Phase Modulation Range Research Articles

Dynamic geometric phase holography based on in-plane switching liquid crystal

ABSTRACT Liquid crystal geometric phase modulation technology is expected to achieve thin, small and achromatic spatial light modulation, which in turn has the potential to achieve holographic displays with large field angles and high clarity. This article presents a continuous dynamic geometric phase liquid crystal spatial light modulation method based on in-plane switching (IPS) mode. The relationship between geometric phase modulation and driving voltage is calibrated by using interferometry. Two hundred fifty-six phase orders within the phase modulation range of 0~π/2 is achieved. Subsequently, a phase hologram with a modulation range of 0~π/2 was designed by combining the GS (Gerchberg-Saxton) algorithm and single Fourier transform, demonstrating the holographic display capability. This article provides a low-cost approach for dynamic geometric phase holographic display.

3D fluorescence imaging through a scattering medium using the transport of intensity equation and Fresnel ping-pong algorithm.

A three-dimensional (3D) fluorescent imaging method through a scattering medium has been presented. The proposed method combines digital phase conjugate reconstruction of the scattered light wave by the transport of intensity equation-based 3D fluorescent imaging to the position of scattering medium and followed phase retrieval method by Fresnel ping-pong algorithm. The effectiveness of the proposed method is experimentally verified by measuring the 3D distribution of fluorescent beads and tobacco-cultured cells through a thin diffuser. Compared to other methods, the reconstructed images show enhancement of the detailed structure of the objects blurred by the diffuser in a wide area. In the simulation, the proposed method is capable of recovering images degraded by scattering in the phase modulation range from 0 to 2π. The capability to identify the depth position is also presented.

Tunable and super-wide angle defocusing lens large phase modulator

Traditional terahertz wavefront modulators exhibit significant limitations in controlling optical wavefronts. In order to address the adjustability issues of terahertz wavefront modulators, we have developed a novel optical device based on the tunability of Fermi energy levels. Structurally, we find the ability to modulate the Fermi energy levels of graphene by modulating the carrier concentration. We can also change the phase of the light, causing the transmission path of the light as it passes through the graphene array to change, affecting the position of the backfocusing focal point, thus realizing a dynamically adjustable backfocusing effect. Theoretically, employing Dirac notation and Matrix analyze, reveals the corresponding relationship between Fermi level and phase shift. Specifically, one group Fermi levels induces a set of modulated phase changes. Experimental results demonstrate that the proposed modulator offers a large phase modulation range, enabling 2π phase adjustments. Moreover, it accurately focuses on a predefined focal point located at 400 um, with a numerical aperture of 0.882, a resolution of 30.39, and a full-width at half-maximum of 38. The substantial numerical aperture and small full-width at half-maximum imply that the lens can focus more light while maintaining high resolution, enhanced optical efficiency, and concentrated light focusing. At the same time, by adjusting the position of the Fermi energy level, we have succeeded in realizing a wide-angle focusing effect up to 90°. This capability extends the terahertz wave’s focusing within a wide range, expanding the application scope of traditional focusing devices.

Molecules structure and viscosity relationship of nematic liquid crystal and BPNN-QSAR model

Nematic liquid crystal materials designed for optics, microwave communication tuning, etc. need high response speed, which is related to the rotational viscosity and the birefringent index of the liquid crystal. In order to achieve a wide tuning range of phase modulation, the nematic liquid crystals often employ large π-electron conjugated systems and large polar groups to enhance the birefringence and dielectric anisotropy of the liquid crystal molecule, which, however, increases the viscosity of the liquid crystal material, deteriorating the response speed of the microwave device. Herein, we explore the viscosity of the nematic liquid crystal from the perspective of liquid crystal compound structure by testing the viscosity of our designed and synthesized forty-two different nematic liquid crystals by using a rotating rheometer at 25 ℃. To the best of our knowledge, the BPNN-QSAR quantitative structure-activity model between nematic liquid crystal molecular structure and viscosity is established for the first time. The correlation coefficient between the predicted value and the experimental value is <i>q</i><sup>2</sup> = 0.607 > 0.5, indicating that the model can be used to predict the viscosity performances of liquid crystal compounds. Besides, the molecular structure descriptors affecting the viscosity properties are explored. Based on the practical application and this model, seven liquid crystal molecules of two series with large birefringent index are designed and tested. The viscosity predicted by the BPNN model is smaller than that of the molecules of the same type and matches with the measured viscosity.

Cascaded optically injection-locked semiconductor laser, rate equations analysis, frequency response, and its application for complex optical signal generation.

In this paper, we introduce the concept of the cascaded optically injection-locked (OIL) semiconductor laser and present its novel rate equations. Then, the new locking range for this configuration has been obtained by mathematical demonstration. Subsequently, we modified a new adjustment for the detuning frequencies (Δ f i n j ) of the cascaded OIL system as well as the linewidth enhancement factors (α) values. Utilizing these tunings, improvements in the steady-state photon number and the phase modulation (PM) range become possible. Afterward, we define the generation of the complex optical signal area and extract the transfer function for investigating the frequency response of the cascaded system. The simulations have been performed once with identical α values and once with the various α values in the slave laser (SL) stages. We conclude that these novel proposed adjustments, combined with a strong injection ratio (Rinj) of 15 dB and a high bias current, can significantly broaden the bandwidth near 700 GHz while maintaining the fair gain available up to 180 GHz. Further, the generation of complex optical signal areas has been boosted for high-quality complex modulation applications. Eventually, we exhibit a novel approach for generating different α values in the SL stages by applying managed temperature variations in the experimental setup of the cascaded system, regardless of employing similar SLs.

Multi-color complex spatial light modulation with a single digital micromirror device.

Spatial light modulators enabling complex light field manipulation has opened up many opportunities in biomedical imaging, holographic display, and adaptive optics. However, traditional spatial light modulators do not allow multi-color operations simultaneously due to their physical constraints, while multi-color modulations are highly desirable in many applications. To overcome this limitation, we demonstrate a multi-color spatial complex light field modulation with a single binary hologram on digital micromirror devices (DMD). This method combines several neighboring micro-mirror pixels into a giant single superpixel, in which the light field's amplitude and phase can be individually determined by internal pixel combinations, and the dynamic range of phase modulation can exceed 2π for the single wavelength. As a result, this extra phase modulation range offers an additional degree of freedom for independent multi-wavelength light modulation. Based on this scheme, multi-color light modulations have been demonstrated in a 2D plane as well as in multiple 3D holographic planes. Moreover, a dual-colored Airy beam has been realized using the same technique. These results bring complex light modulation into a multi-color regime, paving the way for practical applications in information display, imaging, and optical trapping.

Metasurface Patch for Wireless Power Transfer in Implantable Devices

AbstractWireless power transfer (WPT) systems enable the long‐term operation and miniaturization of implantable devices by eliminating the need for battery replacement and wired power supplies. Although wireless power transfer systems for implantable devices are extensively studied, their practical application is still challenging owing to the constraints and requirements of the human body, such as reflection loss owing to differences in the tissue dielectric properties, mm‐sized devices, and electromagnetic (EM) wave attenuation of the tissue. Here, a phase‐gradient metasurface patch is presented to achieve 5.8 GHz EM power focusing at a focal point of depth 10 mm in the tissue via EM wavefront modulation at the skin–air interface. The proposed metasurface patch is fabricated by arranging subwavelength‐thickness (<λ/10) unit cell structures composed of four metallic layers separated by dielectric substrates that exhibit high‐Q resonance properties and a sufficient phase modulation range with enhanced transmission. By applying the fabricated metasurface patch to a wireless power transfer system for implantable devices, it is experimentally confirmed that the transmission coefficient (S21) is improved by 6.37 dB compared with that of a wireless power transfer system without the metasurface patch. Furthermore, it is confirmed that the transmission coefficient can be maintained for an incident angle variation up to 30° from the transmitter to the metasurface patch, resulting in a stable power delivery of the proposed wireless power transfer system.

Terahertz Metasurface Modulators Based on Photosensitive Silicon

AbstractMetasurfaces solve the lack of materials in the terahertz (THz) band and control precisely the amplitude, phase, polarization, and transmission characteristics of THz waves, providing an effective way to realize THz functional devices. This article focuses on the design of THz metasurface modulators with a unit structure consisting of metal square rings, including resonance frequency, phase, and amplitude modulators. By embedding photosensitive semiconductor silicon (Si) in the unit structure, the unit structure is built from meta‐atom to molecularization model under the optical pumping condition, and the resonance frequencies are switched between high and low frequencies. The resonance frequency switchable characteristic is demonstrated using the equivalent LC oscillation circuit model, and the theoretical calculation results agree well with the simulations. Through theoretical calculations, the modulators achieve ultrafast switching times of less than 0.141 ps by the optical pumps, which have significant advantages in ultrafast THz modulators. By continuing to change the embedded position of the silicon in the unit structure, not only is a wide range of THz phase modulation achieved, but also multilevel modulation of the phase is realized. It is found that there is a strong relationship between the modulation depth and phase variation of THz waves, and a reasonable analysis is given. Further the amplitude modulator with a larger modulation depth (MD) is developed, and when the conductivity of photosensitive semiconductor silicon (σSi) reaches 2.5 × 106 S m−1, return loss (RL) is ≈0 dB, and the maximum MD reaches ≈100%; in order to gain insight into the nature of modulation, the modulation mechanism of THz waves under optical pumping conditions is analyzed. In addition, graphene‐based THz metasurface amplitude modulators are designed. When the depth of amplitude modulation is achieved by bias voltage modulation of the Fermi energy level of graphene, the maximum modulation amplitude is 23.42 dB, with a minimal modulation accuracy of 0.05 THz eV−1. In the article, the designed modulators have extremely excellent modulation performance. It has great potential applications in silicon‐based THz photonic devices, ultrahigh frequency electronic devices, high sensitivity sensors, and high‐precision imaging.

Analysis of a vertically aligned liquid-crystal on silicon microdisplay for photonics applications

We present the characterization results for an analogically addressed vertically aligned liquid crystal on silicon microdisplay (VA-LCoS). We show that it covers more than 360º phase modulation range at 1550 nm, and that in the visible the range available enables the generation of multiorder DOEs. There are basically no studies dealing with the characterization of vertically aligned high resolution LCoS devices, since the ones typically found in the literature and commercially available correspond to the parallel aligned. We have also verified that the microdisplay used in this work is free from flicker, which is a very interesting feature for application where the phase stability is of the utmost importance. The results shown here represent a first step in the work underway oriented to the generation of programmable DOEs for telecommunication applications (C-band) and for the visible spectrum.

Full 2\u03c0 tunable phase modulation using avoided crossing of resonances

Active metasurfaces have been proposed as one attractive means of achieving high-resolution spatiotemporal control of optical wavefronts, having applications such as LIDAR and dynamic holography. However, achieving full, dynamic phase control has been elusive in metasurfaces. In this paper, we unveil an electrically tunable metasurface design strategy that operates near the avoided crossing of two resonances, one a spectrally narrow, over-coupled resonance and the other with a high resonance frequency tunability. This strategy displays an unprecedented upper limit of 4π range of dynamic phase modulation with no significant variations in optical amplitude, by enhancing the phase tunability through utilizing two coupled resonances. A proof-of-concept metasurface is justified analytically and verified numerically in an experimentally accessible platform using quasi-bound states in the continuum and graphene plasmon resonances, with results showing a 3π phase modulation capacity with a uniform reflection amplitude of ~0.65.

Arbitrary active control of the Pancharatnam-Berry phase in a terahertz metasurface.

Active phase-control metasurfaces show outstanding capability in the active manipulation of light propagation, while the previous active phase control methods have many constraints in the cost of simulation or the phase modulation range. In this paper, we design and demonstrate a phase controlled metastructure based on two circular split ring resonators (CSRRs) composed of silicon and Au with different widths, which can continuously achieve an arbitrary Pancharatnam-Berry (PB) phase between -π and π before or after active control. The PB phase of such a metasurface before active control is determined by the rotation angle of the Au-composed CSRR, while the PB phase after active control is determined by the rotation angle of the silicon-composed CSRR. And active control of the PB phase is realized by varying conductivity of silicon under an external optical pump. Based on this metastructure, active control of light deflection, metalens with arbitrary reconfigurable focal points and achromatic metalens under selective frequencies are designed and simulated. Moreover, the experimental results demonstrate that focal spots of metalens can be actively controlled by the optical pump, in accord with the simulated ones. Our metastructure implements a plethora of metasurfaces' active phase modulation and provides applications in active light manipulation.

Light-controlled metasurface with a controllable range of reflection phase modulation

The reconfigurable metasurface (MS) with tunable electromagnetic scattering properties has had an impact on innovative communication applications. However, the widely studied MS controlled by direct-current (DC) bias requires complex physical control circuits in array wavefront encoding, while the light-controlled MS does not. In this paper, we report a new approach for a light-controlled MS with a controllable modulation range of reflection phase, and its unit is composed of a reflection phase element based on the varactor and an optical interrogation network (OIN) based on the photoresistor so that the phase distribution of each MS unit could be independently programmed optically. To illustrate our approach, a 2-bit reflection phase MS with 10 × 10 units was fabricated and tested to achieve pencil beam scanning and orbital angular momentum beam at microwave frequencies. The simulated and measured results verify the feasibility of the proposed design. Furthermore, all the OINs on the MS are connected in parallel and powered by the same DC voltage source, which simplifies the difficulty of array expansion in MS design with a large-scale array.

Terahertz dynamic π-phase modulation with high transmittance using graphene-metal metamaterials

Dynamic phase modulation of electromagnetic waves is desirable in many applications such as beaming, tunable focusing, and holography. However, it remains challenging to achieve both large dynamic phase shift and high transmittance based on terahertz metamaterials. Here we propose a double-layered graphene-metal metamaterial for achieving transmissive terahertz phase modulator with large dynamic phase modulation range and meanwhile high transmittance. Numerical results show that a large dynamic transmission phase shift reaching up to 180∘ and relatively high transmittance above 22% can be achieved simultaneously by varying the graphene Fermi level from 0 to 1 eV. We attribute this striking performance to the transition of the resonance type when the graphene Fermi level is tuned across a threshold. We expect that the proposed dynamic phase modulator will find applications in the tunable lens, holographic imaging and phased array antenna in terahertz regime.

Terahertz dynamic π-phase modulation with high transmittance using graphene-metal metamaterials

Abstract Dynamic phase modulation of electromagnetic waves is desirable in many applications such as beaming, tunable focusing, and holography. However, it remains challenging to achieve both large dynamic phase shift and high transmittance based on terahertz metamaterials. Here we propose a double-layered graphene-metal metamaterial for achieving transmissive terahertz phase modulator with large dynamic phase modulation range and meanwhile high transmittance. Numerical results show that a large dynamic transmission phase shift reaching up to 180” and relatively high transmittance above 22% can be achieved simultaneously by varying the graphene Fermi level from 0 eV to 1 eV. We attribute this striking performance to the transition of the resonance type when the graphene Fermi level is tuned across a threshold. We expect that the proposed dynamic phase modulator will find applications in the tunable lens, holographic imaging and phased array antenna in terahertz regime.

Continuous modulation of geometric phase via electrically tuning the in-plane orientation of nematic liquid crystal

ABSTRACT Holographic display requires thinner liquid crystal cell for smaller pixels and fast response. In this paper, a continuous geometric phase modulation method realised by electrically tuning a dual in-plane-switching (IPS) LCD is proposed. Firstly, the function of geometric phase modulation and the driving voltage is obtained via numerical simulation. Secondly, a dual IPS liquid crystal cells is designed and fabricated, and by using it, the experimental data is measured. Lastly, the simulation and test results are matched, which show good consistence with each other, and a phase modulation range of near is achieved. By using the method, the phase modulation quantity is not only independent of the thickness of liquid crystal layer, but can be achieved almost 360° continuous modulation of the geometric phase. This method paves an avenue towards dynamic geometric phase modulation of light, which is meaningful for large field of view and colourful holographic display.

Single-layer phase gradient mmWave metasurface for incident angle independent focusing

Metasurfaces allow the rapid development of compact and flat electromagnetic devices owing to their capability in manipulating the wavefront of electromagnetic waves. Particularly, with respect to the metasurface lenses, wide operational bandwidth and wide incident angle behavior are critically required for practical applications. Herein, a single-layer phase gradient metasurface lens is presented to achieve millimeter-wave focusing at a focal point of 13 mm regardless of the incident angle. The proposed metasurface lens is fabricated by constructing subwavelength-thick (< λ/10) phase elements composed of two metallic layers separated by a single dielectric substrate that exhibits low-Q resonance properties and a wide phase modulation range with satisfactory transmissivity. By controlling the spatial phase distribution, the proposed metasurface lens successfully realises effective wavefront manipulation properties and high-performance electromagnetic-wave-focusing characteristics over a wide operating frequency range from 35 to 40 GHz with incident angle independency up to 30°.

Automatic Inverse Design of High-Performance Beam-Steering Metasurfaces via Genetic-type Tree Optimization.

We introduce a genetic-type tree search (GTTS) algorithm combined with unsupervised clustering for the automatic inverse design of high-performance metasurfaces. With the proposed method, we realize highly directive beam-steering metasurfaces via the cooptimization of the amplitude and phase. In comparison with previous topology optimization approaches, the developed GTTS algorithm optimizes the organization of subwavelength nanoantennas and, thus, is applicable to the design of both passive and active metasurfaces. The optimized beam-steering metasurface specifically exhibits a nearly constant directivity when the steering angle varies from 5° to 30°. Furthermore, the optimized nonintuitive reflectance and phase profiles assist in achieving highly directive beam steering when the phase modulation range is <180°, which was previously challenging. Our approach can diminish the requirements of scattering light properties with substantially enhanced angular resolution of beam-steering metasurfaces, which enables the realization of high-performance metasurfaces that will be promising for a wide range of advanced nanophotonic applications.

Multiple-order correlations and convolutions using a spatial light modulator with extended phase range

We present a new type of optical convolver / optical correlator capable of encoding higher orders. The technique is based on obtaining higher powers of a properly designed hologram. This can be obtained in two ways. First, we can recalculate higher orders of the amplitude and phase hologram in a standard spatial light modulator (SLM). More easily, we can utilize a phase-only hologram and exploit the properties of an SLM with extended phase modulation range. Experimental results agree with theory in both cases.

Design and Characteristics of an Optofluidic Phase Modulator Based on Dielectrowetting

In this paper, an optofluidic phase modulator based on dielectrowetting is designed and fabricated to adjust the optical phase. Two liquids are filled in the device, and a transparent sheet is employed at the liquid interface to keep the interface flat. When different voltages are applied to the modulator, the flat interface moves up and down, leading to the variation of the optical phase. A theoretical model is constructed to predict the optical phase shift quantitatively, and the phase regulation ability is also tested experimentally. Our modulator realizes continuous adjustment of the optical phase in a certain range by the operation of voltage adjustment. When the voltage is increased to 150 V, the optical phase modulation range of our proposed modulator can reach 9.366 π.

Array-Level Inverse Design of Beam Steering Active Metasurfaces.

We report an array-level inverse design approach to optimize the beam steering performance of active metasurfaces, thus overcoming the limitations posed by nonideal metasurface phase and amplitude tuning. In contrast to device-level topology optimization of passive metasurfaces, the outlined system-level optimization framework relies on the electrical tunability of geometrically identical nanoantennas, enabling the design of active antenna arrays with variable spatial phase and amplitude profiles. Based on this method, we demonstrate high-directivity, continuous beam steering up to 70° for phased arrays with realistic tunable antenna designs, despite nonidealities such as strong covariation of scattered light amplitude with phase. Nonintuitive array phase and amplitude profiles further facilitate beam steering with a phase modulation range as low as 180°. Furthermore, we use the device geometries presented in this work for experimental validation of the system-level inverse design approach of active beam steering metasurfaces. The proposed method offers a framework to optimize nanophotonic structures at the array level that is potentially applicable to a wide variety of objective functions and actively tunable metasurface antenna array platforms.