Investigation of transient surface electric field induced by femtosecond laser irradiation of aluminum

Transient surface electric fields induced by femtosecond laser irradiation of an aluminum film were investigated directly by ultrashort electron pulses. At pump intensities of 2.9~7.1 * 10^10 W/cm2, the transient electric fields last at least one nanosecond with a maximum field strength of 3.2~5.3 * 10^4 V/m at 120 micro m above the aluminum surface. The transient electric fields and the associated evolution of photoelectrons were explained by a"three-layer"model. The potential influence of such fields on reflection ultrafast electron diffraction and time-resolved angle-resolved photoemission spectroscopy were evaluated.


Introduction
Transient electric field (TEF) generally exists in femtosecond laser-matter interactions due to thermionic and/or multi-photon emission of electrons [1][2][3][4][5]. The strength and evolution of the TEF is critical in laser ablation mechanism studies [6][7][8], and the formation of early stage plasmas [9,10] after intense laser irradiation. Under moderate laser excitation conditions, it is also a nontrivial influencing factor for photocathode optimizations [11][12][13][14][15] and time-resolved electron scattering studies, such as ultrafast electron diffraction (UED) [16][17][18][19][20] and time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) [21][22][23]. In UED studies, a crystalline sample is first excited by an ultrashort optical pump pulse and then interrogated by an electron probe pulse delivered at a specific delay time. Transient structural information is majorly obtained from the time-dependent evolutions of the diffraction angle and intensity extracted from electron diffraction patterns. However, the existence of the TEF on the sample surface may distort the trajectory of the probe electrons and make the interpretation of diffraction patterns complicated [24][25][26]. For example, in the studies of semiconductors by reflection UED and metals by transmission UED, the deflection angles induced by the TEFs were comparable with the changes of the diffraction angle originated from structural dynamics [27][28][29]. In the transmission geometry, it has been demonstrated that the structural dynamics and the TEF effect can be distinguished by simultaneously tracking the radii and the centroids of the diffraction rings of polycrystalline crystals [29]. However, their separation in reflection UED is still indistinct. In such case, the TEF is nearly normal to the propagation direction of the reflective probe electrons, 3 which may induce additional and notable deflections to the probe electrons.
Furthermore, the field gradient perpendicular to the sample surface may bring nonuniform distortions to different diffraction spots. As a consequence, the convolution of structural dynamics and such TEF is complex. To ultimately separate the TEF effect from structural dynamics in reflection UED, it is crucial to have a better understanding on the origin and evolution of the TEF and its influence on the probe electrons.
Moreover, the TEFs induced by femtosecond pump laser pulses may influence the angular and energy resolution of photoelectrons in TR-ARPES studies [21][22][23]30], which also leads to the necessity of its investigation.
Previously, studies on light induced electron emissions has been focused on the quantum yield or energy spectroscopy of photoelectrons [31][32][33][34], while the temporal evolution of TEFs is sparsely understood. Recently, ultrafast electron deflection and shadowgraph [10,28,[35][36][37] have provided a direct monitor to transient electromagnetic fields, combining the intrinsic field sensitivity of electrons with the ultrahigh temporal resolution provided by a laser-pump electron-probe configuration.
In this contribution, the TEFs generated by femtosecond laser pulse irradiation of a 25nm thick aluminum film have been investigated by picosecond electron deflection.
Under laser intensities on the order of 10 10 W/cm 2 , it is shown that the TEFs at 120 m above the metallic surface last more than one nanosecond with a maximum strength on the level of 10 4 V/m. The experimental results were explained by a "three-layer" 4 analytic model, which indicates that the observed TEFs were mainly attributed to the thermionic emission of electrons with an initial velocity of 1.4 m/ps and a charge density of approximately 10 7 e -/mm 2 . Based on the dynamics of the TEFs revealed in this study, we further evaluated their influence on UED and TR-ARPES.

Ultrafast electron deflection configuration
The laser-pump electron-probe experimental configuration, as shown in Figure 1, includes a Ti:sapphire laser system (1 kHz, 800 nm, 70 fs, 1 mJ/pulse ), a photoelectron gun driven by ultraviolet pulses, a magnetic lens, a sample holder attached to a 5-axial manipulator, an imaging system, and an ultrahigh vacuum chamber. The main laser beam was split into two parts: 90% was used as the pump and directed to a linear translation stage to precisely control its relative time difference (delay time) with respect to the probe beam. The pump beam was focused to a diameter of 0.8 mm (1/e 2 ) and normally impinged onto a freestanding 25 nm thick aluminum sample, which was a paradigm for UED experiments and prepared according to a routine procedure [38].
The pump intensities of interest was varied from 2.9 to 7.1×10 10 W/cm 2 (2~5 mJ/cm 2 fluence), in the same range as generally applied in time-resolved diffraction studies [38][39][40] and well below the ~10 mJ/cm 2 damage threshold of aluminum [41,42]. Under the pump intensities used in this study, the temporal evolutions of the observed deflection angles are repeatable even after a large number of laser shots on the sample. We also inspected the sample after the experiments by an optical microscopy and no observable damage was found on its surface. The remaining 10% of the main beam was converted 5 to 266 nm ultraviolet light through a frequency tripler and directed to the photocathode of the electron gun, a 30-nm silver layer coated on sapphire disc, to generate ultrashort electron pulses. The probe electrons were accelerated to 59 keV, collimated and focused to a diameter of ~200 m (1/e 2 ) by the magnetic lens with their centroid at 120 m above the sample surface. After passing through the sample area, the deflected probe electrons were recorded by the two dimensional imaging system containing a phosphor screen, a multi-channel plate (MCP) and a charge-coupled device (CCD) camera. Each electron deflection image was acquired with 1-s CCD exposure time to accumulate 10 3 electron pulses and the signal-to-noise ratio was further improved by averaging more than 15 independent measurements of the electron deflection pattern at each delay time.
The ~6 ps temporal resolution of the current setup is mainly limited by the travelling time of the 59 keV probe electrons through the laser-aluminum interaction area, which has a dimension similar to that of the pump beam. The time zero was defined as the onset of the observable deflections of the probe electrons. configuration. (b), a detailed view of the sample area. Z axis denotes the sample surface normal direction, while X and Y axis are parallel to the sample surface. The probe electron beam travels along the Y axis before entering the TEF area and its centroid position at the Z axis is Z0 = 120 m.   is the deflection angle of the probe electron beam. The positive and negative deflection angles represent that the probe electron beam is deflected toward and away from the sample surface, respectively.

Electron deflection data analysis
The time-dependent evolution of the TEF was represented by the corresponding deflection angle of the probe electrons at each delay time. In order to calculate the electron deflection angle, we first integrated the 2D deflection pattern (see Figure 2(a)) along the X and Z directions to obtain two 1D intensity profiles, which were fitted by Gaussian functions to derive the peak positions as the centroid coordinates of the probe electrons. Then, the absolute change of the centroid position at each delay time, , was obtained by subtracting the averaged value before time zero.
Because the TEF is mainly perpendicular to the sample surface, only the 1D intensity distribution along the Z-axis is considered in the study presented here.

The evolution of transient electric field
Upon femtosecond laser excitation of aluminum, the optical energy is rapidly deposited into conduction electrons because their heat capacity is several orders of magnitude smaller than that of the lattice [38]. Some energetic electrons overcome the ~ 3.9 eV work function of the nanosized thin aluminum [43], and escape from the sample surface via thermionic and/or multi-photon emission [1][2][3][4][5]. , is described by the following equation:

The evolution of the TEFs explained by a "three-layer" model
The evolution of the TEFs observed above results from the complex nonlinear many-  The distribution function of the total-emitted electrons, (z, t)  , is defined by the following relation: where  is the ratio of the "fallen back" electrons to the total-emitted electrons.  Table 1. Table 1, Symbols used in the "three-layer" model.
The experimental data were acquired from the shifting of the probe electrons centroid, which originally locates at 120 m above the sample surface, therefore, the value of  Table 2 for detailed 15 discussion. In addition, all fitting parameters were set as free variables with no constrains. Because the photon energy of the pump laser is 1.55 eV, much lower than the 3.9 eV work function of the nanosized thin aluminum [43], electrons are expected to be induced by thermionic and/or multi-photon emission instead of single-photon process. We performed the pump intensity dependence experiments to further distinguish these two emission mechanisms. According to Fowler-DuBridge theory [46] that described the electron emission from a solid surface, the electron yield of the n-th order photoemission is proportional to n I , where I is the pump laser intensity. However, the amount of the total-emitted electron charges is found to depend linearly on the pump intensity, as depicted in Figure 4. This linear relation indicated that thermionic emission is the dominant mechanism within the pump intensities applied here and the contribution of multiphoton emission is insignificant, which consists with the previous theoretical prediction [44].  The fitting results also suggest that, the temporal evolution of the electron distribution functions changes slightly with the increasing of the laser intensity, while the "fallen back" ratio  and the initial charge density 0  increase. Therefore, according to Eq. (2) and Eq. which gives rise to difficulties in the interpretation of electron diffraction data [25,28,45].
In this study, the TEF at 120 m above the metallic surface was found to be on the order of 10 4 kV/m under moderate pump intensities, which are generally applied in UED studies. Its influences on the deflection angle, width, and peak intensity of the probe electron beam profile are depicted in Figure 5 and some characteristic parameters are given in Table 3. For the first tens of picoseconds after laser irradiation, the deflection angle of the probe electrons is on the order of tens micro radians, which is comparable 18 to the typical changes of the diffraction angle induced by structural dynamics in reflection UED [24][25][26]. With the fitting parameters obtained under the pump fluence of 7.1×10 10 W/cm 2 , we further calculated the TEF gradient along the Z direction according to the "three-layer" model and evaluated its influence on the broadening of the probe beam profile. The results imply that, the maximum beam width reached at t=92 ps is 2.2 times of that before time zero, which is in good agreement with the 2.3times experimental value. This good agreement suggests that, applying Eq.(3) for the averaged TEF is reasonable and the "three-layer" model is self-consistent. In addition, the broadening of the beam width also induces the attenuation of the peak intensity.
Both the width and peak intensity recover toward their original values before time zero along with the decay of the TEF.
In general, the results presented here indicate that, TEFs widely exist in UED studies, and in a reflection configuration, it can affect the position, width and peak intensities of the probe electron beam profile, which may cause misinterpretations to the structural dynamics extracted from diffraction patterns. In future UED studies, the pump laser induced TEFs should be evaluated in situ for a closer understanding of the structural dynamics and a better resolution. Meanwhile, the strong electric field above the sample surface may also contribute to the transient structure change, which has not been considered in the previous UED studies. Therefore, further efforts are necessary to access the role of TEF effects on transient structure changes.  The TEF effects may also be an important issue in TR-APERS studies emerged recently, which provide a temporal, angular and energy resolution of photoelectrons on the order of sub-picoseconds, tenth of a degree and milli-electronvolts, respectively [23,47]. In these TR-ARPES studies, samples are pumped by a femtosecond laser pulse and probed by ultraviolet (UV) photons to reveal the time-dependent photoemission spectroscopy at the first few picoseconds after laser excitation. It is generally believed that these photoelectrons is induced by the UV photons and the space charge effect of such 20 electrons has been extensively studied [32,48,49] to improve the energy resolution.
However, even at a low fluence/intensity, the femtosecond pump laser pulse can rapidly heat up the electron system and may generate photoelectrons. This additional effect, which could also be an influencing factor to the energy resolution of TR-ARPES, has rarely been assessed [50].
We estimated the averaged electric field strength on aluminum surface by the "threelayer" model under the lowest pump intensity used here. As presented in Figure 6, it indicated that the averaged electric field strength within several micrometers above the sample surface is on the order of 100 kV/m for the first few picoseconds. Its modulation to the photoelectrons is on the order of 10 2 meV, which may influence the understanding of TR-ARPES results and limit the improvement of the energy resolution to better than milli-electronvolts. Although the samples of interest in TR-ARPES studies are mainly superconductors or topological insulators, the study presented here may bring into attention that, it is insufficient to only consider the effect of UV induced photoelectrons, and the TEF induced by the pump laser pulse should also be evaluated to optimize the performance of TR-ARPES.
21 Figure 6, The TEF strength along the Z direction for the first few picoseconds, which is predicted by the "three-layer" model and the fitting parameters under the lowest pump intensity, 2.9×10 10 W/cm 2 .
However, limited by the experimental configuration of this study and the simplified "three-layer" analytical model, we can only obtain some finite insights into the transient electric field on the metallic surface. Inspired by proton radiography [51], in our further efforts, we will experimentally investigate the spatial-temporal evolution of the TEFs by ultrafast electron radiography. A better understanding of the TEFs may help to improve the resolution and accuracy of time-resolved studies that involved with electrons.

Conclusion:
We used ultrashort electron pulse to directly monitor the femtosecond laser induced 22 transient electric field above the metal surface, which was found to build up in hundreds of picoseconds and decay within nanoseconds. Its strength is on the scale of 10 4 V/m at 120 m above the sample surface under the pump intensities on the order of 10 10 W/cm 2 .
The experimental results were explained by a "three-layer" analytic model, and the observed TEFs were attributed to the thermionic emission of electrons with an initial velocity of ~1.4 m/ps and a charge density of approximately 10 7 e -/mm 2 . The study presented here also indicate that, besides deflection, the probe electron beam width, peak intensity and energy dispersion can also been modulated by the transient electric field. Therefore, for time-resolved electron scattering studies, such as ultrafast electron diffraction and time-resolved angle-resolved photoemission spectroscopy, the transient surface electric field should be considered and evaluated in situ for improved resolution and accuracy.