1. Introduction

Nonlinear optical materials have been extensively studied for a variety of applications from ultrafast optical data communication devices to all-optical computational circuits [1–6]. Graphene Oxide (GO), as an ultrathin 2D material, has attracted much research interest in photonics due to its giant third-order nonlinear responses including saturable absorption, optical limiting and nonlinear refraction [7,8]. In practice, GO, which functions as a saturable absorber, has already been demonstrated in femtosecond lasers, ultrafast optical modulators and ultrafast photodetectors [9–11]. Meanwhile, it has been reported that the nonlinearity of GO can be tuned by modifying the oxygen-containing functional groups [12]. As an ultrathin 2D optical material, GO can be fabricated directly onto an integrated circuit through direct laser writing [13]. This tunable easy-to-fabrication method paves the way for a new generation of long-dreamed photonic integrated circuits.

Despite these attractive applications, one of the main obstacles of using graphene based materials as practical devices is the weak absorption, for which a typical layer of graphene can only absorb 2.3% of the incident light [14]. For high performance of ultrafast devices and photonic circuits, the third-order nonlinearity, in particular the saturable absorption, is crucial to the functionalization of ultrashort pulse shaping and manipulation. To further enhance the saturable absorption of GO, photoreduction with the assist of photocatalyst or hybridizing it with metallic materials has been studied to achieve enhanced saturable absorption [14–16]. However, most of these hybrid systems are prepared in the liquid phase, which remains challenging to further reduce the operating laser irradiance for practical applications. Thus, new methods to enhance the saturable absorption of GO are highly desired.

Recently, it has been discovered that silica microspheres can serve as particle lenses, which could generate photonic nanojets (PNJs) and enhance the nonlinear performance of materials [15]. The PNJ is a high-intensity, narrow and non-evanescent electromagnetic beam, which can be obtained by focusing the incident light with a low loss dielectric microsphere of the diameter greater than the illuminating wavelength. It has been studied extensively for its focusing effect and local field enhancement for enhanced Raman scattering, nanolithography and super-resolution imaging [17–22]. However, few studies have been carried out to investigate the enhanced nonlinear properties through the PNJ in solid-state systems. Furthermore, the effect of the concentration on nonlinear performance is still unclear. In this paper, PNJs generated by silica microspheres are adopted to enhance the nonlinearity of the GO film. With the open aperture Z-scan measurement, the normalized peak transmittance of the GO film can be enhanced around 38% after hybridizing it with silica microspheres. The tunability of GO as a saturable absorber is studied through varying the concentration of microspheres. It is observed that increasing the concentration of microspheres leads to enhanced nonlinear performance. While at a high concentration, the enhancement decreases due to the scattering among multi-layered microspheres. This tuning property is crucial to the ultrashort pulse shaping and manipulation. Meanwhile, the hybrid system can be simply fabricated with a uniform GO film over a large area. Thus, this solid-state hybrid system of GO as a tunable saturable absorber offers great potential for integrated photonics as well as tunable ultrafast laser devices.

2. Experiment

2.1 Sample fabrication

The GO solution being used was purchased from Graphenea at a concentration of 4 mg/mL. It was characterized by the Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS), which display its morphology and components respectively as shown in Fig. 1. Silica microsphere dispersion in water was purchased from Thermo Scientific. Most of the microspheres are in spherical shape at an average diameter of 1µm.

 

Fig. 1 (a) Schematics of a silica microsphere GO hybrid system and (b) the photonic nanojet generated by a single microsphere. (c) XPS spectrum and (d) SEM image of GO.

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As shown in Fig. 1, the hybrid system comprising of the GO film and silica microspheres on top was fabricated by drop casting. Firstly, 0.7 mL GO solution was dispersed on a piece of 25 mm × 75 mm cover glass and then spread evenly to cover the whole glass surface. The glass slide was transferred onto a hot plate at a temperature of 105 °C for several minutes to make solid-state GO films. Drop casting offers a simple one-step fabrication method. Large-size sample can also improve the flatness of the coated GO film. Then, the solution containing silica microspheres were diluted with DI water at the ratio of 1:15, 1:10 and 1:5, labelled as MSP1, MSP2 and MSP3, respectively. One drop from each concentration was dispersed on the GO film and put on a hot plate until water evaporated. These three silica microsphere solutions with different concentrations were applied on the same piece of the GO film at different positions to make different hybrid systems. The resulting hybrid systems were characterized by an optical microscope and the average coverage at each concentration was also measured.

2.2 Z-scan characterization

Z-scan technique is an important tool to characterize the nonlinear properties of optical materials attributed to its high sensitivity and simplicity [23,24]. In particular, the sample is scanned through the focal region of a focused laser beam between two focusing lenses. In this experiment, open aperture Z-scan measurement was performed using a femtosecond pulsed laser (Coherent Libra, 50 fs, 1 kH, beam diameter 8 mm) at the wavelength of 800 nm and on-axis peak fluence of 0.19 J/cm² to characterize the nonlinearity of the hybrid system. The absorption spectrum of the hybrid system was characterized by the UV-VIS-NIR spectrophotometer (OceanOptics, HR2000 + ). All the Z-scan experiments were carried out under the same laser fluence and incident light wavelength. Normalized transmittance was plotted against the sample position along Z-axis.

3. Finite-difference time-domain (FDTD) simulation

FDTD simulation using Lumerical FDTD software was carried out to study the local field enhancement of silica microspheres as demonstrated in Fig. 1(b). The PNJ is a high-intensity and narrow electromagnetic beam which propagates into the background medium from the shadow-side surface of a plane-wave illuminated low loss dielectric microsphere or microcylinder with a diameter greater than the illuminating wavelength [17]. The PNJ generated by a dielectric microsphere can be analysed using Mie theory [21]. Since the PNJ generated by a silica microsphere is neither attributed to the resonance nor the coupling effect, the investigation of the field enhancement of a single microsphere would be sufficient. Therefore, in the simulated hybrid system, a single silica microsphere at a diameter of 1 µm was placed on the GO film as the incident light propagating along -Z direction at the wavelength of 800 nm. The refractive indices of air and the quartz substrate were also taken into consideration. As shown in Fig. 2, the silica microsphere functions as a particle lens which focuses the incident light due to the light refraction at the silica-air interface. Based on the simulation result, it can be concluded that the PNJ generated by the silica microsphere enhances the intensity of the incident light by more than 8 times at the central region of the focusing area and it can propagate more than 400 nm after emitting from the bottom of the microsphere.

 

Fig. 2 Simulation of field enhancement of single silica microsphere at a diameter of 1µm.

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4. Results and discussion

Z-scan measurements are carried out for the GO film alone and the GO film hybrid system with MSP1, MSP2 and MSP3, respectively. The normalized transmittance curves as well as the fitting curves are also plotted. For the pristine GO film as shown in Fig. 3(a), the peak in the transmittance curve indicates a strong saturable absorption, which is comparable to the recently published result [12]. The saturable absorption of GO is attributed to the ground state bleaching of the sp² domain [25]. In particular, the band gap of the sp² domain is only around 0.5 eV. Therefore, electrons can be easily excited by photons, resulting in the depletion of valence band and the saturation of conduction band, which prohibit further absorption of photons [7,15]. As a result, the transmittance of the material increases.

 

Fig. 3 Open aperture Z-scan measurement results of (a) GO and GO hybrid system with silica microspheres at an average coverage of (b) 10.2%, (c) 15.6% and (d) 85.8%.

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With the introduction of silica microspheres, the nonlinear performance of the GO film is enhanced significantly. As shown in Fig. 3, under the same laser fluence (0.19 J/cm²) and wavelength (800 nm), the normalized peak transmittance of the GO film alone is 1.32, while the GO film hybrid system with silica microspheres has a peak transmittance of 1.76, 1.82, 1.63 in the case of MSP1, MSP2 and MSP3, respectively. Compared to the GO film alone, the hybrid system demonstrates a significantly enhanced saturable absorption behaviour with up to around 38% increment on the peak transmittance. Silica microsphere does not exhibit any optical nonlinearity at the wavelength of 800 nm [15]. Thus, the giant enhancement is attributed to the PNJ generated by the silica microsphere. The PNJ is a highly focused beam with a subwavelength beam waist on the shadow side of the microsphere. Silica microsphere functions as a particle lens with focusing effect, which induces strong field enhancement as demonstrated in Fig. 2. With a strong localized light intensity, the number of photons interacting with the GO film increases and the saturable absorption process is greatly enhanced.

To better understand the enhancement of silica microspheres, the nonlinearity of the hybrid system at various concentrations of silica microspheres is also studied. The silica microspheres at a diameter of 1 µm are diluted to different concentrations, known as MSP1, MSP2 and MSP3 with increasing concentration. The average coverage of silica microspheres is characterized by an optical microscope. The surface coverage of silica microspheres is estimated by taking the ratio of the total area covered by the microspheres and the area of the whole image taken by the optical microscope. Multiple images are taken at various locations of the same hybrid system and the average value is taken as the average coverage. With increasing concentration of silica microspheres, the average coverage also increases as shown in Fig. 3 at 10.2%, 15.6% and 85.8% for MPS1, MSP2, and MSP 3, respectively. The open aperture Z-scan measurement results for these three systems are plotted, as shown in Figs. 3(b) to 3(d). With increasing concentration of silica microspheres, the nonlinear performance is enhanced first. The peak transmittance increases from 1.76 to 1.82 with around 5% increment on the average coverage of silica microspheres. For MSP1 and MSP2 as shown in Fig. 4, the solution containing silica microspheres is well diluted and most microspheres are dispersed on the GO film as a single layer with negligible microspheres aggregated together. Each microsphere functions as a particle lens which induces local field enhancement. As a result, the PNJ generated by each microsphere is focused onto the GO film surface. Hence, more microspheres at a single layer result in better nonlinear performance. However, when the concentration of silica microspheres is too high as in the case of MSP3, the nonlinear performance worsens with normalized peak transmittance of 1.63. This phenomenon is attributed to the multi-layered microspheres being formed. At a high concentration, silica microspheres aggregate together and display in the form of multilayers as shown in Fig. 4. PNJs generated by microspheres interfere and some focal regions of PNJs cannot reach the GO film surface. As a result, the multi-layered microspheres scatter the incident light and reduce the transmittance of the GO hybrid system. Therefore, the nonlinearity of the hybrid system can be tuned by varying the concentration of silica microspheres. The tunable property of this saturable absorber system is prominent to various applications from passive mode locking to ultrashort laser pulses shaping and manipulation [8–11].

 

Fig. 4 Schematic of the GO hybrid system at different coverages of silica microspheres.

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5. Conclusions

Hybrid system of the GO film coated with silica microspheres is studied for its enhanced nonlinear performance. FDTD simulation is carried out to explain the field enhancement of the PNJ generated by the silica microsphere. The hybrid system is characterized by the open aperture Z-scan measurement. Through hybridizing with silica microspheres, the saturable absorption of the GO film enhances significantly with up to around 38% increase of normalized peak transmittance. The nonlinearity of the hybrid system can also be tuned by varying the concentration of microspheres. The easy one-step fabrication method and local field enhancement of PNJs can also be applied to other 2D materials and solid-state systems to manipulate their nonlinear properties. This research demonstrates the enhanced nonlinear performance of the GO film and the possibility of tunable nonlinearity by flexibly varying the concentration of silica microspheres, which offers much potential on integrated photonics and passively mode-locked lasers.