Hyub Lee

Direct laser writing of graphene oxide patterns using femtosecond laser pulses with different repetition rates

We studied the thermal effect in the photoreduction process for the sub-micrometer thin graphene oxide (GO) films using femtosecond laser direct writing (FsLDW). For efficient photoreduction of GO films, various laser parameters, especially the repetition rate, were examined to identify the photothermal effect with different pulse-to-pulse spacing using a femtosecond laser having a 515-nm central wavelength. The linewidths of photoreduction were measured and the photoreduction degrees were characterized by the optical transmittance of the films. The results show that the heat accumulation due to the time interval between pulses dominantly affects the reduction degree and the linewidth.

Stability test of the silicon Fiber Bragg Grating embroidered on textile for joint angle measurement

Recently, Fiber Bragg Grating (FBG) sensors are being used for motion tracking applications. However, the sensitivity, linearity and stability of the systems have not been fully studied. Herein, an embroidered optical Fiber Bragg Grating (FBG) on a stretchable supportive textile for elbow movement measurement was developed. The sensing principle of this system is based on the alteration of Bragg wavelength due to strain from the elbow movements. The relationship between elbow movements and reflected Bragg wavelength was found to be linear. The dynamic range of FBG sensor on elbow support is between 0 and 120 degree. Finally, the stability of the FBG sensor on the supportive textile was tested during the exercise and the cleaning process with water. The sensitivity of FBG sensors for joint angle measurement and the effect of the movement and cleaning process to signals from FBG sensors after using in the real activity will be the basis knowledge for design and actual implementation of future optical fiber based wearable devices.

Fully laser-patterned stretchable microsupercapacitors integrated with soft electronic circuit components

Stretchable energy storage devices are prerequisites for the realization of autonomous elastomeric electronics. Microsupercapacitors (MSCs) are promising candidates for this purpose due to their high power and energy densities, potential for miniaturization, and feasibility of embedding in circuits; however, efforts to realize stretchable MSCs have mostly relied on strain-accommodating materials and have suffered from limited stretchability, low conductivity, or complicated patterning processes. Here, we designed and fabricated a stretchable MSC based on reduced-graphene-oxide/Au heterostructures patterned by facile and versatile direct laser patterning. An interconnected and stable 3D network composed of vertically oriented heterostructures was realized by high-repetition-rate femtosecond laser pulses. Upon transferring to polydimethylsiloxane (PDMS), the 3D network achieved a high conductivity of ~105 S m−1, and the conductivity could be maintained at ~104 S m−1 even at 50% strain. A fully laser-patterned stretchable electronics system was integrated with embedded MSCs, which will find applications in soft robotics, wearable electronics, and the Internet of Things.Graphene oxide: Spiky structures for stretchy energy storageBy transforming flat graphene oxide films into vertically oriented sheets, researchers have constructed an energy storage device that may power wearable electronics. Graphene oxide’s ability to quickly store and release charge from its surface makes it attractive for supercapacitors, which recharge significantly faster than conventional batteries. Young-Jin Kim and Pooi See Lee from the Nanyang Technological University in Singapore and colleagues have now used ultrafast lasers to controllably heat graphene oxide into a spiky 3D network that accommodates large amounts of mechanical strain. Following deposition of a gold coating and subsequent transfer to an elastomeric substrate, the team’s supercapacitor retained its high energy output even after being rolled, bent, twisted, and crumpled hundreds of times. The laser-based fabrication enabled production of customized patterns, such as wavy electrode ‘fingers’ that can attach to curved surfaces.An intrinsically stretchable and highly conductive graphene electrode based on vertically oriented graphene/Au bilayer flakes are successfully fabricated by a direct-laser-patterning at a very high-repetition rate and fast scanning speed of laser with a femtosecond pulse, allowing micro-supercapacitors to be integrated with the complete soft electronics systems that can be standalone and be customized by user’s circuit designs.

Direct laser writing of tunable diffractive micro-optics on graphene oxide film

In this study, we present diffractive optical elements printed on reduced graphene oxide (rGO) thin film using a femtosecond (fs) laser as the direct laser writing source. Graphene oxide (GO) is an interesting optical material providing unique properties such as the large light absorption modulation level during its laser reduction process; after the photoreduction, the transparent GO become opaque rGO. In order to fabricate the diffractive optical pattern of GO and rGO with sub-micrometer resolution, we use femtosecond laser pulses to induce the photoreduction of GO resulting in the ultrathin binary diffractive optical elements such as a light diffraction grating and a Fresnel lens. In addition, the rGO can play a role as a complaint electrode so that the rGO pattern can be applied to a dielectric elastomer actuation (DEA) after the pattern is transferred to a dielectric elastomer substrate with the few tens of micrometer thickness. As a high voltage is applied to the flexible substrate, it can be stretched, which leads to the expansion of the diffractive element; it means the light diffraction pattern of the element can be tunable as well. Therefore, we could control the optical specifications of the elements, such as grating period and the focal length of the Fresnel lens. Because of the simple fabrication process and arbitrary patterning capability of the femtosecond laser direct writing will provide ultrathin, tunable, compact, cost-effective, and highly efficient optical components over conventional optics.

Flexible and stretchable micro GO/rGO optical structures by femtosecond laser photoreduction

Femtosecond laser direct writing (FsLDW) is an emerging technology enabling arbitrary patterning via non-contact and maskless fabrication processes. The femtosecond pulse irradiation converts the graphene oxide (GO) into reduced graphene oxide (rGO) by removing the oxygen-containing groups of the GO, which is called photoreduction process. Because of its high printing resolution, sub-micron 2D optical structures can be printed based on GO and rGO as the base material. This study describes the fabrication of ultrathin GO/rGO optical components, which can be coated on arbitrary substrates. In the photoreduction using FsLDW, the resultant rGO’s optical properties such as refractive index, absorption coefficient, and surface reflectivity are varied on the input laser parameters, such as light wavelength, intensity, pulse repetition rate, and scan speed. To investigate the effects in detail, we performed parametric studies with different average intensity, pulse energy, and repetition rates at two wavelengths (343 nm and 515 nm). By adjusting the parameters, we could control the optical performances of the printed 2D optical structures. In addition, we studied the morphology changes of the rGO surfaces dependent on the laser parameters. The morphology plays an important role in transferring the rGO layer into the flexible polymer substrate. We will show the critical parameters for the morphology and propose the best conditions for the effective transfer of printed rGO patterns into the various flexible substrate.

Graphene-based ultrathin optical components printed by femtosecond laser direct writing method

We demonstrated various diffractive optical components based on a sub-micrometer graphene oxide (GO) film using a femtosecond (fs) laser direct writing method. The designated GO region exposed to the fs pulses is converted to reduced GO (rGO) with high optical absorbance via photoreduction. The GO/rGO pattern acts as a binary diffractive element.

Nonlinear third harmonic generation at crystalline sapphires

Third harmonic generation (THG) is a nonlinear optical phenomenon which can be applied in diverse research areas including interfacial studies, sub-wavelength light manipulation, and high sensitivity bio-molecular detection. Most precedent studies on THG have focused on dielectric and metallic materials, including silicon, gold, and germanium, due to their high nonlinear susceptibility. Sapphire, a widely-used optical substrate, has not been studied in depth for its third harmonic characteristics, despite its excellent optical transmission in the UV-visible range, high thermal conductance, and superior physical and chemical stability. In this research, we comprehensively studied THG at thin air-dielectric interfaces of sapphire wafers by controlling the wafer cutting planes, focusing depth, incidence angle, laser intensity, and input polarization of the input laser beam. These findings can lead to broader use of third harmonics for high-precision sapphire characterization, such as surface quality inspection, crystallinity determination, interfacial studies, delamination check, and real-time monitoring of crack propagation.