Colloquium: Nanoplasmas generated by intense radiation

Solid, liquid, and gaseous states of matter can exist and acquire unique properties when reduced in size into a nanometer domain. This Colloquium explores the approaches to produce plasmas with nanometer dimensions and the arising physical phenomena and properties associated with this extreme, nonequilibrium state of matter. Analysis of the spatial confinement, coupling, ideality, and degeneracy criteria lead to the possibilities to produce transient nanoplasma states near, in, and from solids by using ultrafast photoexcitation. These states arise through the interplay of nonequilibrium, many-body Coulomb interactions, thermal, and nonthermal effects. Examples include photoexcited electron-hole plasmas in semiconductors, transient solid-to-plasma states including warm dense matter, nanoplasmas produced by interaction of nanoclusters and nanoparticles with intense radiation, nanoplasmas in high-energy ion tracks within solids, nanoplasmas in relativistic regime, and others. Physical phenomena arising due to the localization of high-energy densities to microscales and nanoscales and their potential applications are discussed.

Figure 1

Examples of nanoplasma-related physical systems: (a) plasmas of photoexcited electrons and holes in semiconductors (Sec. 5), (b) transient states from solids to plasmas such as warm dense matter (Sec. 6), (c) nanoplasmas from irradiated clusters and nanoparticles (Sec. 7), (d) photoexcitation of nanostructures to generate hot electrons and strong magnetic fields (Sec. 9), and (e) ion tracks generated by energetic ion projectiles in solids (Sec. 8).

Figure 2

Nanoplasma effects in plasmonic NPs. (a) LSP excitation in a spherical metal NP in an external electric field $E_0$. (b) Peaks in optical absorption spectra corresponding to metals with different electron densities. Adapted from [9]. (c) Higher electron density causes a blueshift in the absorbance spectra. Adapted from [90]. (d) Increasing the aspect ratio of rectangular nanorods leads to redshifted resonance peaks. Adapted from [134]. (e) Enhancement of electromagnetic fields around a silver nanorod polarized along the long axis. Adapted from [49]. The maximum field is near the edges and is shown by an arrow. (f) Quantization of plasmon resonances in a small spherical Au NP. Adapted from [126].

Figure 3

Temporal dynamics of transition from isolated point charges to electron-hole plasmas. (a) Imaginary part of the inverse dielectric function of GaAs and data fit using the Drude model showing the emergence of the plasma resonance at ${\omega }_p/2\pi =14.4\mathrm{THz}$ (60 meV); (b) transition from isolated to correlated and screened charge states. Adapted from [58].

Figure 4

(a) Phase diagram of the electron-hole system in Si. Adapted from [124]. (b) Temporal dynamics of exciton formation from unbound $e\text{−}h$ gas. The dielectric function changes from negative (conducting, $e\text{−}h$ plasma state) to positive (dielectric, exciton-dominated state). The inset shows the experimental scheme. Adapted from [71].

Figure 5

Quasiparticles in direct-gap semiconductors with two types of charge carriers (electrons and holes). (a) The interplay between the nonequilibria, many-body interactions and thermalization effects determines the specific quantum state. (b) Binding energy of $e\text{−}h$ states for a different number of photons in the excitation pulse and the difference between the photon energy and the low-density exciton energy. (c) Computed $e\text{−}h$ pair correlation function $g(r)$ for a quantum droplet with the radius $R=91\mathrm{nm}$. The cylinder represents the droplet shell while the dark area corresponds to the plasma. Adapted from [2].

Figure 6

Responses of semiconductors to fs lasers: (a) bond softening in Si evidenced by the reduction of cohesive energy per bond $E_b$ with the density of the electron-hole plasma ${\xi }_0$ at the same amplitudes of the transverse acoustic phonons, and (b) transition of GaAs to the metallic state evidenced by the temporal dynamics of the electronic energy eigenvalues around the band gap at the $\mathrm{\Gamma }$ point. Adapted from (a) [119], and (b) [31].

Figure 7

A schematic diagram of a uniform slab of nanoplasmas produced by isochoric laser heating of a Au nanofoil.

Figure 8

(a) Lattice disassembly time in a fs-laser heated Au nanofoil. From [3], and [33]. (b) Temporal changes in the Debye-Waller factor ([24]; [130]) of the $⟨220⟩$ electron diffraction peak in fs-laser heated Au obtained experimentally (data points) and by numerical modeling (solid and dashed lines). Adapted from [33].

Figure 9

ac conductivity of nonequilibrium warm dense gold. Adapted from [19].

Figure 10

(a) Temporal evolution of electron temperature of fs-laser heated Cu. Adapted from [20]. For the electron-ion coupling factors, $G_0={10}^{17}\mathrm{W}/{\mathrm{m}}^3\mathrm{K}$ and $G(T_e)$ is calculated using DFT ([20]). The shaded curves include the experimental accuracies in absorbed energy density and the temporal resolution of the detector. (b) Temporal evolution in ac conductivity of fs-laser heated Au. Adapted from [19].

Figure 11

Effects of cluster size and anisotropy in nanoplasma generation. (a), (b) Charge per He atom for He droplets of 2500 atoms with ${\mathrm{Xe}}_n$ cores vs the number (a) $n$ of Xe dopant atoms and number of He atoms (b) $m$ in the droplet for ${\mathrm{Xe}}_{13}$ core, at a laser intensity of $7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. (c) Anisotropic cross section of the Xe core and the nanoplasma around it, within the He droplet; the arrow shows the laser polarization. (a)–(c) Adapted from [88]. (d) Intensity of ${\mathrm{He}}_{\alpha }$ emission line vs initial Ar cluster size for 300 fs-laser excitation with $1.6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$; $N$ is the number of atoms in the cluster. Adapted from [87].

Figure 12

Bistability of the intensity of excitation of a 104.8 nm line of Ar in two different holey and tapered waveguides with submicrometer dimensions of cross sections drilled in Au (an exit aperture with an elliptical cross section is shown in the inset, scale bar is 200 nm). Adapted from [115].

Figure 13

Dynamics of confined microexplosion in (a) sapphire leading to (b) plasmalike state formation in microvoids induced by a fs-laser pulse. Adapted from [44].

Figure 14

(a) Energy spectrum of hot electrons emitted by Si NWs and bulk silica target at $I_{\text{rad}}=3\times {10}^{18}{\mathrm{cm}}^{-2}$. The inset shows similar spectra for bulk silica and silicon at a higher intensity $I_{\text{rad}}=5\times {10}^{18}{\mathrm{cm}}^{-2}$. Adapted from [113]. (b) Dynamics of electron density (expressed in units of $n_{ec}$) in nanoplasmas excited in 55-nm-thin, $15\text{−}\mu \mathrm{m}$-long Ni nanowires. Adapted from [100].

Figure 15

Ultrahigh electron energies and densities achievable through high-intensity irradiation of oriented nanowire arrays in comparison to other high-energy-density (HED) plasmas. Through this approach, the parameter region of ultrahigh-energy densities (UHED) exceeding $\sim {10}^8\mathrm{J}/{\mathrm{cm}}^3$ may be accessible. Approximate parameter spaces corresponding to localized surface plasmons (LSP) of Sec. 4 and nanoplasma examples (a)–(e) summarized in Fig. 1 are also marked for comparison. The sizes of the dashed circles and ovals are not to scale and represent only the approximate locations of the nanoplasma cases considered in this Colloquium, with respect to other plasmas that feature moderate, high-, and ultrahigh-energy densities. Adapted from [100].