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Photonic skins based on flexible organic microlaser arrays

Smart photonic skins based on flexible organic microlaser chips can be used for gesture recognition.


INTRODUCTION
Technologies that render electronic components soft and stretchable led to a new paradigm of electronics-flexible electronics (1)(2)(3)(4)(5)(6), which have accelerated advancements in health monitoring, humanmachine interfacing, and augmented reality. As an analogy, photonics is expected to furnish numerous intriguing opportunities for both fundamental and applied research upon introducing flexible materials (7,8). Taking advantage of the noninvasive nature (9,10), ultrasensitivity to external stimuli (11), and immunity to electromagnetic interference (12) of photons as signal carriers, flexible photonics may enable a plethora of applications beyond the capabilities of its electronic counterpart. Recently, significant progress has been made in the development of flexible photonic devices, including on-chip flexible waveguides, cavities, and filters (13)(14)(15). Such pliable photonic devices have shown extraordinary performance in optical modulation (16) and optical sensing (11,17), showing great potential for fascinating inventions, including artificial smart skins based on soft photonics.
Artificial photonic skin is a collective term for all flexible photonic systems mimicking or enhancing the functionalities (primarily sensing) of human skin (18)(19)(20)(21)(22). Flexible light sources generating photons as signal carriers, including light-emitting diodes and lasers (10,11,(23)(24)(25), are indispensable components of the sensor units in photonic skins. Benefiting from extremely narrow emission peaks, flexible lasers exhibit great superiority in highly sensitive and accurate sensing (11,26). Although large-area sensor networks are highly desired for artificial skin applications (27,28), fabrication of flexible microlaser arrays is still in its infancy: Several examples have been reported (8,14,24), but a general and intrinsically scalable processing technology has yet to be demonstrated. Organic materials combine mechanical flexibility and low-temperature solution processability with outstanding luminescent and optical properties (29)(30)(31), holding great promise for achieving cost-effective, wafer-scale fabrication of flexible laser arrays (32)(33)(34).
In this work, we propose a universal fabrication strategy of largescale organic microlaser arrays and demonstrate their functionalities as a prototype of artificial photonic skins. A bilayer electron-beam direct writing (EBDW) technique was developed to realize the one-step scalable fabrication of three-dimensional pillar-supported organic microdisk arrays on flexible substrates, which otherwise have to be constructed by multiple sophisticated microfabrication procedures. The pillar-supported geometry endowed the microdisk cavities with high mechanical robustness by suppressing the strain interference from the substrate and, thus, could serve as flexible microdisk lasers providing reliable signals for mechanical sensing. Single-mode lasing was achieved through manufacturing the coupled microdisk resonators, which notably enhanced the accuracy and recognizability of the sensing signals. A low-loss microwire waveguide was further fabricated and integrated with the single-mode microlaser to construct a sensing unit in response to the local deformation of the substrate. On this basis, we demonstrated the mechanical photonic modulation and gesture recognition based on the constructed large-scale flexible photonic chips.

RESULTS
The core goal in the field of artificial smart skin is to develop largescale flexible sensor networks that function like the real human skins for applications in wearables, prosthetics, and robotics (27,28). Here, we aim to fabricate flexible photonic chips of sensor arrays to detect human motion (Fig. 1), which is essential for the realization of artificial proprioception and human-machine interfaces (28). The realization of flexible optical sensors with high sensitivity and accuracy requires highly recognizable light signals as sensing information carriers. Microcavity lasers are ideal light sources for such highperformance flexible sensors because their narrow emission linewidth can notably improve the recognizability of the sensing signals (35,36). Moreover, the laser output characteristics of the optical microcavity are very sensitive to environmental perturbation (11,37), which enables one to construct mechanical sensors via integrating microcavity lasers onto elastic surfaces. Such microlaser-based flexible sensors can detect the mechanical deformations of the flexible chip stimulated by external forces. Expanding these mechanical sensors into a two-dimensional array would realize the spatially resolved detection and then distinguish different types of external forces via mapping the local deformations of the flexible chip. When mounted on the human body, the flexible mechanical sensor chip can be used for gesture recognition by converting the human movements to the changes in laser output signals.
We designed an all-organic materials system for the flexible photonic sensor chips ( Fig. 2A) by combining their advantages of inherent flexibility, excellent luminescence/optical property, and high compatibility (14). Highly transparent poly(methyl methacrylate) (PMMA) was selected as the cavity material because its good processability facilitates the scalable fabrication of microcavity arrays for flexible sensor networks. Laser dyes as gain media were doped into the microcavity to achieve low-threshold lasing. The EBDW technique was adopted to directly pattern the organic microlaser arrays on the flexible polymer substrates ( fig. S1). In artificial skin applications, the sensor chips may experience various deformations induced by human motions; hence, high mechanical robustness is crucial for the reliable operation of flexible components (1). To obtain mechanically robust microlaser signal sources, we proposed to insert a buffer layer between the PMMA microcavity and the underlying flexible substrate. With a pillar buffer structure blocking strain transfer from the substrate to microcavity ( fig. S2), the microcavity laser is able to operate continuously on the deformable surfaces; otherwise, it would suffer from output fluctuations and even performance degradations ( fig. S3) (11).
To obtain the abovementioned pillar-supported microstructures, we developed a bilayer EBDW technique ( Fig. 2A). We first successively spin-coated low-molecular mass PMMA and high-molecular mass PMMA doped with laser dyes onto a flexible substrate. Then, a circular array pattern was directly drawn on the PMMA film by a focused electron beam. Because the low-molecular mass polymer experienced a more complete chain scission upon electron beam irradiation (38), more bottom-layer PMMA would be removed during developing (39). Consequently, the bottom and top PMMA films could form small pillar and large disk geometries, respectively, together constituting the pillar-supported microdisk cavities. Besides the microdisk cavities, one-dimensional microwire waveguides can be fabricated as well by using this bilayer EBDW technique ( fig. S4), which contributes to the integration and functionalization of flexible photonic devices (14). Such bilayer EBDW technique can be applied to the fabrication of photonic structures on diverse flexible substrates, such as poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), Kapton, and parylene substrates ( fig. S5). The high substrate compatibility indicates a wide application scope of the all-organic flexible sensor chips in artificial photonic skins. Figure 2 (B and C) presents a freestanding flexible photonic chip comprising a large-scale organic microdisk cavity array on a transparent PET substrate. The cavity size can be readily defined during the EBDW fabrication process with high reproducibility ( fig. S6), which benefits the optimization and modulation of the performance of microdisk lasers toward flexible photonic applications. In the bright-field microscopy image (Fig. 2C), each organic microdisk shows two concentric circles that correspond to the small PMMA pillar and large PMMA disk, respectively. The side-view scanning electron microscopy (SEM) image clearly reveals the disk-on-pillar geometry of the PMMA microcavity ( Fig. 2D), which resulted from the selective removal of more underlying PMMA with lower molecular mass. The pillar-supported microdisk cavity is beneficial for avoiding optical leakage to the substrate, which improves the quality (Q) factors of microcavities (40). Above all, the pillar-supported microdisks can maintain their original circular morphology and smooth surface when the substrate is deformed by an external force ( fig. S7). The mechanical stability of the pillar-supported microdisk cavities ensures robust laser signals for highly accurate and reliable sensing (24). Figure 3A shows the photoluminescence (PL) image of a typical dye-doped microdisk array. The microdisks exhibited much stronger emission at peripheries, suggesting efficient optical confinement and waveguiding in the microdisk cavity. The high-Q microdisk cavity, together with the large optical gain of doped dyes ( fig. S8), would allow for laser action occurring at a low threshold. Optically pumped lasing measurements were carried out on a far-field micro-PL system ( fig. S9). A 430-nm pulsed laser beam, with the beam waist adjusted to be larger than the diameter of each microdisk, was used as a spatially uniform pump source. Upon the gradual increase in the pump energy, a series of narrow lasing peaks appeared in the emission spectra from the dye-doped microdisk (Fig. 3B). The nonlinear response of the light output intensity to pump power (Fig. 3C) confirms the occurrence of laser oscillation, from which the lasing threshold of the dye-doped microdisk was deduced to be ~11.03 J/cm 2 . The laser behavior is highly reproducible for the dye-doped microdisks, exhibiting similar laser spectra with an average threshold of 11.5 ± 1.2 J/cm 2 (fig. S10). The mode spacing between sharp lasing peaks can be finely modulated by changing the size of microdisks (fig. S11) (41). The inversely proportional relationship between the mode spacing and the microdisk diameter indicates a good agreement with the whispering gallery mode resonance (42).
To evaluate the mechanical robustness of the pillar-supported organic microlasers, we measured their lasing properties with respect to the bending radii of the substrate. The flexible photonic chip was mounted on a motorized bending stage that could induce a quantitative deformation (Fig. 3D). As shown in Fig. 3E, the laser spectra showed barely perceptible variation even when the bending radius of the flexible substrate was decreased down to 1.5 mm, which indicates the excellent mechanical stability of the pillar-supported organic microlasers. Furthermore, both the laser mode positions and output intensities remain nearly unchanged after 5000 bending cycles (Fig. 3F), manifesting the superior mechanical durability of our photonic chips. The remarkable mechanical stability and durability of the flexible microlaser arrays make them ideal signal sources for the photonic mechanical sensors. Figure 4A presents the working mechanism of a flexible microlaser-based mechanical sensor, which is composed of a coupledcavity single-mode laser source and a coupled cavity-waveguide sensing unit. The coupled microcavity system with the Vernier effect enables a single-mode laser (43,44), which, as a signal source, can remarkably improve the accuracy and reliability of the flexible sensors by suppressing signal intensity fluctuations and false signaling. Given that a fractional ratio between the cavity lengths is critical for single-mode operation (43), we fabricated the coupled cavities by pairing two microdisks with diameters of 14 and 15 m (Fig. 4B), respectively. Such coupled microdisks supported single-mode operation of lasing (Fig. 4C) and exhibited a lower threshold (~8.74 J/cm 2 ; Fig. 4D) than that of the individual microdisk (~11.03 J/cm 2 ; Fig. 3C) due to the absence of multimode competition (43). With high spectral purity and low energy consumption, these coupled microdisk lasers are superior signal sources for the construction of highly accurate and reliable photonic sensors.
Further integration of the coupled microdisk laser with a cantilever-supported microwire waveguide produces a sensing unit, where the single-mode laser generated in the microdisk pair couples into the microwire waveguide and outputs from its tip (Fig. 4A, middle). The laser output intensity depends on the gap width between the tangential microwire and the microdisk, which, in turn, changes with the substrate deformation (Fig. 4A, bottom, and fig. S12). Therefore, the flexible coupled wire-disk sensor enables the detection of the mechanical deformations and thus the external forces exerted on the flexible chip by monitoring the laser signals outcoupled at the microwire tip. Note that the pillar-supported geometry can enhance the sensitivity of the flexible coupled wire-disk sensors ( fig. S12). Figure 4E shows a typical flexible sensor array, where each sensor is composed of a coupled microdisk laser and a cantileversupported microwire waveguide (Fig. 4F). Because the optical leakage to the substrate is suppressed in the suspended geometry ( fig. S13), the microwires behave as excellent optical waveguides with extremely low optical loss ( fig. S14), which is essential for the effective transmission of sensing signals. When the flexible chip was bent by an external force, the gap width between the microdisk and microwire would gradually become larger ( Fig. 4G and fig. S12), which reduced the laser coupling efficiency from the microdisk to the microwire (fig. S15). Accordingly, the output signal of the single-mode laser from the microwire tip decreased with the increased curvature (Fig. 4H). The remarkable response in the laser signal to external force fully demonstrated the feasibility and high sensitivity of the coupled wiredisk structures as mechanical sensors. Furthermore, with the increase of the cantilever length, the sensitivity of the flexible sensor increases significantly ( fig. S16). With different responsiveness to the substrate bending, such cantilever varied flexible sensors could meet diverse practical applications.
The excellent mechanical sensing performance qualifies the flexible coupled wire-disk sensor chips as smart photonic skins capable of detecting human motions, which is very important for human proprioception restoration and human-machine interaction. As a proof-of-concept demonstration of their application as photonic skins, we used the networks of flexible coupled wire-disk sensors for hand gesture recognition (Fig. 5). The mechanical sensor networks were directly written on the Kapton substrates, which were then attached onto the fingers of a model hand to detect the movements (Fig. 5, A to D). Five flexible sensor chips numbered 1 to 5 monitor the movements of thumb, index, middle, ring, and little fingers, respectively, to get the information conveyed by the hand gestures. On the flexible sensor chips, the coupling directions from microdisks to microwires were set to be parallel to the fingers, which correlated the finger movement with the variation in sensing signal (Fig. 5,  A and B, inset). In addition, the cavity-waveguide sensing units were designed to be in an efficient coupling situation; thus, all flexible sensor chips exhibited maximum laser signal outputs when the fingers were in extension (Fig. 5, A and E, gesture 1). The sensing signal intensities were extracted from the laser output spectra of flexible chips (fig. S17) and plotted versus hand gesture in color scale (Fig. 5E). When each flexible photonic chip bent with the corresponding finger as shown in Fig. 5 (A and E, gestures 2 to 6), the sensing signal experienced a marked decrease resulting from the reduced coupling efficiency from the microcavity to the output waveguide. Furthermore, the gestures with more than one finger in bending state-for example, gestures 7 to 9-can be recognized as well by analyzing the laser output intensities from the five sensor chips. When the model hand made a fist (gesture 10), all flexible sensor chips reached the minimum output state. With five flexible photonic chips monitoring the movements of each finger, we can readily recognize various possible hand gestures with the laser signal output series. Besides the hand gestures, other human movements accompanied by joint motions can be, in principle, read by our photonic chips. The flexible photonic sensor networks, with further optimization owing to their excellent design flexibility, may find numerous applications in the artificial smart skins, human-machine interaction, and robot self-protection systems.

DISCUSSION
We have developed a bilayer EBDW technique for the scalable integration of flexible photonic devices. Such bilayer EBDW technique features high design flexibility, allowing for the fabrication of diverse photonic elements, such as microcavities and waveguides on flexible substrates. Suspended geometry endowed these photonic elements not only with excellent optical confinement and waveguide capability for high photonic performance but also with high mechanical stability and durability toward flexible skin applications. The microcavity lasers exhibited great superiority in high recognizability and accuracy as sensing signal sources due to their extremely narrow emission peaks. The coupled microcavity system enabled singlemode lasers, which, as signal sources, further improved the accuracy and reliability of the flexible sensors by suppressing signal intensity fluctuations and false signaling. Further integration of the coupled microdisk laser with a cantilever-supported microwire waveguide produced a sensing unit in response to the substrate deformations. Note that the pillar-supported geometry, especially the suspended cantilever, significantly enhanced the sensitivity of the flexible coupled wire-disk sensors. As a proof-of-concept demonstration of their application as photonic skins, we used the networks of flexible coupled wire-disk sensors for hand gesture recognition.
Our proposed photonic skin system has high scalability for further performance improvements and functionality expansions. For example, we can realize the detection of biaxial mechanical deformation by the orthogonal arrangement of two mechanical sensors or by integration of two orthogonal microwire waveguides with the microdisk laser ( fig. S18). Besides, we here attempt to propose a fully functional flexible chip integrating flexible light sources, optical sensors, and on-chip spectrometers toward the ultimate smart photonic skins ( fig. S19), which we hope could guide and promote the investigations of flexible photonics.
In summary, we have demonstrated the scalable fabrication of organic flexible integrated photonic devices and their application as artificial photonic smart skins. A bilayer electron-beam direct writing technique was developed to fabricate flexible mechanical sensor networks composed of coupled-cavity single-mode laser sources and coupled cavity-waveguide sensing units. The single-mode organic microlaser array functioned as signal sources with high recognizability and superb mechanical reliability for the sensing applications of flexible photonic chips. The microlaser-based mechanical sensor networks were used as artificial smart skins to recognize hand gestures. Our strategy can be used to produce novel organic flexible photonic systems, and the results in this work might unlock the potential of flexible photonic devices as artificial smart skins.

Flexible bilayer PMMA film deposition
The flexible photonic devices were fabricated through one-step EBDW on bilayer PMMA films. Low-molecular mass PMMA [~20 weight % (wt %)] dissolved in chlorobenzene was first spin-coated at a maximum speed of 1500 rpm for 30 s to produce a 3-m PMMA film on a flexible polymer substrate. The first PMMA layer was baked at 180°C for 120 s to remove the solvent and ensure a clear interface with the following PMMA layer. Then, a mixed chlorobenzene solution of high-molecular mass PMMA (~12 wt %) and 1,4-bis(-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (~0.6 wt %) was spin-coated at a maximum speed of 2000 rpm for 30 s to produce an active PMMA layer on top of the low-molecular mass PMMA film. The mass fraction of the dye to PMMA is set to be 5 wt % to obtain both smooth morphology and excellent luminescence ( fig. S8). The same baking procedure was carried out to remove the residual solvent and ensure flatness and uniformity over the whole substrate. To minimize the pattern distortion caused by the charging effect during the EBDW process, a conductive layer (SX AR-PC 5000/90.2 solution, GermanTech Co. Ltd.) was spincoated on top of the bilayer PMMA film at a maximum speed of 3000 rpm for 30 s and backed at 180°C for 120 s.

Electron-beam direct wiring
EBDW on the bilayer PMMA films was carried out by a scanning electron microscope (FEI Nova NanoSEM 450, electron acceleration voltage at 30 kV, beam current of 84 A) equipped with a Nano Pattern Generator (Raith). After that, the conductive layer was first removed by water flush without affecting the structures and performances of the flexible chip. Then, the exposed bilayer PMMA film was developed in a 1:3 mixture of MIBK and IPA at room temperature for 30 s, followed by a thorough rinse in IPA and blow drying by compressed nitrogen gas.

Morphological characterizations
The morphologies of the as-prepared microdisk arrays were examined with SEM (Hitachi SU8010). Bright-field and fluorescence microscopy images were taken using an inverted fluorescence microscope (Nikon Ti-U) by exciting the samples with the halogen and mercury lamps, respectively.

Spectroscopic studies
The fluorescence quantum yields were measured on an absolute PL quantum yield spectrometer (Hamamatsu C11347). The characterizations of optically pumped lasing and flexible optical sensing were carried out on a far-field micro-PL system equipped with a modelocked Ti:sapphire laser (Spectra physics), an optical microscope (Nikon), a charge-coupled device camera (Nikon), and a spectrophotometer (Princeton Instruments) ( fig. S9) (44).

Finite-element simulations
The numerical simulations of electric field intensity and strain distribution were performed using the Radio Frequency (RF) and solid mechanics modules of COMSOL Multiphysics, respectively.