Optical diagnostics of laser-produced plasmas

Laser-produced plasmas (LPPs) engulf exotic and complex conditions ranging in temperature, density, pressure, magnetic and electric fields, charge states, charged particle kinetics, and gas-phase reactions based on the irradiation conditions, target geometries, and background cover gas. The application potential of the LPP is so diverse that it generates considerable interest for both basic and applied research areas. The fundamental research on LPPs can be traced back to the early 1960s, immediately after the invention of the laser. In the 1970s, the laser was identified as a tool to pursue inertial confinement fusion, and since then several other technologies have emerged out of LPPs. These applications prompted the development and adaptation of innovative diagnostic tools for understanding the fundamental nature and spatiotemporal properties of these complex systems. Although most of the traditional characterization techniques developed for other plasma sources can be used to characterize the LPPs, care must be taken to interpret the results because of their small size, transient nature, and inhomogeneities. The existence of the large spatiotemporal density and temperature gradients often necessitates nonuniform weighted averaging over distance and time. Among the various plasma characterization tools, optical-based diagnostic tools play a key role in the accurate measurements of LPP parameters. The optical toolbox contains optical spectroscopy (emission, absorption, and fluorescence), as well as passive and active imaging and optical probing methods (shadowgraphy, Schlieren imaging, interferometry, Thomson scattering, deflectometry, and velocimetry). Each technique is useful for measuring a specific property, and its use is limited to a certain time span during the LPP evolution because of the sensitivity issues related to the selected measuring tool. Therefore, multiple diagnostic tools are essential for a comprehensive insight into the entire plasma behavior. Recent improvements in performance in laser and detector systems have expanded the capability of the aforementioned passive and active diagnostic tools. This review provides an overview of optical diagnostic tools frequently employed for the characterization of the LPPs and emphasizes techniques, associated assumptions, and challenges. Considering that most of the industrial and other applications of the LPP belong to low to moderate laser intensities (${10}^8–{10}^{15}{\mathrm{W}\mathrm{cm}}^{-2}$), this review focuses on diagnostic tools pertaining to this regime.

Figure 1

Typical properties of LPPs used for various applications.

Figure 2

Various physical processes happening during a nanosecond laser-produced plasma generation and expansion into an ambient gas medium.

Figure 3

(a) Emissions between two electronic levels in an atom. (b) Schematic of an emission spectroscopic system employing a spectrograph-ICCD combination and/or monochromator-PMT combination. (c) Example of a spectrum collected from a LPP. (d) Fundamental properties of the plasma that can be obtained from emission spectroscopy.

Figure 4

Time-resolved plasma spectra recorded from a nanosecond laser-generated Al plasma showing continuum, ionic, atomic, and molecular emissions at various times during its evolution. The gate delays used are marked in the spectral features. The inset in (d) corresponds to the enlarged AlO emission spectral region. Adapted from [128].

Figure 5

Time evolution of the Stark-broadened profile of (a) CaI 585.74 nm and (b) ${\mathrm{H}}_{\alpha }656.28\mathrm{nm}$. The smooth curves represent the spectral fits. Adapted from [35].

Figure 6

Examples of Boltzmann plots obtained from Fe I lines and Fe II lines are given for (a) spatially integrated measurement and (b) spatially resolved measurement and Abel inversion. The temperature values deduced from the linear fit are also given. From [3].

Figure 7

Saha-Boltzmann plot for N lines at a delay of $0.35\mu \mathrm{s}$ during LPP evolution. The LPP was generated using a frequency-doubled Nd:YAG laser in a metal aerosol. From [332].

Figure 8

Comparison of the measured CN violet $\mathrm{\Delta }v=0$ emission band with the spectral simulation. The measured emission spectrum is shown in black in the upper part of the figure, and the inverted simulated spectrum is shown in blue in the lower part. Adapted from [310].

Figure 9

(a) OTOF of ${\mathrm{C}}_2$ emission at 516.5 nm recorded 5 mm away from the target for various laser fluences. Adapted from [133]. (a) $12.7\mathrm{J}{\mathrm{cm}}^{-2}$. (b) $26.7\mathrm{J}{\mathrm{cm}}^{-2}$. (c) $28\mathrm{J}{\mathrm{cm}}^{-2}$. (d) $29.3\mathrm{J}{\mathrm{cm}}^{-2}$. (b) Spatiotemporal emission contours for the U I 591.54 nm line under various background gases (nitrogen, air, and argon) recorded at 100 torr pressure, showing the effect of plasma oxidation on emission persistence. Space- and time-resolved emission contours were generated by combining OTOF profiles recorded at various distances from the target. Adapted from [290].

Figure 10

(a) Absorption process between two electronic levels in an atomic system. (b) Schematic of the laser-absorption spectroscopy experimental scheme for analyzing a LPP. (c) Time evolution of absorbance. (d) Absorption spectrum. (e) Typical fundamental properties of the plasma that can be obtained from absorption spectroscopy. (f) Time-resolved absorption spectrum obtaining using a tunable laser. The selected transition is Al I 394.4 nm. A nanosecond laser was used to produce plasma from an Inconel alloy sample. From [135].

Figure 11

Contour map of the time-resolved optical spectra of ablated Sr atoms in the presence of 2.5 torr He cover gas with a probe beam at 3 mm height. Absorbance units with the strongest absorption are indicated by the darkest shading. Adapted from [38].

Figure 12

(a) Schematic of the AS setup using a broadband arc lamp. (b) The absorption spectrum from a LPP recorded using an arc lamp is given. The absorption spectrum corresponds to the UO 593.55 nm band from a U metal plasma, and the peak marked with a red dotted line corresponds to the UI 593.38 nm atomic transition. From [324]. (c) Schematic of AS employing frequency combs. (d) Broadband single-shot absorption measurement of the laser-induced plasma using frequency combs. Inset: enlargement of the $5S_{1/2}-5P_{3/2}\mathrm{R}\mathrm{b}{D}_2$ line. From [24].

Figure 13

Column density ($nL$) as a function of time calculated from fits to LAS absorbance spectra for two Al transitions at 394.4 and 396.15 nm and a Ca II transition at 393.37 nm. 266 nm, 6 ns pulses were used for producing LPP on a glass target, and LAS spectra were obtained using a cw tunable laser. Adapted from [192]. Comparison of time evolution of kinetic temperatures measured using LAS of nanosecond and femtosecond LPPs. An Inconel sample containing a small amount of Al ($<0.4\mathrm{a}\mathrm{t}\mathrm{.}\%$) was used. The large error bars at early times of nanosecond LPP are a result of uncertainty in the spectral line fitting due to low absorbance. From [135].

Figure 14

(a) Example of a Boltzmann plot generated from DCS absorption spectroscopy of Gd plasma and using Gd lines. (b) Measured excitation temperature evolution. From [323].

Figure 15

TOF absorption signals (blue and black curves) of BaH molecules from a LPP are recorded at different distances from the target surface. The smooth red lines represent the best fit to the data. From [304].

Figure 16

(a) Schematic of LIF excitation and emission. (b) Typical LIF of a LPP experimental scheme. (c) Time-resolved LIF emission from LPP using cw excitation, where the early-time peak seen in the temporal profile is due to thermally excited emission. (d) Excitation spectrum. (e) Potential properties of the plasma that can be gathered using LIF.

Figure 17

LIF spectra of the ${\mathrm{SiO}}^{+}$ O–O band recorded at 80 mtorr ${\mathrm{O}}_2$ pressure. The plasma was produced using a 1064 nm laser beam, and LIF was performed by scanning a pulse dye laser. From [219].

Figure 18

(a) 2D FS of U I and weak thermal emission from K I lines at 45 torr ${\mathrm{N}}_2$ pressure from a nanosecond-laser-produced plasma. (b) Enlarged image of 2D FS of a U I transition. (c),(d) Emission and excitation spectra. From [138].

Figure 19

Density distributions of Bi, Y, and YO from a ${\mathrm{YBiO}}_3$ LPP. Each photo contains a LIF measurement in a 75 mtorr ambient environment. Left image: ${\mathrm{O}}_2$. Right image: Ar. All photos are normalized, with the normalization factor shown at the bottom of each image. The densities shown were measured 35 s after ablation. From [255].

Figure 20

Time-resolved spectrally integrated 2D images of U plasma in 700 torr air. Gate delay times are indicated above each plasma image. Each image is from a single laser shot and was normalized to its maximum intensity. The direction of the incident laser is indicated with a white arrow, and the target position is shown as a gray rectangle in each image. From [170].

Figure 21

(a) LIF images for Gd, Hf, Zr, and Ti species observed with a metal. (b) Mixed oxide samples at $1.5\mu \mathrm{s}$ delay at an ablation energy of 0.5 mJ. The plasmas were produced using 532 nm pulses from a frequency-doubled Nd:YAG laser, and 6 torr He gas was used as a background medium. From [235].

Figure 22

Experimental schematics for (a) direct shadowgraphy and (b) focused shadowgraphy setups. The pump laser is used to generate plasma, and a transverse probe laser is used for generating shadowgram images. (c) Typical LPP shadowgram image recorded using focused shadowgraphy (nanosecond LPP, Al plasma in air, 150 ns after onset). Both primary and secondary shock waves are clearly visible along with material ejection. From [139]. (d) Shadowgrams taken at different times during the evolution of laser-detonated air plasma. From [130].

Figure 23

(a) Schematic of the lens-type Schlieren system with an arc lamp. (b) Folded ($\mathsf{Z}$-shaped) Schlieren setup using mirrors ($F$, filter; $L$, lens; $A$, aperture; $M$, mirror). (c) Schlieren images of laser-created plasma from RDX. A Nd:YAG laser operating at 1064 nm was used for ablation, and a 200 W HgXe lamp served as the illumination source. From [112].

Figure 24

Various laser interferometry configurations for measuring LPP electron density are given. (a) Michelson interferometer. (b) Mach-Zehnder interferometer. (c) Nomarski interferometer. (d) Example of fringe shifts due to refractive index variation caused by a nanosecond LPP recorded using a Nomarski interferometer. $L$, lens; $P1$ and $P2$, polarizers; $M$, mirror; BS, beam sampler.

Figure 25

Typical Thomson-scattering system where the probe laser beam is propagated through a plasma before being focused on an area ($A$) at the Thomson-scattering volume. An aperture stop is imaged onto the plasma to define the Thomson-scattering volume along the propagation of the probe beam ($L$). DPP, distributed phase plate.

Figure 26

(a) High-frequency spectrum calculated from Eq. (32) in the heavily damped noncollective regime $\alpha =0.25$ (red dotted curve), the mildly damped collective regime $\alpha =2.0$ (black dashed curve), and the weakly damped collective regime $\alpha =4.0$ (blue solid curve). The temperature was maintained at $k_B{T}_e=100\mathrm{eV}$, and the density was scaled as $n_e=1\times {10}^{17}$ (red curve), $6\times {10}^{18}$ (black curve), and $2.5\times {10}^{19}{\mathrm{cm}}^{-3}$ (blue curve). The low-frequency spectrum has been suppressed. (b) Low-frequency spectrum calculated from Eq. (32) in the heavily damped noncollective regime $Z{T}_e/T_i=0.5$ (red dotted curve), the mildly damped collective regime $Z{T}_e/T_i=3.5$ (black dashed curve), and the weakly damped collective regime $Z{T}_e/T_i=10$ (blue solid curve). The scattering parameters $\alpha =2$ and $T_e/T_i=0.5$ were held constant. For all calculations, the angle between the incident and scattered light was held constant ($\theta =90°$).

Figure 27

The sensitivity of the spectrum shown in Fig. 26 to (a) electron density in the weakly damped regime, (b) electron temperature in the strongly damped regime, and (c) electron temperature in the mildly damped regime. The parameters were varied around the central value (black dashed curve) by $+10\%$ (blue solid curve) and $-10\%$ (red dotted curve).

Figure 28

(a) Low-frequency spectrum for a single-species nitrogen plasma where $Z{k}_B{T}_e=630\mathrm{eV}$ (red dotted curve), $Z{k}_B{T}_e=700\mathrm{eV}$ (black dashed curve), and $Z{k}_B{T}_e=770\mathrm{eV}$ (blue solid curve), where $T_i=20\mathrm{eV}$. (b) In the mildly damped regime, the width of the ion feature can be used to measure the ion temperature; $k_B{T}_i=18\mathrm{eV}$ (red dotted curve), $k_B{T}_i=20\mathrm{eV}$ (black dashed curve), $k_B{T}_i=22\mathrm{eV}$ (blue solid curve), and $Z{k}_B{T}_e=700\mathrm{eV}$. (c) Introducing 5% nitrogen ($Z=7$) to a hydrogen ($Z=1$) plasma provides two low-frequency modes, and their relative amplitudes provide accurate measures of the ion temperature $T_e/T_i=5$ (red dotted curve), $T_e/T_i=3.3$ (black dashed curve), and $T_e/T_i=2.5$ (blue solid curve); $T_e=100\mathrm{eV}$ was held constant. For all calculations, $\alpha =2$.

Figure 29

Collective Thomson-scattering spectrum simultaneously recorded to reveal (a) the low-frequency ion-acoustic wave spectrum ($Z{T}_e/T_i\sim 3$) and (b) the high-frequency ($\alpha \sim 2.3$) electron-plasma wave spectrum (blueshifted peak only). The drive-laser pulse shape is overlayed. The Thomson-scattering spectrum at 2.8 ns for the (c) ion-acoustic waves and (d) electron-plasma waves. The solid red curves are the measured spectra, and the dashed blue curves are the best-fit spectrum ($k_B{T}_e=0.9\text{keV}$, $k_B{T}_i=0.8\mathrm{keV}$, $n_e=4.4\times {10}^{20}{\mathrm{cm}}^{-3}$).

Figure 30

(a) Typical moiré fringes produced by overlaying two rotationally offset Ronchi gratings. Adapted from [277]. (b) Experimental arrangement for measuring electron density of a LPP. $G$, Ronchi grating; $L$, lens; $F$, filter.

Figure 31

(a) Michelson interferometer as a free-space equivalent to PDV. (b) Scheme for conventional PDV system. (c) Typical PDV signal for time-dependent velocity. From [78].