1. Introduction

Being a bridge between the microscopic and macroscopic systems, nanostructures demonstrate profound impacts in wide fields of, e.g. laser processing [1–3], cancer treatment [4,5], chemical catalysis [6], and climate science [7]. Despite the unique features such as the nanoscale enhanced near field around the nanosystems, the process taken place on the surface of nanoparticles may play a fundamental role during the light-and-matter interactions. For instance, when a nanoparticle is excited by strong femtosecond laser fields, the enhanced near field could be intense enough to cause sufficient ionization of the molecules attached to the surface. As far as the ionization is initiated, the extreme nonlinear response could be possible involving persistent charge generation and avalanching ionization which can give rise to nanoplasma formation. On the other hand, the induced nanoplasma could affect the local field distribution which results in a dramatically altered near field in the nanometer and femtosecond spatial-temporal scales. The determination of the instantaneous near field profile is of fundamental importance in exploring the extreme couplings between intense laser fields and nanostructures. However, relative techniques are yet to be developed at present.

The interaction of nanostructures with ultrashort laser pulses has caught dramatic attentions in the past decade. The characteristic rescattering from small dielectric nanospheres has shown decisive impacts on the electron emission [8–10]. While for large dielectric nanospheres, the propagation effect leads to nanofocusing which allow to manipulate the structure of the near-field in nanometer precision [11–14]. Moreover, the physical property can be significantly influenced in the extreme couplings. For instance, a dielectric nanosphere can be metallized in sub-cycle time regime when induced by intense femtosecond laser pulses [15]. Particularly, when the excitation is strong enough, the nanosystems can be ionized sufficiently thus form nanoplasmas regardless of their constituents. Various experimental apparatus have been constructed for studying the ultrafast dynamics of nanoplasmas [16–18]. Thanks for the development of the advanced imaging techniques, the plasmonic response in nanoparticles have been revealed to a large extent which help people to understand the microscopic world involving many particle interactions [19–22]. The potential influence of the molecules attached to the surface of nanoparticles, who have been ignored in many cases due to the lack of experimental capability have caught people’s attention [18,22–24]. Recently, complex molecular reactions involving proton migration and formation of trihydrogen cations were realized on surface of nanoparticles ignited by intense femtosecond laser pulses [25]. This opens up a new way for tailoring chemical reactions. Nevertheless, the ionization of surface molecules should be taken good use of for the studying of various nanosystems.

Among the above-mentioned works, the target is mostly of the spherical shape which serves as a model system for studying the complicated coupling process between light and nanoparticles. However, with the booming of material science and fabrication techniques, novel nanostructures can be designed and produced giving rise to wide applications. For instance, metallic plasmonic nanocavities that can confine electromagnetic fields in volumes of sub-wavelength scales have been used to promote applications in e.g. nanofocusing [26] and high-sensitive spectroscopy [27,28]. Recently, a tilted plasmonic nanocavity made by silver nanocube successfully shortens the luminescence decay time of a rare-earth-doped nanoparticle to sub-50 ns regime [29]. Looking forward, nanoparticles with various shapes can dramatically promote the functionalities in applications. It is thus essential to explore the dynamics of the coupling of such particles with laser fields.

In this work, we introduce a technique which can help to watch the near field distribution of a gold nanocube when excited by femtosecond laser pulses. The near-field imaging is realized by detection of the momentum distributions of the H⁺ ions generated from surface molecular ionization in metal nanoparticles. When the surface ionization takes place, the produced electrons experience extremely fast dynamics and fly away quickly from the nanosystem before the onset of the ion expansion. The H⁺ ions flying faster than all the other ion products due to its light mass thus can be less affected by other fragments. Moreover, many surface functional groups in nanosystems contain hydrogen, thus the imaging technique based on H⁺ detection can be universally applied to various nanosystems. These make H⁺ a good candidate for imaging the initial near field profiles. The interaction between 90 nm gold nanocubes and near infrared femtosecond laser pulses at 800 nm is measured in a single-shot velocity map imaging apparatus. The ion momentum distributions are obtained for the H⁺ and the Na⁺ ions for two orthogonal laser polarization conditions (x- and y-polarizations shown in Fig. 1). Based on a classical model starting from the enhanced near field distribution in the real space, the final distribution in the momentum space can reach a qualitative agreement with what we observed in the experiment, demonstrating the feasibility of the surface molecular ion imaging technique in detecting the near field profiles of nanostructures.

 

Fig. 1. The schematic experimental setup including the nanoparticle source and the single-shot VMI apparatus. The scanning electron microscope (SEM) image of the gold nanocubes are shown in the enlarged figure here. The side length of the nanocubes used in this experiment is 90 ± 10 nm.

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2. Experimental setup

The experimental setup is schematically illustrated in Fig. 1. The gold nanocubes (with the side length of 90 ± 10 nm) are diluted with distilled water and dispersed into a solution of 0.02 g/L, which is atomized by an atomizer (TSI 3670). The generated aerosol is driven by the carrier gas (argon, 1.5 bar) through a dryer, where most of the moisture can be efficiently absorbed. An impactor is then used to remove the fast-flying particles. The particles are sent to the vacuum chamber by passing through the aerodynamic lens and the differential pumping region, therefore a beam of isolated nanoparticles can be eventually formed. Using the aerosol injection method, the nanoparticles in the gas phase provide a fresh target entering the interaction region for each laser shot, enabling studies in regimes where the targets are irreversibly modified or destroyed after the interaction with the laser [16,30]. Femtosecond laser pulses from a Ti: sapphire amplification system (35 fs @ 800 nm, 1 kHz) are focused on the free-flying nanoparticle beam by a focusing lens (f = 500 mm). The photon energy is about 1.5 eV. To cause sufficient ionization in the gold nanocubes, high intensity is required to initiate multiphoton or tunneling processes. In our work, the excitation laser intensities are on the order of 10¹²∼10¹³ Wcm^-2. These intensities reached by femtosecond pulses cannot be easily realized using continuous beam. Specific ion species generated after the laser-and-nanoparticle interactions are directed to the microchannel plate (MCP)/phosphor detector in the velocity map imaging (VMI) spectrometer. The illuminated images on the phosphor screen are caught by a high-speed CMOS camera at 1 kHz repetition rate.

The nanocubes used in this work are produced by seed-mediated growth [31] and initially dispersed in an aqueous solution. Surface attached sodium citrate is used during the preparation process to reduce the probability of cluster deposition. Besides, some water molecules could be attached to the surface of the gold nanocubes when they enter the interaction region. After being excited by the femtosecond pulses, the ion signals arriving at the MCP detector can be extracted by an RC circuit, and the time of flight (TOF) results can be obtained. As shown in Fig. 2, the H⁺ (m/q = 1) and Na⁺ (m/q = 23) ions (where m/q stands for the mass-to-charge ratio) turn out to be the major products after surface molecular ionization in the gold nanocubes. The dominated signals obtained from ionization of the background airs are the H⁺, H₂O⁺, and N₂⁺ ions, where the highest peak of H₂O⁺ is mainly generated from ionization of water molecules The different ion fragments can be discriminated by applying a high-voltage switch on the MCP at their arriving times. The single-shot momentum distributions of the H⁺ and Na⁺ ions are obtained for the y- and the x-polarized excitation. As being indicated in Fig. 1, the laser pulses propagate along the z-direction, and the excitation laser pulses are linearly polarized along the x- and y-directions, respectively. Atomic units (a.u.) are used throughout this paper unless otherwise indicated.

 

Fig. 2. The time of flight spectra obtained for the gold nanocubes (light green) and the background gases (light blue). The signal intensity for the background gases is magnified by 50 times for better visualization.

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3. Results and discussion

3.1 Ion momentum distributions from surface molecular ionization

In this work, 90 nm nanocubes are excited by linearly polarized femtosecond laser pulses at 800 nm. As schematically shown in Fig. 3 (A), the induced nearfield distribution around the surface of the nanocube is calculated using a Discontinuous Galerkin Time- Domain (DGTD) simulation implemented in the Finite-Difference Time-Domain (FDTD) Solutions software package [32]. The refractive index of gold is given by the Palik model [33]. A three-dimensional (3D) meshing with a grid size of 0.1 nm was applied in the total field domain with perfectly matched layer absorption boundary conditions. For each laser shot, the orientation of the nanocubes may vary with respect to the laser polarization direction since the nanoparticles are flying freely towards the interaction region. After calculating the responses for various nanocube orientations, the geometry with the most efficient ion production is illustrated here. Being a metal material, the coupling with optical laser fields causes screening in the inner region of the nanocube. The resulting field distributions at the surface of the various planes are distinct, where the excitation laser pulses are polarized along the x-axis and the propagation direction is along the z-axis. The fìeld enhancement at the surface of the gold nanocube is concentrated in the red regions, while the center of each plane presents a weak field. A stronger near field enhancement can be obtained in the forward direction at the y-z and the x-z surfaces. Such a propagation effect can not be obtained for gold nanospheres with similar sizes in the same excitation condition. At the surface of the y-z plane, the most intense parts of the near field are close to the four corners, and the strength is gradually reduced along the edge. While the distributions on the x-z plane possess two separated enhanced regions along the polarization direction. The most intense parts are still around the corners, showing a mirror-like symmetry with respect to the z-axis.

 

Fig. 3. (A) The DGTD simulation result of the near field distribution at the surfaces of a gold nanocube. The size of the cube is about 90 nm, and the excitation laser wavelength is at 800 nm. (B) and (C) are the calculation results of the far-field momentum distributions of H⁺ ions for the x- and the y-polarization excitation. The red and green arrows indicate the laser propagation and polarization directions, respectively.

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When a proper excitation strength is used such that the most intense part of the induced near field can reach the ionization threshold of certain surface molecules, the produced ions will be generated in these “most intense” parts. In these cases, the ionization products can be used to image the nanoscale near field distributions [22,34]. To build a bridge between the near field ion distribution to the far-field momentum distribution, which can be detected explicitly, a classical model is used to calculate the far-field momentum distributions of the ionization products from surface molecules. The calculation results are shown in Figs. 3 (B) and (C). Our calculation is operated in two dimensions, involving surface molecular ionization by the induced near field and the expansion dynamics. The initial spatial distribution of the ionization product H⁺ is given according to the near field distribution obtained from the DGTD simulation. The charged particles are then flying towards the detector where the dynamics are calculated with Newtonian equations of motion in consideration of the Coulomb interactions [22]. Here, a positive charge is placed at the center of the nanocube to represent the localized charge ensemble. Figure 3 (B) represents a final momentum distribution evolved from the surface of the x-z plane, where a two-fold distribution can be obtained. Figure 3 (C) showing a ring represents the distribution evolved from the surface of the y-z plane. The distinct profiles of these two cases are mainly attributed to their initial charge distributions. An enhanced ion yield can be observed in the forward direction for both cases. Using the VMI spectrometer, these two cases can be detected explicitly under linear polarized laser pulse excitation in two orthogonal polarization conditions.

3.2 Angle-resolved momentum distributions of different ions from the surface

Ionization of surface molecules has been used to manifest the propagation effect of large silicon nanoparticles in femtosecond laser fields [13]. In our work, the momentum distribution of the H⁺ ions generated from ionization of the surface attached sodium citrate molecules or water molecules are obtained for the x- and the y-polarized excitation and the results are shown in Figs. 4(A) and 4(B). The corresponding integrated angle-resolved energy spectra are presented in Figs. 4(E) and 4 (F) for the x- and the y-polarization, respectively. Among the presented VMI images, Figs. 4(A1) and 4(B1) represent the integrated distribution over many shots. While Figs. 4(A2-A4) and 4(B2-B4) showing the selected single-shot images with representative distributions occupying more than 80% of the effective images. This indicates that the response of a specific particle orientation presented in Fig. 3(A) dominates the interaction. Changing the polarization direction of the excitation laser field in the VMI measurements is equivalent to observe the 3D momentum distributions of the ions from two specific angles. For the H⁺ ions, the momentum distributions are dramatically influenced by the enhanced near-field distribution. The momentum distributions for the x-polarization exhibit left and right lobes. As shown in Fig. 4 (E) the maximum ion yield and cut off energy appears at about 0.45π and 1.55π radius, which is shifted from the polarization direction (pointing at π/2 and 3π/2). The cutoff energy is at about 46 eV and the maximum yield appears at about 19 eV. The enhanced ion yield in the forward direction can be also obtained for the case of the y-polarization excitation. As can be seen in Figs. 4 (B) and 4 (F), the ring-shape profile is slightly elongated at the emission angles of π and 2π which is along the laser propagation direction, and the cutoff energy is at about 40 eV with the maximum yield peak at 15 eV. These energies are smaller than what we obtained for the x-polarization. Qualitative agreements can be reached between the experimental observed momentum distributions and the calculation results shown in Fig. 3. The laser intensity at the focus can reach about 12 TW/cm², which is estimated by examining the momentum shifts of the above-threshold ionization photoelectrons from strong-field ionization of Xe along the laser polarization direction [35]. This intensity does not cause significant ionization of the background gases. As a matter of fact, each nanocube may experience a different laser intensity determined by the exact interaction location within the laser focusing region. The induced near field intensity irradiated on the surface molecules can cover a sufficient large range, even many orders of magnitude. This contributes for the variations among the single-shot VMI images.

 

Fig. 4. The angle-resolved momentum distributions for the H⁺ and the Na⁺ ions from surface ionization of gold nanocubes. (A) The integrated momentum distribution (A1) and three selected single-shot results (A2-A4) obtained for H⁺ ions with x-polarization excitation. (B) Similar to (A) but for the y-polarization. (C) and (D) are similar to (A) and (B) but obtained for the Na⁺ ions. (E) and (F) are angle-resolved energy spectra for the H⁺ ions excited by x- and y-polarization, respectively. The right side show the angle-integrated energy spectrum. The yellow line and the circle with arrow indicate the initial angle (0 degree) and its increasing direction, which is schematically illustrated in Fig. 4 (A1). (G) and (H) are similar to (E) and (F) but for the Na⁺ ions.

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As for the Na⁺ ions, which are probably produced from the surface sodium citrate molecules, the momentum distributions for the x- and the y- polarizations both show forward-focusing features on the round profiles (shown in Figs. 4(C) and 4(D)). The ion distributions are stretched along the laser polarization direction which can be recognized for the x-polarization measurement, while elongated along the laser propagation direction for the y-polarizations. Therefore, the 3D momentum distribution of Na⁺ ions could be an ellipsoidal. Due to the distinct masses of the H⁺ and the Na⁺ ions, the influence of the near field distribution and the dynamical evolvement in the process when the charged particles are flying towards the detectors could be considerably different. By obtaining the projected momentum distributions from two orthogonal directions, the 3D ion momentum distributions can be roughly estimated. More detailed 3D momentum distributions requires can be obtained by measurements based on tomographic method [36]. We can obtain a cutoff energy at about 28 eV with the maximum ion yield at 9 eV. Comparing the momentum distributions of the H⁺ and the Na⁺ ions with the calculation results based on the classical model, the H⁺ ions from surface ionization can serve as a candidate to image the near field profiles of gold nanocubes, when the excitation condition is optimized.

3.3 Intensity-dependent ion momentum distributions

In our work, the spot size of the laser focus is orders of magnitude larger than the size of the gold nanocubes, therefore each nanoparticle could be excited at a distinct intensity depending on its specific location when flying through the laser focus [12]. Former work indicates that the single-shot ion yield can be used to estimate the actual intensity in each interaction event [12,22]. Based on this, we calculate the total counts of each single-shot VMI images obtained for the H⁺ ions and sort the results according to the count rate from each frame. Figure 5(A) show the histograms of H⁺ ions for the x- and the y-polarizations, where the x-axis is the total counts of each VMI image, while the y-axis is the number of frames obtained for a specific count. The profiles are similar for the two polarization conditions. Here, we select three different regions on the histogram, and integrate the single-shot images within each region. Then we get three momentum distributions corresponding to different excitation intensities. The resulting intensity-dependent VMI images are shown in Fig. 5 (B-D) (y-polarization) and Figs. 5 (E-G) (x-polarization). As the laser intensity increases, the ions are gradually expanding for both the x- and the y- polarization. At a relatively high laser intensity, the nanocubes can be sufficiently ionized thus producing a nanoplasma, where characteristic shock wave behaviors can be recognized (e.g. in Figs. 5 (D)), manifested by a sharp ring in the momentum distribution. The angular distribution of the shock waves is almost isotropic, where the less efficient signal around the lower right corner is from the nonuniformity of our detector. This property is different from the shock waves obtained in the SiO₂ nanoparticles exposed to intense laser fields, where distinct angular features can be obtained for each single-shot momentum distribution that was considered to be the results of particle diversity [22]. The underlying formation mechanism of the shock waves can be addressed by a Coulomb explosion model [37]. It was pointed out that the exceeding of the faster moving inner ions with respect to the slower preceding ones could generate a critical sphere with high charge density, and eventually give rise to a leading shock wave. This could also be applied to the present cases.

 

Fig. 5. (A) Histograms obtained for the H⁺ ion at x- and y-polarization excitations. (B-D) momentum distributions obtained by integrate the single-shot images within three distinct histogram regions for the y-polarization, corresponding to different intensity conditions. The red and green arrows indicate the laser propagation and polarization directions, respectively. (E-F) The same to (B-D) but for the x-polarization.

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4. Conclusion

The angle-resolved momentum distributions of the H⁺ and the Na⁺ ions from ionization of the surface molecules adhered to gold nanocubes are detected utilizing a single-shot VMI apparatus. When the excitation intensity is tuned such that the induced near field can just reach the ionization threshold of a specific surface molecule, the resulting momentum distribution of the ion products can be used for imaging the near field distributions. The far field momentum distribution and the near field position profile can be linked by a simple classical model, considering an initial particle distribution which could be dominated by the near field profile and the Newtonian motions of the charged particles when they flying towards the detector. At low laser intensities, the enhanced near field plays a vital role in the ionization process of surface molecules. As we cranking up the excitation intensity, the whole nanosystem can be sufficiently ionized, producing a nanoplasma. There is certain probability that shock waves can be formed in the ionization products from the surface molecules. Our work provides a strategy for visualizing the nanoscale enhanced near field distributions in the extreme coupling between intense laser fields and nanosystems. It is a fundamental breakthrough and is crucial for understanding the following complex dynamics. Looking forward, nanoparticles with various shapes can dramatically promote the functionalities in applications. Our work can contribute to further exploration of the underlying dynamics of unique nanosystems interacting with strong laser fields, and pave the way for novel applications on, e.g. ion source design and tailored chemical reactions.