Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps

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Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps Byung Hee Son, Hawn Sik Kim, Ji-Yong Park, Soonil Lee, Doo Jae Park, and Yeong Hwan Ahn ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00857 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps Byung Hee Son,1 Hawn Sik Kim,1 Ji-Yong Park,1 Soonil Lee,1 Doo Jae Park,2,* and Yeong Hwan Ahn1,* 1

Department of Physics and Department of Energy Systems Research, Ajou University,

Suwon 16499, Korea 2

Department of Applied Optics and Physics, Hallym University, Chuncheon 24252, Korea

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was produced when the gap area was irradiated by the ultrashort pulse laser. The graphene, which has atomically sharp edges with the large damage threshold, enable us to achieve the strong tunneling regime for the subcycle field emission. The photo-induced signals exhibited an anomalous increase in nonlinear order as a function of incident pulse energy in the presence of static electric field. A dynamical analysis of tunneling electrons based on the semi-classical model, which considers the contribution from the recoil electrons, reproduced our observation successfully. The large field enhancement near the graphene edge enabled us to reach the deep tunneling regime with the extraordinary Keldysh parameter of 0.2 in the near-infrared region, which has not been accessible by the conventional metal nanostructures.

KEYWORDS: Strong-field photo-emission, graphene, Keldysh parameter, subcycle wavepacket, attosecond *Electronic mail: [email protected]; [email protected]

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Electron emission by the irradiation of nanostructures using a femtosecond (fs) laser, which relies upon the realization of an ultrashort electron source, is the key to novel imaging systems having unprecedented spatial and temporal resolutions, even down to a few femtoseconds and a few nanometers.1-3 A series of studies have been dedicated to understanding its generation mechanism and dynamic behaviors under the oscillating electric field of the ultrashort laser source.4-12 Specifically, it has been proven that a strong-field tunneling, i.e., a direct tunneling of electrons owing to a strong ponderomotive potential enables a wavepacket with a pulse width of less than half the period of the light wave to be achieved in the regime of very low Keldysh parameter of γ = 1 (practically, γ < 0.5 ).13,14 The Keldysh parameter is defined as γ = ω 2mΦ / eE0 , where Φ is the work function, m is the mass of the electrons, e is the electron charge, ω is the optical frequency and E0 is the optical-field strength. More recently, strong electric fields and the associated field gradients near the sharp metallic nanostructures enabled the successful photoemission based on strongfield tunneling.15-26 In particular, novel phenomena linked to the subcycle electron motions due to the strong-field gradient near the nanostructure have been addressed.12,22-28 The control of electron motions by adjusting the intensity, wavelength ( λ ), or carrierenvelop phase difference has been considered as a key factor for generating an ultrashort pulse and maintaining its pulse width while traveling through free space.29-31 In particular, achieving the sufficiently low Keldysh parameter requires a high-intensity laser field and a longer wavelength that reaches the mid-infrared region. However, most previous studies on the laser-induced electron emissions are based on the metal nanoprobes or similar metal nanostructures32-34 that suffer from thermal damages at the high-intensity regime.16,21,24,25 Until now, the strong-field emission has been mostly attainable for λ > 1µ m due to the wavelength scaling of the Keldysh parameters,23,24,35,36 whereas the subcycle emission with the shorter excitation wavelength (such as in the visible and near-infrared regions) is crucial

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for producing short electron pulses with attosecond duration and with narrow energy spread.25,37 In the near-infrared region (e.g., at λ = 800 nm), the Keldysh parameter of

γ ~ 0.7 has been reported using the gold nanotips;16 however, the strong field tunneling regime ( γ < 0.5 ) has not been accessible with λ < 1µ m . This necessitates the development of novel functional devices based on nanomaterials with the large enhancement factor and the high damage threshold. Low-dimensional carbon materials such as graphene and carbon nanotubes have nanoscale morphology, capable of producing high emission current density owing to their high damage threshold.38-41 In particular, carbon nanotubes have been extensively studied as alternative thermionic emitters, and very recently, the Keldysh parameter of 0.67 has been demonstrated by irradiating fs laser pulses ( λ = 820 nm) on the carbon nanotubes.37 On the other hand, the graphene edge, which has atomically thin morphology, could serve as an ideal platform for the ultrafast field emission devices in the strong tunneling regime, owing to its high aspect ratio, high carrier density, larger carrier mobility, and mechanical stability. Graphene-based field emitters have been demonstrated recently without the light illumination,40-42 and more recently, electron dynamics in the strong-field subcycle regime has been demonstrated using a few-cycle femtosecond laser.43 However, graphene has not been considered for the ultrafast electron emitter especially in the strong-field tunneling regime. In this letter, we demonstrate an efficient generation of subcycle electron wavepackets in a nanogap of the graphene, using near-infrared fs laser pulses. A peculiar power dependence of electron emission ensures the strong-field tunneling regime, consistent with the dynamical analysis based on the quasi-classical model. We extracted the laser field enhancement factor and estimated the Keldysh parameter, confirming the subcycle wavepacket generations.

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Figure 1. Schematic illustration of photo-induced subcycle field emission and DC fieldemission results. (a) Schematic of femtosecond-laser-induced field emission in the nanogap fabricated in graphene and SWNT field-emission devices. (b) SEM image of nanogap (with a width of 100 nm) in the middle of graphene channel with length ( lch ) of 5 µ m . The graphene channel is covered by a Ma-N polymer, which functions as a protective layer. (c)

IDC −VDC characteristics of the graphene nanogap device. (Inset) Fowler–Nordheim plot of the device.

As schematically illustrated in Figure 1, we fabricated graphene field-emission devices with a nanogap in the middle of the conducting channel. The nanogap was fabricated using focused ion-beam (FIB) milling on the graphene devices. We begin with the graphene fieldemission devices as shown in the scanning electron microscopy (SEM) image of Figure 1b. The device has a channel length ( lch ) of 5 µ m with a nanogap (100 nm) in the middle of the graphene channel. It is covered by the negative e-beam resist (Ma-N 2401) used to define the conducting channel (with a width of 0.6 µ m ), which is also used as a protecting layer during

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the FIB processes and the field-emission experiments. The field emission characteristics of the device with the nanogap were measured in a vacuum environment ( ~ 1×10−6 torr ) (without the laser illumination) and are demonstrated in Figure 1c; the current I DC is virtually turned off at a low bias, but increases dramatically at an external bias with VDC > 6.7 V. This rectifying behavior is a strong indication of the field emission in the nanogap, which is also confirmed by the Fowler–Nordheim plot shown in the inset of Figure 1c. From the fitting curve (red line), a field-enhancement factor of ~84.1 was extracted (when the work function of the graphene was assumed to be 5.0 eV). The DC–field enhancement factor reaches 235 in average measured for 39 devices (Supporting Information S1). Here, we are interested in the photo-assisted field emission when we illuminate the gap with the fs laser pulse. A Ti:Sapphire laser centered at 800 nm with a repetition rate of 80 MHz and a pulse width of 30 fs is used to locally photoexcite an field-emission device. First, Figure 2a shows a typical scanning photocurrent microscopy (SPCM)44-46 image of a graphene device that has a channel length of 6.5 µ m without the nanogap in the middle; this image is acquired by scanning using a laser pulse (~3.3 pJ) with a spot size of ~ 1 µ m for VDC = 0 V. Photocurrent spots near the metal contacts (depicted by S and D) are clearly

visible. In this figure, red (blue) indicates a positive (negative) current. The current spots near the metal electrodes originate from the electronic band bending at the electrode-graphene interfaces, as reported in other studies.44-46

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Figure 2. Photo-induced field emission from a graphene nanogap device. (a) A typical photocurrent microscopy image (SPCM) of a graphene device with lch = 6.5 µ m without a nanogap for VDC = 0 V and a laser pulse energy of J ~ 3.3 pJ. (b) SPCM image for the device with the nanogap for VDC = 8 V and J ~ 1.4 pJ. (c) I PE as a function of VDC for J ~ 13.3 pJ (red squares). Shown together is IDC −VDC curve without the laser illumination (black line).

The SPCM images for the device with the nanogap are visualized in Figure 2b for VDC = 8 V. We found a strong signal when the focused laser illuminated the gap area of the device, whereas the metallic signals appearing in Figure 2a are suppressed dramatically. We used a lock-in technique, in which the laser is modulated at 20 kHz; hence, only the current induced by the photoelectron emission ( I PE ) is plotted. Here, two graphene edges facing each other across the nanogap work as an electron emitter and receiver. We observed ~174 pA when the laser pulse energy (J) is 1.4 pJ as compared to the laser-free tunneling current of ~922 pA at VDC = 8 V. The number of photo-induced electrons corresponds to 14 per pulse, considering

the repetitions rate (80 MHz). We plotted I PE as a function of VDC in Figure 2c (red boxes). Shown together as a black line is the IDC −VDC curve without the laser illumination. Clearly,

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the turn-on voltage is reduced significantly with laser irradiation, from 8.8 V to 4.0 V, which is due to the additional field-enhancement supplied by the ultrashort laser pulse.

Figure 3. Field emission characteristics of graphene-metal field-emission devices. (a) SEM image of fabricated graphene-to-metal field emission device. (b) IDC −VDC characteristics of the graphene nanogap device for VDC > 0 (red) and VDC < 0 (black). (Inset) Fowler–Nordheim plots of the device for each VDC polarity. (c) SPCM images for the device with a gap size of 700 nm for VDC = 6 V (top) and VDC = –8 V (bottom) when the laser pulse energy is 20 pJ.

For comparison, we fabricated graphene-to-metal field-emission devices as shown as an SEM image in Figure 3a. Conventional e-beam lithography methods were used to define the

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gap between the metal electrode and graphene. The field emission characteristics of the device were measured in a vacuum environment (without the laser illumination) and are demonstrated in Figure 3b for both positive ( VDC > 0) and negative ( VDC < 0) bias conditions. Obviously, in the positive bias condition, the graphene edge will work as an electron emitter (as in the graphene-graphene devices shown in Figure 2); conversely, in the negative bias condition, the metal edge should work as an emitter. In both cases, we could observe clear DC field-emission characteristics as illustrated by the Fowler–Nordheim plots in the inset of Figure 3b. We note that the field-emission amount is significantly larger for the graphene edge emission ( VDC > 0), although the graphene is much thinner than the metal film (50 nm). When irradiated by the femtosecond laser pulse (J = 20 pJ), we observed a photo-field emission peak at the gap, as in the case of the graphene-graphene devices. Here, we used the sample with a relatively wide gap of 700 nm to clearly locate the emission site which depends on the VDC polarity. The SPCM images confirm that the photo-induced field emission occurs at the edge of the graphene for VDC > 0, whereas, it appears near the metal edge for VDC < 0. This illustrates that Au film edge can also work as a source for the ultrafast electron emission at the higher bias condition; however, the metal nanostructures inherently suffer from the low damage threshold.37 Indeed, the field emission in graphene-metal devices does not persist in general, as compared to the graphene-graphene devices (Supporting Information S2).

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Figure 4. Laser power dependence on the photo-emission from the graphene nanogap device. (a) IPE as a function of pulse energy for VDC = 6V (red circles). Shown as solid lines are the electron emission yield as a function of pulse energy deduced by the Simpleman model with different βAC ’s. (b) and (c) The schematic of the photo-induced electron generation. (d) and (e) Spatio-temporal image for the electron probability, generated by summing the yieldweighted electron trajectories, for those emitted at each birth time with low (d) and high (e) power conditions, respectively, for J = 3.1 pJ and 99.2 pJ.

The laser power dependence on the photo-assisted field emission is plotted in Figure 4a (red circles). The IPE signals increase with the increasing laser pulse energy without demonstrating saturation behaviors, e.g., reaching 2200 e-/pulse (28 nA) at a laser pulse energy of 99.2 pJ for VDC = 6 V. The remarkable amount of the tunneling current was possible owing to the large damage threshold of graphene. Significantly, in the low-power regime, the signal shows a sublinear behavior with an exponent of P ~ 0.45, whereas the

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exponent increases to as high as P ~ 1.27. We also found similar power dependence in the graphene-metal field-emission device shown in Figure 3 (Supporting Information Figure S4). This unique nonlinear relationship, i.e., the increasing exponent with the increasing power, has not been demonstrated before, and is a signature of the subcycle pulse generation assisted by the strong-field tunneling at the high power conditions, as will be shown below. The origin for the photo-induced electron generation is described schematically in Figure 4 b,c. Owing to the oscillating electric fields supplied by the laser pulse, the vacuum potential effectively oscillates such that the tunneling probability increases in the positive phase and decreases in the negative phase of the laser field. A net current induced by the laser field in our experiment is the summation of the contributions from both the positive ( I+ ) and negative ( I− ) phases. In the absence of the external DC bias, those contributions completely cancel each other, resulting in a zero photocurrent. However, owing to the nonlinear increase in the tunneling probability in the presence of a DC bias, the net photocurrent no longer drops to zero (i.e., I+ > I− ), as shown in the plot of Figure 4c. Based on this assumption, the net photocurrent (i.e., the electron yield) IPE can be calculated by applying the Fowler–Nordheim equation for each emission phase and by integrating all electrons, as described below:47

I PE ∝

∫

∞

−∞

dt

e3 E 2 4 2mΦ3/2 exp( ), − 16π 2hΦ 3he | E |

where E is the oscillating electric field (see also Supporting Information S6 and S7 for details). Here, E can be represented by E = βDC ⋅ EDC + βAC ⋅ E0e−4ln 2 t

2

/τ 2 − iωt

e

, where EDC is the

DC electric field, E0 is the peak amplitude of the laser field, τ is the laser pulse width, and

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βDC and βAC denote the DC- and the laser-field enhancement factors, respectively, near the sharp edge of the graphene.18 In the strong-field tunneling regime, the fs laser predominantly works as a source for the AC electric fields that modulate the vacuum potential and subsequently accelerate the photo-emitted electrons, in which case we can neglect the effect of the photoexcited electrons having a nonthermal distribution. More importantly, the dynamic behavior of the electron motion should be considered, because the recoil electrons that do not contribute to the net photocurrent will strongly modify the amount of the collected electrons (Supporting Information S6). Based on this consideration, we applied a modified quasi-classical Simpleman model,15,21,23-25,48 to fully understand the laser-power dependence shown in Figure 4a. The net photocurrent was obtained by integrating the electrons emitted over the entire cycle of the laser field. Shown as a black curve in Figure 4a is a plot of the net photocurrent as a function of incident laser pulse energy, by using this model. For the fixed value of VDC = 6V, the field enhancement factors of βDC = 155 and βAC = 45 have been obtained to best fit to our experimental curve. Evidently, the simulation results reproduced our unique laser power dependence, i.e., from the sublinear to the higher exponent, successfully. Importantly, we note that the presence of recoil electrons, which do not contribute to the net photocurrent, should be considered to fully understand the unique laser-power dependence. In other words, the relative number of recoil electrons decreases in the high-intensity regime, allowing more electrons to escape from the strong near-field region before they are decelerated in the subsequent negative phase. As a result, the exponent will increase with the increasing pulse energy, consistent with our unique power dependence as shown in Figure 4a. To further elucidate the effect of the dynamical motions of the ultrafast electron packets, we added the spatio-temporal image for the electron probability, generated by summing the yield-weighted electron trajectories, for those emitted at each birth time. Here, the electron

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probability was calculated for the laser field cycle with peak electric field (referred to as the main cycle hereafter), and the results are shown in Figure 4d,e for the low ( J = 3.1 pJ ) and high ( J = 99.2 pJ ) pulse energies, respectively. First, for the low-power condition, most of

the electrons are in the quiver-motion regime; therefore, the electron wavepacket spreads over a large time duration. Conversely, in the high-power regime, more electrons exist in the subcycle regime, resulting in the shorter wavepacket as compared to the low-power case. In other words, a transition in the dynamical behaviors occurs from the quiver motion to the subcycle motion with the change in the field intensity. As a result, the high-power condition will allow us to produce the trains of the attosecond electron wavepackets with duration of less than half cycle (~1.3 fs) of the incident laser pulses (i.e., 0.34 fs in terms FWHM for

J = 99.2 pJ ).

Table 1. Comparison of AC field enhancement factors and Keldysh parameters according to field emission devices.

Materials

λ (nm)

Tungsten (tip) Tungsten (tip) Gold (tip) Silicon (tip) CNT

800 800 830 800 410 820 800

Graphene

Max. electron (e-/pulse) 0.5 1000 39.0 2200

βAC

βAC·F0 (GV/m)

γ

Reference

4.0 4 8 14 26.7 21.6 45

6.94 5.5 23.5 15.4 16.8 24.2 77.8

2.37 3.13 ~ 0.7 1.12 1.83 0.67 0.2

20 22 16 34 37

(our data)

Finally, we compare our device parameters such as the photo-electron emission amount, the AC enhancement factor, and the Keldysh parameter with those found in the literatures and the results are summarized in table 1. Including the AC field enhancement factor, the Keldysh parameter can be expressed as γ = ω 2mΦ / eβ AC F0 , where F0 is the incident optical-field strength. As mentioned before, the high damage threshold and the nanoscale

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morphology of graphene edge allowed us to produce large amount of photo-emission electrons (average of 2100). In addition, the Keldysh parameter is one of the dominant factors that determine the dynamical behavior of the photo-electrons. In other words, the strong-field emission dominate the emission process for γ = 1 , whereas γ has been limited to be ~ 0.7 for

λ < 1 µ m . In our graphene nanogap devices, the condition for γ < 0.5 can be reached at the very low pulse energy of J ~ 15 pJ , owing to the large field enhancement factor (Supporting Information S5). At the maximum power condition (for J = 99.2 pJ shown in Figure 4a) in which the photo-emission current exhibit stable operation, the Keldysh parameter reaches as low as 0.2, which is the lowest value reported so far in the near-infrared range. This is sufficiently low value reaching the deep tunneling regime,14 which will allow us to produce the electron emissions consistent with the subcycle wavepackets as illustrated in Figure 4e. To conclude, we demonstrated the generation of subcycle electron wavepackets in graphene-nanogap devices using an fs laser pulse. We observed an efficient field emission from the devices with the nanogap, which is consistent with the Fowler–Nordheim equation. Strong photo-induced currents were produced when the gap area was irradiated by the focused ultrashort pulse laser. A remarkable amount of tunneling current was obtained owing to the large damage threshold of the graphene. More importantly, the photo-induced signals exhibited an anomalous increase in nonlinear order as a function of the incident light intensity, which was successfully reproduced by the dynamic analysis based on the semiclassical model of tunneling electrons under the strong-field regime. The simulated spatiotemporal image for the electron probability revealed the transition of the dynamical behaviors from the quiver motion to the strong subcycle pulse generation with the change in the field intensity. Graphene nanogap structures, which have the atomically sharp morphology and the high damage threshold, enable us to achieve the extremely low Keldysh parameter of 0.2. This confirms that the photo-induced electron emission from the graphene edge is in the deep

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tunneling regime, which has not been obtainable in the near-infrared regime. Therefore, our work will contribute to the development of high-damage-threshold and tunable devices that produce ultrashort (subcycle) electron pulses for future applications of table-top attosecond streaking, spatio-temporal imaging, and high-speed electronics.

Experimental Section Scanning Photocurrent Microscopy on Nanogap Field-emission Device: An fs Ti:Sapphire laser (centered at 800 nm with a repetition rate of 80 MHz and a pulse width of 30 fs) is used to locally photoexcite an FE device. The fs pulse was focused on the samples using the objective lens (60X and N.A. 0.70), which has a full width at half maximum of 1 µ m . A two-axis steering mirror (Newport Corporation, Inc.) is used to manipulate the positions of the focused laser spots. An optical modulator (Boston Micromachines Corporation) is used to optically modulate the pulse at 20 kHz and IPE is measured using a high-speed current preamplifier (FEMTO Messtechnik GmbH) and a subsequent lock-in amplifier (Signal Recovery) operated at the modulation frequency. Throughout the experiments, the device was placed in the home-made chamber in the ~ 1×10−6 torr vacuum environment. Graphene Device Fabrication: Graphene devices were fabricated using conventional chemical vapor deposition (CVD) techniques, followed by photolithography methods.49 Specifically, graphene was prepared utilizing the CVD method on a 25 µ m Cu foil (Alfa Aesar, No.13382) using methane and hydrogen gases. PMMA was used as a graphene carrier from the Cu foil onto the Si substrate that has an oxide layer of thickness 220 nm and heavily doped p-type Si layers of thicknesses 550 µ m (the resistivity was 0.001–0.003 Ω/cm). The Si substrate contained the drain and source electrodes (Cr/Au) fabricated previously with conventional lithography techniques. After the Cu foil was fully dissolved in an ammonium persulfate solution, the PMMA/graphene film was transferred onto the Si substrate. After the transfer process, the PMMA layer was removed using acetone solution for 24 hours at room

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temperature. The graphene channel was narrowed down to 1 µ m using e-beam lithography followed by reactive ion etching (RIE). Nanogap fabrication: Nanoscale gaps (~100 nm) in the middle of the graphene channel were generated by the FIB method using Ga+ ion beam. The acceleration voltage of the FIB was set to 30 kV with the beam current varying from 10 pA to 30 pA, and the exposure time varying from 2–6 s, depending on the samples. The depth of the trench was in the range of 10–100 nm. It is noteworthy that the graphene device should be covered by the protective layer (negative e-beam resist in our case) during the FIB process that causes damage to the device. On the other hand, the nanogap in the the graphene-metal devices were fabricated when we define the graphene channel by using the e-beam lithography method followed by the etching with RIE process.

Acknowledgements This work was supported by the Midcareer Researcher Program (2017R1A2B4009177) and Basic Science Research Program (2015R1D1A1A01061274) through a National Research Foundation grant funded by Korea Government (MSIP) and by Human Resources Program in Energy Technology (20164030201380) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea Government (MOTIE).

Competing of Interests The authors declare that they have no competing interests.

Correspondence. Correspondence and requests for materials should be addressed to D.J.Park ([email protected]) and Y.H.A ([email protected])

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References (1) Zewail, A. H., Micrographia of the twenty-first century: from camera obscura to 4D microscopy. Phil. trans. A 2010, 368, 1191-204. (2) Williamson, J. C.; Cao, J.; Ihee, H.; Frey, H.; Zewail, A. H., Clocking transient chemical changes by ultrafast electron diffraction. Nature 1997, 386, 159-162. (3) Itatani, J.; Quéré, F.; Yudin, G.; Ivanov, M.; Krausz, F.; Corkum, P., Attosecond Streak Camera. Phys. Rev. Lett. 2002, 88, 173903. (4) Corkum, P. B., Plasma perspective on strong field multiphoton ionization. Phys. Rev. Lett. 1993, 71, 1994-1997. (5) Nicklich, W.; Kumpfmüller, H.; Walther, H.; Tang, X.; Xu, H.; Lambropoulos, P., Above-threshold ionization of Cesium under femtosecond laser pulses: New substructure due to strongly coupled bound states. Phys. Rev. Lett. 1992, 69 (24), 3455-3458. (6) Paulus, G. G.; Becker, W.; Walther, H., Classical rescattering effects in two-color above-threshold ionization. Phys. Rev. A 1995, 52, 4043. (7) Varró, S.; Ehlotzky, F., High-order multiphoton ionization at metal surfaces by laser fields of moderate power. Phys. Rev. A 1998, 57, 663-666. (8) Irvine, S. E.; Elezzabi, a. Y., Ponderomotive electron acceleration using surface plasmon waves excited with femtosecond laser pulses. Appl. Phys. Lett. 2005, 86, 264102. (9) Corkum, P. B.; Burnett, N. H.; Brunel, F., Above threshold ionization in the long wavelength limit. Phys. Rev. 1989, 62, 1259-1262. (10) Lemell, C.; Tong, X. M.; Krausz, F.; Burgdörfer, J., Electron Emission from Metal Surfaces by Ultrashort Pulses: Determination of the Carrier-Envelope Phase. Phys. Rev. Lett. 2003, 90 (7), 076403. (11) Krüger, M.; Lemell, C.; Wachter, G.; Burgdörfer, J.; Hommelhoff, P., Attosecond physics phenomena at nanometric tips. J. Phys. B 2018, 51 (17), 172001. (12) Ciappina, M. F., et al., Attosecond physics at the nanoscale. Rep. Prog. Phys. 2017, 80 (5). (13) Keldysh, L. V., Ionization in the field of a strong electromagnetic wave. Sov. Phys. JETP 1965, 20 (5), 1307-1314. (14) Ilkov, F. A.; Decker, J. E.; Chin, S. L., Ionization of atoms in the tunnelling regime with experimental evidence using Hg atoms. J. Phys. B 1992, 25 (19), 4005-4020. (15) Hommelhoff, P.; Sortais, Y.; Aghajani-Talesh, A.; Kasevich, M. A., Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses. Phys. Rev. Lett. 2006, 96, 77401. (16) Bormann, R.; Gulde, M.; Weismann, a.; Yalunin, S.; Ropers, C., Tip-Enhanced Strong-Field Photoemission. Phys. Rev. Lett. 2010, 105, 147601. (17) Vogelsang, J.; Robin, J.; Nagy, B. J.; Dombi, P.; Rosenkranz, D.; Schiek, M.; Groß, P.; Lienau, C., Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons. Nano Lett. 2015, 15 (7), 4685-4691. (18) Hommelhoff, P.; Kealhofer, C.; Kasevich, M. A., Ultrafast Electron Pulses from a Tungsten Tip Triggered by Low-Power Femtosecond Laser Pulses. Phys. Rev. Lett. 2006, 97, 247402. (19) Schröder, B., et al., Real-space imaging of nanotip plasmons using electron energy loss spectroscopy. Phys. Rev. B 2015, 92 (8), 085411. (20) Krüger, M.; Schenk, M.; Hommelhoff, P., Attosecond control of electrons emitted from a nanoscale metal tip. Nature 2011, 475, 78-81. (21) Ropers, C.; Solli, D. R.; Schulz, C. P.; Lienau, C.; Elsaesser, T., Localized Multiphoton Emission of Femtosecond Electron Pulses from Metal Nanotips. Phys. Rev. Lett. 2007, 98, 43907.

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(22) Schenk, M.; Krüger, M.; Hommelhoff, P., Strong-Field Above-Threshold Photoemission from Sharp Metal Tips. Phys. Rev. Lett. 2010, 105, 257601. (23) Piglosiewicz, B.; Schmidt, S.; Park, D. J.; Vogelsang, J.; Groß, P.; Manzoni, C.; Farinello, P.; Cerullo, G.; Lienau, C., Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures. Nature Photon. 2014, 8 (1), 37-42. (24) Park, D. J.; Piglosiewicz, B.; Schmidt, S.; Kollmann, H.; Mascheck, M.; Lienau, C., Strong field acceleration and steering of ultrafast electron pulses from a sharp metallic nanotip. Phys. Rev. Lett. 2012, 109 (24), 244803. (25) Herink, G.; Solli, D. R.; Gulde, M.; Ropers, C., Field-driven photoemission from nanostructures quenches the quiver motion. Nature 2012, 483, 190-193. (26) Park, D. J.; Piglosiewicz, B.; Schmidt, S.; Kollmann, H.; Mascheck, M.; Groß, P.; Lienau, C., Characterizing the optical near-field in the vicinity of a sharp metallic nanoprobe by angle-resolved electron kinetic energy spectroscopy. Ann. Phys. 2013, 525 (1-2), 135-142. (27) Yanagisawa, H.; Schnepp, S.; Hafner, C.; Hengsberger, M.; Kim, D. E.; Kling, M. F.; Landsman, A.; Gallmann, L.; Osterwalder, J., Delayed electron emission in strong-field driven tunnelling from a metallic nanotip in the multi-electron regime. Sci. Rep. 2016, 6, 35877. (28) Ortmann, L., et al., Emergence of a Higher Energy Structure in Strong Field Ionization with Inhomogeneous Electric Fields. Phys. Rev. Lett. 2017, 119 (5). (29) Krausz, F.; Ivanov, M., Attosecond physics. Rev. Mod. Phys. 2009, 81, 163-234. (30) Goulielmakis, E.; Yakovlev, V. S.; Cavalieri, A. L.; Uiberacker, M.; Pervak, V.; Apolonski, A.; Kienberger, R.; Kleineberg, U.; Krausz, F., Attosecond Control and Measurement: Lightwave Electronics. Science 2007, 317 (5839), 769-775. (31) Zherebtsov, S., et al., Controlled near-field enhanced electron acceleration from dielectric nanospheres with intense few-cycle laser fields. Nature Phys. 2011, 7, 656-662. (32) Sivis, M.; Duwe, M.; Abel, B.; Ropers, C., Extreme-ultraviolet light generation in plasmonic nanostructures. Nature Phys. 2013, 9 (5), 304-309. (33) Kim, S.; Jin, J.; Kim, Y.-J.; Park, I.-Y.; Kim, Y.; Kim, S.-W., High-harmonic generation by resonant plasmon field enhancement. Nature 2008, 453, 757-60. (34) Keathley, P. D.; Sell, A.; Putnam, W. P.; Guerrera, S.; Velásquez-García, L.; Kärtner, F. X., Strong-field photoemission from silicon field emitter arrays. Ann. Phys. 2013, 525 (12), 144-150. (35) Wimmer, L.; Herink, G.; Solli, D. R.; Yalunin, S. V.; Echternkamp, K. E.; Ropers, C., Terahertz control of nanotip photoemission. Nature Phys. 2014, 10 (6), 432-436. (36) Colosimo, P., et al., Scaling strong-field interactions towards the classical limit. Nature Phys. 2008, 4 (5), 386-389. (37) Li, C., et al., Carbon Nanotubes as an Ultrafast Emitter with a Narrow Energy Spread at Optical Frequency. Adv. Mater. 2017, 29 (30), 1701580. (38) Eletskii, A. V., Carbon nanotube-based electron field emitters. Phys. Usp. 2010, 53 (9), 863-892. (39) Zhu, W.; Bower, C.; Zhou, O.; Kochanski, G.; Jin, S., Large current density from carbon nanotube field emitters. Appl. Phys. Lett. 1999, 75 (6), 873-875. (40) Kim, T.; Lee, J. S.; Li, K.; Kang, T. J.; Kim, Y. H., High performance graphene foam emitter. Carbon 2016, 101, 345-351. (41) Weiss, N. O.; Zhou, H.; Liao, L.; Liu, Y.; Jiang, S.; Huang, Y.; Duan, X., Graphene: An emerging electronic material. Adv. Mater. 2012, 24 (43), 5782-5825. (42) Chen, L.; Yu, H.; Zhong, J.; Song, L.; Wu, J.; Su, W., Graphene field emitters: A review of fabrication, characterization and properties. Mater. Sci. Eng. B 2017, 220, 44-58. (43) Higuchi, T.; Heide, C.; Ullmann, K.; Weber, H. B.; Hommelhoff, P., Light-fielddriven currents in graphene. Nature 2017, 550 (7675), 224-228.

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(44) Park, J.; Ahn, Y. H.; Ruiz-Vargas, C., Imaging of photocurrent generation and collection in single-layer graphene. Nano Lett. 2009, 9 (5), 1742-1746. (45) Park, J. K.; Son, B. H.; Park, J. Y.; Lee, S.; Ahn, Y. H., Imaging surface charge distribution near carbon nanotube device in aqueous environments. Appl. Phys. Lett. 2014, 105 (22), 223101. (46) Son, B. H.; Park, J. K.; Hong, J. T.; Park, J. Y.; Lee, S.; Ahn, Y. H., Imaging ultrafast carrier transport in nanoscale field-effect transistors. ACS Nano 2014, 8 (11), 11361-11368. (47) Murphy, E. L.; Good, R. H., Jr., Thermionic Emission, Field Emission, and the Transition Region. Phys. Rev. 1956, 102 (6), 1464-1473. (48) Krüger, M.; Schenk, M.; Förster, M.; Hommelhoff, P., Attosecond physics in photoemission from a metal nanotip. J. Phys. B 2012, 45 (7), 074006. (49) Son, B. H.; Kim, H. S.; Jeong, H.; Park, J.-Y.; Lee, S.; Ahn, Y. H., Electron beam induced removal of PMMA layer used for graphene transfer. Sci. Rep. 2017, 7 (1), 18058.

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ACS Photonics

a

c5

Au

-24

4

Graphene

VDC

IDC 2 μm

3

ln(IDC/VDC2)

b

IDC (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

-25 -26 0.10

0.11

1 0 0

2

Figure 1 ACS Paragon Plus Environment

0.12

1/VDC

4

0.13

6

VDC (V)

0.14

8

10

a

S

D 2 μm

b

(nA) 2 1 0 -1 -2

c

Nanogap (pA) 150 100 50 0

25

IDC IPE

20

400

IPE (pA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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IDC (nA)

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(J = 13.3 pJ)

15

200

10 5 0

0 0

2


4

6

VDC (V)

8

10

12

ACS Photonics

a e-

metal e-

graphene

VDC

200 nm IDC

b 60

Graphene ® Metal Metal ® Graphene

40 20

-21.0

ln(IDC/VDC2)

êIDCê (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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VDC>0

-21.5 -22.0 -22.5

VDC 0

graphene graphene

VDC < 0 0

IDC 2 μm

100 200 (pA)


Page 23 of 23

a

b Experiment bDC=155

EF

bAC = 15

Laser field

bAC= 45

+phase

c

bAC= 35 bAC= 65

Pulse Energy (pJ)

0

0.6

1

0.4

2 3

0.2 0

5

10

100

1

+phase

0.5 −phase 0

0

2

4

6

8

10

Voltage (V)

e

0

Distance (nm)

10

Density (arb. unit)

1

E = b DC × EDC + b AC × E0 eiwt

Yield (Norm.)

bAC= 25 1

d

−phase

Φ

2 1 1 2 0 3

Time (fs)

0

5

10

Time (fs)


Density (arb. unit)

IPE (nA)

10

Distance (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Ultrafast Strong-Field Tunneling Emission in Graphene Nanogaps

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