Ultrafast Devices Based on Surface Plasmons on Bulk Metal and Graphene: Switches, Modulators, and Microscope Electron Sources

Optical modulation and switch are crucial in photonics technologies, and microscope electron sources are essential in imaging. However, it is an obstacle to integrating those devices with conventional techniques. Moreover, they cannot function in ultrafast communication and signal processing systems. The surface plasmon resonance phenomenon is of wide interest due to its abundant physics, such as the local field enhancement enabling strong light-matter interactions, breaking the diffraction limit to realize subwavelength structures, and going beyond the speed limit intrinsic to conventional semiconductor materials and devices. Here, by reviewing the various advantages of surface plasmons in different nanostructures or materials, we intend to propose the development trend and designed configuration of such ultrafast devices (usually induced by ultrafast laser) such as modulators, switches, and microscope electron sources. Based on the surface plasmons, ultrafast devices will tend to be more energy-efficient, low-energy consumption, and miniature. We also envision the development of surface plasmons with alternative materials or structures. This review will facilitate the development of ultrafast devices.

Figure 1

(a) Sample structure. (b) Total electron yield versus laser power density [37].

Figure 2

(a) Multiphoton and above-threshold ionization (MPI and ATI); (b) field electron emission; (c) optical field emission (OFE).

Figure 3

(a) Experimental configuration. All equipment is installed in an ultrahigh vacuum chamber. Field emission electrons are emitted from the tip and accelerated onto the microchannel plate detector (MCP), which is located 4 cm away from the tip. (b) The photocurrent varies linearly with power [38]. (c) It exhibits a fourth-order power dependence (four photons absorbed) at zero bias [43]; at 880 V, the electron flux $I\propto P^{1.4}$ and $P=3$ mW.

Figure 4

(a) The designed photoemission-based device. $V_f$ and $V_s$ represent the voltage of the flat port and suspended port, respectively. Emission electrons can be manipulated by adjusting the voltage at the ports. (b) $I$-$V$ curves of the suspended port when the flat port is open circuited. Similarly, the current of a flat port is a function of the voltage when the suspended port is open circuited (without or with laser: $I=5\mathrm{W}{\mathrm{cm}}^{-2}$). (c) The current of the flat port $I_f$ is changed by the laser on the order of $\mathrm{W}{\mathrm{cm}}^{-2}$ with the bias voltage of $V_f=1$ V, while the suspended port is an open state [44].

Figure 5

(a) Plasmon-induced photoemission vacuum-channel device and the gap between cathode and anode is 100 nm. (b) Current versus optical power density [47].

Figure 6

(a) Sketch of an asymmetrical metal nanostructure with emission electron excited by a femtosecond laser pulse. (b) The curve of the electron emission versus $F_{\text{laser}}$ is displayed in a blue line according to the Keldysh theory. Moreover, the Keldysh parameter $\gamma$ is shown on the top axis. The red dashed line means multiphoton absorption for a low $F_{\text{laser}}$, whereas tunneling for a high $F_{\text{laser}}$ is indicated by a green dashed line. (c) Three curves are $I_{\text{dc}}$ versus $V_{\text{bias}}$. The blue is measured before laser illumination and after laser illumination can be drawn with a red curve fitted with a Fowler-Nordheim fit in black [48].

Figure 7

(a) Structure of coupled plasmons with right-angle prism. (b) Functions of the total plasmonic photocurrent about focused laser intensity [spot sizes (FWHM) of $870\text{μ}\text{m}$ (black triangles) and $1400\text{μ}\text{m}$ (red squares) are two independent exemplary scans]. The local slope of the second curve is also plotted (green circles) [46].

Figure 8

(a) Geometry of the nanoplasmonic waveguide. Relative to the excitation fields, $I(r)=|E(r){|}^2$ demonstrates the intensity of the local fields. (b) The vertical cross-section electric field of the waveguide. The radius of the waveguide gradually decreases from 50 to 2 nm along the propagation direction of the SPP [13].

Figure 9

(a) Evanescent plasmon propagation along the taper and energy focus at the tip with $4$–$10$-nm radius [59]. (b) SPPs generated by grating-coupled femtosecond pulses condense adiabatic fields into nanoscale apex volumes [60]. SEM and optical image of a tip with a broadband grating (bottom left).

Figure 10

(a) Imaginary permittivity or optical losses in gold [63, 66]. (b) Transmittance spectrum of single-layer graphene as a function of wavelength (open circles). [Inset: the number of graphene layers dependent on transmittance of white light (squares)] [73]. (c) Infrared absorption spectra of epitaxial graphene. (Inset: locally enlarged view of the mid-IR absorption curves) [74].

Figure 11

(a) Calculated curves of extinction (black), absorption (red), and scattering (blue) spectra of silver nanostructure (note: extinction = absorption + scattering). The shape of the nanostructure is respectively an isotropic sphere in (a), an anisotropic cube in (b), a tetrahedron in (c), an octahedra in (d), a hollow sphere in (e) and a thinner shell in (f) [77].

Figure 12

(a) Schematic image of the absorption structure with monolayer graphene and subwavelength grating. (b) Optical image of the designed sample. (c) SEM image of the top pattern of a fabricated sample. (d) Measured (solid line) and simulated (dot line) absorption spectra of the fabricated sample with different grating periods $d$ for TE polarization [100].

Figure 13

(a) The trajectories of photoelectrons under the excitation of (a) short wavelength, (b) long wavelength. (c) Kinetic energy spreads of photoelectrons with varying intensities at wavelength $3.8\text{μ}\text{m}$. Experimental (circles) and simulated (solid lines); tip radii of 12 nm (solid circles) and 22 nm (open circles) [123].

Figure 14

(a) Simulation of magnitude square of electric field component for a nanostar [an arrow indicates the surface-normal field (20-nm scale bar)]. (b) Light peak intensity for the photoelectron emission rate for single and multiple nanostars [71].

Figure 15

(a) Schematic diagram of a $M$-$I$-$M$ plasmon waveguides for the electro-optic modulator. (b) The voltage-off state $n_{\text{eo-polymer}}=1.6$) (black curve) and the voltage-on state $n_{\text{eo-polymer}}=1.5$) (red curve) [151].

Figure 16

(a) The Mach-Zehnder modulator, close-up of plasmonic phase modulators (PPMs) in both arms on the left and close-up of the photonic-plasmonic converter on the right. (b) Normalized power transmission concerning for the applied dc voltage [152].

Figure 17

(a) All-metal devices involving vertical grating coupler (GC), polarization rotator (PR), and Mach-Zehnder interferometer (MZI). (b) Magnified image of a GC with staircase gratings. (c) A $p$-polarized SPP mode on the top surface. (d) Conversion of the two modes in the PR section. (e) $S$-polarized SPP mode [153].

Figure 18

(a) The optical power transmission spectrum when switching from $-6$ to $+6$ V. (b) The frequency response of the device [153]. (c) Schematic of all-optical modulation [154].

Figure 19

(a) Schematic illustration of a graphene-clad microfiber modulator. (b) The diagrammatic sketch shows pump and probe carriers in the band structure of graphene [164]. (c) The transmittance of the GCM varies by peak power of 220-fs pulses. (d) Differential transmittance of the probe light changes by the pump-probe time delay where the pump power is 200 nW. The inset describes the modulation depth as a function of the pump intensity [5].

Figure 20

(a) Schematic cross section of an all-optical modulator based on graphene on silicon. (b) Maximum phase shift of the modulator versus optical pump intensity [158].

Figure 21

(a) Schematic of the all-optical modulator based on GPSW. (b) Cross section of the designed structure. (c) Changes in the modulation efficiency in dependence of modulation power for the signal light at 1550 nm [159].

Figure 22

(a) Illustration of the designed all-optical modulator. (b) Electric field diagram of the propagating mode in the device. (c) Maximum transmitted power ($T_{\text{max}}$) and the maximum ER (${\mathrm{ER}}_{\text{max}}$) versus the length of the modulator $(L)$. (d) IL and ER:IL ratio versus the modulator length $(L)$. $\lambda =1550$ nm [160].

Figure 23

(a) Schematic of the $M$-$I$-$M$–WG based on graphene. (b) Cross section of the structure at the broken red line. (c) Simulation of the structure’s magnitude square of field profile ($|E{|}^2$). Here $\lambda =1550$ nm and scale bar of 20 nm. (d) All-optical switching with the pump-probe method in the device. (e) The energy of control pulse versus the extinction ratio for the device [175].

Figure 24

(a) Conceptual diagram of single-electron imaging and UEM. Schematics of single-electron packets and random electrons used in imaging (upper left). The electron-pulse envelope and individual electron packets (lower left). Schematic of concept of UEM (right) [180].

Figure 25

(a) Diagrammatic sketch of the femtosecond photoelectron point projection microscope. (b) Enlarged view of the tip and specimen area. The blue and red pulse’s concentrating diameter is $10\text{μ}\text{m}$. (c) Temporal distribution in time-of-flight versus tip-to-specimen distance with various acceleration voltages [193].

Figure 26

(a) Projection image of the same $\mathrm{InP}$ nanowires recorded at a negative time delay. (b) Normalized difference plot after optical excitation [199].

Figure 27

(a) SEM image of the nanofabricated metallic tips with gratings. (b) SPPs on the grating propagating toward the tip apex. (c) The scattered light images for four irradiation positions are indicated in (b). The curves in (d) displays sections through images 2 and 4 corresponding to the dashed line in image 4 [205].

Figure 28

(a) Schematic diagram of electron emission device incited by laser pulses. A microchannel detects emission electrons from a sharp taper. (b) SEM image of a sample. (c) Electron distribution on the MCP phosphor screen for emission by dc excitation (left) and by laser illumination (right) [197]. (d) For certain voltages, the electron emission is only from the taper apex not the grating coupler. Integration of tip and an electron gun assembly enable applications in ultrafast electron imaging [61, 62].

Figure 29

(a) Schematic illustration of fsPPM for imaging an individual $\mathrm{InP}$ nanowire. (b) Simulation of electron pulse duration ${\tau }_{\text{el}}$ at the sample in regard to the distance of tip and sample with different initial energy distributions ${\sigma }_E$ [62].

Figure 30

(a) UPEM imaging of photoemission enhanced by plasmon. (b) SEM image of the device. (c) Shadow image of the central gap region with different time delays. (d) The UPEM image of the sample without optical excitation. (e) Electron transmission concerning different delay times along the dashed white line shown in (b). (f) Electron transmission drops from 90% to 10% in 25 fs. The four curves are points taken equidistantly along the white arrows in $b$ [72].