1. Introduction

Surface plasmon polaritons (SPPs) and cavity mode (CM) induced extraordinary transmission (EOT) behaviors in a nanostructure have been used for environmental sensing, as biosensor, and as chemical sensor [1,2] due to the fact that this phenomenon is highly sensitive to the surrounding refractive index change [3]. With great application potentials, developing the capability to actively and accurately control the behavior of EOT in the temporal domain will further expand future possibilities. Temporal modulation on EOT behaviors was previously demonstrated by electrically controlling the refractive index of a liquid crystal layer [4] or using two standing surface acoustic waves (SAWs) to modulate the refractive index of the substrate [5]. However the modulation speed of the former is limited to a kHz scale owing to the limited response time of liquid crystal (sub-ms) [6]. This problem can be solved by adjusting the relative phase of two standing SAWs to coherently control the refractive index of the substrate, and the modulation frequency of the latter can be extend to a GHz region. Nevertheless, slow damping of the standing waves resulted in a residue modulating signal when two standing waves should cancel each other. We recently used a femtosecond nanoultrasonic technique in a GaN single crystal to confirm the existence of the SPP field distribution below the metal/dielectric interface [7]. The width of the applied longitudinal nanoacoustic pulse was on the nanometer scale. With such a short acoustic pulsewidth, high speed temporal modulation might be achieved through nanoultrasonics. In this paper we report the finding of the effect on the EOT change in gold nanogratings, which is induced by picosecond longitudinal nanoacoustic pulses in the near-field region. In order to explore the near-field acousto-optical effect on a nanometer scale both longitudinally and laterally, we combined a near-field scanning optical microscope (NSOM) with a femtosecond nanoacoustic technique. Therefore measurements with simultaneous high temporal (<100fs), lateral (<200nm) spatial, and longitudinal (<150nm) spatial resolutions were obtained. Our near-field measured results indicate that temporal modulation with a >11GHz bandwidth can be achieved by weak nanoacoustic pulses with 10⁻⁴ order mechanical strain through acoustically modulating the transmission change in CM inside the nanoslit. Our finding is with a potential to improve the temporal resolution on EOT modulation in the nanostructure.

2. Sample preparation and simulation details

In this work we applied gold nanogratings to excite both SPPs and CM for EOT. We first grew a 3.7-μm-thick GaN single wurtzite crystal film by Metal-Organic Chemical Vapor Deposition on a double-side polished c-plane sapphire substrate with a 350μm thickness. The thickness fluctuations for the GaN film and the sapphire substrate were both 10% . A 140-nm-thick E-beam resist layer was coated on the GaN film to define the desired pattern by E-beam lithography. The E-beam resist was Zep520A and the electron dose time was 0.35μs/dot. After developing the resist by a developer, a 70-nm-thick gold film was then coated on the GaN by thermal evaporation. We finally lifted off the remaining resist and finished the sample processing. The fabricated nanostructure covered an area of 300μmX300μm. Figure 1(a) depicts the scanning electron microscopy (SEM) image of our studied sample which consists of 1D gold nanogratings on a GaN single crystal with a sapphire substrate at the bottom. Periodicity of nanogratings is 590 nm while the height and width of the slit are both 70 nm.

 

Fig. 1 (a) SEM image of the studied sample which shows the gold nanogratings sample with a 590nm periodicity. (b) Experimental transmission spectrum and the absolute value of its derivative of the studied sample. (c) Simulated results showing the polarization dependent energy field distribution at 720nm and 670nm wavelength respectively. (d) Simulated position-dependent near-field transmitted power density difference induced by a nanoacoustic pulse at 720nm wavelength while also considering the limited lateral resolution of NSOM.

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Our experimental transmission spectrum was measured by a setup the same as the one disclosed in [1]. Experimental transmission spectrum of the studied gold nanogratings is shown in Fig. 1(b) which suggests that EOT occurred around 700nm. The absolute value of the wavelength derivative of the measured transmission spectrum can roughly provide a guideline to estimate the wavelength dependent transmission modulation strength induced by the refractive index change. From Fig. 1(b), 670nm and 720nm are two most sensitive wavelengths for EOT modulation. In order to study the field intensity distribution at these two wavelengths, we performed a finite-difference time-domain (FDTD) [8] simulation to understand the field intensity distribution at 670nm and 720nm respectively. The refractive index values of gold and GaN were taken from [9] and [10], respectively. The simulation did not consider the sapphire layer due to the fact that the GaN film was thick enough. Since the SPP and CM induced EOT change occurs at the gold/GaN interface, this phenomenon would not be affected by the sapphire substrate.

With an incident wave from the substrate side polarized parallel to the sample surface but perpendicular to nanogratings (x-direction in Fig. 1(c)), the E_x field intensity image shows the interaction result between the incident light wave and the scattered light wave. The field pattern below the nanoslit is the result of interference from these two waves. High field intensity can be observed inside the nanoslit and this phenomenon is attributed to the different effective refractive indices between the upper and lower interfaces of the slit, which causes the so-called “cavity mode.” [11] Since the incident wave is only x-polarized which propagates along the z-axis, thus the E_z field intensity image reflects the induced electric energy field by the incident wave. This induced E_z field intensity can be found to be well-confined at the gold/air and gold/GaN interfaces. Since it is well-known that the SPP field is well confined at the metal/dielectric interface and exponentially decays in the z-direction into the substrate, we refer this field as the SPP field [12].

From the FDTD simulations shown in Fig. 1(c), we can observe that the SPP field exists both at gold/air and gold/GaN interfaces for the 720nm case. However, the SPP field exists only at gold/air interface for the 670nm case. Since there is no field below the gold grating for the 670nm case, the SPP field is only sensitive to the environment change above the gold. In order to acoustically modulate the refractive index of GaN and to maximize the EOT modulation through the opto-acoustic effect [13], we chose the operating probe wavelength as 720nm.

Figure 1(d) shows the simulated near-field transmitted power density difference of the nanoacoustic effect in our sample as a function of x-axis. To simulate the effect, we simply modified the refractive index value of GaN in the FDTD simulation (Δn = ± 0.001) at 720nm while also considering the limited lateral resolution of NSOM. Due to the weak acousto-optical effect in gold [14], the refractive index change in gold has been ignored. The modulated region is chosen to be the same as the width of the nanoacoustic pulse, which is 150nm. It can be observed that the major near-field transmitted power density change is concentrated inside the nanoslit rather than the SPP field at gold/air interface. The near-field transmitted power density modulated region in the nanoslit is wider than the slit size due to the considered aperture size, which is 200nm for our adopted NSOM probe. This result indicates the possibility of directly modulating EOT in the nanogratings through modifying CM field inside the nanoslit by nanoacoustic pulses.

2. Experimental details

To modulate the refractive index of a GaN crystal, we launched nanoacoustic pulses by femtosecond laser excitation. A femtosecond nanoultrasonic system [15–17] was used to generate both nanoacoustic pulses and EOT in gold nanogratings. The light source of our experiment is a mode-locked Ti:sapphire laser (Coherent Mira 900). This laser generated 720nm 100fs pulses with a 76MHz repetition rate. There were two laser beams with different wavelengths in our system: one was the frequency-doubled pump beam (360nm), which was used to generate nanoacoustic pulses; the other was the probe beam (720nm), which was responsible for EOT generation and acousto-optical detection. In order to excite EOT, optical probe beam was chosen to be TM polarized (the polarization is on x-axis in Fig. 1(c)). A translation stage was adopted to control the optical path length difference between pump and probe beams. The transmitted probe light was then directed into a photodetector to measure the far-field signal. (Fig. 2 ) The diameters of pump and probe beams at focus were 10μm and 25μm, respectively. With a nanostructure area covering 300μmX300μm, the focused laser spot sizes were much smaller than the nanostructure size and we made sure that all the measured signals were through the nanostructure. The pump and probe average powers at the sample surface were 50mW and 10mW, respectively.

 

Fig. 2 A combination of femtosecond laser excitation system and NSOM, which has high temporal (<200fs), lateral (<200nm), and longitudinal (<150nm) resolutions.

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There are many mechanisms responsible for the optical excitation of nanoacoustic pulses, including thermal expansion [18–20], deformation potential coupling [21,22], and piezoelectric coupling [23–25]. For a GaN single crystal, deformation potential is the dominant mechanism for nanoacoustic pulses generation [26,27]. Once the above-bandgap pump light was absorbed, free carriers were excited and caused a mechanic strain/stress in the semiconductor [28], leading to the launch of nanoacoustic pulses from the GaN/sapphire interface. Since the acoustic impedances of GaN crystal and sapphire substrate are with a similar value, thus the width of the single nanoacoustic pulse is equal to the penetration depth of pump beam in the GaN single crystal, which was 150nm.

To further investigate the effect on the EOT change induced by nanoacoustic pulses in the near-field region, we combined a NSOM into our femtosecond nanoultrasonic system (Fig. 2) to measure the transmitted probe light. The NSOM applied in this work is with a shear-force feedback, which allowed us to measure the optical field and sample topography at the same time. The NSOM probe was made from an optical fiber by a pulling machine and coated with a 50nm thick silver film. With a NSOM probe aperture diameter of 200nm, a measurement with high temporal (<100fs), lateral (<200nm), and longitudinal (<150nm) resolutions were thus obtained. Compared with the far-field measurement setup, the only difference in the near-field study is that the probe beam was collected through a NSOM probe (Fig. 2).

3. Results and discussion

With the femtosecond nanoultrasonic system, excited free carriers generated stress with a mechanic strain on the order of 10⁻⁴, which is calculated based on Eq. (18) in [26]. Figure 3(a) shows the far-field measured transient transmissions change with different linear-polarized probe beams. The transient transmission change at zero time delay is due to carrier excitation by the pump beam and the exponentially-decaying background is caused by carrier relaxation. The transient transmission difference in the TM case is higher than the TE case, which suggests that EOT is induced by TM polarized incident light. A bipolar shaped signal appears between 400ps and 500ps, which existed only in the TM case. Figure 3(b) shows the carrier-dynamics background-removed bipolar shaped signal from Fig. 3(a). The corresponding delay time of this bipolar shaped signal agrees well with the acoustic traveling time from the GaN/sapphire interface to enter and leave the SPP field below the gold respectively (Fig. 3(c)) [7]. Based on the far-field measurement, we observed that the transmission was first decreased and then increased by the nanoacoustic pulses. The transmission change induced by nanoacoustic pulse was close to zero at ~465ps (Fig. 3(b)) due to the fact that the center of the nanoacoustic pulse was near gold/air interface. The nanoacoustic pulse was reflected from this interface with a sign change [29], which causes the sign change of the refractive index change and thus the sign change of the transmission modulation.

 

Fig. 3 (a) Far-field measured transient transmission in TM and TE polarized incident light. (b) Background removed far-field measured transient transmission change between 300ps to 600ps. (c) Schematic showing the locations of nanoacoustic pulse at different time delays.

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In order to understand the effect of EOT change induced by nanoacoustic pulses in the near-field region, we combined the NSOM into our femtosecond nanoultrasonic system. With a NSOM probe aperture diameter of 200-nm, the position dependent near-field transmitted power density change image can be obtained by positioning a near-field aperture on the top surface of our sample. We acquired the position dependent transmitted power density NSOM images at specific time delays, which were selected based on the far-field measurement results (400ps, 450ps, 465ps, 480ps, and 500ps). In order to image the transmitted power density change induced by nanoacoustic pulses only, we took images obtained at other time delays to subtract the image taken at 465ps, thus removing the unrelated zero-strain background signal. Figure 4(a) shows thus normalized transmitted power density NSOM images taken at different time delays. Since the one-dimensional nanogratings were designed to be with no variation along the y-axis, we also integrated our obtained images in Fig. 4(a) along y-axis for improved signal-to-noise ratio. Figure 4(b) shows thus integrated transmitted power density change versus x-axis for different time delays. By comparing the shear-force atomic force topography (also shown in Fig. 4(b)) measured by the same NSOM tip of our femtosecond time resolved NSOM system, we find that the transmitted power density change induced by the launched nanoacoustic pulses is dominated by modifying the CM inside the nanoslit rather than the SPP field at the gold/air interface, agreeing well with our theoretical prediction shown in Fig. 1(d). This phenomenon is especially obvious for time delays at 450ps and 480ps, since the peak of the nanoacoustic pulse was closed to the nanoslit (at the gold/GaN interface) at these two time delays. It is important to notice that the observed normalized transmitted power density changed signs for 450 and 480ps. This is due to the sign-change of strain of the traveling acoustic waves at the sample surface [29], thus changing the sign of the refractive index modulation and the sign of transmitted power density change, as calculated in Fig. 1(d). Due to the fact that the 200-nm NSOM tip is much wider than the 70-nm slit, the observed slit size measured by NSOM was much greater than the real slit size. Figure 4(c) shows the image that reveals the real topography measured by a tapping mode atomic force microscope (AFM) with a 5nm tip. From the high resolution AFM image, we partially attribute the observed transmitted power density fluctuation along the y-axis in Fig. 4(a) to the fluctuation of the manufactured slit width, which strongly affects the CM/SPP behavior. Finally, we conclude that the EOT in gold nanogratings can be modulated through nanoacoustic pulses modulating the CM field inside the nanoslit.

 

Fig. 4 (a) Normalized near-field transmitted power density change images taken at different time delays between 400 and 500 ps. (b) Normalized near-field transmitted power density change at different time delays and shear-force AFM data measured by the same NSOM, both are integrated and averaged along y-axis. (c) Tapping mode AFM topography measured by a 5-nm tip. (d) Near-field measured transient transmitted power density changes between 350ps to 600ps by positioning the NSOM probe on different nanoslits. Carrier dynamics background was removed. (e) Fourier transform of the near-field measured transient signal in Fig. 4(d), which suggests that a temporal modulation with a >11GHz bandwidth can be achieved by nanoacoustic pulses.

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With a NSOM probe, the position-specific transient transmitted power density difference can also be obtained by positioning the near-field aperture at a specific location on the sample top surface. We placed the NSOM probe on top of the nanoslit to measure the transient near-field transmitted power density change induced by the nanoacoustic pulse. A bipolar signal, similar to those observed in far field, appeared in our near-field measurement. Figure 4(d) shows the near-field measured transient transmission change for three different locations, all centered at nanoslits. The time shift of the centers of the bipolar signals between these measurements reveals different traveling distances for nanoacoustic pulses from the bottom to the top of the GaN film, attributing to the film thickness fluctuation (3.7μm ± 10%). It is interesting to notice that in the far-field measurement as shown in Fig. 3(b) where the signal was contributed from the whole illuminated area, the local difference would thus be averaged and the center of the bipolar signal appeared in between of the measured near-field signals while the integrated bipolar signal appeared broadened. Our near-field measured results once again confirm our conclusion that the EOT in gold nanogratings can be modulated through propagating nanoacoustic pulses, which modulates the CM field inside the nanoslit. Figure 4(e) shows the Fourier transform of different time-domain data from Fig. 4(d), which indicates a high temporal modulation bandwidth >11GHz [30] of the CM field, achieved by the weak nanoacoustic pulse.

4. Conclusion

In this paper we report that nanoacoustic pulses can modulate the EOT in gold nanogratings with a high frequency bandwidth. With the combination of NSOM and a femtosecond nanoultrasonic system, we found that this near-field modulation is through modifying the CM inside the nanoslit. Further details indicate that the propagating longitudinal nanoacoustic pulses changed the refractive index of the GaN substrate and thus modulated the near-field CM behavior of the gold nanogratings. Our finding suggests that temporal modulation with a >11GHz bandwidth can be achieved, with a high potential for future temporal and high speed control on EOT behavior in nanostructures.