Tunable Optical Gratings Research Articles

Dielectric Elastomer Actuator-Based Multifunctional Smart Window for Transparency Tuning and Noise Absorption

Soft actuators are compliant material-based devices capable of producing large deformation upon external stimuli. Dielectric elastomer actuators (DEA) are a type of soft actuator that operates on voltage stimuli. Apart from soft robotics, these actuators can serve many novel applications, for example, tunable optical gratings, lenses, diffusers, smart windows and so on. This article presents our current work on tunable smart windows which can regulate the light transmittance and the sound absorption. This smart window can promote daylighting while maintaining privacy by electrically switching between transparent and opaque. As a tunable optical surface scatters, it turns transparent with smooth surfaces like a flat glass; but it turns ‘opaque’ (translucent) with the micro-rough surface. The surface roughness is varied employing surface micro-wrinkling or unfolding using dielectric elastomer actuation. Moreover, this smart window is equipped with another layer of transparent micro-perforated dielectric elastomer actuator (DEA), which acts like Helmholtz resonators serving as a tunable and broader sound absorber. It can electrically tune its absorption spectrum to match the noise frequency for maximum acoustic absorption. The membrane tension and perforation size are tuned using DEA activation to tune its acoustic resonant frequency. Such a novel smart window can be made as cheap as glass due to its simple all-solid-state construction. In future, they might be used in smart green buildings and could potentially enhance urban livability.

(Invited) Tunable Elastomeric Optical Gratings Based on Triboelectric Nanogenerators

For people in life, the existence of triboelectrification usually is a kind of negative effect, especially in winter. However, the team of professor Zhong Lin Wang of Georgia Institute of Technology in the United states invented Triboelectric nanogenerator (TENG) to fully utilize this kind of effect. TENG can convert mechanical energy in the environment into electrical energy, which based on the principle of triboelectrification and charge induction, such as waves, sounds, muscle contractions, mechanical vibrations, etc. The charge separation between the two layers of polymer films will lead to the formation of potential difference. In addition, not only the fabrication process of TENG is simple and low cost, it also can be used in large-scale production. At the same time, it has excellent durability and plasticity, so TENG can be easily combined with other devices. Therefore, TENG and its applications are an important research direction of nanoenergy. Here we have designed TENG for different applications. Listed below is two applications in electrostatic manipulations based on TENG: 1. Integrate a TENG with electrostatic actuation system (EAS) to manipulate the movement of micro-droplets and fine solids. The mechanical triggering of the TENG produces a very high electrostatic field inside the electrostatic actuator, producing a Coulomb force that drives tiny objects (liquid or solid) in the electrostatic actuator. Therefore, the tribomotion of TENG can be used as a driving source and control signals of electrostatic actuators.2. Based on dielectric elastomer actuator (DEA), tunable optical gratings (TOGs) and TENG combined so that the grating period can be controlled by dielectric elastomer actuator. Therefore, a self-powered TOGs system can be implemented, with TENG as the power and control module for its system. In combination with unique output performance of the TENG, three different TOGs systems were designed and used to compress or expand the spacing of the grating array by driving the dielectric elastomer actuator.

Wrinkling of silicon nanoribbons on shape memory polymers

Shape memory polymers (SMPs) have recently emerged as the substrate for surface wrinkling with potential applications in smart adhesion and tunable optical gratings. In this paper, wrinkling of single-crystal silicon nanoribbons on epoxy SMP substrate is investigated experimentally and theoretically. With the unique shape fixing capability of SMPs, the well-defined wavy profile of silicon nanoribbons is obtained. A simple theoretical mechanics model, which bypasses the complex thermomechanical properties of SMPs, based on the existing wrinkling mechanics is proposed to explain the underlying physics behind the wrinkled silicon nanoribbons. Theoretical predictions on wavy profile agree very well with experiments. It is found that the wavy profile is independent of the recovery temperature, which is the high temperature TH above the transition temperature Tg. The material properties of SMPs at the transition temperature play a key role in determining the wavy profile. These results provide a scientific basis to design SMP-supported surface morphologies with important applications in smart surface engineering.

Tunable Optical Grating Based on the Flexoelectric Effect in a Bent-Core Nematic Liquid Crystal

Liquid-crystal display technology is ubiquitous, but these materials can also be used to make electrically tunable components for optics and photonics. The authors demonstrate voltage control of the wavelength of flexoelectric domains in a transmissive diffraction grating, which enables dynamic light steering. They prove that the mechanism of pattern onset differs from that of switching between flexodomain states, and they explain the surprising finding that the response to increasing voltage is much slower than that to decreasing voltage.

Self-powered modulation of elastomeric optical grating by using triboelectric nanogenerator

Tunable optical gratings (TOGs) based on dielectric elastomer actuator (DEA) are combined with triboelectric nanogenerator (TENG), where the actuated strain of DEA under the drive of TENG can regulate the grating period. Hence, a self-powered TOG system can be achieved and TENG can serve as both power supplier and control module for this system. Three different TOG structures have been delicately designed to cooperate with the unique output performance of TENG and the actuated strain of DEA can be utilized to either compress or expand the spacing between grating array. The separation motion of the TENG with a contact surface of 120cm2 can induce a decrease of 16.5% or an increase of 9.4% for the grating period. Both one-dimensional and two-dimensional grating matrixes are designed for the TOG system, while TENG can control two kinds of grating systems to realize several functionalized modulations. The TOG systems based on TENG-DEA conjunction can achieve a fast-speed, highly efficient and multifunctional modulation of the grating period, while the breakdown problem of these TOG devices is largely suppressed since the maximum transferred charges from TENG device is under a limited value. Therefore, the demonstrated self-powered TOG may have tremendous application prospects for various display system, optical sensors, optical communications and so on.

Fabrication of transmissive dielectric elastomer actuator driven tunable optical gratings with improved tunability

We present an electrically actuated tunable optical grating based on dielectric elastomer actuators. A simple fabrication protocol is presented, which integrates the grating with the actuating mechanism both made of soft elastomers, improving the tunability of the grating. The device is designed to be operated in the transmission mode. It exhibits a continuous period tunability of 34.4% at an actuation voltage of 5.5 kV, which is an improvement over reported tunable optical gratings.

Micro-strain sensing using wrinkled stiff thin films on soft substrates as tunable optical grating

We report a strain sensing approach that utilizes wrinkled patterns on poly (dimethylsiloxane) (PDMS) as an optical grating to measure thermally-induced strain of different materials. The mechanism for the strain sensing and the effect of PDMS grating on strain sensing are discussed. By bonding the PDMS grating onto a copper or silicon substrate, the coefficient of thermal expansion (CTE) of the substrates can be deduced by measuring the diffraction angle change due to the change in PDMS grating periodicity when thermal strain is introduced. The measured CTEs agree well with the known reference values.

Novel optical devices based on the tunable refractive index of magnetic fluid and their characteristics

As a new type of functional material, magnetic fluid (MF) is a stable colloid of magnetic nanoparticles, dressed with surfactant and dispersed in the carrier liquid uniformly. The MF has many unique optical properties, and the most important one is its tunable refractive index property. This paper summarizes the properties of the MF refractive index and the related optical devices. The refractive index can be easily controlled by external magnetic field, temperature, and so on. But the tunable refractive index of MF has a relaxation effect. As a result, the response time is more than milliseconds and the MF is only suitable for low speed environment. Compared with the traditional optical devices, the magnetic fluid based optical devices have the tuning ability. Compared with the tunable optical devices (the electro-optic devices (LiNbO3) of more than 10GHz modulation speed, acoustic–optic devices (Ge) of more than 20MHz modulation speed), the speed of the magnetic fluid based optical devices is low. Now there are many applications of magnetic fluid based on the refractive index in the field of optical information communication and sensing technology, such as tunable beam splitter, optical-fiber modulator, tunable optical gratings, tunable optical filter, optical logic device, tunable interferometer, and electromagnetic sensor. With the development of the research and application of magnetic fluid,a new method, structure and material to improve the response time can be found, which will play an important role in the fields of optical information communication and sensing technology.

Tunable optical gratings based on buckled nanoscale thin films on transparent elastomeric substrates

This letter reports a tunable optical grating based on buckled thin film with periodic sinusoidal patterns on a transparent elastomeric substrate. Submicron scale sinusoidal gratings have been fabricated with nanometer thick Gold/Palladium film coated on 30% pretensioned polydimethylsiloxane substrates. Due to competition between the soft elastomeric substrates and relatively stiff films, periodic wavy profiles are created upon releasing the pretension. The buckling profiles can be easily tuned by mechanically stretching or compressing. Optical transmittance diffraction testing has been conducted, and 85 nm peak wavelength shifts of the first order diffraction have been achieved by stretching the grating up to 30% of its original length.

Temperature tunable optical gratings in nematic elastomer

We have investigated the behaviour of temperature dependent periodic-index gratings fabricated in a nematic elastomer. The gratings have been obtained by photopolymerisation under a microscopy apparatus. Contraction properties, as well as diffraction properties, have been studied as a function of temperature. Unidirectional contraction has been demonstrated by means of circular figure deformation and the polarisation dependency of the diffraction by the gratings has been determined.