Micro/nanoscale photonic barcodes have attracted extensive attention because of their potential applications in anti-counterfeiting^[1], information security^[2,3], personnel tracking^[4], etc. In this regard, various barcode-based microstructures with optical signals have been developed in both encoding and detection applications, such as photonic crystals^[5], robust lanthanide luminescence^[6,7], multiblock Ln-MOF microrods^[8–11], and DNA nanoarchitectures^[12,13]. Unfortunately, the broad photoluminescence (PL) band of different coloring materials is prone to be overlapped, resulting in limited coding capacity.

Whispering gallery mode (WGM) microcavities with their narrow linewidth and brightness, where the PL spectrum has a series of sharp peaks originating from the WGM modulation^[14,15], exhibit huge potential in miniaturized photonic barcodes^[16,17]. Moreover, their compact size, complicated structure, and unique optical properties allow them to be used for advanced identification and make them difficult to tamper with. Researchers are exploring micro/nanolaser-based barcodes with different laser-based micro-structures^[18–21]. However, the tunability of lasers is limited, restricting encoding capacity and a high security level.

Herein, we demonstrate a facile strategy to achieve polymer dye-doped microspheres with switchable WGM signals through photomerization manipulation based on an excited-state intramolecular proton transfer (ESIPT) process^[22–25]. The WGM microspheres are composed of highly polarized organic intramolecular charge-transfer (ICT) dye molecules. The active materials have two cooperative gain states including trans-excited state and cis-excited state. The WGM spectra can be switchable by light manipulation. Moreover, the covert photonic barcodes have further been obtained through an ESIPT energy-level process between the trans-excited state and cis-excited state^[26–28]. Furthermore, the fingerprint of the corresponding microsphere can be modulated through tuning the size of the WGM microspheres. The well-established photomerization manipulation WGM lasing enables the development of high-security covert photonic barcodes, which improve the security level of the information anticounterfeiting labels.

The design principle for the realization of WGM microsphere high-security covert photonic barcodes based on photoisomerization manipulation is schematically displayed in Fig. 1. The dye-doped microspheres with perfect boundaries and smooth surfaces could be applied as WGM resonators^[29–31]. When the microsphere doping with dye materials is pumped locally under a laser beam, it would emit the strong PL spectra with a series of unique and distinguishable peaks. Therefore, the photonic barcodes could be obtained by adjusting the parameters of the microcavities. Under light-manipulated stimulus, the switchable covert photonic barcodes can be obtained via photoisomerization manipulation as shown in Fig. 1 (top), which endows the designed photonic barcodes with extra security features.

For achieving high-security covert photonic barcodes, the WGM microcavity is built by highly polarized organic intramolecular charge transfer dye molecules. With a typical active material, Disodium 4, 4’-Bis (2-sulfonatostyryl)biphenyl (S420), the ICT dye molecules can be transferred from trans-S420 state to cis-S420 state under irradiation, owing to the principle of photoisomerization manipulation, which enables the cultivate of a dynamically switchable wide wavelength range lasing device. Therefore, the lasing emission spectra can be switched from short wavelength (trans-S420) to long wavelength (cis-S420) in the same microcavity, as shown in Fig. 1 (middle). The two cooperative gain states of ICT dye molecules can be switched by light manipulation, and the WGM lasing emission can be reversibly switched under photoisomerization activation. The samples were encapsulated in a PDMS solution, which ensures the stable output of laser signals. The position and width of photonic barcodes correspond to the mode peak and intensity of the WGM spectra, as shown in Fig. 1 (bottom).

Thereby, the covert photonic barcodes can be obtained by directional selection. When a dye-doped microsphere was pumped by a pulsed laser beam (343 nm, 200 Hz, ∼1ns) with a microphotoluminescence system, the WGM lasing action could be obtained. Figure 2(a) shows the schematic of the WGM microcavity. We simulate the electric field intensity distribution in the transverse cross-section using the commercial software COMSOL Multiphysics as shown in Fig. 2(b), which indicates that the microsphere would serve as an excellent WGM feedback resonant microcavity. A bright ring-shaped pattern was observed at the outer boundary of the microsphere as displayed in Fig. 2(c). Their size can vary flexibly depending on the experimental conditions (in Fig. S1 in the Supplementary Material). These results indicated that the fabricated dye-doped microspheres would support strong optical confinement.

As shown in Fig. 2(d), the lasing profiles were obtained with different pump densities. The theoretical calculation demonstrates that the peak wavelength belongs to the first-order transverse magnetic (TM) modes and transverse electric (TE) modes^[32], and the corresponding mode count is from 389 to 392 (in Fig. S2 in the Supplementary Material). Furthermore, the relationship between output intensity and full width at half-maximum (FWHM) under different pump fluences is shown in Fig. 2(e). The threshold behavior of the WGM microsphere with 41.3μJ/cm2 was demonstrated. The FWHM of WGM lasing was less than 0.1 nm.

To create a diverse range of photonic barcodes, it is crucial to understand the correlation between the WGM signals and the dimensions of WGM microspheres. As shown in Figs. 3(a)–3(c), it provides the PL images of microspheres with distinct diameters. The WGM modulation spectra and corresponding photonic barcodes of three microspheres are as shown in Figs. 3(d)–3(f). The free spectrum range (FSR) is 1.79, 1.03, and 0.84 nm, respectively. The relationship between FSR and the diameter of microspheres was demonstrated as shown in Fig. S3 in the Supplementary Material, indicating that the WGM lasing spectrum fingerprint would be modulated through tuning the dimensions of microspheres. The Q factor can be tuned by changing the size of microspheres^[33,34], and Q factors are over 10000 as shown in Fig. S4 in the Supplementary Material. The results demonstrate that photonic barcodes have high-resolution signal identification, improving the overall information anticounterfeiting security level.

The dye-doped WGM microspheres are composed of highly polarized organic ICT dye molecules, which have two cooperative gain states including trans-state (E-state) and cis-state (Z-state) as shown in Fig. 4(a). The output WGM lasing spectra can be switched via photomerization manipulation. The trans-state needs to cross a very high energy barrier to reach the excited intermediate state (at a twist angle of 90°) as shown in Fig. 4(b). However, the energy barrier for cis-state in the excited state (S1) to reach the excited intermediate state is greatly reduced (twist angle of 90°). Moreover, the process from the excited enol state to the excited ketone state is achieved by photoisomerization manipulation^[35]. The covert photonic barcodes have further been obtained through an ESIPT energy-level process between E-state and Z-state by light manipulation as shown in Figs. 3(c)–3(e), which improves the security level of information anticounterfeiting labels. The WGM lasing spectra and corresponding photonic barcodes are displayed in Figs. 3(f)–3(h). With light manipulation, the switchable WGM emission spectra in different molecular excited states including E-state, E + Z-state, and Z-state are presented as shown in Fig. S5 in the Supplementary Material. Consequently, based on the above encoding rule, we can acquire another photonic barcode that corresponds to the PL spectrum after photoisomerization manipulation, different from the initial one. The excited states would be reversible. The results indicated that dye-doped WGM microspheres provide a platform to achieve the reversibly covert photonic barcodes through photoisomerization manipulation.

Responsive mode-dependent covert photonic barcodes based on dye-doped microspheres offer a chance to explore their application for information anticounterfeiting. As shown in Fig. 5, the PL spectra of both E-state and Z-state from photoisomerization manipulation microspheres are used to explore the covert coding strategy. We can obtain the WGM singles (Barcode-1) when the microcavity is pumped. Upon heating, we can obtain another WGM single (Barcode-2). In order to increase the anticounterfeiting safety, the real barcode (Barcode-3) is designed by directional selection from Barcode-1 and Barcode-2. The bank card with the security tag to be tested enters circulation. If the resulting Barcode-3 matches the one stored in the cloud, it confirms the authenticity of the bank card. Conversely, any discrepancies indicate that the card is counterfeit.

In summary, we realize the photoisomerization manipulation of high-security covert photonic barcodes in dye-doped WGM microspheres. The WGM microcavities are composed of highly polarized organic intramolecular charge-transfer dye molecules with two cooperative gain states. The PL spectra constitute the fingerprint, which is dependent on the diameter of the microsphere. Moreover, the covert photonic barcodes have further been obtained through photoisomerization manipulation between trans-excited state and cis-excited state, which improves the information anticounterfeiting label’s security level. The composite microspheres might offer a promising route for data recording and information security.