The light-induced emission of electrons from metal has long been a subject of study because it allows one to probe the physical properties of metals.1,2 Now, the advent of ultrashort femtosecond (fs) high-intensity laser pulses has made it possible to observe new photoelectric phenomena such as multiphoton and above-threshold photoemission. Here, we describe work showing how surface plasmons (SPs, collective electron oscillations along a surface) enhance photoelectron emission from metals.3 Our study has implications for ultrashort x-ray sources and laser cathodes for free-electron lasers, because they are all based on electron emission.4
We recently discovered a unique type of surface phenomenon: nanostructure-covered, laser-induced periodic surface structures (NC-LIPSSs) on metals following irradiation with an fs-laser pulse.5 NC-LIPSSs induce significant changes in the optical properties of metal surfaces. Furthermore, the nanostructures on LIPSSs can also change the so-called energy-momentum dispersion relationship of the SP modes.
Amplified titanium:sapphire fs-laser systems are used to produce NC-LIPSSs and study photoelectron emission. Once NC-LIPSSs have been generated on a metal surface, we characterize the mirrorlike specular reflectance and photoelectron emission of the surface with fs-laser pulses. To find the right resonant angle for exciting SPs on an NC-LIPSSs surface, we vary the incident angle and polarization of the laser beam. We have observed a minimum reflectance for p-polarized light (having a component perpendicular to the surface), but not for s-polarized light (parallel to the surface) at an incident angle of 11°, indicating that this minimum angle could be caused by resonant coupling of SPs. In fact, the minimum reflectance angle (θ) by SP excitation can be deduced from the period (d) of NC-LIPSSs measured in our experiments with the following condition:6
where is the real part of the effective refractive index for SPs, ε is the dielectric function of the metal, and λ is the wavelength of the incident light.
The groove period d of the NC-LIPSSs is approximately 530nm. Consequently, if we use η = 1.0075 for platinum (Pt) at λ = 810nm,7 we will obtain θ ≈ 31°. However, in our measurements, the minimum reflectance angle for p-polarized light is 11°, which is significantly different from the calculated angle. We know that surface roughness causes an increase in the modulus of the SP wave vector8 that corresponds to an increase in the real part of the effective refractive index. According to Equation (1), an increase in η from 1.0074 to 1.338 will cause the SP resonant angle to reduce to 11°. This change in η also reduces the NC-LIPSS period to the observed value of ~530nm.5
Once we determine the SP resonant angle on NC-LIPSSs, we examine photoelectron emission in a high-vacuum chamber at the shifted SP resonant angle of the fs-laser beam incidence. For Pt, four-photon energy is required to overcome the work function of 5.64eV.1 Therefore, the doubly-logarithmic plot of the photoelectron current versus laser intensity should show a slope of 4, indicating the order of the multiphoton process for Pt. Figure 1 shows that the photoelectron-emission yield induced by p-polarized light from NC-LIPSSs is much higher than that induced by s-polarized light from NC-LIPSSs and by both p- and s-polarized light from a smooth Pt surface. We note that p polarization is most efficient in exciting SPs and that the excitation of SPs can enhance the electric field on NC-LIPSSs for p polarization. We believe that this explains the overall enhanced photoelectron yield. For photoelectron-emission yield induced by the p-polarized light from NC-LIPSSs, that slope is ~4 over an intensity range of 5–15GW/cm2. However, for intensities above 15GW/cm2, the slope begins to increase and finally reaches 5.3 at 27GW/cm2. For intensities lower than 15GW/cm2, the pure four-photon process dominates. Above this value, the effect of nonequilibrium heating of hot electrons by fs pulses appears sufficiently strong to increase the slope of the total photoelectron current to above 4.4 Thermally assisted photoemission current may also contribute to the total current.
Figure 1. Averaged photoelectron current versus laser intensity, with p and s polarization on both the nanostructure-covered, laser-induced periodic surface structure (NC-LIPSS) and smooth platinum surfaces with an incident angle of 11°. s-pol, p-pol: Parallel, perpendicular to the surface.
In summary, we have shown that the SP resonant angle on NC-LIPSSs differs significantly from the angle calculated from regular periodic grooves, indicating that a simple theory cannot explain our observations. We also note that excitation of SPs significantly enhances photoelectron emission on NC-LIPSSs. We conclude that NC-LIPSSs are a unique type of structure for enhancing and controlling photoelectron emission. In the future, we plan to further study SP effects on various physical processes in materials.
University of Rochester