Friday 12th August 2022

Holographic metasurface gas sensors for instantaneous visual alarms


Design of gas-responsive LC cells

The molecular ordering of LCs, an anisotropic medium with long-range molecular ordering, has been proven to be controllable through a variety of external stimuli. The preferred orientation of LCs is described by the director , and the degree of orientation is expressed by the order parameter S=〈12(3 cos2θ−1)〉, where θ is the angle between and the molecular axis (33, 34). S is temperature dependent and decreases as the temperature increases, inducing a more randomly ordered state. In a perfectly ordered crystal, S = 1 as the molecular axes are aligned parallel to , while S = 0 for an isotropic liquid. Generally, in an LC phase, S is in the range of 0.2 ≤ S ≤ 0.8, with the nematic phase in the range of 0.5 ≤ S ≤ 0.7 (35). The phase retardation of light passing through the LC medium is modulated by the LC ordering. The reordering of LCs changes the effective refractive index (∆neff) in the LC cell as Δneff=(none/ne2cos2α+no2sin2α)−no, where ne and no are the extraordinary and ordinary refractive index of the LC, respectively, and α represents the angle between and the substrate normal (α = 0° indicates vertical orientation). In general, the phase retardation τ can be expressed as τ=∫0l2πΔneff(z)/λdz, where l is the thickness of the LC cell and λ is the wavelength of light. Consequently, the desired output beam polarization state can be adjusted by τ of the LC cells (3337).

To realize the volatile gas sensing LC systems that transform the polarization of transmitted light, we first observe and characterize the gas responsiveness of the LCs in the simplest geometry, where a microwell structure (thickness of about 20 μm) is filled with nematic LCs, i.e., 4-cyano-4′-pentylbiphenyl (5CB) as described in Fig. 1B. The bottom glass substrate was coated with a polyimide and then rubbed to obtain a unidirectional tangential orientation with a pretilted angle of 3.8°. As the air interface causes the LCs to be orientated vertically (69), the LC cell has the hybrid anchoring configuration (Fig. 1B) exhibiting a bright optical texture between crossed polarizers with the rubbing direction of 45° from the optic axes of polarizers (Fig. 1C). The initial hybrid configuration is designed to transmit right circularly polarized (RCP) light for incident RCP light by setting τ ≈ 2πN, where N is integer. Using a Bereck compensator, the retardance of our cell is measured to be 1637.52 nm corresponding to 6π for λ = 550 nm at 25°C (a more detailed explanation can be found in Materials and Methods) (33, 36).

As a target hazardous gas for detection, we use an isopropyl alcohol (IPA) gas because it is typically used as a cleaning solution in the industry (38). The toxicity of IPA is well known to cause stomach pain, confusion, dizziness, and slowed breathing. In the semiconductor industry, IPA is suspected to be one of the main causes of leukemia (39). The current permissible exposure limit (PEL) of IPA is 400 parts per million (ppm) by the Occupational Safety and Health Administration of the United States.

Upon the exposure of IPA gas at the constant concentration (CIPA) of 200 ppm in a closed chamber, we observe the LC cell to exhibit the transition from white (Fig. 1C) to colored (Fig. 1, D to F) within a few seconds and end up with black optical appearances 140 s after the exposure of IPA gas (Fig. 1, E and F, and movie S1). It indicates that the IPA gas molecules had diffused into the LCs, thus lowering the LC ordering. Consequently, the nematic to isotropic phase transition occurs from the air interface and the resulting isotropic layer expands toward the glass interface, as shown in Fig. 1 (C to E). These results demonstrate the capability of the LC cell to promptly sense a toxic gas and convert the polarization of the transmitted light, as evidenced by the measure of τ to gradually decrease over exposure time (Fig. 1F). The thickness of the induced isotropic layer is also extracted from the measure of τ, which is in a good agreement with the optical transition (Fig. 1, C to E); the director of 5CB is tilted to about 26.5° at the nematic-isotropic interface (4042). We make two additional observations to investigate the optical behavior of 5CB film with respect to the type and dosage of gas. First, the response rate of the LC cell is dependent on CIPA. While the entire 5CB film becomes an isotropic phase at ~140 s for CIPA = 200 ppm, the phase transition ends at shorter time periods for higher CIPA of 400 ppm (within 40 s; movie S2). The CIPA-dependent optical response in LCs is responsible for the fact that the diffusion of more gas molecules into the LCs lowers their molecular ordering faster. We note that the polarization conversion of transmitted light from RCP to left circularly polarization (LCP) (i.e., when ∆τ = λ/2 for λ = 633 nm) for the switching of holographic image occurs at ~18 s for CIPA ≈ 200 ppm (Fig. 1, C to F) and at ~7 s for CIPA ≈ 400 ppm (movie S2). Second, the optical response of LCs is also dependent on the type of gas exposed, as each gas molecule has different diffusion coefficients and intermolecular interactions with LCs. To demonstrate the response rates of LCs depending on the type and dosage of gas, we conduct experiments with seven different gases generated by evaporating solvents in an ambient condition [IPA, acetone, toluene, chloroform, dimethylformamide (DMF), methanol, and para-xylene] with different dose conditions (see the Supplementary Materials and table S1). According to the type and dosage of gas, we confirm the 5CB film (20 μm thick) to exhibit different rates of RCP-to-LCP conversion. Under a low-dose condition (one gas source), for instance, we measure the conversion to occur at around 1.3 s (chloroform), 1.6 s (acetone), 5.2 s (toluene), 13.9 s (IPA), 22.7 s (para-xylene), 58.3 s (methanol), and 136 s (DMF) (table S1). For higher dosage, faster response rates are observed.

Design of spin-encoded metaholograms with asymmetric spin-orbit interaction

The spin-encoded metasurface is designed on the basis of conventional Pancharatnam-Berry (PB)–phase modulation technique to exploit the inherent symmetry of spin and degrees of orbital interactions (43, 44). This demands two different unit cells for the metasurface to be created in separate spaces, resulting in a total efficiency loss. The optical energy utilization becomes inefficient because only 50% of the unit cell structures of the metadevice are useful for a specific helicity of incident illumination. As a result, the maximal theoretical total efficiency of a conventional PB-phase–based metasurface device is only 50%. To overcome the optical energy loss, we designed the metasurface with spin encoding via the asymmetric coupling between the orbit degrees of photons and their spin rather than symmetric coupling. This allows all unit cell structures of the metadevice to operate for both LCP and RCP light, which, in turn, helps to break the conventional efficiency limit. The asymmetric coupling can be stimulated by defying the innate behavior of PB-phase metaatoms, i.e., breaking the symmetry of the geometric phase reversal using a specifically designed array of metaatoms for both LCP and RCP illumination. Therefore, an extra phase delay that is independent of the geometrical parameters is required (i.e., the rotation angle of the metaatom). Here, we use a retardation phase delay (also known as propagation phase) for certain metaatoms to achieve that. Thus, the total accumulated phase delay of the scattered electromagnetic light can be written as the linear combination of the retardation phase delay ρ(x, y) and the PB-phase delay 2σ.ʒ(x, y), i.e., ρ(x, y) + 2σ.ʒ(x, y), where ʒ(x, y) is the orientation angle of the deployed metaatom and the helicity of the light defines the value of σ to be −1 and +1 for LCP and RCP, respectively. Note that the first term of the total phase delay is helicity independent. The careful optimization of individual metaatoms not only ensures the full independent control of the phase delay but also promises the generation of noncentral symmetric and distinct wavefronts, which maximize the optical energy usage of the metasurface designs. The concept of asymmetric coupling is achieved by optimizing a set of eight unit cells consisting of distinct nanoantennas made of a-Si:H on top of a glass substrate. The variables that…


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