Achromatic linear retarder with tunable retardance

Tunable broadband polarization retarders

We theoretically propose a type of tunable polarization retarder, which is composed of sequences of half-wave and quarter-wave polarization retarders, allowing operation at broad spectral bandwidth. The constituent retarders are composed of stacked standard half-wave retarders and quarter-wave retarders rotated at designated angles relative to their fast polarization axes. The proposed composite retarder (CR) can be tuned to an arbitrary value of the retardance by varying the middle retarder alone while maintaining its broadband spectral bandwidth intact.

Composite acousto-optical modulation

We propose a composite acousto-optical modulation (AOM) scheme for wide-band, efficient modulation of CW and pulsed lasers. We show that by adjusting the amplitudes and phases of weakly-driven daughter AOMs, diffraction beyond the Bragg condition can be achieved with exceptional efficiencies. Furthermore, by imaging pairs of AOMs with opposite directions of sound-wave propagation, high contrast switching of output orders can be achieved at the driving radio frequency (rf) limit, thereby enabling efficient bidirectional routing of a synchronized mode-locked laser. Here we demonstrate a simplest example of such scheme with a double-AOM setup for efficient diffraction across an octave of rf bandwidth, and for routing a mode-locked pulse train with up to f_rep = 400 MHz repetition rate. We discuss extension of the composite scheme toward multi-path routing and time-domain multiplexing, so as to individually shape each pulses of ultrafast lasers for novel quantum control applications.

Negative dispersion of a form birefringence in subwavelength gratings

An achromatic response is required in most optical systems for wideband and straightforward configurations. The chromatic response of the optical system depends on the optical dispersion of the elements in the system. Here we study the dispersion of subwavelength grating (SWG) known to have a form birefringence. The birefringence of SWG was numerically analyzed with Bloch wave analysis (BWA) and finite element method (FEM). The sandwiched SWG with two identical substrates was studied for practical applications. We successfully demonstrated the negative dispersion form birefringence of SWG with an optimal duty cycle. This extraordinary dispersion was also shown considering the intrinsic dispersion of materials. Dispersion- and the angular response were in a tradeoff relationship while they depended on periodicity. The optical interference between the grating and the substrates can be eliminated by controlling the duty cycle. Our analysis offers optimal SWG with achromatic birefringence and high transparency, promising in the widespread applications of polarization control devices.

Superachromatic polarization modulator for stable and complete polarization measurement over an ultra-wide spectral range

The polarization measurement system deals with polarized light-matter interactions, and has been a kind of powerful optical metrology applied in wide fields of physics and material. In this paper, we address several general theoretical aspects related to the system model and optimization for linear polarization systems from a view of the matrix algebra. Based on these theories, we propose a new framework of superachromatic polarization modulator (PM) by combining a linear polarizer and a sequence of parallel linear retarders (LRs) for a typical kind of linear polarization system based on the rotating compensator (RC) principle. In the proposed PM, the LRs are made of quarter-wave plates and as a whole act as the RC. Compared with conventional achromatic/superachromatic composite waveplates, the LR sequence has general axis orientations and is optimized by the condition number of the instrument matrix of the PM, which thereby provide much more flexibility to achieve uniform, stable and complete polarization modulation over ultra-wide spectral range. The intrinsic mechanisms, including the working principle, optimization strategy and in-situ calibration method of the proposed PM, are presented and revealed mathematically by the matrix algebra. Results on several prototypes of the PM demonstrate the validity and capability of the proposed methods for applications in broadband polarization measurement systems. The fabricated PM is further applied to a home-made dual RC Mueller matrix ellipsometer, and the accuracy and precision in the full Mueller matrix measurement are better than 2‰ and 0.6‰ respectively over the ultra-wide spectral range of 200∼1000 nm. Compared with existing techniques, the proposed PM has advantages due to superachromatic performances over ultra-wide spectral ranges, stable and complete modulation of the polarized light, and convenience for adjustment and calibration.

Composite achromatic quartz wave plate with adjustable retardation and temperature insensitivity

In order to obtain a composite achromatic wave plate with adjustable retardation, temperature insensitivity, and achromatic bandwidth of 500 nm, a five-element composite achromatic quartz wave plate is designed based on particle swarm optimization. The total phase retardation can be adjusted by rotating the azimuth angle of the central wave plate. The results show that the total phase retardation is adjustable with the range of 90°-180° when the range of temperature and the achromatic wavelength is -20-80^∘C and 750-1250 nm, respectively. The absolute value of the relative deviation of the maximum phase retardation is less than 3.6%, which meets the design requirements of the wave plate. The temperature insensitivity of the five-element wave plate is better than the three- and four-element wave plates. This method is of great reference value for designing this kind of composite achromatic wave plate.

Tunable Transmissive Terahertz Linear Polarizer for Arbitrary Linear Incidence Based on Low-Dimensional Metamaterials

In this work, we propose a structure consisting of three metamaterial layers and a metallic grating layer to rotate the polarization of arbitrary linearly polarized incidence to the y-direction with high transmissivity by electrically tuning these metamaterials. The transfer matrix method together with a harmonic oscillator model is adopted to theoretically study the proposed structure. Numerical simulation based on the finite difference time-domain method is performed assuming that the metamaterial layers are constituted by graphene ribbon arrays. The calculation and simulation results show that the Drude absorption is responsible for the polarization rotation. Fermi level and scattering rate of graphene are important for the transmissivity. For a polarization rotation of around 90°, the thickness of either the upper or lower dielectric separations influences the transmission window. For a polarization rotation of around 45° and 135°, the lower dielectric separations decide the frequency of the transmission window, while the upper dielectric separations just slightly influence the transmissivity.

Composite polarization systems for independent controlling polarization of two beams with different wavelengths

The usage of independent and simultaneous control of the state of light polarization at different wavelengths can expand the capabilities of polarization methods for biomedical application. Unfortunately, all known methods of polarization conversion cannot convert the state of light polarization at different wavelengths independently. We propose a method and device for independent and simultaneous control of the polarization state at two wavelengths. We have theoretically proved the possibility of maintaining the phase shift at the first wavelength unchanged while simultaneously and independently changing the phase shift at the second wavelength from 0 to 180 degrees. The capabilities of the method were for the first time demonstrated for radiation with wavelengths λ = 632.8 nm and λ = 488 nm. At the wavelength λ = 632.8 nm, the phase shift remained equal to 180° whereas at the wavelength λ = 488 nm, it varied in the range from 121° to 136°.

Terahertz Broadband Polarization Conversion for Transmitted Waves Based on Graphene Plasmon Resonances

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.

Arbitrary polarization control by magnetic field variation

We propose a universal scheme for the construction of a device for arbitrary-to-arbitrary polarization transformation, which consists of two quarter-wave plates and two Faraday rotators. Using this device, one can continuously change the retardance and the rotation angle simply by changing the magnetic fields in each Faraday rotator.

Wavelength-adaptable effective q-plates with passively tunable retardance

Wave retarders having spatially varying optical axes orientations, called q-plates are extremely efficient devices for converting spin to orbital angular momentum of light and for the generation of optical vortices. Most often, these plates are designed for a specific wavelength and have a homogeneous constant retardance. The present work provides a polarimetric approach for overcoming both these limitations. We theoretically propose and experimentally demonstrate q-plates with tunable retardance, employing a combination of only standard q-plates and waveplates. A clear prescription is provided for realizing wavelength indepedent q-plates for a desired retardance, with a potential for ultrafast switching. Apart from the potential commercial value of the proposed devices, our results may find applications in quantum communication protocols, astronomical coronography, angular momentum sorting and in schemes that leverage optical vortices and spin to orbital angular momentum conversion.

Individual selective rotation of the linear polarization of single light beam in a bundle

An approach for aligning of linear polarized light beams with different wavelengths into a single beam with parallel linear polarization is presented and experimentally demonstrated. The main idea is based on the application of recently developed composite half-wave plates, working as linear polarization filters.

Wide field-of-view angle linear retarder with an ultra-flat retardance response

We propose a universal method for optimal design of a wide field-of-view (FOV) angle linear retarder with an ultra-flat retardance response based on paired positive/negative birefringent plates. The mechanism and rules for the FOV effect compensation are revealed to determine the structure of the proposed retarder by introducing the partial derivative of refractive index with respect to the incident angle. A concrete example based on quartz/sapphire plates is designed and manufactured as a proof-of-concept of the proposed method. The maximum deviation in the retardance of the optimally designed quartz/sapphire wide FOV angle retarder over the concerned FOV angles (incidence: -20°-20°; azimuth: 0°-360°) is only 0.08°, which has been reduced by 3 orders of magnitude compared with the common quartz retarder.

Optical wave retarder based on metal-nanostripe metamaterial

Wave retarders, including quarter- and half-wave plates, are used in many optical systems for polarization conversion. They are usually realized with anisotropic crystalline materials. However, much thinner and possibly also less expensive wave plates can be made of micro- and nanostructures. We present a new way to create thin-film optical retarders based on a highly birefringent metamaterial. The wave plate is capable of low-loss, broadband operation, which we verify both numerically and experimentally. Owing to the remarkable simplicity of our design, the wave plates operating on the proposed principle can meet the requirements of large-scale production and find widespread application in optics and photonics.

Polarization-insensitive tunable terahertz polarization rotator

A rotation-angle variable polarization rotator is proposed and demonstrated using an all-dielectric metasurface doublet. Such a transmissive device can conveniently rotate the polarization of incident light by any desired angles by mechanically changing the relative angle of the double metasurface layers regardless of the incident state of polarization. Under certain conditions the device acts as a full phase modulator for the circularly polarized incident wave. Hence, it has a promising application in polarization and phase control.