Exploration of responsive photonic arrays fabricated by two photon lithography

. Responsive hydrogel photonic structures were fabricated using two-photon lithography. The design versatility of two-photon lithography provides for unprecedented manipulation of transmittance and reflectance spectra, producing distinct structural color. Hydrogel photonic structures have potential for wide range applications, in this instance we demonstrate examples of color transformation, reversible vapor sensing, and pH detection.


Introduction
People have never stopped pursuing the design and manufacture of smaller, faster, stronger and more environmentally friendly optoelectronic components.Preparation technology and material innovation drive developments within the sensor industry.Compared with non-renewable rare metals or alloys, responsive hydrogels are characterized by energy saving, ready availability, and high response sensitivity, making them attractive candidates for sensing applications.In recent decades, the advancement and development of two-photon polymerization technology has made the preparation of complex micro-nano photonic structures a topic of intensive research.Submicron polymeric photonic structures have shown great advantages in sensing, anticounterfeiting, information encryption, optical steganography, and micro-nano optical device fabrication [1][2][3][4][5].
We present several photonic array designs based on responsive hydrogels, and demonstrate their novel applications, such as vapor sensing, information encryption, and pH detection.FDTD simulations were used as design and analysis tools, and fast two-photon lithography (TPL) was used to fabricate complex photonic arrays.

Photonic structure design 2.1 2D photonic structures
A grating structure was designed, as shown in Fig. 1a.The design linewidth, d. is 1 μm and the gap (a) between the lines is fixed at 2 μm.The structure height (h) was varied from 1.5 μm to 4.0 μm.TPL combined with the commercial resin IPL-780 was used to fabricate the grating arrays and the observed structural colors are shown in Fig. 1b.From Fig. 1c, red shift of the zero order transmission peaks with increasing structure height is observed.The dashed lines in Fig. 1c, corresponding to the peak wavelengths, are calculated using: where n and na are the refractive indices of the polymer and air gap, respectively, h and ha are the height of structure and air gap, respectively, and m is an integer.
A distinct color transformation can be achieved by soaking samples in IPA solvent (nIPA = 1.3772), as shown in Fig. 1b.The transmission spectra are presented in Fig. 1d.The change in refractive index in the gap results in transmitted color transformation.A FDTD solver simulates the reflectance spectra of this 3D photonic structure for cube array height, hc, and polymer film thickness, hf, varying from 0.3 μm to 0.5 μm.A clear red shift of the reflectance peak can be observed in Fig. 2c.The corresponding structural colors are shown on the CIE plot in Fig. 2d.Compared with a 2D array, 3D structures offer advantages in manipulating and generating reflective structural color.The spectral intensity characteristic is similar to that of a Bragg reflector.The greater the number of pairs in the z direction, the higher the reflected intensity.

Novel applications of PhCs
We demonstrate several novel applications of submicron 2D photonic arrays based on novel responsive hydrogels, such as vapor sensing, information encryption, and pH detection [1][2][3].
Fig. 3a shows color change of vapor responsive hydrogel grids exposed to water vapor.Fig. 3b demonstrates an 11 lines spiral structure operating as a vapor sensor.The sensitivity is highly dependent on the solvent polarity: water > ethanol > isopropanol > acetone.The modulation of the transmittance increases with higher flowrate corresponding to a higher vapor concentration.In this demonstration the spectral redshift arises due to the structure expansion in the presence of the vapors.The results clearly indicate the enormous potential of the 3D hydrogel photonic arrays for vapor sensing [1][2][3].
Fig. 3c gives an example of 2D gratings acting as solvent pH detectors.A clear color change is observed in acid (pH~2.3)and in base (pH~11) solutions, with a spectral blue shift with increasing pH, see Fig. 3d.Structural expansion contributes to the color modification.
AFM measurements confirmed the hydrogel swells by 10.5% and 46% in the xy direction and 5.7% and 13.2% in the z direction in acid and base solutions, respectively.

Conclusion
FDTD simulations were used to design 2D and 3D photonic arrays for generation and manipulation of structural color.The combination of responsive hydrogel and TPL greatly enriches the design possibilities and application potential.We have successfully demonstrated the submicron responsive arrays for vapor sensing and pH detection.
This work was supported by the Chinese Science Council and SFI award 21/FFP-P/10187 and 17/RC-PhD/348.