Fig. 1. Lycurgus Cup in the 4th century^[21]. (a) Light reflected by gold-silver colloid in glass; (b) light transmitted by gold-silver colloid in glass

Fig. 2. Fluorescence emitted by quantum dots of different sizes (size of quantum dot from left to right increases in turn)

Fig. 3. Transmission electron micrograph (TEM) images of SiO₂ nanoparticles with different diameters^［39］. (a) Diameter of nanoparticle is 50 nm; (b) diameter of nanoparticle is 184 nm; (c) diameter of nanoparticle is 280 nm

Fig. 4. TEM images of AuNPs and gold nanoshells^[39]

Fig. 5. Schematic of aerosol generation system (system is consisted of evaporator, silica drying tube, impactor, and pressure equalizer)^[40]

Fig. 6. Schematic diagram of aerodynamic lens [nanoparticles enter the atmosphere from the left side through a 150-μm glass hole under vacuum, through a series of lens (aperture) groups]^[39]

Fig. 7. Experimental arrangements of TL-VMI^[47]. (a) Diagram of TL-VMI setup; (b) photo of TL-VMI setup

Fig. 8. TL-VMI geometric structure used in SIMION simulations and electric field isopotential lines in lens^[47]. (a)(b) Distributions of voltages along electrodes of TL-VMI for repeller and focusing voltages of -10 kV and -8.05 kV, respectively (total distance from repeller to detector is 133.6 mm, and gray lines mark locations of repeller, extractor, focusing electrode, and exit ring at ground potential); (c) starting positions within interaction region for 7 electron trajectories with identical initial momentum vectors (central position is at x=0 mm, y=0 mm, and z=6 mm)

Fig. 9. Overview of SPIM^[59]. Velocity-resolved photoemission signal is collected as a function of sample position as stage is scanned in two dimensions. 50 fs, 75 MHz, 1.7 W pulsed laser with a center wavelength of 800 nm is output from a titanium sapphire oscillator and pumped into an optical parametric oscillator (OPO) through a second harmonic pump. The signal light of the OPO is continuously adjustable between 515-775 nm. The tuned excitation pulse is introduced into a high numerical aperture microscope via a reflector and focused on the sample stage of the VMI electrostatic lens reflector. The excited photoelectrons are accelerated by a VMI electrostatic lens group and directed towards the signal receiving end, which comprises a microchannel plate (MCP) and a fluorescent screen. The camera records the momentum imaging information of the photoelectrons. The upper left image depicts the photoelectron energy spectrum of a gold nanoshell, which has been magnified and scanned on a single nanoparticle

Fig. 10. Schematics of double-sided VMI spectrometer^[59]. (a) Geometric dimensions (length unit: mm) of double-sided VMI and typical voltages for spectrometer electrode and detector (left is electronic detector, and right is ion detector); (b) SIMION equipotential lines and simulated trajectories of ions with a kinetic energy of 15 eV and electrons with a kinetic energy of 120 eV emitted into different directions, towards and away from the detector and at 0, 45, 90, and 135 degrees with respect to the spectrometer axis, at three positions (green, red, and blue lines, respectively) along the direction of the molecular beam

Fig. 11. Reaction nanoscope^[60]. Nanoparticles are delivered by an aerosol generator and pass an aerodynamic lens and a set of skimmers for differential pumping. The few-cycle laser pulses cross the focused nanoparticle beam in the center of the reaction nanoscope. The SiO₂ nanoparticles and molecular surface adsorbates are ionized during the interaction. Fragments arising from molecular photodissociation are accelerated towards the ion detector [bottom is MCP and delay-line detector (DLD)] by a homogeneous electric field. Electrons are accelerated towards the opposite side of the spectrometer and are detected with a channeltron (top). Electrons and ions are recorded in coincidence

Fig. 12. Diagram of SMM^[3]

Fig. 13. M³C model^[3]. (a) Illustration of model (linear near-fields generated by non-resonant excitation via XUV and NIR laser pulses. White dots mark ionization sites of electron trajectories); (b) visualization of M³C model

Fig. 14. Electron emission from SiO₂ nanoparticles under intense few-cycle pulses^[5]. (a)(b) CEP maps of the photoelectron momenta in the propagation polarization (x-y) plane measured from SiO₂ nanoparticles (diameter is 100 nm) and as predicted by M³C model. The dashed circles indicate the cutoff momentum circle corresponding to an energy of 10U_p. Shaded areas visualize a 50° full opening angle along the laser polarization axis for upward and downward emissions. Electron kinetic energy spectra are extracted via integration of the data in these regions. (c) CEP-averaged photoelectron energy spectra for xenon gas and nanoparticles;(d)-(f) diagrams of the relationship between the asymmetric parameters A and CEP extracted from the two triangular cones in Fig. 14(a) [A=(Y_up-Y_down)/(Y_up+Y_down), in which Y_up and Y_down correspond to the electron yields of up-emission and down-emission, respectively. Fig. 14(d) shows the experimental results, Fig. 14(e) shows simulation results by M³C model, and Fig. 14(f) shows xenon gas as background]. (g) Intensity-dependence of measured cut-off energies of electrons emitted from d=(100±50) nm silica spheres

Fig. 15. Strong-field photoelectric emission from a dielectric nanosphere^[81]. (a) Maximum field enhancement of SiO₂ nanosphere with diameter of 100 nm excited by a near-infrared few-cycle pulse. Plus symbols indicate the positive surface charges induced by the residual ions resulting from tunnel ionization in the enhanced surface field. Spheres represent a bunch of emitted backscattered electrons. Coulomb explode occurs due to space charge repulsion. Inset is schematic of potential induced by the free charges leading to an attractive trapping potential near the surface and a repulsive component in the bunch. (b) Energy spectra of emitted electrons from simulations excluding and including charge interaction. Grey areas represent full spectra, dashed and solid lines show selective spectra for directly emitted electrons and backscattered electrons, respectively. The cutoff energies (symbols) are defined as the energies U_p where the backscattering spectra drop by three orders of magnitude. Vertical lines indicate the classical 2U_p and 10U_p cutoff energies for direct emission and backscattering, respectively. (c) Evolution of electron cutoff energy with time under both actions, including energy difference resulting from the trapping fields and the additional energy accumulated during the escape phase due to Coulomb explosion of the escaping electron bunch

Fig. 16. Quenching and enhancement of the electron emission from surfaces induced by trapping field^[81]. (a)-(c) Cutoff energies of optimal trajectories varying with field strength E₀and extension α of a triangular trapping field [Fig. (a) shows optimal trajectories of electrons emitted directly, Fig. (b) shows trajectories of electrons after elastic backscattering, Fig. (c) shows trajectories of recollision electrons, and dashed black lines in Figs. (b) and (c) indicate the optimal parameters for the maximal cut-off and recollision energies]

Fig. 17. Quenching of the material dependence in strong-field emission from nanospheres^[73]. (a)-(d) Measured cut-off energies of electrons emitted from small SiO₂, ZnS, Fe₃O₄ and PS nanospheres in dependence of the peak intensity Iloc at the surface and results predicted by M³C simulations (dashed curves represent excluding charge interaction and solid curves represent including charge interaction). Experimental error bars reflect uncertainties of the laser intensities and in the cutoff evaluation. Dark gray indicates areas with cutoff energy less than or equal to 10Uploc, and light gray indicates areas with cutoff energy less than or equal to 20Uploc. (e) Cut-off energies of photoelectrons from d=100 nm spheres under few-cycle pulses (5 fs, 720 nm) in dependence of ionization energy and peak intensity of the enhanced linear near-field as predicted by M³C. The vertical line marks the surface field intensity corresponding to a laser intensity of 3×10¹³ W/cm² for SiO₂. Horizontal lines indicate the ionization energies of different dielectric nanoparticles

Fig. 18. Electron cutoff energies measured from different nanoparticles varying with incident laser intensity^[34]

Fig. 19. Streaking spectrograms and extracted straking delays^[75]. Attosecond streaking spectrograms (experimental data) obtained from angular integration of projected momentum maps around the laser polarization direction for (a) gas and (b) SiO₂ nanoparticles. To extract the streaking delays, energy-dependent frequency-filtered isolines were fitted with few-cycle waveforms as described in the text. The appropriate carrier phases define the respective streaking delays Δt. (c) Attosecond streaking spectrograms of SiO₂ nanoparticles from the simulations; (d) streaking delays extracted in the high energy range of the measured gas and nanoparticle streaking spectrograms. Left and right curves show delays predicted by gas simulations and nanoparticle simulations for the experimental parameters, respectively. The curve in the middle represents the delay extracted from a mixed spectrogram. The dark gray shaded area reflects the maximal variation of the extracted streaking delay when performing the nanosphere simulations with charge interaction enabled or anisotropic elastic collisions; (e) ratio of nanoparticle signal to gas signal in dependence of energies extracted from the experiment and from the simulations

Fig. 20. Theoretical framework for attosecond fringe spectra in media^[75]. (a) Streaking delays in dependence of elastic and inelastic scattering time; (b) streaking delays as functions of the material’s attenuation factor α=1/ε_r, where ε_r is the relative permittivity at the wavelength of the NIR field. Gray shaded areas in both panels show the variations of the streaking delay in dependence of the elastic scattering time

Fig. 21. CEP-averaged projected momentum distributions measured from SiO₂ nanospheres with 95, 313, 400 and 550 nm diameters^[4]

Fig. 22. Size dependent photoelectron cutoff energy values from SiO₂ nanoparticles^[82]. Photoelectron cutoff energy values are normalized in units of the ponderomotive potential U_p. Three hollow circle symbols show the size-dependent cutoff energies of the three different laser intensities (I₀=8.8×10¹² W/cm²) . Square data are short period pulse data from Süßmann et al.^[4]

Fig. 23. Electron emission from SiO₂ nanoparticles under intense few-cycle pulses^[3-4]. CEP-averaged projected momentum maps of photoelectrons emitted from spheres with (a) d=95 nm and (b) d=550 nm under light intensity of 3×10¹³ W/cm² predicted by M³C; (c) corresponding CEP-averaged energy spectra (two dot symbols indicate respective cut-off energies E_c); (d)-(g) directionality and phase-dependent results of photoelectron emission [Figs. (d) and (e) show the relationship between cutoff electron emission angle θ_e and CEP for SiO₂ nanospheres with d=95 nm and d=550 nm measured experimentally, and color coordinates are cutoff electron yield Y(θ_e, φ_ce). Critical emission angles θ_crit (vertical dashed lines) and phase offsets Δφ (solid black curves) are defined via the amplitude and phase of harmonic fits of the data for each vertical slice. Figures (f) and (g) show the prediction results of the experimental parameters in Figs. (d) and (e) obtained by M³C]

Fig. 24. Critical emission parameters [three characteristic emission parameters (the critical emission angle θecrit, the critical phase φcecrit, and the cut-off energy E_c) varying with nanoparticle size]^[3-4]. (a)-(c) Evolutions of measured characteristic emission parameters with nanosphere diameter, and simulation results of SMM and M³C [critical emission angles of the radial and full (dashed line) linear near-fields varying with nanoscale. Fig. (b) shows critical phase varying with particle size. Fig. (c) shows the change of cutoff energy with particle size under the energy gain contributed by different external field. Shaded areas represent the additional energy gains due to TRAB, CE, and tangential field]. (d)(e) Time evolutions of photoelectron cutoff energies for two sphere sizes (the results include the kinetic energy evolution predicted by SMM and the time evolution of photoelectron cutoff energy calculated by M³C model under various field gains. Shaded areas indicate the energy gains from TRAB, CE, and tangential field effect)

Fig. 25. All-optical directional control of the photoemission from SiO₂ nanospheres in ω-2ω laser fields^[83]. (a)-(c) Measured angular and relative phase-resolved electron cutoff energies for different sphere diameters and different light intensity ratios. The IR intensity is I_ω=3×10¹² W/cm². Energies are normalized to the active potential of the incident IR field. (d)-(f) SMM simulation results; (g) typical momentum spectrum for selected emission angle and relative phase, which has been fitted with a Fermi function; (h) experimental and simulated results of the optimal emission angle of photoelectron emission for 300 nm SiO₂ nanospheres at the second harmonic intensity ratio of 0.5

Fig. 26. Intensity ratio-dependent critical emission angles for upward emission obtained from measurement and SMM simulations for 300 nm SiO₂ nanospheres^[83]

Fig. 27. Photoelectron momentum distributions of four different nanomaterials^[33]. (a)-(f) Photoions from 100 nm NaCl crystals are ejected in the laser propagation direction, suggesting a focusing effect; (g)-(l) ions from TiO₂ aggregates are typically centered around zero kinetic energy, indicating formation of symmetry of most plasmas; (m)-(r) photoions from 50 nm gold nanoparticles imbedded in a larger PVP sphere eject ions with directions that depend on the orientation of the nanostructure; (s)-(x) photoions from 17 nm gold-PVP nanostructures eject ions in the direction opposite to the laser propagation direction, indicating absorption of laser energy on the front [Figs. (f), (l), (r), and (x) correspond to 70, 869, 419, and 108 particles, respectively, reveal general trends, but conceal the diversity of the individual particle, especially in the case of the 50 nm gold sample]

Fig. 28. Forward-concentrated effect of dimer of SiO₂ nanoparticles^[85]. (a)-(c) Proton momentum distributions, in atomic unit (a. u.), in the polarization-propagation plane for dispersed SiO₂ nanoparticles with mass concentrations of 0.2, 0.4, and 1.5 g/L, respectively, and data was taken at a peak intensity of 5×10¹³ W/cm²; (d) momentum distribution regions (I region is the momentum distribution region of monomer nanoparticles, and II region is the momentum distribution region of dimer); (e) angular distributions of counts of nanoparticles

Fig. 29. Ionization momentum spectra of SiO₂ and surface molecules (single frame momentum distribution of ions ejected from the nanoparticle system under low light intensity excitation, and color coordinate represents the intensity distribution obtained on the CMOS detector)^［86］. (a)-(d) Momentum distributions of H⁺ excited by light fields with different polarizations and wavelengths at excitation intensity of 17 TW/cm²; momentum distributions of Si⁺ ions excited by p polarized (e) 800 nm and (f) 400 nm femtosecond laser pulses at a light intensity of 30 TW/cm²

Fig. 30. Observation of a shock wave from an individual nanoplasma^[87]. (a) Ion momentum spectrum of individual NH₄NO₃ nanoparticles under 400 nm and 800 nm double pulse lasers; (b) if the particle size, laser intensity, and laser pulse time delay are tuned appropriately, a sharp shock wave appears in addition to the broad ion distribution; (c) radial energy distribution of the typical nanoplasma explosion can be fitted by a single broad Gaussian function; (d) shock wave manifests as an additional sharp peak, which can be fitted by a second narrower Gaussian function

Fig. 31. Shock waves in H⁺ and Si⁺ ion emissions^[86]. Momentum distributions of H⁺ ion at the excitation intensity of 100 TW/cm² at excitation wavelengths of (a) 800 nm and (b) 400 nm; momentum distributions of Si⁺ ion at the excitation intensity of 100 TW/cm² at excitation wavelengths of (c) 800 nm and (d) 400 nm; (e)(f) radial momentum distributions of the proton is fitted by the double Gaussian fitting function, the experimental results are shown by the scattered points, the fitting results are shown by the dashed lines, and the figure contains the Gaussian fitting results of the plasma expansion and shock wave

Fig. 32. Experimental results^[60]. (a) Numbers of detected electrons after the interaction of few-cycle pulses with background gas only and with 110 nm SiO₂ particles; (b) average ion time-of-flight spectrum of shots containing nanoparticle hitting on a mass/charge (m/q) axis (the indicated ionic fragments arise from ionization of argon and dissociative ionization of ethanol and water. The inset shows the enlarged peak of H⁺ on a momentum scale along the polarization direction, for events with SiO₂ particles and with just background gas. The dashed lines indicate a momentum of ± 40 a.u.)

Fig. 33. Time flight spectra of ions emitted from the surface of nanoparticles^[89]. (a) Comparison of TOF spectra of two experiments in terms of nanoparticle sizes (100 nm and 300 nm), sample mass concentrations (3 g/L and 1.5 g/L), and laser intensity (2×10¹⁴ W/cm²), as evidence of H₃⁺ formation; (b) comparison of ion emissions on the surface of 100 nm silica nanoparticles adhered to H₂O and D₂O molecules at a mass concentration of 3 g/L under 2×10¹⁴ W/cm² intensity laser irradiation

Fig. 34. Comparison between the field enhancement in the plane z=0 [Figs. (a),(c), and (e)] and the projection of the proton momenta onto the xy-plane [Figs. (b), (d), and (f)] for 300 nm silica nanoparticles. Gray symbols indicate the polarization state for each panel. First column is results for linear polarization light. Second column is results for an elliptic polarization light. Third column is results for circular polarization light. The pulse energy is 18 μJ in all cases, corresponding to a peak intensity of about 2×10¹³ W/cm² in the linearly polarized case) ^[90]

Fig. 35. Schematic illustrations of the experimental setups of reaction nanoscopy and phase-locked two-color interferometer^[91]. (a) Upper-left inset illustrates the asymmetric proton emission from the surface of nanoparticle driven by a linearly polarized two-color laser pulse. The bottom-left inset shows a transmission electron micrograph (TEM) image of 300 nm SiO₂ nanoparticles. Opical devices at bottom-right including dichroic mirror (DM), lithium niobate (LiNB), half-wave plate (HWP), quarter-wave plate (QWP), wire-grid polarizer (WGP), piezo-driven translation stage (PZT), charge-coupled device (CCD), silver mirror (SM), microchannel plate (MCP), delay-line anode detector (DLD), and focusing mirror (FM). CW represents continuous wave. Results of a two-color laser field with different polarization states at relative phases (b) (d) (f) 0 and (c) (e) (g) π [Figs. (b) and (c) are spatial distributions of the near-field enhancement in the laser polarization plane (y-z plane) for 300 nm SiO₂ nanoparticles, Figs. (d) and (e) are results of the reversed two-color circular polarization laser pulse, and Figs. (f) and (g) are results of the co-rotating bicircular two-color (2 µm and 1 µm) laser fields with relative phases of 0 and π, respectively. Figs. (d)-(g) are obtained from FDTD simulations

Fig. 36. Measured and simulated distributions of protons under different laser intensities^[80]. (a)-(c) Momentum distributions of protons generated from gold nanoparticle surface at intensities of 5, 75, and 100 TW·cm^-2 at 400 nm, respectively; (d) polar plots of the proton momentum distributions at 5 and 100 TW·cm^-2; (e) near-field |E| distribution of interaction of gold nanospheres based on FDTD simulation; (f) calculated far-field momentum distributions of protons. Inset is the initial spatial distribution of the protons from gold nanospheres. Plus signs at (-40 nm, -15 nm) and (40 nm, -15 nm) represent the effective charges of localized plasmas

Fig. 37. Identification of orientation of nanostructures through the detected photoelectron angular distribution (PIAD)^[92]. (a) Circular PIAD originates from nanotubes perpendicular to the detector (in laboratory coordinate along the z-axis direction); (b) strip PIAD originates from nanotubes parallel to the detector (in laboratory coordinate along the y-axis direction); (c) circular PIAD originates from a nanocube, with its side perpendicular to the detector; (d) strip PIAD originates from nanocubes parallel to the detector on the side; (e) circular PIAD originates from a dimeric system perpendicular to the detector; (f) PIAD originates from a dimeric system parallel to the detector. The detector is fixed on the x-y plane

Fig. 38. DGTD simulation results of near-field distribution on surface of gold nanocubes^[93]. (a) Size of the cube is about 90 nm, and the excitation laser wavelength is 800 nm; calculation results of the far-field momentum distributions of H⁺ under (b) x- and (c) y- polarization excitations

Fig. 39. Angular resolved momentum distributions of H⁺ and Na⁺ ions ionized on nanocube surfaces^[93]. (a1) Total momentum distribution and (a2)-(a4) three single frame results of H⁺ ions under x-polarized excitation; (b1) total momentum distribution and (b2)-(b4) three single frame results of H⁺ ions under y-polarized excitation; (c1) total momentum distribution and (c2)-(c4) three single frame results of Na⁺ ions under x-polarized excitation; (d1) total momentum distribution and (d2)-(d4) three single frame results of Na⁺ ions under y-polarized excitation; angular integral energy spectra of (e)(f) H⁺ ions and (g)(h) Na⁺ under x-polarization and y-polarization excitations, respectively. Yellow lines and circles with arrows represent the initial angle (0°) and its increasing direction, respectively