We developed an ink-jet printing method for fabricating inorganic microdisks at room temperature, which is much lower than the melting point of solid-state inorganic oxide, and have fabricated an organic-inorganic hybrid microdisk laser. Silica was used as the inorganic disk material, and microdisk-shaped aggregates were formed by the ink-jet printing method using a solution in which nanosilica particles were dispersed in propylene glycol monomethylether (PGME) solvent. Then, a microdisk capable of laser oscillation was also prepared by preliminarily adding the laser dye rhodamine 6G to the ink to form a mixed organic material. The structural evaluation of the printed microdisk was first conducted using an optical microscope, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The results of laser oscillation evaluation by optical excitation showed that the printed microdisk sufficiently functions as an optical resonator with a low optical loss. In these evaluations, excellent values such as a surface roughness of 5.83 nm from root mean square (R. M. S.) which is one forth smaller than the particle diameter, and a laser oscillation threshold of 4.76 µJ/mm2 at a wavelength of 601.4 nm were obtained. To the best of our knowledge, this is the first time that an inorganic microdisk has been fabricated at room temperature to realize an organic-inorganic hybrid microdisk laser.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The optical microdisk cavity is a type of optical microcavity having a diameter of about several micrometers to several hundred micrometers. It has a high quality factor (Q factor) owing to the formation of whispering-gallery modes (WGMs) in which light propagates stably in the circumferential direction. Therefore, related studies, such as on integrated optics, low threshold lasers, nonlinear optics, optical signal processing, and highly sensitive biosensors are actively conducted [1–19]. In the latest research on microdisk, a high-Q integrated microdisks was also reported . Among various shapes such as spheres, rods, and rings, the microdisk has a planar shape, so light is localized only on the edge. Therefore, it is easy to obtain pure basic mode WGM and higher Q factors with smaller mode volume. In addition, since it can be fabricated onto a flat surface, a curved surface, or a large area [3,4,6,8–12,20], it is attractive in mounting and application [2,6,8,12–14]. The fabrication methods and material combinations have been widely reported. As mainstream “subtractive methods,” the photolithography method [2–4,6,9,10,12,20], electron beam/laser beam exposure method [4,9,11], and sol-gel method  are used. Among them, the photolithography method is frequently used, and it is suitable for simultaneous, large-quantity fabrication with a high precision. However, it requires dedicated equipment for each process, and chemical treatment with a strong stimulus or thermal treatment with high temperature are also necessary in the fabricating process. These treatments are likely to cause stress or damage of materials. Thus materials that can be used are limited in these subtractive methods, since durability at chemical treatment like gas etching and temperature at which material flows such as melting point (e. g., 1600°C on silica) or glass transition temperature (e. g., 160°C on PMMA) are different for each material. For example, the preparation of microdisks of silica [2,3,6,10,20] involves four processes: (1) silica layer formation by thermal oxidation on a silicon substrate, (2) exposure and development, (3) substrate etching, and (4) heat flow and annealing. Actually, thermal treatment is required in two processes, including for melting the material. The electron beam/laser beam exposure method and sol-gel method also include a thermal treatment process. To overcome these limitations, we recently developed a new “ink-jet printing method [5, 16].” This is an “additive method” for fabricating organic microdisks using spherical polymer and can fabricate microdisks on-site or on demand. As ink ejection is possible at room temperature under atmospheric pressure, which is a feature of the ink-jet technique, the extent of deterioration and distortion of the organic materials and thermal stress on them are not significant. In addition, the process can perform with low cost since material and energy consumption are extremely low. Furthermore, to our knowledge, the fabricated organic microdisks exhibited relatively ultra-low lasing threshold and high Q factor. The preparation of microdisks by these subtractive/additive techniques by using many materials have been previously reported [1–6,8–12]. In the organic category, there are microdisks produced using linear polymers (e.g., polymethyl methacrylate (PMMA)) [4,9], a spherical polymer [5,16], and laser dyes doped in these polymers [4,5,16]. In the inorganic category, there are microdisks prepared using silica [2,3,6,10,12,20], silicon , and rare earth ions doped in them [10,12]. Focusing on the cavity and dopant material combination, organic-organic or inorganic-inorganic microdisks have been developed, but organic-inorganic microdisks have not yet been developed, although there are reports on applications in which other materials are coated on a single material disk  or composition is used as microdisk material synthesized of organic element and inorganic element . In other words, to the best of our knowledge, there are currently no organic-inorganic hybrid microdisks freely combined with gain material and host material each other. This is because the required process temperature is different between inorganic and organic materials due to the difference between the melting point and the glass transition temperature in the subtractive method. On the other hand, the ink-jet printing additive method is limited to the use of the organic materials, since the technique is carried out at temperatures much lower than the melting point of inorganic materials. In this study, we have established a method for fabricating inorganic microdisks by the ink-jet printing method at room temperature environment, which is much lower than the material melting point, and have developed an organic-inorganic hybrid microdisk. When silica–an inorganic material–was used, microdisk-shaped aggregates were formed by the ink-jet printing method using a solution in which nanosilica particles were dispersed in PGME solvent. As a mixed organic material, a microdisk capable of laser oscillation was also prepared by preliminarily adding the laser dye rhodamine 6G to the ink. The structural evaluation of the printed microdisk was first conducted. The evaluation of laser oscillation by optical excitation showed that the printed microdisk sufficiently functions as an optical resonator with a low optical loss. To the best of our knowledge, this is the first time that an inorganic microdisk was fabricated at room temperature to produce an organic-inorganic hybrid microdisk.
2. Ink-jet printing method for fabricating inorganic microdisks
2.1. Ink-jet printing method for fabricating inorganic microdisks
The room temperature ink-jet printing method for fabricating inorganic microdisks was developed with a concept inspired by ceramic fabrication techniques for laser media. In the fabrication of laser ceramics, microcrystalline aggregates are used in both chemical [21,22] and mechanical methods [21,23,24]. Then, the initial microcrystals are sintered to reduce the defects between the fine grains; the subsequent finishing step results in a laser ceramic with low light propagation loss. In large size laser ceramics for high power lasers, sintering for loss reduction is important. However, in the microcavity, since the resonator itself is extremely small, even in the case of aggregation before sintering, the influence of light loss on defects between microcrystals may be insignificant. Therefore, we hypothesized that “if aggregates are constructed, they will function as optical resonators.” Figure 1 shows the process and protocol for ink-jet printing for producing silica microdisks. FN-107M, a solution doped with nanosilica particles, was novelty developed as an ink solution. FN-107M also contains slightly acrylic-based oligomer for structural stability. The concentration of these dopants was totally 20 wt.% in PGME solvent. The particle diameter and refractive index of FN-107M are about 20 nm and 1.29, respectively. After ink droplets with dispersed silica were discharged on the substrate, (1) the volume of the edge part of droplet increased in a ring shape due to Marangoni convection of the solvent. After that, (2) only the silica particles remained following solvent evaporation, and toroidal disk aggregates were formed. Due to the fluid in the solution as the surface tension, the surface remained extremely smooth with the maximum roughness of the order of the particle size. To evaluate a typical microdisk aggregate, a simple process of one shot direct printing to a substrate was conducted. In the ink-jet printing, the ink-jet head (MD-K-130, Microdrop Technologies) with a piezo actuator with controlled voltage was used. The discharge port diameter of the ink-jet head is 50 µm. The piezo driver (uDropC-140, HANTEC Ltd.) controls the pulse voltage and its width. The meniscus suction pressure is applied at the discharge port by the pressure driver (SF-100, MICROJET Corporation). The distance between the ink-jet head and the substrate is kept 1 mm.
2.2. Fabrication of silica microdisk by ink-jet printing method
First, substrate screening was carried out to obtain a fine microdisk with high circularity, a small diameter, and a thick edge. In particular, the large thickness of the disk edge is important for confining the light to form the WGM. Five sample substrates of the following were evaluated: PMMA, slide glass (10127101P, AS ONE Corporation), indium tin oxide (ITO), polyethylene terephthalate (PET) film, and fluorinated ethylene propylene (FEP, NF-0100, DAIKIN INDUSTRIES, LTD.) film. All the substrates were carefully cleaned with ethanol. Microdisks were fabricated with the same parameters for all substrates: the pulse voltage was 123.4 V, the pulse width was 26 µs, the meniscus suction pressure was −1.3 kPa, and all process were conducted at 25°C. Figure 2 shows optical microscope images of printed microdisks. Microdisks on PMMA, glass, and PET as shown respectively in Figs. 2(a), 2(b), and 2(d) seem to have thin, warped edges and large diameters. On the other hand, microdisks on the ITO and FEP as shown respectively in Figs. 2(c) and 2(e) have thick edges and high circularity. However, the diameter of the microdisk on ITO is large. Therefore, the FEP substrate was selected as the suitable substrate to form the microdisk structure with high circularity, a small diameter, and a thick edge. On the FEP, the radius of printed microdisks is fluctuated within a range of about 14%, mainly due to the low flatness of FEP. This repeatability can be improved using substrate with fine flatness.
For further evaluation of structures, the 3D structure was analyzed by SEM (VE-7800, KEYENCE CORPORATION) and AFM (VN-8000, KEYENCE CORPORATION). Figure 3 shows the SEM and AFM images of the microdisk fabricated using the FEP substrate. For the SEM image in Fig. 3(a), the accelerating voltage was 5 kV, working distance was 30.8 mm, and magnification was 390x. For the image in Fig. 3(b), the accelerating voltage was 5 kV, working distance was 29.1 mm, and magnification was 1000×. In these SEM images, the edge of the microdisk is clearly rising because of the strong coffee-ring effect on the FEP film. From the SEM image of Fig. 3(b), the taper angle of the edge is estimated to be 30°. This strong coffee-ring effect and large taper angle lead to toroidal-like structure, and can form a stable WGM. According to the AFM image in Fig. 3(c), the R. M. S. corresponding to the surface roughness at the edge is approximately 5.83 nm, which is one forth smaller than the particle diameter of FN-107M.
3. Experiments and discussion
3.1. Experimental setup for WGM lasing
WGM lasing was performed to show that a silica-based microdisk printed by the ink-jet printing method functions as an optical resonator. Figure 4 shows the experimental setup for the measurement of WGM lasing. A Q-switched and frequency-doubled Nd:YAG laser (PNG-002025-040, Nanolase Corp.) was used for the excitation as a pumping source. The pulse width and repetition rate were ~ 0.5 ns and 10 Hz, respectively. The pumping light was guided by high-reflectivity mirrors, and focused onto the microdisk sample by a plano-concave lens with the focal length of 2 mm. The WGM lasing signal from the microdisk was recorded by the optical microscope (ECLIPSE TE2000-U, Nikon) with 100× magnification after the blocking filter of the pumping light. This lasing signal was observed from the bottom of the microdisk since the observation from the bottom is efficient as shown in a simulation of Ref. . The collected signal was collimated onto the optical fiber (AFS105/125Y, Thorlabs Inc.) to couple to the spectrometer (MS7504, SOLAR TII). The incident slit width and the exposure time were set at 0.1 mm and 100 s, respectively. To generate the lasing faction, 5 mmol/l of the laser dye rhodamine 6G (Rhodamine590 Perchlorate, Exciton corp.) was added into the FN-107M ink solution as the gain material, resulting in an approximately 21 mmol/l after the solvent PGME was evaporated. The microdisk sample for measurement was prepared with these parameters: a pulse voltage of 82.2 V, pulse width of 57 µs, and meniscus suction pressure of −0.9 kPa. The microdisk sample was removed from the FEP film substrate by using a transparent cellophane adhesive tape to avoid light leakage to the FEP substrate, since the refractive index of the fabricated microdisk is lower than that of FEP film.
3.2. WGM lasing and Q factors of silica microdisk
Figure 5(a) shows spectrum while silica-based microdisk was excited by optical pumping. The edge of excited microdisk was clearly brilliant at circumferential direction by WGM lasing as shown in inset of Fig. 5(b). The lasing spectrum was observed in the wavelength region over 595 nm as shown in Fig. 5(a). According to this comb-like lasing spectrum, WGM laser oscillation was clearly obtained. The mode spacing was 0.441 nm, which agrees with the theoretical WGM fundamental free spectral range (FSR) of 0.467 nm represented as the following function :Fig. 5(a). Then, the Q factors were calculated by the following equation Q = λ/d λ, where the λ is the center wavelength of the mode and d λ is the full width at half maximum (FWHM), which equals 2σ of the Lorentzian function. The Q factor was 6.6 × 103 in the best fitting result. Compared with Q factors (103–108) of previous reported works [9–12,20] involving microdisks, this value is acceptable and reasonable for the Q factor.
Finally, the input-output characteristics of WGM lasing were evaluated as shown in Fig. 5(b). These characteristics were obtained using the relative intensity of the spectra fitted using the Lorentzian function in Fig. 5(a). The lasing threshold was obtained around 6 µJ/mm2 by determining the superlinear output onset. As a minimum lasing threshold, a lasing threshold of 4.76 µJ/mm2 was obtained at a wavelength of 601.4 nm. A comparison of this threshold with previous reported values [5,16] shows that this lasing threshold is about 30 times higher than the result of near infrared oscillation in the etched TZ-001 polymer microdisk . On the other hand, this is slightly higher–about 3 times higher–than the result of red oscillation in the non-etched TZ-001 polymer microdisk . It showed the same level of performance as the oscillation in the wavelength region where light leakage to the substrate side was large and the light absorption loss due to the material was large without etching. As a result, it can be concluded that the silica microdisk fabricated by the ink-jet printing method can function as an optical resonator with low optical loss. In addition, it is expected that disk-shaped microstructure aggregates of other inorganic materials can be formed by the ink-jet printing method, and these are expected to be useful as optical resonators.
In summary, we have developed an ink-jet printing method for fabricating an inorganic microdisk at room temperature, which is much lower than the melting point, and achieved an organic-inorganic hybrid microdisk laser. Silica was used as the inorganic material, and microdisk-shaped aggregates were formed by the ink-jet printing method using a solution in which nanosilica particles were dispersed. Then, rhodamine 6G was used as a mixed organic material, and a microdisk capable of laser oscillation was also prepared by preliminarily adding the laser dye to the ink. Structural evaluation of the microdisks was conducted. The results of laser oscillation by optical excitation showed that the printed microdisk functions adequately as an optical resonator with low optical loss. In these evaluations, excellent values such as an R. M. S. of 5.83 nm which is one forth smaller than the particle diameter, and a laser oscillation threshold of 4.76 µJ/mm2 at the wavelength of 601.4 nm were obtained. While printed microdisk has relatively high R. M. S. value, the achievement of laser oscillation is meaningful in a use of organic-inorganic hybrid microdisk laser. To the best of our knowledge, the fabrication of an inorganic microdisk at room temperature to realize an organic-inorganic hybrid microdisk laser has been achieved for the first time. Therefore, new applications such as sensors in which biomolecules such as antigens and antibodies are directly mixed into silica microdisk aggregates with high chemical stability can be expected.
Japan Society for the Promotion of Science (JSPS) KAKENHI (JP16K04980, JP16K17531).
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