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Abstract
We have novelly, to the best of our knowledge, developed the liquid flow microetching method that can treat a single microdisk in a microregion with precise position control for inkjet-printed microdisk lasers. The injection-drain wet etching setup consisted of two microneedles that successfully performed a formation of a fine undercut structure of an inkjet-printed microdisk on a pre-pedestal layer through the individual wet etching process. Then measurement of the undercut structure using scanning electron microscopy and lasing characteristics with whispering gallery modes were carried out to demonstrate performance of the etched microdisks. The measured lasing threshold decreased by half compared with that of the unetched microdisk directly printed on a fluorine-type film. A point to note is that this etching method exhibits an excellent undercut and lasing characteristics even when using a clad pre-pedestal layer having a refractive index higher than that of core microdisks. This technique, combined with inkjet printing, offers a powerful tool for individually designing a microdisk and can help develop novel devices that comprise several inkjet-printed microdisks being evanescently coupled.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
As microcavities [1] can strongly confine light supported by whispering gallery modes (WGMs) with a high quality factor (${ Q}$ factor) and small mode volume, they have generated a considerable interest for applications in biosensors [2–6], bio-imaging [7], frequency combs [8–10], signal processing [11,12], and low-threshold lasers [13,14]. Although various shapes for microcavities, such as a sphere [15] and a toroid [16], have been reported with laser oscillation by WGMs, microdisk lasers [13,17–20] have been especially studied due to their good compatibility with photonic integrated circuits in terms of its geometry. Then subtractive process such as lithographic techniques [18,21–26] has been traditionally used for its fabrication. On the other hand, we previously demonstrated the inkjet-printing method [13,17,27] of an additive method that is promising for this fabrication under open-air atmospheric pressure and room-temperature conditions. Since the formation of a microdisk structure from ejected ink droplets is performed by self-assembly, well-shaped microdisks can be fabricated with a perfectly smooth surface. Furthermore, this technique is an additive method; thus, it is possible to create microdisks on any substrates using different inks or even layered structures in a simpler way. Therefore, the inkjet-printing method becomes an important tool to design a single, individual microdisk as on-demand fabrication for application in multicolor lasing [28,29], as robust and highly selective biosensing [2,5], and as add-drop filters [12]. Therefore, assembly of inkjet-printed microdisks can develop new functional devices such as organic printable full-color laser displays [30].
Recently, many researchers have reported integrated optical devices using microcavities [9,11,31]. For instance, coupling of two microresonators through a tapered fiber can enable low-power synchronization of signals with mode locking [9]. Another study revealed that input control light in one coupled microcavity induced Kerr effect tuning the refractive index and changed the evanescently coupling state, which thus realized optically tunable buffering [11]. This study is noteworthy because it shows that a quick response of signals can be obtained only by manipulating light. Hence, reciprocal coupling of microcavities has recently been gaining increasing attention. However, all these cavities are horizontally aligned and simply coupled to each other. In contrast, an inkjet-printed microdisk can explore a new area of evanescently coupling research because the cross-sectional shape of the edge has a wedge structure rather than the rectangle structure like slab waveguides and the light propagates along the sidewall through WGMs in the wedge [32]. As this mode is controllable by tuning the geometrical features such as taper angle and diameter, it cannot be only handled by the light propagation but also more precisely manipulate the specific mode coupling. Furthermore, owing to propagation of light along a slight internal smoothness of an inkjet-printed microdisk, coupling through the wedge structure can be expected for reducing scattering loss and contribute to a high ${Q}$ factor compared with horizontally coupled microcavities. For realization of these integrated devices, however, the etching process is significantly important to form an undercut structure that functions as a floating optical waveguide of the microdisk laser without any contact with the substrate. Although dry and wet etching by etchant such as plasma and solution are common processes in practical use, targeting a single sample and treating it under favorable conditions is difficult. This situation applies not only to the traditional subtractive process but also to the fabrication of microdisks by the inkjet printing method.
In this paper, we have novelly developed the liquid flow microetching method, wherein a single microdisk can be treated individually in a microregion under precisely controlled conditions without any influence on other microdisks around the target. This process provides several controllable etching parameters such as etchant concentration and etching duration for a single microdisk and enables design of inkjet-printed microdisks. In the experimental part, the liquid flow microetching method was demonstrated for inkjet-printed microdisks on a pre-pedestal layer, and etched microdisks were evaluated through measurements of precise three-dimensional structure and WGM lasing characteristics. A scanning electron microscope (SEM) was used to reveal that the undercut structure was successfully formed. Furthermore, by laser excitation measurement, it was confirmed that the undercut inkjet-printed microdisk showed laser oscillation through WGMs, and its lasing threshold was at maximum twice as small as that of a microdisk directly printed on a fluorinated ethylene propylene (FEP) substrate. A point to be noted is that this etching method exhibits an excellent undercut and lasing characteristics even when using a clad pre-pedestal layer having a refractive index higher than that of core microdisks. Hence, these results open the initial door toward the design of novel devices involving integrated wedge microdisks evanescently coupled with WGMs.
2. FABRICATION AND STRUCTURAL ANALYSIS
A. Material and Ink Preparation
Since hyperbranched polymers (HBPs) have lower viscosity when dissolved in a solvent than linear polymers, these are good candidates for inkjet material. In our previous study, a triazine-based HBP, TZ-001 (Nissan Chemical Corp.), which has a high refractive index ($n = 1.78$), was demonstrated for inkjet-printed microdisks, and an ultralow lasing threshold was reported [13]. However, this polymer shows large absorption in the visible region, and there is a limitation in terms of available wavelengths for laser oscillation. In this research, to extend the visible wavelength region of lasing, FZ-001 (Nissan Chemical Corp.), which is a kind of fluorine-based HBP [33], was used for the core microdisk material. This polymer shows high transparency in broad wavelength in the visible region as shown in Fig. 1. Furthermore, not only does this material expand the available wavelengths but also shows high water repellency due to existence of carbon-fluorine bonds. This property makes an inkjet-printed microdisk well shaped, forming a thick waveguide when it is fabricated on a substrate. For these reasons, fluorine-based HBPs could be expected for excellent properties as inkjet materials for fabrication of optical microcavities. For ink preparation, the following steps were carried out: (i) FZ-001 was dispersed into cyclohexanone at a concentration of 20 Weight %; (ii) 5 mM of laser-dye pyrromethene 597 [34] (Exciton Corp.) was doped as a gain medium for laser oscillation; (iii) ultrasonication was applied to the ink to disperse homogeneously.
Fig. 2. Scheme of an undercut inkjet-printed microdisk. Formation of undercut structure decreases a refractive index of the clad layer and thus works as an optical waveguide along the edge of the microdisk.
B. Fabrication of Inkjet-Printed Microdisks
After preparation of the ink, the FZ-001 microdisk printing procedure was carried out with a single droplet ejection condition. In this research, two substrates for the clad layer were used: (i) the FEP ($n = 1.33$) film to confirm if laser oscillation of WGMs was possible for the FZ-001 microdisk laser and (ii) the triazine-based HBP, TZ-001 ($n = 1.78$) coating layer to perform the etching process for undercut microdisks. Here the laser oscillation cannot be theoretically obtained without formation of the undercut structure, considering the refractive index profile as shown in Fig. 2. Hence, for the TZ-001 substrate, successful formation of the undercut structure by etching would lead to laser oscillation under pumping and become evidence of the undercut microdisk laser. The TZ-001 layer for the pedestal was spin coated on the FEP substrate to have a film thickness of about 1 µm to aim a gap suitable for future evanescent coupling applications. For the printing of microdisks, an inkjet head (PIJ-60ASET, Cluster Technology Corp.) with a controllable piezo-actuator used by applying voltage and meniscus suction pressure was applied by the pressure driver to prevent the ink from spreading out around a hole in the nozzle. The printing position was controlled via a precise positioning robot (SHOTmini 200 $\Omega$, Musashi Engineering Inc.), and the nozzle was set at a height of 1 mm from the substrate. More detailed procedures were reported in previous works [13,17].
Fig. 3. (a) Setup of the liquid flow microetching method. The inset images show optical microscope images when the twin needles were approaching the microdisk. The red dashed line shows an area where the etchant flowed from the injection needle to the drain needle. (b) Schematic diagram of the liquid flow microetching method. The target was being observed by the optical microscope during the etching process.
Fig. 4. (a) SEM image of the undercut microdisk. The red dashed lines highlight magnification of a part of the sidewall and existence of the air gap underneath the microdisk. (b) An AFM image of the undercut microdisk. The microdisk and the pedestal are highlighted in the red and orange region, respectively. (c) Comparison of cross sections of the microdisk before and after the etching process along the red dashed line.
C. Liquid Flow Microetching Method
The liquid flow microetching method performs individual etching on a single microdisk. The setup of the liquid flow microetching method and the schematic diagram are shown in Fig. 3. To precisely control a flow of the etchant, a twin-needle system was developed: one as the injection and the other as the drain. This etching setup is enabled to inject the etchant into the injection needle and remove it from the drain needle smoothly. Furthermore, a pressure driver (SF-100, Microjet Corp.) was installed into the system to control the volume and duration of etching. In addition, this system provides accurate position control of the twin needles so that etching individually on a single microdisk can be performed.
After fabrication, each microdisk was individually treated with the liquid flow microetching method. The etching demonstration is shown in the video (see Visualization 1 in the Supplementary Material). At the beginning of this video, the twin metal needles with larger inner radius (internal diameter 100 µm, outer diameter 230 µm) than the microdisk are moving toward the target and are set at the center. This arrangement provides a good etchant flow without convection to the target. In this process, N,N-dimethylformamide (DMF) diluted with water was used as the etchant capable of selectively dissolving the clad layer. By optimization of etching parameters, it results that a concentration of 84% and etching duration of 1.2 s were preferable conditions for well-shaped undercut microdisks. If the concentration or etching time were too much, the microdisks got slightly distorted due to swelling of the etchant, especially in the sidewall, which could lead to scattering loss when light was propagated inside it.
D. Structural Analysis
After the etching process, structural analysis was performed using an optical microscope (OM), an atomic force microscope (AFM), and a SEM. Figure 4 shows the (a) SEM and (b) AFM images of the sample. The SEM image was taken at a tilt angle of 35° and shows successful formation of the undercut structure because of the existence of the shade beneath the sidewall. The roughness of the surface near the edge for both the unetched and etched microdisk on the TZ-001 layer were 1.5 nm and 4.1 nm, respectively, as calculated by root mean square (RMS) from the AFM image. Although the wet etching process can affect the microdisks by swelling, the surface still possessed high smoothness. The diameter, thickness, and taper angle of the treated microdisk were estimated to be 94 µm, 0.6 µm and 3.0°, respectively. Figure 4(c) shows comparison of cross sections of the microdisk before and after the etching process along the red dashed line. Fringe patterns were observed for both microdisks, and their patterns were slightly different in the edge area. Thus, the etched depth toward the center reached 4.6 µm in average from the edge. The undercut thickness can be obtained by comparing the AFM measurements of microdisks before and after etching and can be estimated to be 0.92 µm as shown in Fig. 4(c). This thin undercut of about 1 µm can be expected to be a pedestal suitable for evanescent coupling applications.
3. LASER EXCITATION MEASUREMENT AND RESULTS
A. Experimental Setup for Laser Excitation Measurement
To evaluate lasing performance of the inkjet-printed microdisks, laser excitation measurement was performed using the schematic of the setup shown in Fig. 5. The $Q$-switched second harmonic generation (SHG) of the Nd:YAG laser (PNG- 002025-040, Nanolase Corp.) with a pulse width of 0.5 ns and repetition rate of 20 Hz was used as the pumping source. The pumping laser was focused on the microdisk through a plano-concave lens with focal length of 20 mm, and the spot size was approximately 130 µm. While pumping the sample, the lasing signal of WGMs, which slightly leaked from defects of the microdisk around the edge, was spatially collected using the optical fiber and coupled to the spectrometer (MS7504, SOLAR TII) with blocking of the pumping light through the filter. The incident slit width and the exposure time were set at 0.2 mm and 10 s, respectively. These parameters were optimized as much as possible to detect the tiny signal of WGMs lasing that slightly leaked from the defects on the edge surface.
Fig. 5. Setup of laser excitation measurement. The microdisks were irradiated by the SHG of the Nd:YAG laser. Lasing signals from the microdisks were collected by the spectrometer through the optical fiber.
Fig. 6. Lasing characteristics of inkjet-printed microdisks. (a) Lasing spectrum of WGMs with ${Q}$ factors and (b) input–output characteristic of a directly printed microdisk on FEP film. (c) Lasing spectrum of WGMs with ${Q}$ factors and (d) input–output characteristic of an etched microdisk with an undercut. The inset of the microscope images in the graphs shows WGM lasing of each microdisk when the sample was irradiated by the pumping source.
B. Lasing Characteristics of WGMs
Figures 6(a) and 6(c) show lasing spectra of WGMs for the directly printed microdisk on the FEP film and the etched microdisk with an undercut on TZ-001, respectively. The insets of each optical microscope image show the microdisks lased by WGMs. While the directly printed microdisk was very lossy due to the FEP film [Fig. 6(a)], very few of the speckles were observed in the case of the undercut microdisk as shown in Fig. 6(c). Both spectra exhibited characteristic comb-like spectra, which is typical for WGMs. The free spectral range (FSR) was estimated to be 1.05 nm for the directly printed microdisk ($\phi = 72\;\unicode{x00B5}{\rm m}$) at the wavelengths of 588.9, 589.9, 590.9, and 592.0 nm [Fig. 6(a)], and 0.67 nm for the undercut microdisk ($\phi = 94\;\unicode{x00B5}{\rm m}$) at the wavelengths of 590.0, 590.6, 591.3, and 592.0 nm [Fig. 6(c)]. These are in good agreement with theoretical values determined by Eq. (1):
where $\Delta \lambda$ is the FSR, $\lambda$ is the resonating wavelength, $n$ is the refractive index of a microdisk, and $R$ is a microdisk radius. Therefore, it is clear that these lasing signals originate from WGMs. However, the lasing signals leaked from the defects on the surface of the undercut microdisk were very weak compared with the directly printed one. The measured WGMs were fitted based on the Lorentzian function, which is shown as orange curves, and $Q$ factors (red points) were calculated from the peak point and FWHM based on the fitting curves. The $Q$ factors were estimated to be in the order of ${10^3}$ (at maximum, $6.2 \times {{10}^3}$ at $\lambda = 592.0\;{\rm nm}$) for the undercut microdisk, though it was in the order of ${{10}^4}$ (at maximum, $2.5 \times {{10}^4}$ at $\lambda = 590.9\;{\rm nm}$) for the directly printed one. The reason why the ${Q}$ factors of the undercut microdisk did not increase, even though the formation of the undercut structure would enhance light confinement, is that the clad layer just below the edge of the microdisk was not fully removed due to the thin pedestal layer oriented for evanescent coupling. In other words, it results that the residual component of the clad polymer at etching remains on the surface of the microdisk. It is also possible that the swelling of the etching solution caused a slight damage to the microdisk. In fact, the roughness of the surface around the microdisk edge was increased from 1.5 to 4.1 nm after the etching process and would increase the scattering loss on the surface. It is reasonably understood that the etching process is different from the inkjet-printing process in that the etchant is immiscible with the microdisk polymer. The ${ Q}$ factor of WGMs of a microcavity can be expressed in terms of different loss channels that contribute to reduce the overall photon lifetime ($\tau$) as shown in Eq. (2), where ${Q_{\rm total}} = 2\pi \nu \tau$ and ${Q_{\rm mat}}$, ${Q_{\rm rad}}$, ${Q_{\rm ss}}$ are the contributions of material absorption, radiative loss, surface scattering loss, respectively. In the experiment, therefore, ${Q_{\rm ss}}$ became dominant as optical loss inside the microdisk and results in lower ${Q}$ factors than those of the directly printed microdisk. Although the roughness was somewhat large, the successful laser oscillation from the core microdisk formed on the clad pedestal layer having a high refractive index is a very impactful result:4. CONCLUSION
In this study, we successfully demonstrated the formation of the undercut structure of an inkjet-printed microdisk individually treated with the liquid flow microetching method developed for the first time to our knowledge. According to the SEM measurement, an air gap underneath the microdisk was observed, proving the successful formation of the undercut structure. In addition, the AFM was used to evaluate its roughness, which showed that it still maintained its smooth texture even after the wet etching process. In the laser excitation measurement, laser oscillation of WGMs was achieved by removing the clad pedestal layer using the liquid flow microetching, although this would not have been possible without a cutout of the clad pedestal considering the refractive index profile. Furthermore, the lasing threshold was decreased by half compared with the microdisk directly printed on the FEP film. A point to be noted is that this etching method exhibits an excellent undercut and lasing characteristics, even when using a clad pre-pedestal layer having a refractive index higher than that of core microdisks. These experimental results support the strong evidence of formation of the undercut structure, and the liquid flow microetching method becomes a powerful tool to design inkjet-printed microdisks. This technique might open doors to a new research field such as photonic circuits integrated by different types of inkjet-printed microdisks based on evanescent coupling by WGMs at the wedge boundary and be capable of controlling the specific modes inside them.
Funding
Japan Society for the Promotion of Science (JP18K14149, JP19KK0379).
Acknowledgment
The authors thank Takaaki Kanemaru (Kyushu University Hospital) for SEM image measurements and Nissan Chemical Corporation (Planning and Development Department, Performance Materials Division) for providing the hyperbranched polymers used in this study.
Disclosures
The authors declare no conflicts of interest.
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