Plasmon Enhanced Photoemission

Abstract

Next generation ultrabright light sources will operate at megahertz repetition rates with temporal resolution in the attosecond regime. For an X-Ray Free Electron Laser (FEL) to operate at such repetition rate requires a high quantum e ciency (QE) cathode to produce electron bunches of 300 pC per 1.5 µJ incident laser pulse. Semiconductor photocathodes have su cient QE in the ultraviolet (UV) and the visible spectrum, however, they produce picosecond electron pulses due to the electron-phonon scattering. On the other hand, metals have two orders of magnitude less QE, but can produce femtosecond pulses, that are required to form the optimum electron distribution for high e ciency FEL operation. In this work, a novel metallic photocathode design is presented, where a set of nano-cavities is introduced on the metal surface to increase its QE to meet the FEL requirements, while maintaining the fast time response.Photoemission can be broken up into three steps: (1) photon absorption, (2) electron transport to the surface, and (3) crossing the metal-vacuum barrier. The first two steps can be improved by making the metal completely absorbing and by localizing the fields closer to the metal surface, thereby reducing the electron travel distance. Both of these e↵ects can be achieved by coupling the incident light to an electron density wave on the metal surface, represented by a quasi-particle, the Surface Plasmon Polariton (SPP).The photoemission then becomes a process where the photon energy is transferred to an SPP and then to an electron. The dispersion relation for the SPP defines the region of energies where such process can occur. For example, for gold, the maximum SPP energy is 2.4 eV, however, the work function is 5.6 eV, therefore, only a fourth order photoemission process is possible. In such process, four photons excite four plasmons that together excite only one electron. The yield of such non-linear process depends strongly on the light intensity.In this work, the structure consisted of rectangular nano-grooves (NGs) arranged in a subwavelength grating on a metal surface is presented that provides a dramatic increase in the metal's absorption, field localization, and field enhancement. When light is polarized perpendicular to the orientation of the grooves a standing SPP wave is excited along the vertical walls in the NGs, that act as Fabry-Perot resonators. By adjusting the geometry of 2 the NGs and the period of the subwavelength grating the resonance can be fine tuned to a desired position, for example, the laser fundamental wavelength, anywhere from the UV to the near infrared (NIR).Two types of gratings are presented: (a) a gold grating with period of 600 nm, and (b) an aluminum-gold grating with a period of 100 nm; both with resonance at 720 nm. In each case, strong on-resonance absorption was observed, with over 98% for grating (b). Unlike the grating-coupled SPP waves, where the angle is well defined by the momentum matching condition, the resonant NGs allow coupling to the standing modes at a range...

Cosmic fluctuations from a quantum effective action

“…with x µ (τ ) on a geodesic parametrized by an invariant parameter τ . The propagator in flat space (37) has indeed the form…”

“…(investigations of the effects of interactions in a non-flat geometry can be found in refs. [27,[30][31][32][33][34][35][36][37][38][39][40][41].) In our simple approximation interactions appear in the form of the inflaton potential.…”

Primordial cosmic fluctuations for variable gravity

“…refs. [41][42][43][44][45][46][47][48][49][50][51][52][53][54]. The issue is similar to the understanding of thermalization of arbitrary initial correlation functions in flat space, for which powerful functional methods have been developed [55][56][57][58][59][60][61][62][63][64][65].…”

Effect of plasmonic near field on the emittance of plasmon-enhanced photocathode

“…The metal photocathode has been researched for a long time and used in many facilities, due to its virtue of high brightness, long lifetime, and low cost. And it has been indicated that it holds great promise to eject a uniform 3D ellipsoid electron bunch with only linear internal space charge fields because of its ultrafast response characteristic, which is an ideal distribution for a high-brightness FEL operation [4][5][6]. But, in order to ensure a high photoemission yield, the metal photocathode usually needs to be pumped by high power ultraviolent laser, which undoubtedly increases the demand and cost for the drive laser [7].In recent years, the plasmon-enhanced photocathode (PEP) attracts more attentions, which introduces the surface plasmon polarizations (SPPs) to enhance the interaction between laser and materials [8].…”