ZnO-Based Microcavities Sculpted by Focus Ion Beam Milling
© The Author(s). 2016
Received: 17 May 2016
Accepted: 24 June 2016
Published: 30 June 2016
We reported an easy fabrication method to realize ZnO-based microcavities with various cavity shapes by focused ion beam (FIB) milling. The optical characteristics of different shaped microcavities have been systematically carried out and analyzed. Through comprehensive studies of cathodoluminescence and photoluminescence spectra, the whispering gallery mode (WGM) was observed in different shaped microcavities. Up further increasing excitation, the lasing action was dominated by these WGMs and matched very well to the simulated results. Our experiment shows that ZnO microcavities with different shapes can be made with high quality by FIB milling for specific applications of microlight sources and optical devices.
Recently, micro/nanoscience made a great progress and attracted extensive research efforts because they have potential applications in optoelectronic devices, such as microlight sources, photo-switches, and optical integrated circuits [1–4]. ZnO is considered to be one of the promising materials for making microsize devices, which would be able to operate at ultraviolet (UV) region due to its wide bandgap of about 3.37 eV and large exciton binding energy of about 60 meV at room temperature . In addition, the specific crystal facets of single crystalline wurtzite ZnO bulk parallel to the c-plane have a naturally hexagonal cross section, which would be able to readily serve as a high-quality whispering-gallery mode (WGM) resonator owing to its relatively high reflective index (~2.4) in comparison to the surrounding air. The high-quality factor (Q) of WGM microcavity (MC) could be achieved by the total internal reflection (TIR) that could facilitate to further reduce the lasing threshold. Over the past decades, the ZnO-based WGM optical resonator was first reported by Nobis et al.  and the corresponding WGM lasing action was observed in a ZnO nanonail .
In terms of fabrication of ZnO MCs, less studies utilized top-down etching technique [8, 9] because it might require complex fabrication steps as well as subsequent precise patterning procedure. In addition, it was difficult to define the sample position and the material quality was limited by the substrate. Therefore, the fabrication of low dimensional laser resonators by top-down approach remains a challenging task. In contrast, bottom-up synthesized nanostructures that usually form hexagonal symmetry of crystal morphology have inherent advantages over top-down fabricated structures such as high material quality, smooth facet, and high assembly throughput. ZnO nano/microstructures have a perfect hexagonal cross section, and WGM in such hexagonal structure has been experimentally studied in detail in ZnO micro- and nanowires and disks [4, 7, 10–12]. Light propagating around the WGM resonator due to the total internal reflection effect has been investigated by cathodoluminescence (CL) [10, 13, 14] and photoluminescence (PL) [2, 11]. Various bottom-up fabrication methods have been reported to realize ZnO nanostructures, such as hydrothermal [3, 4], chemical vapor deposition (CVD) [11, 12, 15], and vapor phase transport (VPT) [2, 13, 14] method. However, the challenge of bottom-up synthesized ZnO MC is the controllability in position of a single microcavity because the spatial isolation is important for optically investigating one single nanoscale cavity . The spatial distribution of samples made by the bottom-up synthesis method usually appears clustering arrangement. To study optical characterization of an individual object could be tedious process. Recently, many researchers have demonstrated the capability to fabricate various types of polygonal microcavities, but the morphology and size of the microcavities are difficult to control [9, 12, 16–18]. On the other hand, the top-down approach could benefit from the ready single crystalline ZnO material that could provide very good optical characteristics and laser gain. However, the ZnO-based single crystalline resonators are rarely reported because of the difficult fabrication process, which strongly depends on the availability of substrate [15, 19].
Recently, we have successfully fabricated the membrane-type ZnO MC. The ZnO membrane was cut from a single crystalline ZnO substrate by using focused ion beam (FIB) milling. However, this fabrication process only allows us to realize a square-shaped ZnO MC . In this work, we developed an easy method to fabricate ZnO MCs with a controllable submicrometer spatial resolution to realize various shape ZnO MCs in which whispering-gallery mode lasing can be achieved. To obtain a high-quality MC, the starting material was the ZnO bulk substrate. Then, the FIB milling and glass tip technique were applied for the cavity formation on the ZnO bulk substrate and position to the targeted substrate. The narrow linewidth WGM mode lasing was observed in circular, hexagonal, pentagonal, and square resonators, verified by using the microphotoluminescence (μ-PL) system. Detailed characteristics of whispering-gallery mode lasing microcavities have been discussed and analyzed.
Figure 2a is a schematic of the ZnO MC put on the SiO2/Si substrate. The 2-μm-thick SiO2 was grown by the wet oxidation method, and this SiO2 layer was used to serve as a low index layer to properly support the high Q WGM. The ZnO MC was excited by the 355-nm third-harmonic generation of an Nd:YVO4 pulse laser with 0.5-ns duration and 1-kHz repetition rate, and the pumping spot size was approximately 30 μm. The resultant fluorescence emitting from the ZnO MC was collected through an optical microscope with an objective lens of ×100 and then coupled to a spectrometer through an optical fiber. Figure 2b to g displays the scanning electron microscopy (SEM) images of suspended ZnO MCs with square, pentagon, hexagon, octagon, circle, and even hexagram shapes, fabricated by the FIB technique. Specifically, the various shaped ZnO MCs shown in Fig. 2 were realized with a perfect symmetry, which were considered to be difficult by using aforementioned conventional fabrication methods. From these SEM images, the sidewalls of ZnO MCs have smooth facets that are benefit to form the WGM resonance. Therefore, we have clearly demonstrated symmetric or even exotic ZnO MCs which can be realized by this technique.
Results and Discussion
The transverse-magnetic-polarized (TM) WGMs can also be obtained in a similar way. For the ZnO MC that contains c-plane facets, the WGM modes mainly occur in the c-plane of ZnO which is parallel to the normal surface of as-prepared samples because the TE fields in the c-plane of ZnO exhibits a greater optical gain than the TM-polarized emission . However, in some cases, the TM-polarized WGMs could be observed if the proper excitation is applied on the MCs.
where N c is the number of allowed optical modes within the gain bandwidth, n th is the threshold carrier density, a is the differential gain, υg is the group velocity, τp is the photon lifetime, and n tr is the transparency carrier density. It can be seen that the threshold carrier density would be increased as more lasing modes participating the same exciton reservoir. In contrast, the limited cavity mode numbers in the pentagon ZnO MC actually resulted in a lower threshold power.
We presented a novel bulk nanomachining technique for carving various polygonal MCs. The different shapes of single crystalline ZnO MCs were realized with circle, square, pentagon, hexagon, octagon, and even hexagram shapes. The fabrication process is achieved by FIB milling and subsequent utilizing a glass tip to control the exact position of samples. The lasing characteristics of circular, square, pentagonal, and hexagonal MCs were further measured and analyzed. The experimental results showed the pentagon cavity has potential to achieve WGM lasing with the lowest threshold power density of 0.26 MW/cm2 owing to the cavity exhibiting limited resonant modes. Our study revealed that the FIB milling could be a handy process to design and fabricate different kinds of MCs and the inherent good crystal properties could be simultaneously preserved for further practical coherent microlight sources in integrated photonic devices and optical biosensor applications.
Authors acknowledge the help of Prof. H. C. Kuo at National Chiao Tung University for the technical support. This work was partially supported by the Ministry of Education Aim for the Top University program and by Minister of Science and Technology (MOST) under Contract Nos. NSC102-2221-E-009-135-MY3, NSC102-2221-E-009-156-MY3, and MOST 104-2221-E-009-096-MY3.
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