Laser-induced ultrafast electron emission from a field emission tip

We show that a fi eld emission tip electron source that is triggered with a femtosecond laser pulse can generate electron pulses shorter than the laser pulse duration (100  fs). The emission process is sensitive to a power law of the laser intensity, which supports an emission mechanism based on multiphoton absorption followed by over-the-barrier emission. Observed continuous transitions between power laws of different orders are indicative of fi eld emission processes. We show that the source can also be operated so that thermionic emission processes become signifi cant. Understanding these different emission processes is relevant for the production of sub-cycle electron pulses. The temporal resolution of ultrafast electron diffraction (UED)[1, 2], ultrafast electron microscopy (UEM) [2]–[4] and ultrafast electron crystallography (UEC) [2] is limited to the duration of the electron pulse. The ultrafast electron source most commonly used in these applications is based on electron emission induced by focusing an amplifi ed femtosecond laser pulse on to a surface [1, 5]. Due to the high particle density per pulse, space-charge broadens the pulse duration to 500 fs [5]. An interesting electron source implementing a Ti:sapphire oscillator has been used to generate 27 fs electron pulses [6] using impulsively excited surface plasmons. However, this method has a large kinetic energy spread (ΔE ~ 100 eV) in the emitted electrons, which would cause the pulse to expand temporally as it propagates. An electron gun has been proposed that could produce sub-fs electron pulses [7]. This 10 keV source would have an initial energy spread of ΔE ~ 1 eV, but this spread would be reduced by injecting the electrons into an RF cav2 BARWICK ET AL. IN NEW JOURNAL OF PHYSICS 9 (2007) ity to compensate for the different velocities, allowing the electron packet to arrive at a target in a sub-fs time window. A promising and experimentally realized source relies on the combination of a fi eld emission tip with a low power femtosecond oscillator [8]–[10]. The low laser powers required allow the production of few electrons per pulse with high repetition rates. This gives useful average electron count rates and overcomes space-charge broadening. Field emission tip sources also have small energy spreads (ΔE < 1 eV) [11, 12], so their temporal expansion is suppressed. Assuming that the emission process from the nanometer tip is due to optical fi eld emission, the electron bunches were claimed to be sub-cycle [9]. In this paper, we study the emission of electrons from a nanometer tip due to femtosecond laser pulses. Our pump–probe data shows that the laser-induced electron pulses are shorter than 100 fs justifying the claim of Hommelhoff et al [9] and Ropers et al [10]. It is important to note that a (nonlinear) autocorrelation spectrum such as that shown by Ropers et al (their fi gure 2(a)) and Hommelhoff et al (their fi gure 2(a)) does not provide any information on the electron pulse duration. The pulse duration can be determined by the absence or presence of the additive nature of the electron emission process (as Hommelhoff et al [9] points out but does not show data). A detailed discussion of the autocorrelation spectrum and its additive nature will be given below. The next important question is: “How much shorter than 100 fs is the electron pulse duration?” No direct measurement of the electron pulse duration is available to date. However, identifying the emission mechanism can help predict the pulse duration, and assist in operating the source in an experimental parameter range where shorter electron pulses could be obtained. In this context, it is highly interesting that the mechanisms suggested in [9] and [10] are not identical. Hommelhoff et al [9] claim a pure optical tunneling mechanism. Ropers et al [10] consider a multiphoton mechanism and also an optical tunneling mechanism from an adjustable nonequilibrium carrier distribution in the metal tip [10]. This suggests that the agreement in [9] with data could be fortuitous, given the alternative emission mechanisms in [10]. We investigated the emission process by considering thermionic emission [13], optical fi eld emission [9] and multiphoton emission [10]. Multiphoton absorption can be followed by electron emission over a barrier lowered by the Schottky effect [14]. Optical fi eld emission is based on the instantaneous laser electric fi eld lowering the potential barrier, thus allowing electrons to tunnel out of the tip. An analysis of our experimental data shows characteristics of both mechanisms. This is in accordance with the value of our Keldysh parameter [6, 15, 16]. The nature of the emission process is not uniquely established and depends critically on the experimental parameters. This stimulates further debate on the detailed emission process and its consequence for the ultimate electron pulse duration for this source. Only direct experimental evidence, such as diffraction in time experiments [17]–[19], would unambiguously support the claim that the electron emission is sub-cycle. To help determine the nature of the emission process we study two mechanisms in detail (fi gure 1). The optical fi eld emission process is electron-tunneling through a barrier V that has been lowered by Ftot, the sum of a dc and laser fi eld. Tunneling is most likely for electrons close to the Fermi level, EF. Multiphoton absorption can lead to over-the-barrier emission. Upon absorption of four or more photons the gained electron energy exceeds the workfunction φ and direct emission can occur. Here we do not distinguish between coherent and incoherent multiphoton absorption [6]. An applied DC fi eld reduces the workfunction to φeff (Schottky effect [20]) thus lowering the number of photons required for over-the-barrier emission. We also consider the possibility of photon absorption followed by tunneling. These models do not include any band structure [14], LASER-INDUCED ULTRAFAST ELECTRON EMISSION FROM A FIELD EMISSION TIP 3 collision dynamics in the tip [21], or dynamic polarizability in the tip [22]–[26], and cannot be expected to describe the detailed dynamics of the emission process. However, our simple models agree well with experiment. We now turn our attention to a more detailed description of the model. For n-photon absorption the electron emission rate Jabs,n is proportional to the (2n)th power of the laser fi eld (θ is the angle between the laser fi eld polarization and the tip axis),