August 15, 2014 / Vol. 39, No. 16 / OPTICS LETTERS
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Fiber-optic ferrule-top nanomechanical resonator with multilayer graphene film Jun Ma, Wei Jin,* Haifeng Xuan, Chao Wang, and Hoi Lut Ho Department of Electrical Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China *Corresponding author: [email protected] Received May 22, 2014; revised July 4, 2014; accepted July 8, 2014; posted July 10, 2014 (Doc. ID 212620); published August 7, 2014 Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN∕Hz1∕2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (120.2230) Fabry-Perot; (160.4236) Nanomaterials. http://dx.doi.org/10.1364/OL.39.004769
Fiber-optic micro-mechanical resonators, with resonant frequency varying as a function of the measurand, have been investigated for pressure, displacement, and hydrogen sensing applications [1–4]. The resonators are excited into vibration optically with an intensitymodulated laser beam, and the vibration is interrogated by optical interferometry. Light beams for optical excitation and interrogation are transmitted via a single optical fiber cable. Compared to their electrical counterparts, fiber-optic resonators offer a number of advantages, such as electromagnetic immunity, remote detection, and multiplexing capability. Thickness is one of the key parameters that determine the resonating characteristics of mechanical resonators. Resonating films or beams with smaller thickness would provide higher sensitivity to mass, force, and displacement, and the ultimate limit would be a one-atom-thick resonator. However, conventional vibrating elements are made of materials such as silica and metal [5,6] and generally have thicknesses of micrometer/ submicrometer scale. Further reduction of the material thickness is limited by the mechanical stability and stiffness of the materials [7]. Among all the elastic materials known, graphene exhibits excellent mechanical properties, such as a high Young’s modulus of approximately 1 TPa, and arguably the highest intrinsic strength of ∼130 GPa. Recently, nanometer-thick graphene films have been studied and successfully employed for building novel nanomechanical resonators [8–10]. By transferring mechanically cleaved graphene sheets onto predefined trenches in SiO2 substrates, Bunch et al. demonstrated the world’s thinnest two-dimensional nanomechanical resonators [8]. Monolayer graphene films with a thickness of ∼0.335 nm have very low mass and an extremely large surface area, and this makes the graphene-based mechanical resonators couple strongly to the environment and be promising for high-sensitivity sensing applications [7]. In addition, the reasonably large light absorption coefficient over the entire visible and nearinfrared wavelength range (∼2.3% for monolayer 0146-9592/14/164769-04$15.00/0
graphene) also benefits the optical excitation of the nanometer-thick graphene resonators [11,12]. In this Letter, we report a compact fiber-optic nanomechanical resonator with multilayer graphene (MLG) film. By transferring a graphene film onto the end face of a ceramic ferrule instead of a SiO2 ∕Si substrate as reported [8–10], the resonance of the graphene film can be easily excited and detected via an optical fiber telemetry system. The compact resonator structure and the use of the optical fiber for the excitation and interrogation avoid the need for careful alignment of bulky optical components and make the graphene-based nanomechanical resonator more practical for real-world sensing applications. The fabrication process of the ferrule-top nanomechanical resonator is schematically shown in Fig. 1. We start with the commercial bilayer graphene grown on the copper (Cu) foil by chemical vapor deposition and fabricate polymethyl methacrylate(PMMA)/MLG/Cu foil by a repeated stacking process by following a similar procedure as described in [13]. The Cu foil of the PMMA/ MLG/Cu [refer to Fig. 1(a)] is then etched away, and the
Fig. 1. Schematic showing the process of transferring a MLG film to a ferrule top. (a) PMMA/graphene film/Cu in FeCl3 solution. (b) PMMA/graphene film transferred on the ferrule end face. (c) Removing PMMA layer from the PMMA/graphene film. (d) Ferrule end face covered with graphene film only. (e) Ferrule with an SMF inserted into its borehole. © 2014 Optical Society of America
OPTICS LETTERS / Vol. 39, No. 16 / August 15, 2014
PMMA/MLG is transferred onto a clean silica/silicon substrate. The PMMA is subsequently removed, and the thickness of the MLG film is measured by the use of an atomic force microscope. The average thickness of the same stacked MLG film is found to vary from 10 to 20 nm. The variation may arise from the wrinkles formed during the stacking process and possible PMMA residuals on the MLG. MLG films instead of single-layer graphene (SLG) are used to build the resonator for the following reasons: first, the transfer of MLG films is easier as compared with SLG, which requires post-processing, such as high-temperature heating or critical-point drying [10]. Second, the optical absorption coefficient of SLG is ∼2.3%, and it scales up with the increasing number of graphene layers [11]. A larger optical absorption coefficient requires a lower level of light power to excite the resonator and thus improves the efficiency for the resonator excitation. The detailed fabrication process of the ferrule-top MLG resonator is described as follows: first, a thin layer of PMMA is spin-coated onto a MLG/Cu foil, and the PMMA/graphene/Cu sample is dried at 90 °C for 30 min. The Cu layer of the PMMA/graphene/Cu sample is then etched away by immersing it into a ferric chloride (FeCl3 ) solution with a concentration of 0.05 g∕mL [Fig. 1(a)]. After the Cu foil is etched off, the PMMA/graphene film is washed by de-ionized water several times to remove the residual ions. This is followed by transferring the PMMA/graphene film onto the end face of the ferrule with a bore diameter of 125 μm [Fig. 1(b)]. The ferrule head is then repeatedly carefully immersed into acetone for several minutes several times to dissolve the PMMA layer [Fig. 1(c)] but leave the graphene film attached to the ferrule end [Fig. 1(d)]. Finally, a cleaved single-mode fiber (SMF) is inserted into the ferrule toward the graphene film, as shown in Fig. 1(e). Figure 2(a) shows the typical microscope images of the ferrule end face covered with the MLG film. PMMA residuals are observed on the surface of the graphene, and further washing in the acetone shows no further reduction of the residual. Heating up the ferrule with the MLG film to 400 °C in an argon environment is attempted, and the graphene surface is substantially cleaned. However, wrinkles on the surface of the graphene are found, which makes the MLG film easily broken during the process of inserting the fiber into the ferrule. The PMMAassisted transfer technique described here might provide a better means for fabricating the MLG-based acoustic sensors with thin diaphragms [14]. After being transferred onto the ferrule end face, the MLG film on the ferrule top is post-processed into a beam shape by using
Fig. 2. Microscope images of the ferrule covered with (a) circular graphene and (b) fs laser post-processed beam-shape graphene film.
a femtosecond (fs) laser. The post-processing is carried out by using a Ti:sapphire regenerative amplifier system (Spectra-Physics) with pulses of a central wavelength of 800 nm, a repetition rate of 1 kHz, and a duration of 120 fs. The fs pulses are focused onto the surface of the MLG diaphragm by a microscope objective and have a spot size (diameter at 1∕e2 intensity) of ∼2 μm. The intensity of the laser pulses is controlled by a half-wave plate in combination with a polarizer. The samples to be processed are mounted on a three-axis translation stage with a positioning resolution of 0.1 μm. The parameters for the fs fabrication are as follows: objective (×20, NA 0.5), pulse energy (20 nJ), and moving speed (10 μm∕s). Figure 2(b) shows the fabricated beam-shape diaphragm with a length of ∼125 μm and a width of 25 μm. The cavity beneath the beam-shape diaphragm is completely exposed to the external environment. The MLG film and the fiber end face form a Fabry–Perot cavity interferometer, and the cavity length is measured by monitoring its reflection spectrum with an optical spectrum analyzer (OSA). Figure 3 shows a typical reflection spectrum of the resonators measured with a broadband source and an OSA, as described in [14]. The interference fringe contrast is ∼13 dB, attributed to the larger reflectivity of the stacked MLG compared with that of the SLG. For the MLG film with a thickness of 10 nm, the calculated reflectivity is ∼8%, which is higher than the SMF end face [14]. In our experiments, the reflectivity of the MLG varies from sample to sample, but the fringe contrast of the interference patterns is sufficiently large, which varies from ∼10 to -10
-15 Reflection (dB)
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Fig. 3. Typical reflection spectrum of the MLG-based Fabry–Perot cavity.
Fig. 4. Schematic of the all-fiber system for excitation and interrogation of the graphene-film-based resonator. PD, photodetector; EDFA, erbium-doped fiber amplifier; IM, intensity modulator.
August 15, 2014 / Vol. 39, No. 16 / OPTICS LETTERS
over 20 dB. Figure 4 shows the all-fiber system for optical excitation and interrogation of the ferrule-top resonator. Two lasers with different center wavelengths (λ1 and λ2 ) are used, respectively, for the opto-thermal excitation of the MLG film and for detecting the induced mechanical vibration. For the excitation, the light from a semiconductor distributed feedback (DFB) laser operating at a wavelength of λ1 (1531 nm) passes through the intensity modulator (IM) and is delivered to the resonator via a fiber coupler, a circulator, and an optical fiber cable. The broadband absorption spectrum of the graphene offers more flexibility for the selection of the light wavelength for excitation [12]. For the detection, the light from another laser working at a wavelength of λ2 (1510 nm) is injected into the system through the second port of the same coupler. The phase difference between light beams reflected by the silica fiber end and the MLG surface will be modulated by the vibration of the MLG film. This results in modulation of the reflected light intensity at λ2 , which is detected, via the circulator, by a photodetector (PD) and is further processed by a network analyzer. To avoid interference between the excitation light λ1 and the interrogation light λ2 , the reflected light signals from the resonator pass through a bandpass filter before being detected by the PD. The network analyzer analyzes the frequency components of the electric signal from the PD. The resonant frequency of the MLG resonator corresponds to the point where the largest PD signal appears when the frequency of intensity modulation of the excitation light at λ1 is tuned. To ensure that sufficient excitation power is being delivered to the resonator, an erbium-doped fiber amplifier (EDFA) is used just after the IM. The resonator is placed in a vacuum chamber (PreiffeR, TSV-P1), and measurements are performed at room temperature in the vacuum of 1 × 10−4 mbar. Figure 5 shows the measured frequency spectrum of a MLG-film-based resonator, with multiple resonant peaks observed at the frequency of 135, 165, and 235 kHz. The resonant peak at 165 kHz might be due to the imperfect edges of the MLG beam or the edge/transverse modes due to nonuniform strain [10]. For a clamped–clamped beam with a length of L and a thickness of t, its fundamental resonant frequency f 0 may be expressed as [15] Resonant peak 2
1.0
0.6 0.4
Resonant peak 1
Amplitude (a.u.)
Amplitude (a.u.)
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Fig. 5. Amplitude versus frequency of the ferrule-top resonator in the vacuum (1 × 10−4 mbar); inset: enlarged image for the resonant peak at the frequency of 135 kHz.
v u s !2 2S u E t 0.57A f0 t A ; ρ L2 ρL2 t
4771
(1)
where E and ρ are the Young’s modulus and mass density of the beam, respectively; S is the tension per width of the beam; and A is equal to 1.03. If the peak with a frequency of 135 kHz is regarded as the fundamental mode of the resonator, for a MLG with a thickness of ∼15 nm, the stress in the film is estimated to be S 0.015 N∕m, which is smaller than that for the SLG film [16]. The excitation power incident onto the graphene film of the resonator in the experiment is small and on the level of 100 μW, which benefits from the enhanced light absorption coefficient of the MLG over SLG. For force detection, the detection limit of the resonator can be expressed as [8] s 4kB TM eff · 2πf 0 ; SF Q
(2)
where kB is the Boltzmann constant, T is temperature, and M eff is the effective mass of the graphene film. The force detection limit S F for the sample with a resonant frequency of 135 kHz and a Q value of 81 may be calculated to be ∼3.8 fN∕Hz1∕2 , which is on the same level with the value (∼0.9 fN∕Hz1∕2 ) reported in [8]. The resonant frequencies and Q values of the ferruletop MLG resonators are found to vary from 60 to 204 kHz and 81 to 103, respectively. Because of the extremely small thickness of MLG, the resonant frequency and quality factor are significantly influenced by the stress in the film. For the experimental procedure as described in this Letter, the stress is mainly induced during the transfer of the PMMA/graphene film onto the ferrule and the washing process for the removal of the PMMA. The force sensitivity of the resonator might be further improved by increasing the Q value of the resonator by stress engineering of the MLG [17]. Figure 6 shows the dependence of the Q value and the resonant frequency of the resonator on pressure from 1 × 10−4 mbar to atmospheric pressure. As shown in Fig. 6(b), the frequency for Resonant Peak 1 (refer to the frequency spectrum in Fig. 5) decreases to 88 kHz, which corresponds to a relative frequency shift of 34.8%, which is over one order of magnitude larger than that (∼1%) of the conventional resonators reported [18,19]. This large shift of the resonant frequency is due to the small thickness of the MLG diaphragm, which significantly amplifies the damping effect of the air when the diaphragm vibrates. Resonant Peak 2 at 165 kHz shows a similar trend with Resonant Peak 1, and the peak becomes difficult to observe in the frequency spectrum when the pressure is above 100 mbar because of the significant reduction in its amplitude. By detection of the pressure-induced resonant frequency shift, the resonator may be used as a novel vacuum gauge with advantages such as digital output and no need for a sealed cavity. In summary, a ferrule-top nanomechanical resonator with a MLG film is integrated with an all-fiber optical
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for possible biological/medical applications in atmospheric or liquid environments.
Unit: mBar
80 60
0.9
Q value
Signal Amplitude(mV)
1.2
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The authors acknowledge the support of NSF of China through Grant No. 61290313 and Hong Kong Polytechnic University through Grant G-YM16. The authors also thank Mr. Liyong Niu and Ms. Tingting Gao from the Institute of Textiles and Clothing, Hong Kong Polytechnic University, for their helpful discussion on graphene preparation and transfer.
20 0
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(b) Fig. 6. (a) Resonant spectra of the ferrule-top resonator at different air pressures. Inset: Q values of the fundamental resonant mode as a function of the air pressure. (b) Resonant frequency of the fundamental resonant mode as a function of the air pressure.
system for the first time to the best of our knowledge. Both the excitation light and the interrogation light are delivered by the same optical fiber telemetry cable, which makes real-world remote interrogation possible. The MLG is obtained by stacking the bilayer graphene grown on the Cu foil and is placed onto the ferrule top by a PMMA-assisted transfer process. A resonator with 10-nm-thick MLG demonstrates a theoretical force sensitivity of ∼3.8 fN∕Hz1∕2 . Because of the tiny mass of the graphene film, the resonator could also be used for high-sensitivity mass detection for the study of the dynamics of atoms and molecules that interact with graphene, as well as the specific detection of gas or bio molecules with functionalized graphene films [6,20]. However, the state-of-the-art performance of the resonator in terms of reproducibility, robustness, and the Q factor is still not as superior as the ferrule-top and fiber-top cantilevers reported previously [4,5]. The Q value of the resonator is low at atmospheric pressure due to the effect of air damping, which could be enhanced by means of (for example) stress engineering [16]. Further investigations on this will be conducted
References 1. D. Uttamchandani, Z. N. Li, L. M. Zhang, and B. Culshaw, Opt. Lasers Eng. 16, 119 (1992). 2. A. A. Said, M. Dugan, S. de Man, and D. Iannuzzi, J. Micromech. Microeng. 18, 035005 (2008). 3. D. Iannuzzi, S. Deladi, M. Slaman, J. H. Rector, H. Schreuders, and M. C. Elwenspoek, Sens. Actuators B 121, 706 (2007). 4. G. Gruca, S. de Man, M. Slaman, J. H. Rector, and D. Iannuzzi, Meas. Sci. Technol. 21, 094033 (2010). 5. N. A. D. Stokes, R. M. A. Fatah, and S. Venkatesh, Sens. Actuators A 21, 369 (1990). 6. K. B. Gavan, J. H. Rector, K. Heeck, D. Chavan, G. Gruca, T. H. Oosterkamp, and D. Iannuzzi, Opt. Lett. 36, 2898 (2011). 7. R. A. Barton, J. Parpia, and H. G. Craighead, J. Vac. Sci. Technol. B 29, 050801 (2011). 8. J. S. Bunch, A. M. van der Zande, S. S. Verbridge, I. W. Frank, D. M. Tanenbaum, J. M. Parpia, H. G. Craighead, and P. L. McEuen, Science 315, 490 (2007). 9. J. W. Suk, A. Kitt, C. W. Magnuson, Y. Hao, S. Ahmed, J. An, A. K. Swan, B. B. Goldberg, and R. S. Ruoff, ACS Nano 5, 6916 (2011). 10. A. M. van der Zande, R. A. Barton, J. S. Alden, C. S. Ruiz-Vargas, W. S. Whitney, P. H. Q. Pham, J. Park, J. M. Parpia, H. G. Craighead, and P. L. McEuen, Nano Lett. 10, 4869 (2010). 11. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. Peres, and A. K. Geim, Science 320, 1308 (2008). 12. K. F. Mak, L. Ju, F. Wang, and T. F. Heinz, Solid State Commun. 152, 1341 (2012). 13. Z. Liu, J. Li, and F. Yan, Adv. Mater. 25, 4296 (2013). 14. J. Ma, H. Xuan, H. L. Ho, W. Jin, Y. Yang, and S. Fan, IEEE Photon. Technol. Lett. 25, 932 (2013). 15. W. Weaver, Jr., S. P. Timoshenko, and D. H. Young, Vibration Problems in Engineering, 5th ed. (Wolfenden, 2007). 16. C. Lee, X. Wei, J. W. Kysar, and J. Hone, Science 321, 385 (2008). 17. Y. Oshidari, T. Hatakeyama, R. Kometani, S. Warisawa, and S. Ishihara, Appl. Phys. Express 5, 117201 (2012). 18. F. R. Blom, S. Bouwstra, M. Elwenspoek, and J. H. J. Fluitman, J. Vac. Sci. Technol. B 10, 19 (1992). 19. Y. Xu, J.-T. Lin, B. W. Alphenaar, and R. S. Keynton, Appl. Phys. Lett. 88, 143513 (2006). 20. J. Fritz, M. K. Baller, H. P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H.-J. Guntherodt, C. Gerber, and J. K. Gimzewski, Science 288, 316 (2000).
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Fiber-optic ferrule-top nanomechanical resonator with multilayer graphene film Jun Ma, Wei Jin,* Haifeng Xuan, Chao Wang, and Hoi Lut Ho Department of Electrical Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China *Corresponding author: [email protected] Received May 22, 2014; revised July 4, 2014; accepted July 8, 2014; posted July 10, 2014 (Doc. ID 212620); published August 7, 2014 Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN∕Hz1∕2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (120.2230) Fabry-Perot; (160.4236) Nanomaterials. http://dx.doi.org/10.1364/OL.39.004769
Fiber-optic micro-mechanical resonators, with resonant frequency varying as a function of the measurand, have been investigated for pressure, displacement, and hydrogen sensing applications [1–4]. The resonators are excited into vibration optically with an intensitymodulated laser beam, and the vibration is interrogated by optical interferometry. Light beams for optical excitation and interrogation are transmitted via a single optical fiber cable. Compared to their electrical counterparts, fiber-optic resonators offer a number of advantages, such as electromagnetic immunity, remote detection, and multiplexing capability. Thickness is one of the key parameters that determine the resonating characteristics of mechanical resonators. Resonating films or beams with smaller thickness would provide higher sensitivity to mass, force, and displacement, and the ultimate limit would be a one-atom-thick resonator. However, conventional vibrating elements are made of materials such as silica and metal [5,6] and generally have thicknesses of micrometer/ submicrometer scale. Further reduction of the material thickness is limited by the mechanical stability and stiffness of the materials [7]. Among all the elastic materials known, graphene exhibits excellent mechanical properties, such as a high Young’s modulus of approximately 1 TPa, and arguably the highest intrinsic strength of ∼130 GPa. Recently, nanometer-thick graphene films have been studied and successfully employed for building novel nanomechanical resonators [8–10]. By transferring mechanically cleaved graphene sheets onto predefined trenches in SiO2 substrates, Bunch et al. demonstrated the world’s thinnest two-dimensional nanomechanical resonators [8]. Monolayer graphene films with a thickness of ∼0.335 nm have very low mass and an extremely large surface area, and this makes the graphene-based mechanical resonators couple strongly to the environment and be promising for high-sensitivity sensing applications [7]. In addition, the reasonably large light absorption coefficient over the entire visible and nearinfrared wavelength range (∼2.3% for monolayer 0146-9592/14/164769-04$15.00/0
graphene) also benefits the optical excitation of the nanometer-thick graphene resonators [11,12]. In this Letter, we report a compact fiber-optic nanomechanical resonator with multilayer graphene (MLG) film. By transferring a graphene film onto the end face of a ceramic ferrule instead of a SiO2 ∕Si substrate as reported [8–10], the resonance of the graphene film can be easily excited and detected via an optical fiber telemetry system. The compact resonator structure and the use of the optical fiber for the excitation and interrogation avoid the need for careful alignment of bulky optical components and make the graphene-based nanomechanical resonator more practical for real-world sensing applications. The fabrication process of the ferrule-top nanomechanical resonator is schematically shown in Fig. 1. We start with the commercial bilayer graphene grown on the copper (Cu) foil by chemical vapor deposition and fabricate polymethyl methacrylate(PMMA)/MLG/Cu foil by a repeated stacking process by following a similar procedure as described in [13]. The Cu foil of the PMMA/ MLG/Cu [refer to Fig. 1(a)] is then etched away, and the
Fig. 1. Schematic showing the process of transferring a MLG film to a ferrule top. (a) PMMA/graphene film/Cu in FeCl3 solution. (b) PMMA/graphene film transferred on the ferrule end face. (c) Removing PMMA layer from the PMMA/graphene film. (d) Ferrule end face covered with graphene film only. (e) Ferrule with an SMF inserted into its borehole. © 2014 Optical Society of America
OPTICS LETTERS / Vol. 39, No. 16 / August 15, 2014
PMMA/MLG is transferred onto a clean silica/silicon substrate. The PMMA is subsequently removed, and the thickness of the MLG film is measured by the use of an atomic force microscope. The average thickness of the same stacked MLG film is found to vary from 10 to 20 nm. The variation may arise from the wrinkles formed during the stacking process and possible PMMA residuals on the MLG. MLG films instead of single-layer graphene (SLG) are used to build the resonator for the following reasons: first, the transfer of MLG films is easier as compared with SLG, which requires post-processing, such as high-temperature heating or critical-point drying [10]. Second, the optical absorption coefficient of SLG is ∼2.3%, and it scales up with the increasing number of graphene layers [11]. A larger optical absorption coefficient requires a lower level of light power to excite the resonator and thus improves the efficiency for the resonator excitation. The detailed fabrication process of the ferrule-top MLG resonator is described as follows: first, a thin layer of PMMA is spin-coated onto a MLG/Cu foil, and the PMMA/graphene/Cu sample is dried at 90 °C for 30 min. The Cu layer of the PMMA/graphene/Cu sample is then etched away by immersing it into a ferric chloride (FeCl3 ) solution with a concentration of 0.05 g∕mL [Fig. 1(a)]. After the Cu foil is etched off, the PMMA/graphene film is washed by de-ionized water several times to remove the residual ions. This is followed by transferring the PMMA/graphene film onto the end face of the ferrule with a bore diameter of 125 μm [Fig. 1(b)]. The ferrule head is then repeatedly carefully immersed into acetone for several minutes several times to dissolve the PMMA layer [Fig. 1(c)] but leave the graphene film attached to the ferrule end [Fig. 1(d)]. Finally, a cleaved single-mode fiber (SMF) is inserted into the ferrule toward the graphene film, as shown in Fig. 1(e). Figure 2(a) shows the typical microscope images of the ferrule end face covered with the MLG film. PMMA residuals are observed on the surface of the graphene, and further washing in the acetone shows no further reduction of the residual. Heating up the ferrule with the MLG film to 400 °C in an argon environment is attempted, and the graphene surface is substantially cleaned. However, wrinkles on the surface of the graphene are found, which makes the MLG film easily broken during the process of inserting the fiber into the ferrule. The PMMAassisted transfer technique described here might provide a better means for fabricating the MLG-based acoustic sensors with thin diaphragms [14]. After being transferred onto the ferrule end face, the MLG film on the ferrule top is post-processed into a beam shape by using
Fig. 2. Microscope images of the ferrule covered with (a) circular graphene and (b) fs laser post-processed beam-shape graphene film.
a femtosecond (fs) laser. The post-processing is carried out by using a Ti:sapphire regenerative amplifier system (Spectra-Physics) with pulses of a central wavelength of 800 nm, a repetition rate of 1 kHz, and a duration of 120 fs. The fs pulses are focused onto the surface of the MLG diaphragm by a microscope objective and have a spot size (diameter at 1∕e2 intensity) of ∼2 μm. The intensity of the laser pulses is controlled by a half-wave plate in combination with a polarizer. The samples to be processed are mounted on a three-axis translation stage with a positioning resolution of 0.1 μm. The parameters for the fs fabrication are as follows: objective (×20, NA 0.5), pulse energy (20 nJ), and moving speed (10 μm∕s). Figure 2(b) shows the fabricated beam-shape diaphragm with a length of ∼125 μm and a width of 25 μm. The cavity beneath the beam-shape diaphragm is completely exposed to the external environment. The MLG film and the fiber end face form a Fabry–Perot cavity interferometer, and the cavity length is measured by monitoring its reflection spectrum with an optical spectrum analyzer (OSA). Figure 3 shows a typical reflection spectrum of the resonators measured with a broadband source and an OSA, as described in [14]. The interference fringe contrast is ∼13 dB, attributed to the larger reflectivity of the stacked MLG compared with that of the SLG. For the MLG film with a thickness of 10 nm, the calculated reflectivity is ∼8%, which is higher than the SMF end face [14]. In our experiments, the reflectivity of the MLG varies from sample to sample, but the fringe contrast of the interference patterns is sufficiently large, which varies from ∼10 to -10
-15 Reflection (dB)
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1540 1560 Wavelength (nm)
1580
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Fig. 3. Typical reflection spectrum of the MLG-based Fabry–Perot cavity.
Fig. 4. Schematic of the all-fiber system for excitation and interrogation of the graphene-film-based resonator. PD, photodetector; EDFA, erbium-doped fiber amplifier; IM, intensity modulator.
August 15, 2014 / Vol. 39, No. 16 / OPTICS LETTERS
over 20 dB. Figure 4 shows the all-fiber system for optical excitation and interrogation of the ferrule-top resonator. Two lasers with different center wavelengths (λ1 and λ2 ) are used, respectively, for the opto-thermal excitation of the MLG film and for detecting the induced mechanical vibration. For the excitation, the light from a semiconductor distributed feedback (DFB) laser operating at a wavelength of λ1 (1531 nm) passes through the intensity modulator (IM) and is delivered to the resonator via a fiber coupler, a circulator, and an optical fiber cable. The broadband absorption spectrum of the graphene offers more flexibility for the selection of the light wavelength for excitation [12]. For the detection, the light from another laser working at a wavelength of λ2 (1510 nm) is injected into the system through the second port of the same coupler. The phase difference between light beams reflected by the silica fiber end and the MLG surface will be modulated by the vibration of the MLG film. This results in modulation of the reflected light intensity at λ2 , which is detected, via the circulator, by a photodetector (PD) and is further processed by a network analyzer. To avoid interference between the excitation light λ1 and the interrogation light λ2 , the reflected light signals from the resonator pass through a bandpass filter before being detected by the PD. The network analyzer analyzes the frequency components of the electric signal from the PD. The resonant frequency of the MLG resonator corresponds to the point where the largest PD signal appears when the frequency of intensity modulation of the excitation light at λ1 is tuned. To ensure that sufficient excitation power is being delivered to the resonator, an erbium-doped fiber amplifier (EDFA) is used just after the IM. The resonator is placed in a vacuum chamber (PreiffeR, TSV-P1), and measurements are performed at room temperature in the vacuum of 1 × 10−4 mbar. Figure 5 shows the measured frequency spectrum of a MLG-film-based resonator, with multiple resonant peaks observed at the frequency of 135, 165, and 235 kHz. The resonant peak at 165 kHz might be due to the imperfect edges of the MLG beam or the edge/transverse modes due to nonuniform strain [10]. For a clamped–clamped beam with a length of L and a thickness of t, its fundamental resonant frequency f 0 may be expressed as [15] Resonant peak 2
1.0
0.6 0.4
Resonant peak 1
Amplitude (a.u.)
Amplitude (a.u.)
1.0
0.8
-4
Pressure: 10 mBar Q~81
0.8 0.6 0.4 0.2 0.0 120 125 130 135 140 145 Frequency (kHz)
0.2 0.0
120
140
160 180 200 220 Frequency (kHz)
240
260
Fig. 5. Amplitude versus frequency of the ferrule-top resonator in the vacuum (1 × 10−4 mbar); inset: enlarged image for the resonant peak at the frequency of 135 kHz.
v u s !2 2S u E t 0.57A f0 t A ; ρ L2 ρL2 t
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(1)
where E and ρ are the Young’s modulus and mass density of the beam, respectively; S is the tension per width of the beam; and A is equal to 1.03. If the peak with a frequency of 135 kHz is regarded as the fundamental mode of the resonator, for a MLG with a thickness of ∼15 nm, the stress in the film is estimated to be S 0.015 N∕m, which is smaller than that for the SLG film [16]. The excitation power incident onto the graphene film of the resonator in the experiment is small and on the level of 100 μW, which benefits from the enhanced light absorption coefficient of the MLG over SLG. For force detection, the detection limit of the resonator can be expressed as [8] s 4kB TM eff · 2πf 0 ; SF Q
(2)
where kB is the Boltzmann constant, T is temperature, and M eff is the effective mass of the graphene film. The force detection limit S F for the sample with a resonant frequency of 135 kHz and a Q value of 81 may be calculated to be ∼3.8 fN∕Hz1∕2 , which is on the same level with the value (∼0.9 fN∕Hz1∕2 ) reported in [8]. The resonant frequencies and Q values of the ferruletop MLG resonators are found to vary from 60 to 204 kHz and 81 to 103, respectively. Because of the extremely small thickness of MLG, the resonant frequency and quality factor are significantly influenced by the stress in the film. For the experimental procedure as described in this Letter, the stress is mainly induced during the transfer of the PMMA/graphene film onto the ferrule and the washing process for the removal of the PMMA. The force sensitivity of the resonator might be further improved by increasing the Q value of the resonator by stress engineering of the MLG [17]. Figure 6 shows the dependence of the Q value and the resonant frequency of the resonator on pressure from 1 × 10−4 mbar to atmospheric pressure. As shown in Fig. 6(b), the frequency for Resonant Peak 1 (refer to the frequency spectrum in Fig. 5) decreases to 88 kHz, which corresponds to a relative frequency shift of 34.8%, which is over one order of magnitude larger than that (∼1%) of the conventional resonators reported [18,19]. This large shift of the resonant frequency is due to the small thickness of the MLG diaphragm, which significantly amplifies the damping effect of the air when the diaphragm vibrates. Resonant Peak 2 at 165 kHz shows a similar trend with Resonant Peak 1, and the peak becomes difficult to observe in the frequency spectrum when the pressure is above 100 mbar because of the significant reduction in its amplitude. By detection of the pressure-induced resonant frequency shift, the resonator may be used as a novel vacuum gauge with advantages such as digital output and no need for a sealed cavity. In summary, a ferrule-top nanomechanical resonator with a MLG film is integrated with an all-fiber optical
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OPTICS LETTERS / Vol. 39, No. 16 / August 15, 2014
for possible biological/medical applications in atmospheric or liquid environments.
Unit: mBar
80 60
0.9
Q value
Signal Amplitude(mV)
1.2
40
The authors acknowledge the support of NSF of China through Grant No. 61290313 and Hong Kong Polytechnic University through Grant G-YM16. The authors also thank Mr. Liyong Niu and Ms. Tingting Gao from the Institute of Textiles and Clothing, Hong Kong Polytechnic University, for their helpful discussion on graphene preparation and transfer.
20 0
0.6
10
-5
-3
-1
1
10 10 10 Pressure (mBar)
100 0.3
50
-4
2.0
1000 0.0
3
10
1x10
75
100 125 Frequency (kHz)
150
175
(a)
180
Frequency (kHz)
160 140 120 100
Resonant peak 1 Resonant peak 2
80 10
-4
10
-3
10
-2
-1
0
10 10 10 Pressure (mBar)
1
10
2
10
3
(b) Fig. 6. (a) Resonant spectra of the ferrule-top resonator at different air pressures. Inset: Q values of the fundamental resonant mode as a function of the air pressure. (b) Resonant frequency of the fundamental resonant mode as a function of the air pressure.
system for the first time to the best of our knowledge. Both the excitation light and the interrogation light are delivered by the same optical fiber telemetry cable, which makes real-world remote interrogation possible. The MLG is obtained by stacking the bilayer graphene grown on the Cu foil and is placed onto the ferrule top by a PMMA-assisted transfer process. A resonator with 10-nm-thick MLG demonstrates a theoretical force sensitivity of ∼3.8 fN∕Hz1∕2 . Because of the tiny mass of the graphene film, the resonator could also be used for high-sensitivity mass detection for the study of the dynamics of atoms and molecules that interact with graphene, as well as the specific detection of gas or bio molecules with functionalized graphene films [6,20]. However, the state-of-the-art performance of the resonator in terms of reproducibility, robustness, and the Q factor is still not as superior as the ferrule-top and fiber-top cantilevers reported previously [4,5]. The Q value of the resonator is low at atmospheric pressure due to the effect of air damping, which could be enhanced by means of (for example) stress engineering [16]. Further investigations on this will be conducted
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