1. Introduction

Ultrafast modulation by the plasmon polaritons on the optical reflection or transmission can be utilized to achieve optical switching devices [1–6]. Localized surface plasmon resonance (LSPR) [7–10] and surface plasmon polaritons (SPPs) [11–14] have been the physical mechanisms for the plasmonic optical switching effects. Spectral shift of the plasmonic resonance modes due to the strong optical excitation has been the main mechanism for the optical switching effects. Coupling of the LSPR or SPPs with the photonic resonance modes in periodic lattices of metallic or hybrid metallic/dielectric structures is generally the main factor that influences the efficiency of the optical switching process. The pure electronic oscillation dynamics at the plasmonic resonance determines the speed of the switching.

Various optical switch devices have been demonstrated in the spectral range from the visible to the terahertz bands. Nanophotonic structures based on dielectric materials show a response time in the order of picoseconds or even nanoseconds [15–18], however, plasmonic structures exhibit femtosecond time-scale relaxations in their modulation on the optical electric fields. Therefore, plasmonic nanostructures are usually employed for achieving ultrafast optical switch devices [19–25]. Optical switching effect based on propagating SPPs at the aluminum/silica interface was observed with a speed shorter than 200 fs and a modulation depth of 7.5% [19]. Optical nonlinearity of plasmonic metamaterials was employed to achieve an optical switching device with a speed of 190 fs and an efficiency of 9% [20]. Coupling between photonic and plasmonic resonance modes in a hybrid microcavity was used to achieve ultrafast all-optical switching process, showing a transmission modulation larger than 100% and a switching speed of 3.4 ps [21]. We explored optical switching functions in the waveguide gold nanowire grating and achieved a working speed shorter than 200 fs [22]. Interference between plasmonic electronic oscillation and ultrafast optical electric field have been investigated, which is important for the design and optimization of ultrafast optical switching devices [23]. Recently, we demonstrated ultrafast optical switching devices using cross-stacked gold nanowire networks [24] and hemispherical gold nanoshell arrays [25], we have achieved a switching speed as fast as 270 fs and a switching efficiency higher than 27.8%. In these devices, we have enhanced the optical switching effect by coupling the photonic resonance modes in the photonic structures into different types of LSPRs.

In this work, we construct an ultrafast optical switching device by coating a silver layer onto the surface of a nano-grating structure that was produced by etching a photoresist mask grating into the conductive thin film of indium tin oxide (ITO). Two different surface plasmon polaritons working symmetrically and asymmetrically in terms of the oscillation of the free electrons were excited on the bottom and top surfaces of the silver nanoshell, which have different lifetimes in transient absorption dynamics.

2. Optical switching device based on periodically modulated silver film

Figure 1 shows the design and the principles of the optical switching device. In the fabrication, a silver film as thick as 30 nm was deposited on an indium-tin-oxide (ITO) grating. The ITO layer had an original thickness of about 200 nm. A photoresist (PR) grating with a period of 400 nm was produced on the top of the ITO layer by interference lithography. Then, inductively coupled plasma (ICP) etching enabled transfer of the template PR grating into the ITO layer and produced an ITO grating with a modulation depth of about 40 nm. Thus, the thickness of the ITO grating varies periodically between 126 and 160 nm.

 

Fig. 1 (a) SEM image for top view of the Ag-coated ITO grating. (b) SEM image for the cross-sectional view of the silver shell and the ITO grating with structural parameters included. (c) Transmissive optical extinction spectra measured on the pure ITO grating (blue) and on that with the 30-nm Ag shell.

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A 325-nm He–Cd laser was used as light source and S1805 photoresist was selected as the recording medium in interference lithography. Figure 1(a) shows the scanning electron microscope (SEM) image of the silver-film grating and Fig. 1(b) shows the cross-sectional profile. Structural parameters of the grating are included in Fig. 1(b). Adjusting the exposure and development time, we can tune the duty cycle and the modulation depth of the grating structures. Meanwhile, controlling the ICP etching time, we can adjust the thickness of the remaining ITO layer.

A waveguide forms between the silver layer and the ITO/glass-substrate interface. High reflection by the silver film and total reflection at the ITO/glass interface layer are responsible for the formation of the waveguide modes. This can be verified by a comparison between the spectroscopic response of the ITO grating with and without the Ag shell, as shown in Fig. 1(c), by the blue and red optical extinction spectra for normal incidence, respectively.

As shown by the blue curve in Fig. 1(c), a weak waveguide resonance mode is observed at about 600 nm, where the waveguide was supplied by the remaining ITO layer. This waveguide mode is also observable in the red spectrum, when a layer of silver was deposited, as indicated by the vertical double arrows in Fig. 1(c). However, two additional resonance modes with strong spectroscopic response can be observed at 440 and 702 nm after the ITO grating was coated with an Ag shell, which are observed as a dip and a peak in the red spectrum and highlighted by a blue and red triangle, respectively, in Fig. 1(c). As will be verified in section 3, these two resonance modes correspond to the SPPs propagating along the top and bottom surfaces of the Ag thin-film shell. The sharp spectral peak of the SPP mode at 702 nm is the main mechanism for the efficient ultrafast optical switching effect.

3. Propagating plasmon polaritons on the top and bottom interfaces of the periodically modulated Ag shell

To verify and distinguish between the two plasmon polariton modes, we measured the reflective optical extinction spectra from the top and bottom surfaces, as well as the transmissive modes, as shown in Fig. 2(a). At normal incidence, the + 1 and −1 orders of diffraction take place at a same wavelength, implying degeneracy of the two resonance modes. To avoid degeneracy of the resonance modes, we carried out the measurements at an incident angle of 10 degrees, so that the diffractions at + 1 and −1 orders excite resonance modes at different wavelengths. Figuire 2(b) shows the optical extinction spectrum measured on reflection from the top surface (RT), where a single narrow spectrum peaked at about 500 nm, whereas, splitting of the resonance mode centered at about 700 nm is observed in Fig. 2(c) due to the breaking of the degeneracy when the reflection was measured from the bottom surface (RB). Meanwhile, an additional spectral feature centered at about 470 nm with a bandwidth of about 100 nm is observed for the reflection from the bottom. Apparently, the RT and RB modes are independent on each other and there is no interaction between them in Figs. 2(b) and 2(c). Thus, in transmission, we observe overlap of the two resonance mode in Figs. 2(b) and 2(c), which was plotted by the black spectrum in Fig. 2(d). For comparison, we also included in Fig. 2(d) the spectra in Figs. 2(b) and (c), as shown by the red and blue spectra, respectively. Clearly, the resonance mode at 700 nm is simply a transfer from the RT to the transmission spectrum. However, we observe Fano-like coupling between the resonance modes in Figs. 2(b) and 2(c) at about 500 nm, which turns up as an asymmetric spectral feature. All of the measurement results in Figs. 2(b)-2(d) were based on TM polarization of the light, which is perpendicular to the grating lines.

 

Fig. 2 (a) Geometric scheme for the measurements on the optical extinction spectra for bottom-surface (RB) and top-surface (RT) reflection and for transmission (T) modes. (b), (c), and (d): Optical extinction spectra measured at an incident angle of 10° for RT, RB, and T modes, respectively. (e) Schematic illustration of the plasmon polaritons excited on the top (blue) and the bottom (red) surface of the Ag shell. (f) and (g): Distribution of the oscillating charges and the optical electric field intensity, respectively, in the Ag shell at an excitation wavelength of 450 nm. (h) and (i): Distribution of the oscillating charges and the optical electric field intensity, respectively, at an excitation wavelength of 730 nm.

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Considering the spectroscopic response in Figs. 2(b)-2(d), we can assign the top-surface resonance mode as a surface plasmon polariton excited by the diffraction anomaly (Rayleigh anomaly) along the air/Ag interface, as shown by the blue arrow in Fig. 2(e). This mechanism can be verified by theoretical simulations on the distribution of charge carriers in Fig. 2(f) and that of the optical electric field in Fig. 2(g) for excitation at about 450 nm. Furthermore, we can assign the resonance mode at the bottom surface or at the Ag/ITO interface as a surface plasmon polariton excited by the waveguide mode propagating inside the ITO layer, as shown by the curved red arrow. The corresponding charge-carrier and optical-electric-field distributions are shown in Figs. 2(h) and 2(i) for excitation at 730 nm. Figures 2(f) and 2(g) show an asymmetrically propagating plasmon polariton, where the charge-carrier density oscillates in the perpendicular direction to the Ag film. In contrast, Figs. 2(h) and 2(i) show a symmetric SPP, where the charge-carrier density oscillates along the Ag shell. These two SPP modes support independently the operation of the optical switching processes.

4. Ultrafast optical switching performance

In the investigation on the ultrafast optical response of the structures, we pumped the silver nanoshell grating using 150 fs pulses at 800 nm from a Ti:sapphire laser amplifier from Coherent Co. and probed the excited system using supercontinuum pulses generated by focusing a portion of 800-nm pulses into heavy water with a thickness of 3 mm. The pump pulses at 800 nm have a repetition rate of 1 kHz and a maximum pulse energy of 1 mJ. The delay of the pump pulse from the probe was adjusted with a resolution of 0.1 μm and a range as large as 1 ns. However, as shown in Fig. 1(c), the strong resonance spectrum is peaked at about 702 nm, which is far from the pump laser pulses in their spectral positions. To make the excitation more efficient, we have tuned the resonance spectrum from about 702 nm to longer than 730 nm by increasing the thickness of the unetched ITO layer from 126 to 140 nm, as shown in the inset of Fig. 3 by a SEM image of the cross-sectional profile. The optical extinction spectra for TE (red) and TM (black) polarization are shown in Fig. 3. The peak of the SPP excited on the bottom surface of the silver shell by the waveguide mode is peaked at 730 nm. Meanwhile, the intrinsic waveguide resonance mode of the ITO grating is located at about 612 nm. These features are highlighted by the downward arrows.

 

Fig. 3 Optical extinction spectra measured on the silver nanoshell grating for TM and TE polarizations by the black and red curves, respectively. Inset: SEM image measured on the cross-sectional profile of the silver nanoshell grating.

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Figure 4(a) shows the geometry for the pump-probe measurement, where the probe beam of supercontinuum pulses were incident at the normal of the substrate. The pump pulse was delayed with respect to the probe, as indicated by the double arrow. Figure 4(b) shows the directly acquired transient absorption (TA) data by a 3D plot of ΔA as function of wavelength and time delay, where ΔA denotes the value of the pump-pulse excitation induced transient absorption of the probe pulses. In our measurement results, ΔA is defined as –log₁₀[I_on(λ,τ)/I_off(λ,τ)], where I_on and I_off denote the spectral intensity of the probe pulse passing through the sample with the pump pulse switched on and off, respectively. λ is the wavelength of the probe pulse and τ is the time delay between the probe and the pump pulses. Three features are marked roughly at 450, 529, and 740 nm. Apparently, red-shift of the resonance modes centered at 450 and 740 nm took place as a result of the excitation by the pump pulse, so that left-negative (blue) and right-positive (red) TA spectra can be observed at these two spectral locations. This can be more clearly observed if we plot the TA spectrum at a delay defined by the dashed line in Fig. 4(b). As has been discussed in section 2, the resonance mode at about 450 nm resulted from the SPP on the top surface of the silver shell. Negative and positive TA peaks are observed at 445 and 457 nm, respectively, as shown in Fig. 4(c) by blue triangles. Similar effects can be observed around 740 nm, which resulted from the SPP mode on the bottom surface of the silver shell and are indicated by red triangles in Fig. 4(c). These features verify the red-shift performance of both of the SPP resonance modes under femtosecond optical excitation.

 

Fig. 4 (a) The schematic geometry for pump-probe measurement. (b) A 3D plot of the directly measured TA dynamics by ΔA as a function of time delay and wavelength. (c) TA spectrum at a time delay with the chirp compensated, as highlighted by the dashed line in (b). (d) TA dynamics at 433, 439, 445, 451, 457, and 463 nm. (e) TA dynamics at 730 and 753 nm. Dashed lines in green: guidelines showing dynamic stages with different lifetimes.

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Figure 4(d) shows the TA dynamics in the spectral range from 433 to 463 nm in steps of 6 nm. In the first 650 fs, all of the dynamic curves exhibit negative transient absorption, which can be understood as bleaching of the bulk plasmon in silver film and is not dependent on wavelength, as indicated by a yellow triangle. However, due to the modulation by the SPP resonance mode, negative dynamics are observed at shorter wavelengths of 433, 439, and 445 nm, whereas, positive ones are observed at 451, 457, and 463 nm. Redshift of the SPP under strong optical excitation is responsible for this negative to positive transition, as highlighted by the dashed arrow in Fig. 4(d).

Figure 4(e) shows the TA dynamics at 730 and 753 nm, corresponding to the dip and peak TA spectral features highlighted by red triangles in Fig. 4(c), respectively. Thus, negative and positive TA dynamics are observed at 730 and 753 nm, respectively. These two TA processes consist of similar evolution stages with nearly the same lifetimes, therefore, the two dynamic curves exhibit a vertically mirror-like relationship. Three stages are clearly resolved for the dynamics, as guided by dashed green lines, which have a lifetime of about 300 fs, 1 ps, and longer than 500 ps. According to time scales of the plasmonic dynamics [26–28], coherent electron oscillation and electron scattering processes take place within the first 100 fs after excitation, electron-phonon interaction takes place in the next 100 fs to 5 ps, and the pure phonon process generally takes place after 1 ps. Therefore, the three stages resolved in Fig. 4(e) can be roughly attributed to the electronic scattering, electron-phonon interacting, and pure phonon processes, respectively. Thus, the TA dynamics in Fig. 4(e) demonstrates optical switching performance with a speed of about 300 fs at full width at half maximum and a modulation of 3~4% on the transmission.

We still need to understand the negative TA spectral feature at about 529 nm, as indicated by a green triangle in Fig. 4(b). This is a broad-band feature, as shown in Fig. 4(c). This also resulted from the bleaching of the bulk plasmon in silver.

5. Conclusions

We report femtosecond optical switching effects based on surface plasmon polaritons excited on the top and bottom interfaces of a single silver nanoshell coating on a grating etched into a layer of metal oxides, which exhibit unidirectional performance in the reflective spectroscopic response. Strong optical excitation induces redshift of the resonance spectrum of SPP modes, modulating the light reflection and transmission at the resonance spectra. Symmetric and asymmetric oscillation of free electrons in the silver nanoshell are responsible for these two SPPs, where are resonant at longer and shorter wavelengths, respectively. These two SPP resonance modes evolve with different lifetimes, corresponding to different speeds of the optical switching processes. The simple fabrication method and the clear physics enable easy design and achievement of practically applicable optical switching devices.