Thermoplasmonic Activated Reverse-Mode Liquid Crystal Gratings

Abstract

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 μm by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the “grating” is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the plasmonic absorption of the GNRs. This heating induces a refractive index mismatch between the PLC and the NLC as the latter undergoes a phase transition from nematic to isotropic, finally resulting in a transmission diffractive structure activated over a few seconds exhibiting an efficiency of about 70%. The same HLCPDLC samples have also been tested as thermosensitive waveplates, enabling a new, fast methodology for quantifying the photoinduced plasmonic heating with a thermal sensitivity of ∼0.04 °C. Moreover, thermoplasmonic driven waveplates represent a new avenue in the field of light controllable optical phase modulators.

Thermoplasmonics with Gold Nanoparticles: A New Weapon in Modern Optics and Biomedicine

“…[ 64 ] In a different approach, we used the thermoplasmonic heating of AuNRs to activate and deactivate the diffractive properties of a reverse‐mode diffraction grating (Figure 5e) made of NLC and a polymerizable liquid crystal (PLC). [ 65,66 ] The grating is completely “hidden” and appears as a transparent optical window in the off state because of the initial refractive‐index match between the PLC‐rich and NLC‐rich regions. Figure 5f shows the temporal behavior of the first‐order diffraction efficiency acquired for different values of the pump laser's intensity (from 87 to 500 mW cm −2 ) while the probe laser intensity is kept constant.…”

“…Photo‐exciting the AuNR‐containing structure with a resonant laser beam (λ = 808 nm) causes thermoplasmonic heating, there is a change in the refractive index between the PLC and NLC, which results in the formation of a highly efficient diffractive structure. In addition, we showed [ 65 ] that the same structure can be used to realize a variable waveplate, thus learning an innovative methodology for measuring the photo‐induced temperature changes and controlling a laser source's polarization state. This new class of optical components does not require optical power in the diffraction‐off state, thanks to the reverse‐mode working principle.…”

Contactless Pulsed and Continuous Microdroplet Release Using Photothermal Liquid Crystals

“…LCs were previously hybridized with fluorescent dyes, [ 27–29 ] azobenzene molecules, [ 30–32 ] gold nanoparticles, [ 33,34 ] and magnetic nanoparticles [ 35,36 ] to design platforms responsive against noncontact electromagnetic sources including ultraviolet light, near infrared (NIR) light, or magnetic field. NIR light presents particular advantages that include localized exposure, high penetration depth, and low power intensity, which hold significant importance in biomedical fields.…”

Orthogonal Reconstruction of Upconversion and Holographic Images for Anticounterfeiting Based on Energy Transfer

“…As a rational alternative, holographic images reconstructed through photopolymerization-induced phase separation show remarkable advantages for anticounterfeiting, − since both amplitude and phase information can be stored. Not only naked-eye recognizable three-dimensional (3D) images can be achieved − but also more functions like photoluminescence and electro-optic response can be added, which can technically prevent illegal copying. ,− For instance, Goldenberg and Tomita have achieved fluorescent fringes within holographic images by introducing Ce 3+ and Tb 3+ doped LaPO 4 nanoparticles or CdSe quantum dots, respectively. Sutherland, Peng, Xie, and others have fabricated holographic images based on liquid crystals (LCs), ,− where the LC is not only able to provide a high refractive index difference with the polymer-rich phases and consequently generate a high diffraction efficiency but is also capable of affording added electro-optic response and thermal sensitivity. ,, Quite recently, Peng and co-workers have also enabled orthogonal storage of holographic and fluorescent images within the same polymer film .…”