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Abstract
A light-driven diffraction grating incorporating two grating patterns with different pitches atop a photothermal actuator (PTA) has been proposed. It is based on graphene oxide/reduced graphene oxide (GO/rGO) induced via femtosecond laser direct writing (FsLDW). The rGO, its controllable linewidth, and transmission support the formation of grating patterns; its noticeably small coefficient of thermal expansion (CTE), good flexibility, and thermal conductivity enable the fabrication of a PTA consisting of a polydimethylsiloxane layer with a relatively large CTE. Under different intensities of light stimuli, diffraction patterns can be efficiently tailored according to different gratings, which are selectively addressed by incident light beam hinging on the bending of the PTA. This is the first demonstration of combining gratings and PTA, wherein the GO plays the role of a bridge. The light-driven mechanism enables the contactless operation of the proposed device, which can be efficiently induced via FsLDW. The diffraction angle could be changed between 2° and 6° horizontally, and the deviation of side lobes from the main lobe could be altered vertically in a continuous range. The proposed device may provide powerful support for activating dynamic diffraction devices in photothermally contactless schemes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tunable diffraction grating has been extensively utilized in applications such as beam steering, switching, and shaping [1–7]. Conventional tunable gratings, which are based on mechanical stretching, advanced optical materials, or actuators [6–11], have been widely used for optical modulation. However, the mechanical damage caused by stretching [5,6] as well as the sophisticated fabrication and operation required for advanced optical materials [8–10] pose critical challenges. The actuators have been used to convert external stimuli to mechanical motions through various energy conversion mechanisms [11–16]. Stimuli are mostly physically delivered via wiring circuits to actuators, as in the case of electrically driven ones [11–13]. A photothermal actuator (PTA), which is simple to implement and works in a contactless manner, is perceived as a promising candidate to facilitate the practical application of tunable optical devices. The PTA typically has a bilayer configuration comprised of a photothermally active layer as well as a thermal expansion layer, exhibiting distinct coefficients of thermal expansion (CTE). The photothermally active layer, with a high optical absorption and relatively low CTE, converts the optical energy into heat, causing an increase in local temperature and the conduction of the generated heat to the entire structure. In the meantime, the thermal expansion layer, with a higher CTE, undergoes volume expansion when the active layer is heated. Therefore, the PTA structure tends to bend toward the photothermally active layer in response to the strain mismatch between the two layers. Consequently, a large bending motion can be achieved through light irradiation in a simple and contactless operation [14]. To date, there has been no report on a tunable grating driven by such a PTA.
Recently, reduced graphene oxide (rGO) has garnered increasing attention as it features excellent characteristics such as high thermal and electrical conductivities, flexibility, and high chemical/physical stability [17–21]. It is clear that GO can be reduced to rGO via thermal reduction, chemical reduction, and photoreduction. Notably, the photoreduction of GO based on femtosecond laser (Fs-laser) direct writing (FsLDW) can create various diffraction GO/rGO patterns. The related process is simple and maskless, and the removal of oxygen groups from GO by controlling the Fs-laser power can be exploited to efficiently tailor the properties of rGO [22–25]. For the existing actuators, there has been no effort on the use of the GO/rGO-based actuator in diffraction devices because the GO films are relatively too thick to facilitate high optical transmission [12–14]. Using FsLDW, an ultra-thin GO/rGO film that renders high optical transmission as well as good flexibility and thermal conductivity can be adopted to embody the PTA and diffraction grating.
In this work, we propose a light-driven diffraction grating (LDDG) based on a PTA, in which a pair of grating patterns with different pitches are inscribed on top of a GO/rGO film via FsLDW. A polydimethylsiloxane (PDMS) film, which has a relatively large positive CTE, serves as the thermal expansion layer for the PTA and the substrate for the GO/rGO film that exhibits a significantly smaller negative CTE. The photothermal actuation is enabled by the substantial disparity in CTE between PDMS and GO/rGO [13,14,26–28]. Incident light is rendered to undergo either of the two gratings depending on the bending of the PTA. With the aim of precisely creating the grating in conjunction with the PTA as intended, the FsLDW process for inducing rGO patterns has been intensively explored in terms of their linewidth and optical transmission/absorption. The optical response of the LDDG could be effectively tailored by virtue of light stimuli assisted heating; thus, the proposed device can be a viable candidate facilitating beam steering and modulation in a contactless manner [2–7,29].
2. Design and result
2.1 Proposed LDDG and its operation
As depicted in Fig. 1, the proposed LDDG consists of two diffraction gratings of different pitches atop a PTA. The GO/rGO layer, with a negative CTE of −6 × 10−6/K [13], and PDMS layer, exhibiting a positive CTE of +3.1 × 10−4/K [28], serve as a photothermally active layer and thermal expansion layer for the PTA, respectively. The radical CTE contrast initiates the bending motion toward GO/rGO due to the strain mismatch in volume between the two layers as a result of light heating. Two gratings, Gratings A and B, are inscribed on the GO film by FsLDW to diffract light according to $\Lambda \sin {\theta _m} = m\lambda $, where Λ is the grating period; λ (650 nm) and θm are the wavelength of incident light beam and the mth order diffraction angle, respectively. Considering that we are primarily interested in the first order, θ will be used to represent θ1 hereafter. When the white light stimulus is deactivated, the proposed LDDG attains a flat state. Incident light beam impinges on Grating A to produce a diffraction pattern, characterized by an angle θA according to a pitch ΛA. When the light stimulus is activated, the bending motion of the LDDG toward the GO/rGO side renders an oblique incidence of the beam on Grating B, with an angle θB corresponding to a pitch ΛB. Hence, the diffraction angle changes from θA to θB in response to the bending, entailing variations in the corresponding diffraction patterns. The incident angle effectively varies in the direction perpendicular to the grating depending on the bending; thus, the side lobes of the diffracted wave deviate from the main lobe in the x-direction [30,31]. The diffracted wave establishes a curved spot distribution as a result of a deviation between the side and main lobes in the xy-plane. A light-driven area based on GO/rGO is constructed at one side of the PTA to absorb and convert incoming light to heat, which will be transported to the entire PTA in a non-contact manner.
Fig. 1. Operation of the proposed LDDG for (a) the flat state with the light stimulus deactivated and (b) the bending state with the stimulus activated.
To elucidate the modulation of the incident light beam mediated by the LDDG, an analytical model has been developed, as illustrated in Fig. 2. Gratings A and B inscribed on the GO/rGO film are marked in black and gray, respectively, where a0 and b0 corresponding to Gratings A and B are the points of irradiation for the incident beam, respectively. When the light stimulus is turned on, the LDDG attains the bending state, and points a0 and b0 displace to a and b, respectively. As the PTA bends, the incident beam passes through point b instead of point a0, as compared with the case of no stimulus. As a result, the bending alters the diffraction angle. For both the flat and bending states, the beam should be routed to go through either of Grating A or B. The distance l0 between the vertical line OT and the boundary of the two gratings, the incident beam diameter d, and the bending angle α should satisfy the following condition:
2.2 Fabrication of the LDDG
The fabrication process includes the preparation of a GO film on a PDMS substrate, accompanied by the photoreduction thereof by Fs-laser, as delineated in Fig. 3(a). PDMS was spin-coated on a transparent plastic substrate at 800 rpm and baked at 80°C for 2 h. GO solution was subsequently spin-coated on the PDMS substrate at 600 rpm for 60 s and then heated at 50°C for 30 min. The GO solution with a concentration of 5 mg/ml was prepared using the Hummers method [32]. Here, a 700-nm thick GO film was prepared on a 90-µm thick PDMS substrate. The combination of ultra-thin GO and PDMS in the proposed device yielded a relatively high transmittance as well as excellent durability compared with previous thermal actuators [12–14]. The photoreduction process to convert GO into rGO was fulfilled by an Fs-laser system with a pulse width of 250 fs. As illustrated in Fig. 3(b), the GO/rGO patterning was executed through direct laser writing at λ = 520 nm. GO exhibits a high absorption in the visible band [19,22], and the green laser (acquired through a second harmonic generation system from a 1040-nm Fs-laser beam) initiated an efficient GO-to-rGO photoreduction. Finally, the processed films peeled off the plastic substrate.
Fig. 3. (a) Procedure for fabricating the LDDG. (b) Schematic of the FsLDW adopted for the photoreduction of GO.
The linewidth, optical transmission, and absorbance of rGO were controlled by manipulating the average Fs-laser power (PFs) during the photoreduction, while the repetition rate and scanning speed were set at 500 kHz and 0.5 mm/s, respectively. An objective lens of 40× magnification, with a numerical aperture of 0.6, was used to tightly focus the Fs-laser beam onto the surface of the GO film. Figure 4(a) shows the microscope image of the GO film after line scanning under different PFs. The area irradiated by the laser beam was transformed from golden-brown GO to black rGO, by virtue of the increased absorption in the visible band. As plotted in Fig. 4(b), the linewidth increased with stronger PFs. A series of rGO patterns, with dimensions of 1.5 × 1.0 mm2, were inscribed to measure the optical transmission and absorbance, with the intention of discovering the optimum PFs for fabricating the diffraction grating and driving PTA, respectively. Figure 4(c) shows the optical transmission of GO and rGO in the spectral range between λ = 450 nm and 900 nm, which was checked by a spectrometer (USB 4000, Ocean optics). The optical absorbance indicates the capacity of the rGO and GO regions to absorb light. The observed absorbance spectra of rGO via the same spectrometer, as plotted in Fig. 4(d), a PFs of 23.8 mW was revealed to yield the highest absorption. Thus, this one PFs was selected for producing a light-driven area leading to enhanced photothermal conversion for the PTA. To mimic the ideal diffraction grating model, rGO exhibiting a low transmission was necessary to ensure efficient diffraction, in collaboration with a highly transmissive thin GO film [33]. The case of PFs of 23.8 mW led to the lowest transmission at λ = 650 nm as depicted in Fig. 4(c). However, the corresponding rGO line was observed to be undesirably surrounded by irregular transition regions of brighter color, according to Fig. 4(a). The transition regions might adversely affect the diffraction gratings because its optical transmission remained unknown. The condiction of PFs = 3.2 mW, corresponding to a linewidth of 2 µm, has been finally selected to inscribe the grating patterns for the proposed LDDG.
Fig. 4. Characterization of rGO after FsLDW. (a) Microscope image of surface of the GO/rGO film after line scanning with average Fs-laser power PFs. (b) Inscribed linewidth as a function of PFs. (c) Measured optical transmission and (d) absorbance spectra of the GO/rGO film formed on a PDMS substrate with respect to PFs.
To verify the photothermal properties of rGO, an rGO pattern (1.5 × 1.5 cm2) was inscribed on a GO film formed on a PDMS substrate and a thermal imaging camera (E5-XT, FLIR) was used to monitor the temperature. As shown in Fig. 5(a), with the white light stimulus turned on, the rGO region recorded a higher temperature compared to the GO and PDMS substrate. As shown in Fig. 5(b), the temperature of rGO increased from room temperature (25 °C) and reached the maximum within tens of seconds after the light was turned on. The temperature variation depended on the distance from the rGO film to the stimulus. The temperature was maintained to be constant, which gradually dropped to room temperature when the stimulus was turned off. As the G and D bands in the Raman spectra for graphene are associated with the E2g mode of sp2 carbon atoms and sp3-hybridized carbon vibrations, respectively, the ratio of the D peak intensity to the G peak intensity (ID/IG) can be used to identify graphite-like materials. As shown in Fig. 5(c), overall changes in the D and G bands implied a decrease in the structural defects and re-establishment of sp2 network after Fs-laser illumination, due to the removal of oxygen-containing groups within the GO basal planes [13,34]. Although GO presented a poor thermal property, the restoration of ordered sp2 domain improved the thermal conductivity after Fs-laser based photoreduction, which cooperated with the high optical absorption to contribute to the high light-to-heat conversion efficiency of rGO [13]. To further investigate the chemical composition of the GO/rGO film, samples were inspected via X-ray photoelectron spectroscopy, as plotted in Fig. 5(d). The ratio of the carbon peak intensity to the oxygen peak intensity (IC/IO) increased because of the photoreduction, suggesting that the oxygen groups were eliminated and laser-induced rGO formed as predicted.
Fig. 5. Photothermal properties of rGO on a PDMS substrate. (a) Thermal image captured by an infrared camera, when white light stimulus irradiated the sample. (b) Measured temperature of rGO surface when distances between the rGO film and the stimulus were 15 and 20 cm. (c) Raman and (d) X-ray photoelectron spectra of GO and rGO.
Next, we examined the variations of the bending angle in terms of the footprint of the LDDG, by referring to Eq. (1). The configuration of PTA is shown in Fig. 6(a). The length of the light-driven area was set at 1.5 mm. The length of Grating B, lB, was fixed at 2 mm to ensure a larger area than the incident collimated beam, with a diameter of d = ∼1 mm, which was provided by a supercontinuum laser (SuperK compact, NKT Photonics). Thus, the length of the device, L, was determined by that of Grating A, lA. In addition, Gratings A and B exhibited pitches of 6 µm and 18 µm, respectively, as depicted in Fig. 6(c). The PTA was driven by shining the light-driven area with the white light stimulus from an LED lamp (SLDH-21-W, Seiwa Optical), placed 3 mm away from it. The light intensity of the stimulus was adjusted by altering the electrical power with the help of a pulse power supply. Under the peak electrical power of 9 W, the angle of bending was estimated from captured images of the PTA in action, as plotted in Fig. 6(b). With d = 1 mm and l0 = (L – 2) mm, αmin is the minimum to satisfy Eq. (1), alluding to the weakest bending required to allow the incident beam to transmit each grating individually. As shown in Fig. 6(b), αmeas of 45° was only larger than αmin of 41.9° for the case of L = 12.5 mm (lA = 9 mm). Thus, the proposed device was designed to be 12.5-mm long. The bending increased with L to a certain extent and then decreased. This could be due to the positive correlation between the bending and the multiplication of the increased temperature and length difference between PDMS and GO/rGO, resulting from the distinct CTEs between them [28]. When the temperature was elevated enough to maximally elongate the two layers, the length difference was governed only by the initial length of PDMS (L), under negligibly expanded GO/rGO. Consequently, the length disparity and the bending angle were expected to increase with L. Moreover, for a constant light stimulus, the temperature change was chiefly affected by the dimension of the PTA. According to the law of heat transfer, a finite amount of heat can invoke a smaller increment in temperature for an object exhibiting a larger volume. For a small temperature increment, the PDMS would stop elongating prior to reaching its maximum length of the expansion. It was finally expected that the increase in temperature and the resulting bending would decrease with L.
Fig. 6. Influence of the dimension of the LDDG. (a) Illustration showing the size of each part for the proposed device. (b) Calculated minimum and measured maximum bending angles for different lengths of the device. (c) Microscope images of the diffraction gratings corresponding to pitches of 6 µm and 18 µm.
2.3 Characterization of the fabricated LDDG
The tunability of the diffraction patterns generated by the LDDG was explored with respect to the intensity of the light stimulus, as delineated in Fig. 7(a). A white LED lamp was used to activate the light-driven area, while the incident beam from the supercontinuum laser was launched to the created gratings via a spectral filter (FB650-10, Thorlabs), centered at λ = 650 nm with a 10-nm bandwidth. It was experimentally revealed that in the case of a beam diameter of 1 mm at λ = 650 nm, the incident visible light beam should be practically limited to ∼20 mW in power, in order not to invoke unwanted photothermal bending of the LDDG. Diffraction patterns were monitored on the receiving screen, placed 50 cm away from the LDDG. Gratings A and B pertaining to the LDDG were deemed to yield different diffractions corresponding to 6° and 2°, respectively. Considering that the pitch of Grating B was three times larger than that of Grating A, the 1stA and 3rdB order diffracted waves were located at the same position, where mthA and mthB denote the mth order diffraction for Gratings A and B, respectively. The 0th order beams from the two gratings overlapped as desired. The positions and intensities of the 1st and 3rd order beams were regarded as the key indicators of LDDG performance. The bending angle α could be tailored by controlling the intensity of the stimulus, while the diffraction characteristic was assessed in terms of the diffraction angle θ and the deviation angle δ. θ was defined by projecting the 0th and 1st order diffracted beams in the horizontal yz-plane. For the obliquely incident beam, δ was used to estimate the deviation of the side lobes from the main lobe [30,31]. δ is the angle which the projection of the 0th order beam creates with the 3rdB beam for Grating B in the vertical xz-plane. α and δ were determined to be positive when the device bent toward the GO/rGO. With the light stimuli deactivated, the device bent toward the PDMS with an initial bending angle of −45°, as shown in the inset of Fig. 7(b). This was ascribed to the residual stress of LDDG resulting from the fabrication process and peeling-off of the device from the plastic substrate. Both the demonstrated maximum of +45° and the initial α were found to be larger than αmin = 41.9°, as expected. Consequently, three distinct states were identified as portrayed in Fig. 7(a).
Fig. 7. Investigation of the diffraction characteristics of LDDG according to the intensity of the light stimulus. (a) Experimental setup and (b) recorded diffraction patterns when the electrical powers of the stimulus light were 0, 3, 4.5, 6, and 9 W, respectively. Insets show the LDDG at each state.
The influence of the intensity of the light stimulus, which was controlled by the electrical power of the LED, is shown in Fig. 7(b). In the absence of the stimulus, the incident beam was impinging upon Grating B with an 18-µm period, and the LDDG was in a bending state according to the initial α. A curved spot distribution was observed to exhibit small diffraction corresponding to θ = 2° and a slightly negative deviation equivalent to δ = −0.45°. For stronger optical stimuli, the curve relating to the spot distribution was straightened. The intensities for the 1stB and 2ndB order beams for Grating B gradually decreased and disappeared almost completely for an electrical power of 3 W. The intensity corresponding to the 3rdB spot, which was supposed to diminish, appeared to be slightly strengthened owing to the coexistence of the 1stA spot, when the incidence changed from Grating B to A. When the power was increased to 4.5 W, the device assumed the flat state. The transition of states was invoked by the photothermal conversion of rGO, resulting in the mismatch between the expansion of the GO/rGO and PDMS layers. A larger θ equivalent to 6° emerged when all the spots belonging to Grating B disappeared, while only the diffraction spots associated with Grating A were observed on the screen. All the diffraction spots were linearly located, resulting in δ = 0°. The diffraction pattern was vertically reversed for the power of 9 W such that α = 45° when light absorption and conversion increased in the rGO region. Accordingly, the diffraction angle returned to θ = 2°, while the deviation angle increased to +0.45°. The migration of the higher-order diffraction spots toward the negative x-direction (i.e., downward) originated from the obliquely incident light beam. The demonstrated beam tuning transpired within a period of 10 s, which was reversible with the help of the light stimuli (see Visualization 1). As indicated in Visualization 1, the stimulus light was witnessed to tenuous background noise. From the perspective of the transmitted laser light, degradation in the signal-to-noise ratio was experimentally estimated to be ∼0.7 dB. With the aim of assessing its lifetime and stability, the proposed LDDG was operated by repeatedly imposing the bending thousands of times. Alongside the optical power of the 1stB order beams under two different bending states as delineated in Figs. 7(a)(i) and (iii), the measured diffraction angle θ and deviation angle δ are shown in Figs. 8(a) and 8(b) under the electrical powers associated with the stimulus light of 0 W and 9 W, respectively. Considering no significant degradations in the device performance and no physical distortions or damages were witnessed even after as many as 3,000 operation cycles, the proposed device based on flexible GO/rGO was validated to be highly stable and sufficiently robust. The proposed device is expected to facilitate facile two-dimensional beam steering.
Fig. 8. Stability test of the fabricated LDDG according to the number of operation cycles. The diffraction angle θ and deviation angle δ as well as the optical power of the 1stB order beams were measured for electrical powers relating to the stimulus light of (a) 0 W and (b) 9 W, respectively.
3. Conclusion
The proposed LDDG was a PTA with two grating patterns exhibiting different periods inscribed by FsLDW on the surface; it was composed of GO/rGO and PDMS layers. The difference in CTEs between GO/rGO and PDMS enabled the photothermal activation of the LDDG. The linewidth, optical transmission, and absorption of rGO with different powers of Fs-laser were studied in detail for forming gratings and light-driven area. When changing the intensity of light stimuli, the diffraction angle was controlled by the bending of the PTA, due to different gratings irradiated by the incident light beam. In the direction perpendicular to the change of diffraction angle, the varying angle of the incidence caused the tunable deviation between the side and main lobes. Characteristics of simple manufacturing and non-contact activation of the proposed device can be utilized to produce flexibly configurable diffraction components, which can be applied to tunable integrated optical devices.
Funding
National Research Foundation of Korea (2018R1A6A1A03025242, 2020R1A2C3007007).
Disclosures
The authors declare no conflicts of interest.
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