Near-perfect microlenses based on graphene microbubbles

. Microbubbles acting as lenses are interesting for optical and photonic applications such as volumetric displays, optical resonators, integration of photonic components onto chips, high-resolution spectroscopy, lithography, and imaging. However, stable, rationally designed, and uniform microbubbles on substrates such as silicon chips are challenging because of the random nature of microbubble formation. We describe the fabrication of elastic microbubbles with a precise control of volume and curvature based on femtosecond laser irradiated graphene oxide. We demonstrate that the graphene microbubbles possess a near-perfect curvature that allows them to function as reflective microlenses for focusing broadband white light into an ultrahigh aspect ratio diffraction-limited photonic jet without chromatic aberration. Our results provide a pathway for integration of graphene microbubbles as lenses for nanophotonic components for miniaturized lab-on-a-chip devices along with applications in high-resolution spectroscopy and imaging. dimensional transition metal dichalcogenides (TMDs). In particular, his group studies the optical and electronic properties of different phases of 2D-TMDs. He has demonstrated that it is possible to induce phase transformations in atomically thin materials and utilize phases with dispa-rate properties for field effect transistors, catalysis, and energy storage. His interests include synthesis of graphene and 2D nanomaterial-based soft materials and their applications in nanoionics, molecular/ion separation, electrochemi- cal energy storage and conversion, and biomedicine. and nanomaterial interaction. laser manipulation of two-dimensional materials and fabrication of functional nanostructures and nanomaterials for effective har- nessing and storage of clean energy from purifying and air for clean environment and imaging and spectroscopy, and nanofabrica- tion using ultrafast laser toward fast-speed all-optical communications and intelligent manufacturing.


Introduction
Microbubbles are widely applied as actuators in microfluidic devices 1 for microfluidic mixing, 2 ink-jet printing, 3 and logic circuitry. 4 More importantly, microbubbles have demonstrated broad applications in therapeutic and medical imaging, 5 biomedicine, 6 and cell and DNA trapping and manipulation. 7 In addition, applications of microbubbles in photonics for lithography 8 and optical resonators have recently been demonstrated. 9 For applications in imaging, trapping, and photonics, creation of bubbles at accurate positions with a controllable volume, curvature, and stability is essential. However, currently microbubbles can be generated with ultrasonic waves or laser pulses only when substrates are immersed in liquids, 10 which are unstable and occur at random locations. For integration into biological or photonic applications, the generation of well controlled and stable microbubbles fabricated using a technique that is compatible with current processing technologies is highly desirable but has not yet been demonstrated. In this paper, we demonstrate the generation of microbubbles on the surface of a solid material using femtosecond laser pulse induced gas generation in a layer of graphene material for the first time. The position, volume, and curvature of the microbubbles are precisely controlled. Such a high-quality bubble can be used for advanced optoelectronic devices with high-precision requirements. As a showcase, nearperfect lenses for focusing broadband white light into an ultrahigh aspect ratio diffraction-limited photonic jet without chromatic aberration are demonstrated in this paper. Our results provide a pathway for integration of graphene microbubbles as dynamic and high-precision nanophotonic components for miniaturized lab-on-a-chip devices, along with applications in high-resolution spectroscopy and medical imaging.

Materials and Methods
The impermeability of graphene has been used to generate random microbubbles 11 and tune their properties by electric fields. 12 We irradiate graphene oxide (GO) with a focused laser [13][14][15] in precise locations using a commercial direct laser writing system (Innofocus NanoPrint 3D ) to create microbubbles in highly localized regions, as shown in Fig. 1(a). In this study, we use the low repetition rate (1 kHz) femtosecond laser writing technique, which is considered to be a "cold" process, 16 because the highinstant temperature achieved by a femtosecond laser pulse decreases to room temperature on the timescale of microseconds, before the irradiation of the next femtosecond pulse. Thus the GO film cannot be heated up by the femtosecond laser during the reduction process. The laser irradiation leads to a reduction of GO via evolution of oxygen functional groups. 17,18 These evolved gaseous species 19,20 are trapped within the reduced GO, forming near-perfect microbubbles at precise locations [see Figs. 1(b)-1(d) and Sec. S1 in the Supplementary Material for more details]. The amount of gas, and therefore the volume, and the size of microbubbles can be finely tuned by varying the laser power and exposure area during the one-step process (see Secs. S2 and S3 in the Supplementary Material).
The microbubbles can be identified by the observation of Newton's rings that are formed by the interference of light reflected by interfaces between the trapped gas and graphene and between the gas and substrate interface, as indicated in Figs. 1(d) and 1(e). The lateral size of the microbubble is defined by the outermost dark ring that is formed via destructive interference at the edge of the microbubble. The height of the microbubbles can be calculated from the number of rings by a simple equation: 2hn gas ¼ mλ, where n gas is the refractive index of the trapped gas (∼1), λ is the illumination wavelength (650 nm), and m is an integer corresponding to the number of rings. The highly symmetric Newton's rings observed in Fig. 1(e) and their close agreement with the theoretically calculated pattern in Fig. 1(d) suggest that the produced microbubble has a near-perfect spherical curvature, which is attributed to the exceptional mechanical strength of graphene.
The ability to generate and eliminate microbubbles of various lateral dimensions and heights is demonstrated in Fig. 2. The pristine area before irradiation is shown in Fig. 2(a). As shown in Figs. 2(b)-2(d), the GO film was irradiated in locations indicated by the squares of different sizes. It can be clearly seen that the lateral dimensions and the number of Newton's rings (height of the microbubble) increase with increasing laser exposure area (see Sec. S4 in the Supplementary Material). This is expected since the laser irradiation of a larger area leads to a greater reduction of GO and the release of more gas. The microbubbles can be eliminated by increasing the laser power to ablate the graphene film to release the gases inside the bubbles [Figs. 2(e)-2(h)]. In Fig. 2(h), the surface of the reduced GO film after the removal of microbubbles is shown. Our analysis reveals that the film is largely flat after release of the gases, due to the high elasticity of graphene. The process for eliminating the bubbles takes about 1.5 s and depends on the volume of enclosed gas and the area of the ablated film. Microbubble generation and gas release can be viewed in Videos S1 and S2.
The surface curvature and the volume of microbubbles can be accurately calculated by tracing the position of the rings and reconstructing the surface profile, as shown in Fig. 3. In Fig. 3(a), the area of the microbubble is marked with a white dashed circle and the cross section of the Newton's rings is marked with a yellow dashed line. The measured radial intensity distributions of the rings in different sized microbubbles shown in Fig. 2 are plotted in Fig. 3(b) and appear as a series of bright and dark fringes. The corresponding reconstructed surface curvatures are shown in Fig. 3(c) and indicate that the height of the microbubbles increases with the size of irradiated area.
The three-dimensional (3D) profile of a typical microbubble is shown in Fig. 3(d) and reveals that it exhibits a highly symmetric spherical shape. The uniform thickness of the graphene film, which ensures the isotropic surface tension along all directions, is a key parameter for achieving a highly symmetric spherical surface of the bubble. In order to confirm that the near-perfect spherical surface is achieved, we have fitted the surface curvature with a perfect circle function, which shows a good match (Sec. S6 in the Supplementary Material). This means that the surface profile of the microbubbles is very close to an ideal spherical shape.

Results
The highly uniform surface quality and near-perfect spherical shape of the microbubbles make them ideal reflective microlenses for focusing optical energy. A microscopic imaging system combined with a CCD camera, as shown in Fig. 4(a), was used to measure the intensity distributions in the focal plane of microlenses so that key parameters could be extracted. This was done by placing the graphene microbubble sample on a 1D piezoelectric scanning stage, which scans along the direction of the optical axis (z direction) to record a layer-by-layer series of 2D intensity distributions of the focal spot (the characterization process can be viewed in Videos S3 and S4). To evaluate the focal length of the microbubble, the radius (R) of the sphere was calculated from the measured radius (r) and height (h) of the microbubble, as shown in Fig. 4(b). The focal length of the microbubble can then be obtained from f ¼ R∕2. The numerical aperture (NA) of the microbubble, a parameter that directly determines the resolution of the bubble focus, can be determined using NA ¼ r∕f. In addition, the full-width at half-maximum (FWHM) of the focal spot (d) is d ¼ λ∕ð2 × NAÞ, which can  be used to characterize the focusing performance of the microlenses. The focal intensity distribution of the microbubble illuminated with a He-Ne laser (633 nm) shows a surprisingly strong 3D focal spot (see Sec. S9 and Fig. S8 in the Supplementary Material), indicating the high quality of the microbubble. Furthermore, from the measured radius and height of the microbubble, NA is estimated to be 0.27, which corresponds to d ¼ 1.18 μm, matching well with the measured FWHM in the y direction. This demonstrates that diffraction-limited focusing can be achieved by the graphene microbubble.
Compared with diffractive graphene flat lenses, [21][22][23][24] a key advantage of reflective graphene microlenses is that the reflection occurs at the gas-reduced GO interface and therefore material dispersion does not play a role. This means that graphene microbubble microlenses are able to focus light at different wavelengths without dispersion, similar to the reflection lenses in Fourier transform infrared spectrometers. 25 To verify this unique feature experimentally, an ultrabroadband white light source (a halogen lamp) was employed for focusing measurements.
The resulting intensity distributions in the x-y plane (focal plane) and the x-z plane are shown in Figs. 4(c) and 4(d), respectively, and can be seen to exhibit photonic jet-like features. The corresponding intensity plots along the y direction and z direction are shown in Figs. 4(e) and 4(f). Negligible broadening of the FWHM (∼1.31 μm) is observed along the y direction despite the incoherence of the light source. In contrast, the FWHM along the z direction is significantly reduced to 30 μm due to the broad spectral distribution of the light source, which increases the resolution. These results demonstrate that a 3D focal spot can be achieved with ultra-broadband white light illumination, as shown in Fig. 4(g). The focal spots of different wavelengths overlap extremely well with each other at all normalized intensity levels (0.8, 0.6, and 0.5), as indicated by different colors [ Fig. 4(g)].

Discussion
This microbubble photonic jet created by reflective graphene microlenses is technologically significant and has clear advantages over a photonic jet that is generated by transmission through a glass microsphere. 26 The reflection mode operation of our microlenses means that they are insensitive to material dispersion 27 and therefore can generate diffraction-limited focus for broad wavelengths. Second, the size and radius of the microbubbles are readily tunable in situ, allowing adjustable focusing properties. Therefore, GO microbubbles offer a flexible and in situ tuning mechanism for focal length without degrading the focal resolution of the photonic jet. The large focal length leads to long working distances, which is highly desirable for 3D biological imaging in lab-on-a-chip devices. 28 As a result, graphene microbubbles are expected to find broad applications in integrated devices for imaging, spectroscopy, and sensing by generating microlenses at arbitrarily desired positions directly on substrates.
dimensional transition metal dichalcogenides (TMDs). In particular, his group studies the optical and electronic properties of different phases of 2D-TMDs. He has demonstrated that it is possible to induce phase transformations in atomically thin materials and utilize phases with disparate properties for field effect transistors, catalysis, and energy storage.
Dan Li is an Australian Laureate Fellow and professor in materials science and engineering in the Department of Chemical Engineering at the University of Melbourne, Australia. His current research interests include synthesis of graphene and 2D nanomaterial-based soft materials and their applications in nanoionics, molecular/ion separation, electrochemical energy storage and conversion, and biomedicine.
Baohua Jia received her PhD in 2007 from Swinburne University of Technology, Australia. She is a full professor and a founding director of the Centre for Translational Atomaterials and Research Leader at Swinburne University of Technology. Her research focuses on the fundamental light and nanomaterial interaction. In particular, her work on laser manipulation of two-dimensional materials has led to the design and fabrication of functional nanostructures and nanomaterials for effective harnessing and storage of clean energy from sunlight, purifying water and air for clean environment and imaging and spectroscopy, and nanofabrication using ultrafast laser toward fast-speed all-optical communications and intelligent manufacturing.