonance frequencies. In addition, the far-field radiation pattern of the bowtie antenna is measured, and it shows dipole-like characteristics with large beam width. Such a broadband antenna will be useful for a myriad of applications, ranging from photonic electromagnetic wave sensing to wireless communications.

Single-Shot Smartphone-Based Quantitative Phase Imaging Using a Distorted Grating.

Abstract: Blood testing has been used as an essential tool to diagnose diseases for decades. Recently, there has been a rapid developing trend in using Quantitative Phase Imaging (QPI) methods for blood cell screening. Compared to traditional blood testing techniques, QPI has the advantage of avoiding dyeing or staining the specimen, which may cause damage to the cells. However, most existing systems are bulky and costly, requiring experienced personnel to operate. This work demonstrates the integration of one QPI method onto a smartphone platform and the application of imaging red blood cells. The adopted QPI method is based on solving the Intensity Transport Equation (ITE) from two de-focused pupil images taken in one shot by the smartphone camera. The device demonstrates a system resolution of about 1 μm, and is ready to be used for 3D morphological study of red blood cells.

Synthesis of focused beam with controllable arbitrary homogeneous polarization using engineered vectorial optical fields.

Abstract: The propagation and focusing properties of light beams continue to remain a research interest owning to their promising applications in physics, chemistry and biological sciences. One of the main challenges to these applications is the control of polarization distribution within the focal volume. In this work, we propose and experimentally demonstrate a method for generating a focused beam with arbitrary homogeneous polarization at any transverse plane. The required input field at the pupil plane of a high numerical aperture objective lens can be found analytically by solving an inverse problem with the Richard-Wolf vectorial diffraction method, and can be experimentally created with a vectorial optical field generator. Focused fields with various polarizations are successfully generated and verified using a Stokes parameter measurement to demonstrate the capability and versatility of proposed technique.

Creation of radially polarized optical fields with multiple controllable parameters using a vectorial optical field generator

Abstract: A vectorial optical field generator (VOF-Gen) based on two reflective phase-only liquid crystal spatial light modulators enables the creation of an arbitrary optical complex field. In this work, the capabilities of the VOF-Gen in terms of manipulating the spatial distributions of phase, amplitude, and polarization are experimentally demonstrated by generating a radially polarized optical field consisted of five annular rings, the focusing properties of which are also numerically studied with vectorial diffraction theory. By carefully adjusting the relative amplitude and phase between the adjacent rings, an optical needle field with purely longitudinal polarization can be produced in the focal region of a high numerical aperture lens. The versatile method presented in this work can be easily extended to the generation of a vectorial optical field with any desired complex distributions.

Nonlinear polarization evolution of hybridly polarized vector beams through isotropic Kerr nonlinearities.

Abstract: Structured intense laser interacting with matter will result in a variety of novel nonlinear optical effects, modulate the light propagation behavior, and change the structural property of a material In this work, we theoretically investigate the spatial self-phase modulation (SSPM) effect, nonlinear ellipse rotation, and spin angular momentum (SAM) flux redistribution of hybridly polarized vector beams through isotropic Kerr nonlinearities Experimentally, we observe the SSPM effect of the femtosecond-pulsed hybridly polarized vector beam in carbon disulfide at 800 nm, which is in agreement with the theoretical predictions Our results show that the SSPM intensity pattern, the distribution of state of polarization (SoP), and the SAM flux of a hybridly polarized vector beam could be manipulated by tuning the isotropic optical nonlinearity, which may find interesting applications in nonlinear mechanism analysis, nonlinear optical characterization, and SAM manipulation

Integrated Broadband Bowtie Antenna on Transparent Silica Substrate

Abstract: The bowtie antenna is a topic of growing interest in recent years. In this letter, we design, fabricate, and characterize a modified gold bowtie antenna integrated on a transparent silica substrate. The bowtie antenna is designed with broad RF bandwidth to cover the X-band in the electromagnetic spectrum. We numerically investigate the antenna characteristics, specifically its resonant frequency and enhancement factor. Our designed bowtie antenna provides a strong broadband electric field enhancement in its feed gap. Taking advantage of the low-k silica substrate, high enhancement factor can be achieved without the unwanted reflection and scattering from the backside silicon handle, which is the issue of using a silicon-on-insulator (SOI) substrate. We simulate the dependence of resonance frequency on bowtie geometry, and verify the simulation results through experimental investigation, by fabricating different sets of bowtie antennas on silica substrates and then measuring their resonance frequencies. In addition, the far-field radiation pattern of the bowtie antenna is measured, and it shows dipole-like characteristics with large beamwidth. Such a broadband antenna will be useful for a myriad of applications, ranging from photonic electromagnetic wave sensing to wireless communications.

Enhanced sensitivity of the Z-scan technique on saturable absorbers using radially polarized beams

Abstract: We develop the Z-scan technique on a saturable absorber under the excitation of radially polarized beams. It is shown that the sensitivity of the Z-scan measurements on a saturable absorber using radially polarized beams has the great improvement compared with those using the scalar light beams such as Gaussian beams, Laguerre-Gaussian beams, and top-hat beams. As the experimental evidence, we investigate the saturable absorption properties of layered WS2 nanosheets in aqueous suspension by performing the radially polarized-beam Z-scan measurements with femtosecond laser pulses in the near infrared region.

Experimental Demonstration of a Highly Efficient Fan-out Polarization Grating.

Abstract: Highly efficient fan-out elements are crucial in coherent beam combining architectures especially in coupled laser resonators where the beam passes through the fan-out element twice per round trip. Although the theoretical efficiency is usually less than 86%, the Dammann gratings are ubiquitously utilized in a variety of types of coherent beam combining systems due to the facile design and fabrication. In the current paper, we experimentally demonstrate a highly efficient fan-out polarization grating. It is the first time to our knowledge that all the three space-variant parameters of a polarization grating are simultaneously optimized to achieve the function of multi-beam splitting. Besides the high fan-out efficiency, the ability to control the polarization states of individual split beams is another advantage of this polarization grating. The novel polarization grating is promising to find applications in laser beam combining systems.

Multi-channel orbital angular momentum detection with metahologram.

Abstract: Orbital angular momentum (OAM) is an intrinsic property of light that has attracted increasing attention recently. In a wide range of applications that involve OAM, it is often crucial to discern the OAM states with high fidelity. In this Letter, we propose a novel method to extend the detectable range of the OAM states by adopting a multi-sector metahologram. The incident light carrying OAM would be focused by the metahologram into surface plasmon waves with separated propagation directions that are spatially sampled by multiple subwavelength detectors. Through quantizing and mapping the detector signals into a lookup table, a wide range of OAM states could be distinguished. The principle reported in this Letter may find important applications in optical communications and information processing with the OAM states.

Focusing Through High–Numerical Aperture Objective

Abstract: High-numerical aperture (NA) optical objective lenses are widely used in high-spatial-resolution optical imaging and fabrication techniques. In optical microscopy, these high-NA objectives produce tightly focused optical fields as probes to interrogate the sample properties within the focal volume and generate the contrast for imaging. In optical microfabrication and nanofabrication, they are used to concentrate the optical energy to produce changes of materials properties within the intended dimensions. Hence, controlling the optical field characteristics within the focal volume plays a critical role in determining the function and performance of these optical systems. Consequently, it is very important to understand the physics of optical focusing through high-NA objective lens. In this chapter, a brief review of the diffraction theory that can handle high-NA focusing of both scalar and vectorial optical illuminations is presented. Various focus engineering techniques that can be used to create optical focal fields with many exotic distributions and characteristics are briefly discussed. Aberrations and mitigation methodology are also mentioned toward the end of this chapter.

Topological insulators based passively Q-switched fiber laser with cylindrical-vector beam emission

Abstract: Based on a few-mode fiber Bragg grating as polarization-selective output coupler and topological insulators Bi2Te3 as the saturable absorber, we propose a passively Q-switched fiber laser with cylindrical vector beam output. Both radially and azimuthally polarized beams can be readily generated, and the output polarization can be switchable through tuning the polarization controllers inside the laser cavity. The laser operates at the wavelength of 1557.5 nm with a 3 dB linewidth of less than 0.04 nm. The repetition rate of the Q-switched laser can be tuned from 31.54 kHz to 49.40 kHz when the pump power increases from 103.5 mW to 139.5 mW.

Manipulating plasmonic nanoparticles with tailored optical focal field

Abstract: Since Ashkin and colleagues reported the first stable three-dimensional (3D) optical trapping, or optical tweezers, created using radiation pressure from a single focused laser beam, optical tweezers have become an important tool for research in the fields of biology, physical chemistry and soft matter physics. Plasmonic nanoparticles have attracted increased attentions in recent years due to various applications of plasmon resonance. For example, the local field enhancement offered by the resonant noble metal nanoparticles enable the amplifying of the Raman signal in surface-enhanced Raman scattering spectroscopy (SERS). Due to the noncontact and “holding” nature, optical trapping is well suited to be combined with SERS, potentially enabling ultrasensitive molecular recognition in liquids. However, the plasmonic nanoparticles are generally considered difficult to manipulate due to the large scattering force and severe optical heating effect, especially when the trapping wavelength approaches the resonant wavelength of the plasmonic nanoparticle. We develop a novel strategy to form a stable 3D manipulating of plasmonic nanoparticles even under resonant conditions through careful and purposeful engineering a vectorial optical field as the illumination. The required optical illumination is created by sculpting the amplitude and phase of a cylindrical vector beam. When strongly focused by a high numerical aperture objective lens, this specially engineered vectorial optical field offers the optical pulling feature of the tractor beam, a recently reported technique to generate a scattering force that points against the optical power flow. The main difference is the tractor beam pulls the particles all the way towards to the light source with no equilibrium point, while the particles conveyed by the engineered vectorial optical beam in this work gets stably trapped at an equilibrium position at the beam axis. The additional degree of freedom provided by the spatial distribution of the illumination enables the manipulation of the plasmonic nanoparticle behavior, meeting specific needs of different applications. For example, the particle can be stably trapped at the beam axis when illumination has concentric binary phase at the beam cross section [1]. However, if the illumination carries a spiral phase simultaneously, which is also known as orbital angular momentum (OAM), the particle would orbit around the beam axis due to the conservation of angular momentum. The particle's motion trajectory in terms of rotating radius and direction can be holistically manipulated by adjusting the OAM of the illumination [2, 3]. Besides, the particle can also be stably trapped off the beam axis, which is realized by introducing a sinusoidal varied phase to the illumination [3]. In addition, the high polarizability of these resonant metallic nanoparticles that normally becomes a problem due to the large scattering force can be taken advantage of to reduce the amount of required power, and hence decrease the heating effect and eliminate the ultimate obstacle in 3D stable optical manipulation of resonant plasmonic nanoparticles. This versatile trapping method may open up new avenues for optical manipulation and their applications in various scientific fields.

Silicon nitride on-silicon 3-dimensional photonic circuits for integrated photodetection

Abstract: We develop a new technique for integrating waveguide detectors in a silicon nitride photonic circuit. Fabrication relies on a novel method of selective oxidation as an alternative to chemical-mechanical-planarization with higher yield and tolerance.

Multi-channel detection of photonic orbital angular momentum states using metahologram

Abstract: A multi-channel orbital angular momentum (OAM) detection scheme is proposed. Numerical simulations demonstrate that 13 OAM states can be distinguished with high fidelity by using a 4-sector metahologram with 4 integrated spatially distributed subwavelength detectors.