Rikkyo University

May 22, 2020PRESS RELEASE

Development of an unreplicable micro-optical authentication device
- Every pixel with a different spectral fingerprint -

Keyword:RESEARCH

OBJECTIVE.

A group of researchers that included Prof. Yohei Yamamoto of the University of Tsukuba Faculty of Pure and Applied Sciences, Dr. Daichi Okada of the University of Tsukuba Graduate School of Science and Technology (now a researcher at RIKEN), Prof. Masakazu Morimoto of the Rikkyo University College of Science, Prof. Masahiro Irie, guest researcher and deputy director of the Rikkyo University Research Center for Smart Molecules, and Drs. Tadaaki Nagao, Takeo Minari, and Satoshi Ishii, who are group leader, group leader, and senior researcher, respectively at the National Institute for Materials Science, teamed up with scientists at the Leipnitz Institute of Photonic Technology in Germany to develop an unreplicable micro-optical authentication device.

Key points of the study

1. Using molecules that can switch their photoluminescence (PL) characteristics with photoirradiation, the researchers constructed an optical authentication device that uses a 2-step verification system consisting of 2-dimensional data (QR code) and spectral information that is essentially unreplicable.

2. Each microsphere created with these molecules exhibits a unique PL spectrum that can be switched on and off by irradiation with ultraviolet and visible light.

3. These molecules easily self-organize into microspheres on a substrate. Therefore, by optimizing the conditions it is possible to create arrays with various structures.
Diarylethene*1) is a photoresponsive molecule whose PL can be turned on and off by irradiation with ultraviolet and visible light. In this study, the group produced microsphere optical resonators*3) from self-organizing diarylethene molecules*2) and achieved the switching of whispering gallery mode (WGM) resonant photoluminescence*4) by confining photons inside the spheres. Microhemispherical arrays*5) produced with these molecules possess a memory function in which information can be written (PL on) or erased (PL off) using ultraviolet and visible light, and each pixel*6) has a unique spectra (spectral fingerprint) from WGM. With these features, 2-step authentication can be performed on each pixel by (1) verifying micro-sized 2-dimensional data (QR code*7)) and (2) authentication of the spectra, proving this to be an optical authentication device that is essentially impossible to counterfeit or replicate. High-density assemblies of these PL pixels could be used as a writeable optical memory.

Figure 1. This was used for the back cover image of the journal "Materials Horizons."

This study's findings were published online and in the May 6, 2020, issue of the Royal Chemistry Society's journal "Materials Horizons." An image from the study was also used on the back cover of the issue (Figure 1).

* This study was conducted using grant-in-aid for scientific research on innovative areas from the Ministry of Education, Culture, Sports, Science and Technology as part of the π-System Figuration program for "Confining photons in π electron spheres and modulation resonant photoluminescence through various excitation processes," basic research A into "Development of photonic metamaterials by higher linking of photofunctional polymer spheres," bilateral joint research, basic research S, young researcher A, research fellow grants, JST-CREST, NEDO, University of Tsukuba Pre-strategic initiative “Ensemble of light with matters and life,” TIA Kakehashi's "TIA partnership for creating an international research center for cutting-edge optical materials and optical technology," the Asahi Glass Foundation research grant for young researchers, and the Ogasawara Foundation.

Background of the study

No two people in the world have the same fingerprints. Fingerprint identification is based on this principle and is often used as a tool for identifying people. Moreover, authentication methods that are difficult to counterfeit or replicate are essential to the secure use of devices such as IC cards. Most of the simple authentication methods currently in use are based on reading patterns of ones and zeros (binary data), such as on barcodes and QR codes. If more information could be embedded in each piece of binary data, even more secure authentication devices that are difficult to counterfeit or replicate could be built. In fact, in today's semiconductor chip market, a method called the physical unclonable function (PUF)*8) is used to determine whether a chip was actually manufactured at a particular factory by recording the variation in performance of each transistor on the chip in advance.

Content and findings of the study

Figure 2. (A) Molecular structure of DAE in closed (left) and open (right) forms. (B) Fluorescence micrographs of DAE microspheres in closed and open forms. Excitation light: 400-440 nm. (C) PL spectra of a single DAE microsphere in closed and open form. Excitation light: 470 nm (picosecond laser).

The research group focused on whispering gallery mode (WGM) resonant photoluminescence, which is able to display complex spectral patterns even among optical resonators[1]. WGM is a mode in which light is confined in a circular resonator, and its spectral waveform varies depending on the size, shape, and materials of the resonator. In this study, the group used fluorescence-switchable oxidized diarylethene (DAE) (Figure 2A) as the material for the resonator[2]. DAE emits almost no light in the open form, but when irradiated with ultraviolet light its chemical structure changes to a closed form that emits yellow light. When irradiated with visible light, it returns to the open form and the PL disappears, which is how its PL can be repeatedly switched on and off.

When the DAE molecules were allowed to self-organize in solution, they formed spheres several micrometers in diameter. Photoexcitation of microspheres composed of closed DAE showed yellow PL (Figure 2B, left). Next, irradiation with visible light changed the microspheres back to the open form and the PL disappeared (Figure 2B, right), confirming irradiation with ultraviolet and visible light could switch the PL on and off. Further, the PL spectra of a single microsphere upon excitation showed a clear WGM pattern (Figure 2C, left), which demonstrated that light was actually confined inside the sphere generating WGM. Continuing to irradiate this particle with visible light caused the WGM PL to nearly disappear, showing that the PL of each particle could be switched on and off (Figure 2C, right).

Figure 3 (A, B). Schematic representation and SEM micrograph of oblate ellipsoids self-organized on a substrate. (C) PL spectra of a single oblate ellipsoid particle.

Microspheres even formed when the DAE molecule solution was drop cast on the substrate and the solvent was evaporated slowly. Careful observation of these spheres revealed distortion along the vertical axis (Figure 3A). Measuring the PL spectrum of these oblate ellipsoids during photoirradiation showed even more complex WGM patterns (Figure 3B). This is thought to be because the reduced shape symmetry caused each WGM mode to split. In addition, none of the WGM patterns from the resulting structures were the same, which shows that this pattern can function as a "fingerprint" for each individual sphere.

Figure 4 (A). Schematic diagram of the microdisk array fabrication process by drop casting, drying, and solvent vapor annealing of the DAE acetone solution on a substrate surface with a hydrophobic/hydrophilic pattern. (B) SEM micrograph of solvent vapor annealing time dependence in surface self-organization. (C-G) Fluorescence micrographs of an open form DAE microdisk array (C) after 1 second and 30 seconds of ultraviolet light irradiation (D, E), 60 minutes of visible light irradiation (F), and 30 seconds of ultraviolet light irradiation (F). (H–L) Fluorescence micrographs of ultraviolet and visible light irradiation of an open form DAE microdisk array to write and erase a specific pixel.

The method of surface self-organization described above is simple and convenient. A hydrophobic/hydrophilic micropattern was created in advance on the substrate surface, upon which a DAE solution was drop cast to form a thin film. Solvent vapor annealing*9) caused a DAE microdisc array*10) to form spontaneously in a period of about 5 micrometers (Figure 4A,B). Irradiating this open form DAE microdisk array with ultraviolet light can change the PL state of the entire array (Figure 4C-E) or locally (Figure 4H-J). This on/off PL switch allowed the group to render microscale fluorescent pixels (Figure 4F,G,K,L).

Figure 5 (A) Schematic diagram of the microhemispherical array fabrication process by drop casting, drying, and immersion in a water/acetone mixed solvent of a DAE toluene solution on a substrate surface with a hydrophobic/hydrophilic pattern. (B) SEM micrographs showing the shapes of the self-organized material with different water/acetone mixing ratios.

Further, optimizing the self-organizing conditions confirmed the formation of arrays with hemispherical microstructures (Figure 5).

Figure 6 (a) Fluorescence micrograph of an array drawn on a microhemispherical array composed of open form DAE using a photomask (wavelength 350-390 nm, irradiated for 3 minutes, area: 1.6x2.7 mm2). (B) Enlarged image of the cheek of the Mona Lisa (4x7 pixels). (C) PL spectra of each pixel.

Because light is confined inside the hemispheres, each pixel functions as an WGM resonator, but because each has a slightly different size and shape, all the resonators exhibit different WGM patterns (Figure 6).

Figure 7 (A, C) Pictures drawn using the same photomask. (B, D) PL spectra of individual pixels in the fingertips.

In other words, the differences in optical resonator characteristics caused by structural variations can be used as a PUF. For example, when an open form DAE hemispherical array is irradiated with ultraviolet light at a specific site using a photomask, it renders a micrometer-resolution image, and while the images may look the same at first glance, individual images can be distinguished by differences in their spectral patterns (Figure 7). In other words, it is virtually impossible to produce a perfect replication that includes the spectral pattern.

Future prospects

Figure 8. Shapes, SEM micrographs, and height/diameter ratios of the molecular assemblies formed with each self-organizing method.

In this study, the group proposed a new optical authentication device by applying a PUF to an optical resonator. By applying this method of 2-step authentication—(1) identification of 2-dimensional PL on/off patterns (micro QR codes), (2) identification of the WGM fingerprint pattern for each pixel (spectrum)—optical authentication devices can be constructed that are virtually impossible to counterfeit or replicate. In addition, because long-term photoirradiation is needed to erase the data (PL status), it could also be used as a form of optical memory that can hold data for a long period with a single writing. Because the self-organizing methods developed in this study can be used to produce arrays of various structures, it could someday be a key technology for fabricating optical integrated circuits on substrate surfaces (Figure 8).

Glossary

*1: Diarylethene
It was first synthesized and reported in 1988 by Masahiro Irie, who was then at Kyushu University. By appropriately modifying the structure, it is possible to change the color of the open and closed ring structures and the wavelength of light required to induce this change. Because the photochromic phenomenon is reversible even in the crystal state, applications to high-density memory media that use light to read and write reversibly are thought to be possible.

*2: Self-organization
A process in which molecules and other substances spontaneously assemble to form structures.

*3: Microsphere optical resonator
A resonator that confines light inside micrometer-scale spherical structures. A resonator is a container in which standing waves only form and intensify with light of a specific wavelength.

*4: Whispering gallery mode (WGM) resonant photoluminescence
The "Whispering Gallery" is a circular hall at St. Paul's Cathedral in London, where when a whisper is sent along the wall of the inside of the dome, overlapping amplification of multiple sound wave reflex pathways makes the whisper clearly audible to someone on the other side. WGM resonant photoluminescence is when the sound waves in this phenomenon are replaced by light. Because the light wavelengths are much shorter than sound waves, light propagates by total internal reflection inside micrometer-sized spheres or disks, and light resonance is generated at wavelengths that match the phase after one lap, increasing the intensity.

*5: Array
In this paper, an array refers to a structure in which micrometer-scale structures are arranged on a substrate surface.

*6: Pixel
The smallest unit of a digital image. When a digital image is enlarged to the maximum limit, it consists of individual dots containing color data.

*7: QR code
A matrix-type 2-dimensional code invented by the development department of the auto parts manufacturer DENSO Corp. QR stands for "quick response."

*8: Physical unclonable function (PUF)
A function that outputs device-specific values using difficult-to-replicate physical properties. These include variations in the performance of transistors, the fiber structures of paper, the diffusion patterns of a laser passing through a light-transmissible device containing impurities, and the changes in the capacitance of a dielectric scattered with fine particles.

*9: Solvent vapor annealing
A process in which a sample is exposed to the vapor of a solvent, which promotes spontaneous changes in the integrated and assembled structures of molecules.

*10: Microdisk array
A micrometer-scale disk arranged on a substrate surface.

References

[1] Y. Yamamoto, “Spherical resonators from π-conjugated polymers” Polym. J. 2016, 48, 1045–1050.
[2] M. Irie and M. Morimoto, “Photoswitchable Turn-on Mode Fluorescent Diarylethenes: Strategies for Controlling the Switching Response” Bull. Chem. Soc. Jpn. 2018, 91, 237–250.

Published article

Title: Optical microresonator arrays of fluorescence-switchable diarylethenes with unreplicable spectral fingerprints
Authors: Daichi Okada, Zhan-Hong Lin, Jer-Shing Huang, Osamu Oki, Masakazu Morimoto, Xuying Liu, Takeo Minari, Satoshi Ishii, Tadaaki Nagao, Masahiro Irie, Yohei Yamamoto
Journal: Materials Horizons (DOI: 10.1039/D0MH00566E)

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