Spatial- and energy-resolved photoemission electron from plasmonic nanoparticles in multiphoton regime

Peng Lang; Xiaowei Song; Boyu Ji; Haiyan Tao; Yinping Dou; Xun Gao; Zuoqiang Hao; Jingquan Lin

doi:10.1364/OE.27.006878

1. Introduction

Since the discovery and theoretical explanation of photoelectric effect by Hertz [1] and A. Einstein [2], respectively, photoemission has been widely exploited from mapping the electronic band structure to optoelectronics [3,4]. Photoemission from metal surface irradiated by UV source has been often preferred to be the photocathode as radio frequency photo-injector for free electron lasers and advanced accelerator applications due to its fast response, durability and vacuum compatibility. However, the development of UV-based photoemission faces drawback due to the difficulty in obtaining high performance UV laser as well as transport optics in UV wavelength [5]. Benefiting from the development of near infrared femtosecond laser, photoemission has accessed the regime of multiphoton ionization (MPI) and tunnel ionization (TI) [6]. The investigation of tunnel ionization in gas phase has attracted considerable interest in researching underlying physics and generating attosecond pulse [6–8]. Meanwhile, multiphoton process induced by femtosecond laser oscillator is especially important for metal photoemission because in this regime high repetition rates are easily accessible, which can lead to bright photoemission electron sources while avoiding laser induced damage [8]. Therefore, research on the MPPE from metal as ultrafast, laser driven photo-injector is of importance for free electron laser and ultrafast electron microscopy [7–9]. In the meantime, the ability to control electron emission with unprecedented spatial localization is required, which is of critically importance from free electron laser to microelectronics devices [9–11].

Spatial resolution of photoemission from flat metal surface is limited by optical diffraction, which set an upper limitation for spatial controllability of electron bunches [9–12]. Metallic nanoparticle can sustain localized collective oscillation of conduction electrons through the application of external optical field, which exhibits a pronounced resonant behavior with the combination of plasmonic effect known as localized surface plasmons (LSPs) [13]. The electric field can be strongly localized to the region beyond optical diffraction in metallic nanostructure, resulting in a strong field enhancement with additional resonant and geometric degrees of freedoms [13]. These properties make metallic nanostructure a promising platform for generating spatially controllable and ultrafast electron bunches with high quantum efficiency [14,15]. It has been demonstrated that photoemission yield and kinetic energy of the liberated electrons can be dramatically modified by the resonant mode of plasmonic field in both flat metallic surface and nanostructure [16–25]. The possible underlying mechanisms responsible for those photoemission electrons, depending on the near field intensity enhanced by the excitation of LSPs, include multi-photon photoemission [26], above-threshold photoemission [27] in the perturbation region, field emission [28,29] in the strong regime and other effects such as thermionic emission [30]. The inhomogeneous intensity distribution of the electromagnetic field on the nanostructured sample surface will occur due to plasmonic effect [14–19]. As a result, different photoemission mechanisms, induced by the significantly-varied near field intensity across the nanostructured sample, will be involved in the observed electron spectra when femtosecond laser pulse is used for photoelectron generation. Therefore, it is necessary to clarify an electron’s origin in the electron spectra, i.e., where the electron come from and the corresponding physical mechanism dominates for the photoemission [31]. This clarification is of importance for deeply understanding photoemission process of a metallic nanostructure as well as to the design of a metallic nano-emitter for photocathode [26–31]. However, up to now, investigation on the spatial resolved electron spectra from plasmonic nanostructure is scarce for the photoemission process. Therefore, simultaneously spatial- and energy- resolved photoelectron investigation from a plasmonic nanoparticle is strongly desired.

In this paper, we have observed spatial resolved photoelectron spectra from Au film and Au bowtie in multiphoton regime with Time-of-Flight Photoemission Electron Microscopy (ToF PEEM). ToF-PEEM is a powerful tool for characterizing photoemission in nanometric scale. It provides a non-invasive method for simultaneously detecting the spatial distribution and kinetic energy of the liberated electron, which is of importance for investigating the mechanism for emission process in metallic plasmonic nanostructure [32,33]. Photoelectron spectra from flat surface of Au film and Au bow-tie are measured at different incident wavelengths. Enhanced photoemission yield and kinetic energy of the liberated electron is observed in both flat metal surface and Au bowtie due to the excellent performance of LSPs. In Au bowtie multiphoton process is confirmed as the dominating process, and meanwhile, above threshold photoemission also contributes, after excluding other completing processes as space charge, thermionic emission and field emission. The spatial resolved electron spectra are investigated for flat metal surface and bow-tie nanostructure. The overall photoemission electron spectra can be well interpreted by the results of spatial resolved photoelectron spectrum. In particularly, spatial resolved photoelectron spectrum from Au bowtie shows the occurrence of high energy electron in weak field region near the plasmonic hot spot, which can be explained by the drifting of liberated electron from plasmonic hot spot by field gradient in inhomogeneously distributed near field in Au bowtie.

2. Experimental setup

The laser system used in this study is a commercial Mira-900F mode-locked Ti: Sapphire laser oscillator with wavelength tunable from 700nm to 900nm and pulse duration of 120fs operated at a repetition rate of 76MHz. The incident pulse illuminates sample at 65 degree to the surface normal and the beam is focused to a spot of $\sim 30 \times 60 μ m^{2}$ . The experimental setup and the oblique incidence illumination geometry as well as laser polarization are schematically depicted in Fig. 1(a) and Fig. 1(b), respectively.

Fig. 1 Schematic diagram of experimental setup (a) and oblique incidence illumination geometry (b), inset of (a) is the SEM image of Au bowtie.

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A ToF-PEEM from Focus GmbH, Germany, was used in this study [33]. The electron emitted from the sample is collected by the + 12kV positively voltage, 1.6mm distance cylindrical extractor. The PEEM is operated at a base pressure of $10^{- 10}$ mbar. Electric field strength generated by the extractor of PEEM is around 7 × 10⁻³V/nm, well below the threshold of field emission [6]. In addition, no photoemission image can be observed without the incident laser in this study. Therefore, DC field emission induced by the high extractor voltage can be eliminated. Photoelectron is accelerated by electrostatic lens mode and then disperses when passing through drift tube. The flight time of electrons propagating through the drift tube are recorded by Delay-Line-Detector (DLD3636, Surface Concept [33]) for obtaining their spatial and energy resolved distribution. DLD is performed in single electron counting mode within the time interval between the adjacent trigger signals and the threshold is 1M counts-per-second, i.e., the electrons accumulating for one second should be less than 1 million. The spatial and spectral resolution of ToF-PEEM are 30nm and 100meV, respectively [33]. All the experiments are carried out at room temperature.

The investigated samples are Au-film in the shape of island and Au bowtie lying on the top of indium tin oxide coated fused silica substrate with about 1nm of Ti as the adhesion layer. The samples are fabricated by electron beam lithography (EBL) process. Au bowtie is fabricated with thickness of 40nm, side length of 357nm and tip-tip distance of 100nm. Au bowtie arrays are arranged by space larger than 1.5μm to get rid of interaction between adjacent nanoparticles. Au film is selected from a fraction region of the residual Au film in the shape of island (residues from lift off process) on the sample. The residual Au-film was from a piece of the incompletely broken Au-film floating in the acetone in supersonic cleaning process of EBL, as a minimum supersonic cleaning time was applied to avoid the fall-off of the delicate nano-prisms of the bowtie. The floating piece of the Au-film redeposited on the ITO substrate and formed the island shape Au-film when taking the sample out of the acetone.

3. Result and Discussion

We select a flat surface from the Au film and well-fabricated Au bowtie on the sample for photoemission investigation. Figure 2 displays the PEEM images of Au film and Au bowtie irradiated by mercury lamp and 850nm femtosecond laser, respectively. Fig. 2(a) is PEEM image of the Au island by illuminated Hg lamp and Fig. 2 (b) is Atomic Force Microscope (AFM) image of the Au-film surface from the Au-island. The AFM image shows that the Au-film surface is full of bumps with several nm height, which has been found from the surface roughness of the ITO substrate.

Fig. 2 PEEM images of Au island and Au bowtie under the irradiation of Hg lamp (a, d) and with 850nm femtosecond laser oscillator (c, e). (b) Surface topography of the Au-film measured by AFM. The in plane polarization direction and wave vector are indicated by the red arrow marked by E and k in (e). Au film and bowtie are indicated by the green dash circle and square in (c) and (e), respectively. The contour of bowtie is indicated by blue dash line in (e). Note that (a, d) were taken with mercury lamp (CW source) in the imaging mode of PEEM, (c, e) were taken with femtosecond laser in the ToF mode of PEEM.

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The full of bumps spreading over the surface of Au-film as shown in Fig. 2(b) is consistent with hot spots in MPPE image [Fig. 2(c)] under the illumination of 850nm femtosecond laser, corresponding to the regions where LSPs are excited by surface roughness on Au film. Meanwhile, only a dominating hot spot appears on the apex of the right nanoprism in the Au bowtie [Fig. 2(d)] at 850nm [Fig. 2(e)], consistent with FDTD simulating result [23,24].

3.1 Photoemission from Au film

Semi-logarithmic plots of photoelectron spectra from Au film are shown in Fig. 3. Photoelectron spectrum records the photoemission yield versus the final energy of liberated electrons with respect to Fermi level, i.e., E-E_F, for Au film. In the photoelectron spectra of Au film, a common spectral peak occurs at the energy E-E_F≈4.2eV after a rapidly increase at the energy level above vacuum level. It can be explained that photoemission is cutoff by the vacuum level of sample, i.e., electron can be liberated if it is excited to the energy state above the vacuum level, otherwise it is still bound in the metal, resulting to a steeply rise above the vacuum level ( $E - E_{F} = 3.72 e V$ ). The resulting photoelectron spectra rapidly increases from zero at vacuum level to the spectral peak since most liberated electrons with this kinetic energy can be all collected by ToF PEEM [17]. Above the cutoff of vacuum level, the number of photoelectrons is determined by the product of electron density of state and the quantum efficiency of photoemission with the incident pulse.

Fig. 3 Photoelectron spectra of Au-film at excitation of 700nm (a), 750nm (b), 800nm (c), 850nm (d) and 900nm (e), respectively. The red dash lines are the fits of Fermi-Dirac distribution with the black bars indicating the Fermi levels at E-E_F≈5.35eV (700nm), 4.99eV (750nm), 4.69eV (800nm), 4.36eV (850nm) and 4.14eV (900nm), respectively. The insets are the linear fits of log-log plots of photoemission yield versus laser power at respective wavelengths.

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Noticeable that clearly rapid decay in the photoelectron spectra can be observed after the spectral peaks in Figs. 3(a)-3(e). The drastic decays exhibit the sign of Fermi edge, a truly metallic feature, which can be fit by the Fermi-Dirac distribution [18] of $\frac{1}{1+ \exp [(E - E_{F}) / k_{B} T_{e}]}$ as indicated by the red dash lines in Figs. 3(a)-3(e), where E the electron kinetic energy, $E_{F}$ the Fermi level after absorption of multiphoton energy, k_B the Boltzmann constant, T_e the electron temperature. Meanwhile, the rapid decay is dimming but still distinguishable and shifts toward lower energy with increasing the incident wavelength. The shifting of Fermi edge $Δ E_{F}$ is approximated to 3 times of the deviation of photon energy, i.e., $Δ E_{F} \approx 3 Δ ℏ ω$ , indicating shifting of Fermi edge originating from electron emitted by absorbing the minimum necessary number of photons to overcome the work function, i.e., 3PPE. To confirm the nonlinearity of photoemission, we measure the photoemission yield versus laser power in logarithmic coordinate as shown in the insets of Figs. 3(a)-3(e).

The consistence of experimental data with linear slope confirms multiphoton process. The slopes of the log-log plots are 3.09, 3.06 and 3.15 at 700, 750 and 800nm, respectively, confirming 3PPE process at these wavelengths. Taking the fitting Fermi edges of 3PPE in the photoelectron spectra, the work function is 3.72eV, lower than the typical work function of gold. This phenomenon is similar to the work by J. Kupersztych who observed the anomalous photoemission in the gold film on a conducting substrate [34]; it is explained that the required energy for MPPE is reduced since surface plasmon can transfer energy to conduction electrons. Meanwhile, the slopes increase to 3.56 at 850nm and 3.8 at 900nm, implying 4PPE also contribute to the emission process.

It is noted that above the Fermi edge appears a small tail whose intensity increases with the incident wavelength. The contribution of this tail can’t be neglected especially at 850nm and 900nm. It can be ascribed to the intense emergence of higher order multiphoton process induced by the strong coupling of surface plasmon with surface roughness of Au film with the increasing of the incident wavelength. Meanwhile, it is found that, for similar photoemission yield, the required incident power is dramatically decreased at 850nm and 900nm [e.g., from 100mW at 800nm reducing to 25mW at 850nm and to 30mW at 900nm], which is contrary to the expectation that photoemission yield should decrease with the incident photon energy [9]. Moreover, it is shown that there are increased number of electrons with energy above the Fermi edge corresponding to 4PPE under the incident laser wavelengths at 850 and 900 nm, which requires larger field intensity for occurrence in perturbative regime [17–19]. This phenomenon can be ascribed to the enhanced localized field induced by the excitation of LSPR on random surface roughness at these wavelengths. The much more sensitivity of photoelectron spectral profile to the incident wavelength reveals the critical role of LSPR on photoemission process.

For understanding the detailed mechanism of the observed phenomenon, localized photoelectron spectrum is studied by measuring spatial- and energy- resolved photoemission signal as shown in Fig. 4. Photoemission spectra of entire flat surface as in the selected region in Fig. 2(b) from the Au film, hot spot and dark point at the same laser power of 40mW but different wavelengths of 700nm [Fig. 4(a)] and 850nm [Fig. 4(c)] are measured as shown in Figs. 4(b) and 4(d). All of the spectra contain a common low energy peak around E-E_F≈4.2eV originating from cutoff of vacuum level of gold. Meanwhile, the spectra of the plasmonic hot spots exponentially decay in high energy regime without obvious sign of Fermi edge. The spectrum from dark point decays rapidly around E-E_F≈5.4eV [Fig. 4(b)] at 700 nm illumination, coincident with Fermi edge feature of metal surface at this wavelength.

Fig. 4 Localized photoelectron spectra from Au film. Spatial resolved MPPE image of Au film at 700nm(a) and 850nm(c), where hot spot and dark spot are marked by green circles and the overall photoelectron spectra of Au film is taken within the whole image. The spectra of Au film (black), hot spot (red) and dark point (magenta) are excited at power of 40mW by 700nm (b) and 850nm (d), respectively.

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At the illumination of 850nm [Fig. 4(c)], the spectrum from dark point experiences a rapid decay around E-E_F≈4.4eV, also coincident with the Fermi edge of 3PPE at 850nm. Note that the dark sport spectrum in Fig. 4(d) clearly shows a second high-energy peak possibly originating from the 4PPE process. However, the separation to low spectral peak is much less than the photon energy. The reason is explained as the following.

In Fig. 4(d), there are two spectral peaks of E-E_F≈4.2eV and E-E_F≈5eV in the photoelectron spectrum of dark spot at 850nm laser illumination. Here, the 3PPE Fermi edge at 850nm is E-E_F = 3ℏω≈4.4eV, so photoelectrons from lower and higher energy peak in the spectrum mainly originate from 3PPE and 4PPE process, respectively. In Fig. 4(b), the observed photoelectron yield increases with the reduction of the final energy of photoelectron, indicating large electron density for low initial state. The lower energy peak occurs at around 4.2eV both for 700nm and 850nm as a consequence of the cutoff of vacuum level because only the electron above the vacuum level can be detected. Therefore, the low energy peak at 4.2 eV in Fig. 4(d) doesn’t correspond to the maximum of electron density and the larger electron density of state is with initial state at E = E_peak-E_F-3ℏω≈-1.2eV based on the spectrum at 700nm in Fig. 4(b), where E_peak the energy of spectral peak in Fig. 4(b), which correspondes to initial state at 1.2eV below Fermi level. For higher energy peak at 5eV for 850 nm laser illumination in Fig. 4 (d), the liberated electrons are mainly emitted from the initial state with larger electron density around 1.2eV below Fermi level through 4PPE process, and for electrons at lower energy peak at 4.2eV, they are mainly from initial states close to the Fermi edge through 3PPE process. Therefore, the separation between two spectral peaks of dark spot at 850 nm laser illumination is smaller than single photon energy.

Both of the spectra for dark points at these two wavelengths exhibit the sign of metallic Fermi edge. This result shows that Fermi edge become prominent at bulk metal surface if we choose the dark region as regions of interest. Meanwhile, it is noted that entire photoelectron spectrum of Au film is similar to photoelectron spectrum of the plasmonic hot spot at 700nm [Fig. 4(b)] and 850nm [Fig. 4(d)]. Therefore, it is experimentally demonstrated that the photoelectron spectrum profile from Au film surface is dominated by plasmonic hot spots induced by surface roughness that contributes most electrons to the overall spectra, which simultaneously takes the sign of the Fermi edge from metal surface, but Fermi edge fades at the intensified excitation of LSP with increasing the incident wavelength. In addition, there are more bright hot spots appearing on the PEEM images at 850nm in Fig. 4(c) than that at 700nm in Fig. 4(a), confirming the enhanced excitation of LSPs on random surface roughness under the irradiation of 850nm.

3.2 Photoemission from Au Bowtie

To further disclose the influence of plasmonic effect on MPPE in metallic nanostructure, Au bowtie is introduced as a classical plasmonic nanoantenna [23,24]. At similar incident intensity, photoelectron spectra from Au bowtie [as indicated by the dash square in Fig. 2(d)] differ from Au flat surface from the film in that Fermi edge is completely smearing out with constant exponentially decay at all wavelengths as shown in Figs. 5(a-e). Therefore, photoemission yield versus the absolute kinetic energy of liberated electron at different wavelengths are shown in Fig. 5 (a-e). Compared with Au flat surface, the spectra of Au bow-tie case show that the photoelectron yield is greatly increased and the cutoff energy of the liberated electrons extend toward higher end. Apparently, these phenomena can be attributed to the introduced bowtie structure with much better performance of field enhancement of LSP than Au flat surface.

Fig. 5 Photoelectron spectra from Au Bowtie at the excitation wavelengths of 700nm (a), 750nm (b), 800nm (c), 850nm (d) and 900nm (e). (f) The 1/4th power of photoemission yield (black dots, right Y axis) from PEEM measurement and the simulating absorption spectrum (red line, left Y) of Au bowtie obtained by FDTD. The on-resonant plasmonic mode is around 850nm.

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It is necessary to clarify the dominating mechanism for photoemission of Au bowtie. Photoelectron with kinetic energy above the photon energy is reminiscent of higher order multiphoton process [35–37]. Semi-logarithmic plots of the spectra as a function of laser intensity are shown in Figs. 6(a)-6(e). Among all the incident wavelengths, the maxima of photoelectron yield and kinetic energy of liberated electron occurs at 850nm for Au bowtie, consistent with the simulated absorption spectrum of LSP on-resonant mode obtained by FDTD as well as the 1/4th power of photoemission yield from experimental measurement by PEEM at the same incident laser power because photoemission yield is averagely proportional to the forth power of field intensity obtained from power dependence within the tunable spectrum of laser oscillator as shown in Fig. 5(f). It is noted that, compared to Au flat surface, the photoemission yield and the maximum kinetic energy of electron from Au bowtie are increased by 5 times and 1eV at the same power of 25mW at the plasmonic on-resonant mode of 850nm. Moreover, for Au bow-tie nanostructure, Fig. 6 shows that LSP effect plays much more influential role in photoemission process. For instance, at the same excitation power of 35mW, the maximum kinetic energy of photoelectron increases from 1.7eV at 700nm (off-resonant mode) to 3.2eV at 850nm (on-resonant mode), together with one order of magnitude increasing of the photoemission yield. The slopes from linear fits of Figs. 7(a)-7(e) are recorded in Table 1. Noticeably, the slopes increasing above 1 from spectral peak to high energy cutoff, indicates the high energy electron is emitted by the absorption of excess photon for emission, i.e., electron emission through above-threshold-photoemission (ATP) process at high kinetic energy regime [35]. Figure 8 records the log-log plots of the spectrally separated photoelectron yield at peak and high energy cutoff indicated by the black and red arrows in Fig. 6, respectively.

Fig. 6 Photoelectron spectra as a function of laser intensity at excitation of 700nm (a), 750nm (b), 800nm (c), 850nm (d) and 900nm (e), respectively. Peak and maximum energies marked by black and red arrows are as following: (a) 0.42eV, 1.55eV; (b) 0.55eV, 2.13eV; (c) 0.55eV, 1.80eV; (d) 0.58eV, 1.60eV; (e) 0.67eV, 1.76eV.

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Fig. 7 Linear fits of log-log plots of photoemission yield of peak and high energy cutoff indicated in Fig. 6 versus laser power at 700nm (a), 750nm (b), 800nm (c), 850nm (d) and 900nm (e), respectively.

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Table 1. Slopes of linear fitting in Fig. 7.

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Fig. 8 Localized photoelectron spectra of Au Bowtie. (a) PEEM image of Au Bowtie at the illumination of mercury lamp with 850nm fs laser pulse. The entire bowtie, hot spot and dark spots close to and far from hot spot are marked, respectively. The localized spectra from Au Bowtie are measured at 30mW (b). The black, red, blue, cyan, green dot lines display the spectra of entire, hotspot, dark spots 1, 2, 3, respectively. (c) Field gradient of right nanoprism of Au bowtie obtained by FDTD simulation, the unit of color bar is the scale of V/m.

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To assure the ATP photoemission mechanism of the high energetic end in the spectra in Fig. 5 and 6, it’s necessary to rule out disturbing effect and other competing mechanisms of these observations, such as space charge [38], thermionic emission [30] and field emission [28,29]. In order to clarify the underlying mechanism, we have to check whether these processes play major roles.

It is known that a large number of electrons confined to a small region in vacuum can mutually repel due to the Coulomb repulsion, i.e., space charge effect, which leads to an enlarged electron kinetic energy [38]. The stability of linear relation of photoemission yield versus laser power in Fig. 7 can exclude the space charge effect, otherwise there should be depressed in the high laser power regime as occurred in [38]. In addition, we carefully checked the PEEM image as we increased the incident laser power much higher than the value applied in this study, and we did not observe any image blurring. Therefore, space charge effect can be safely eliminated in this study.

Thermionic emission is another possible mechanism for electron generation. Electrons can be emitted from the high energy tail of the Fermi-Dirac distribution of hot, thermalized electrons above the vacuum level. If emission is governed by thermionic emission, the resulting electron temperatures estimated by Fermi-Dirac distribution ranges from 3200K (700nm30mW) to 3850K (850nm35mW) for the exponentially decay at high energy regime in photoelectron spectra in Fig. 6. As a result, electron can be excited by $k_{B} T_{e} < 0.4 e V$ above the Fermi level, which is far below the work function of gold. Thermal assisted MPPE can also be ruled out since the measured photoelectron spectra exponentially decay and extend by about 1eV in high energy regime, far above the magnitude of thermal energy extension of $k_{B} T_{e} \sim 0.4 e V$ . In addition, the oscillation time of LSP in Au Bowtie is about 30fs according to our previous measurement [39]. If transient electron heating is the responsible for emission, the thermalized electrons should persist for picosecond timescale [19]. Therefore, thermionic emission cannot be the dominating process for photoemission from Au bowtie.

Another possible channel of electron emission is field emission [28,29]. The electrons can periodically tunnel into the vacuum as free electrons with the oscillation of optical field when the field intensity is comparable or larger than the Coulomb attraction from atomic core. The onset of field emission can be identified by the adiabatically Keldysh parameter $γ$ [10]:

γ = \sqrt{\frac{Φ}{2 U_{P}}} = \frac{\sqrt{2 m Φ}}{e} \times \frac{ω}{f E}

where

U_{P}

is the quiver energy of electron oscillating in the optical field, m the mass of electron, f the near field enhancement factor of LSPR,

ω

the angle frequency and

E

the electric field amplitude of the laser. Multiphoton process is favored for

γ ≫ 1

and field emission for

γ ≪ 1

[6]. The maximum near field intensity (incident laser power of 35mW, corresponding to 0.46nJ per pulse with the duration of 120fs) during our experiment is

\sim 6 .9 \times 10^{13} W / {cm}^{2}

with local field enhancement factor

f \sim 30

obtained by FDTD simulation, corresponding to localized field density of

E_{n e a r} \sim 0.8 V / n m

and

γ \sim 50

with the typical work function of gold as

Φ \approx 5 .1eV

. The localized field intensity in Au bowtie nanostructure is far below the threshold of field emission.

Figure 6 shows that the photoemission is still dominated by the energy peak regime (with electron energy less than 1 eV) resulting from multiphoton photoemission within the bow-tie structure. Therefore, MPPE is confirmed as the dominating mechanism for photoemission from the Au bowtie. Meanwhile, ATP process should be responsible for photoemission in high energy regime.

For getting insight to how the photoelectron governed by LSP field, localized photoelectron spectra are measured as shown in Fig. 8. Similar to the case of Au film, photoemission process of the Au bowtie is determined by the electrons emitted from plasmonic hot spot and the spectra from dark points exhibit different behaviors in high energy regime. It is noted that within 120nm from the center of hot spots as shown in Fig. 8(a), the spectra from dark points 1 and 2 [Fig. 8(b)] occurs high energy electron that can be only obtained by ATP process from strongly enhanced field region. Meanwhile, around 200nm away from the center of hot spot, dark point 3 consists of only low energy electrons distribution with a spectral peak around 0.5eV as shown in Fig. 8(b).

As to this bow-tie structure, the strong enhancement of local field is restricted in the tip of the right nano-prism as hot spot in Fig. 8(a) and Fig. 8 (c) supported by the FDTD simulation [23,24], and the rest area such as dark points 1-3 have similar local field intensity, where the local field is not much enhanced. Accordingly, photoemission electrons from those three dark points are expected to have similar spectral contour and with similar shape in the high energy end. It is apparent that electron from all the dark points have the common spectral peak around 0.5eV at 30mW [Fig. 8(b)], which should be attributed to the emission from the dark points themselves. Surprisingly, the spectra from the dark points 1 and 2 also contain large numbers of electrons with unexpected high energy, which can’t originate from the local position based on the above discussion. Moreover, it is noted that, for energy distribution contours in high energy regime of the photoelectron spectra from the dark points 1 and 2, they show the same decay trend as well as cut off energy with those from the hot spot. This reminds us that the high energy electron component from the dark points 1 and 2 are most probably originating from the hot spot, i.e., the observed high energy electrons in dark points 1 and 2 are the drifted electrons from the tip of the right nano-prism of bow-tie, and they are the ones driven by the strongly field gradient of LSP as shown in Fig. 8(c) [31]. Although the number of drift electrons is low, it is still comparable to photoemission from weak field regions, such as dark spots 1&2 since only few electrons can be emitted by MPPE process by the not much enhanced field intensity. The resulting spectra have second peaks for dark spot 1 and 2.

Meanwhile, photoemission through MPPE with low energy is dramatically increased due to the strongly enhanced field intensity at hot spot. The number of electron emitted through ATP process is still much less than that from MPPE at hot spot. The separation of different orders multiphoton photoemission, i.e., Fermi edge of photoemission due to the limited number of Fermi electrons, now is smearing out because it is mixed with electron below Fermi level emitted through ATP process. The resulting photoelectron spectrum at hot spot shows an exponential decay after the spectral peak and the contribution of ATP electron can slow down the decay at high energy regime. As a result, there is no obvious second spectral peak observed in photoelectron spectrum of hot spot.

In the following, we will estimate the drift distance of a liberated electron from hot spot driven by the gradient of the LSP field to explain the appearance of high energy electrons at dark spot 1& 2. In the near field of hot spot on Au bowtie, the momentum of liberated electron can be changed as a consequence of Coulomb interaction with the oscillating near field of LSP. After escaping from near field region, electron is collected by the extractor of PEEM due to + 12kV extractor voltage with a drift distance. The momentum change is determined by [40]:

Δ p_{/ /, ⊥} = e \int_{t_{0}}^{t_{1}} E_{n e a r} (r, t) \cdot n_{/ /, ⊥} d t,

where E_near(r,t) is the near field of Au bowtie,

n_{/ /, ⊥}

is the unit vectors either parallel or perpendicular of sample surface, t₀ and t₁ correspond to the electron emission time and escaping time from the near field of Au bowtie, respectively. Due to the nonlinearity of photoemission yield, electron is mainly emitted at the maximum of the near field intensity determined by

E_{n e a r} (r, t) = R (r, t) \cdot E_{i n c} (r, t),

where R(r,t) is the response function of the LSP field. Photoemission yield is proportional to the 4th power of localized field intensity. The emission time can be estimated at the maximum of carrier envelope of near field. We set t₀ = 0fs as the time of electron emission, corresponding to the peak of the temporal evolution of the laser pulse. Meanwhile, t₁ is mainly determined by the time that electron escape from the decay length of near field of Au bowtie.

According to the simulation of localized field components that in parallel and in perpendicular directions of the bow-tie surface by FDTD, the decay length of normal component (E_z) and parallel component (E_x, at centre of bowtie where E_y = 0) is 3.3nm and 2.6nm, respectively. In this case, the amplitude of quiver motion of liberated electron in the near field is smaller than 0.001nm, which is far below decay length of electric field. The liberated electron can’t escape the decay length of near field by the oscillating localized field [17]. Under the high extractor voltage of around 7 × 10⁻³V/nm, the time for liberated electrons to escape the near field can be estimated as $t_{2} = \sqrt{2 l_{d, z} m_{e} / e E_{D C}} \approx 73 f s$ by the DC field generated from the extractor voltage, where m_e the mass of electron, E_DC the DC field strength. corresponding to the time of electron with the kinetic energy of E_kin≈0.002eV to escape the near field region of LSP. Therefore, the time for liberated electron escaping the near field region is mainly determined by the kinetic energy of photoelectron.

Here, we focus on the electron escaping the near field of parallel direction, $t_{1} = l_{d, x} / \sqrt{E_{k i n} / m}$ with the assumption that velocity of electron is equal on x and z direction. Then, the drift distance of photoelectron is the parallel displacement of liberated electron at the entrance of the extractor of PEEM. The time for an electron travels the distance of 1.6mm from the sample surface to the extractor can be estimated by $t_{3} = l_{e x t} / \sqrt{2 E_{e x t} / m} \approx 49.2 p s$ , where l_ext the distance between sample and extractor and E_ext = eU_ext the kinetic energy of electron at extractor. The drift distance of l_drift can be calculated by

l_{d r i f t} = \frac{t_{2} Δ p_{/ /}}{m} = \frac{e t_{2}}{m} \int_{0}^{t_{1}} f E_{0, x} \exp [- {(\frac{t}{τ})}^{2}] \cos (ω t + φ) d t,

where f is the spatially average field enhancement factor in the near field of LSP which is around 20 obtained by FDTD simulation, E_0,x is the amplitude of electric strength at x direction of the incident laser pulse, which is about 3 × 10⁴V/cm in this case, τ the pulse duration, ω the angle frequency of laser and φ the CEP of incident laser pulse. Since the pulse duration is 120fs, much larger than the laser cycle, φ doesn’t influence the momentum charge of liberated electron. It is notable that we don’t take the fast decay time of plasmonic hot spot into account in formula (4), leading to the calculated drift length larger than the real one due to fast degradation of the enhancement factor, especially for low energy electron with larger time for escaping the near field of plasmon.

The maximum drift distance of electron with kinetic energy of 3eV is estimated to be 170nm based on formula (4), which is below the distance of dark spot 3 with respect to hot spot, and it is consistent with no high energy electron appeared in the photoelectron spectrum of dark spot 3. Correspondingly, the second spectral peak at about 1.4eV for dark spot 1corresponds to about 130nm of the calculated maximum drift distance, and the second spectral peak at 1.7eV of dark spot 2 to a maximal drift distance of around 140nm. The calculated results show that the photoemission electron can be drifted to hundreds of nanometer distance from the hotspot driven by the inhomogeneously distributed field of LSP in Au bowtie and the observed high energy components in photoelectron spectra of localized dark spots 1and 2 in Au bowtie can be explained by the drifted electron.

4. Summary

In conclusion, we have demonstrated MPPE generation from both the flat metal surface and metallic nanoparticle irradiated by femtosecond laser oscillator with ToF PEEM. It shows that electron spectral profile from the flat metal surface with the sign of Fermi edge and it is fading with increasing the incident wavelength, which can be attributed to the enhanced excitation of higher order multiphoton photoemission induced by LSPs via random nanoparticles on the surface. It is found that the near field enhancement induced by the excitation of LSP in the bow-tie nanostructure can greatly enhance multiphoton photoemission, and the occurrence of above-threshold photoemission is confirmed in the electron spectra.

Spatial and energy resolved photoemission measurements have revealed that overall electron spectral profile from both the flat metal and bow-tie structure are determined by photoemission from plasmonic hot spots and some electrons with high kinetic energy emitted from LSP “hot spot” can be drifted to the surrounding by the inhomogeneous distribution of strong local field of metallic nanoparticle. The enhanced understanding of the photoemission by plasmonic nanoparticles in this work is of importance for establishing nanoscale femtosecond electron source, which is a promising platform for several ambitious research endeavor ranging from the ultrafast electron diffraction microscopy to the free electron laser.

Funding

National Natural Science Foundation of China (NSFC) (61775021, 91850109, 11474040, 61605017); Department of Science and Technology of Jilin Province (20170519018JH); Education Department of Jilin Province (JJKH20181104KJ and JJKH20190555KJ) and “111” Project (D17017).

Acknowledgment

The authors acknowledge helpful discussion with Prof. Toshihisa Tomie at Changchun University of Science and Technology.

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Wavelength(nm)	700	750	800	850	900
Peak	3.51	3.64	3.82	4.56	4.81
Cutoff	4.41	4.98	4.65	5.78	6.37

Spatial- and energy-resolved photoemission electron from plasmonic nanoparticles in multiphoton regime

Abstract

1. Introduction

2. Experimental setup

3. Result and Discussion

3.1 Photoemission from Au film

3.2 Photoemission from Au Bowtie

4. Summary

Funding

Acknowledgment

References

Cited By

Figures (8)

Tables (1)

Equations (4)

Optics Express