Since the 1930′s, investigators have developed increasingly capable pressure generating devices, such as lever-arm presses [1], piston-driven presses [2], and opposed anvil presses ^[3][4][3,4]; this has enabled materials to be studied at successively higher static pressures over time [5]. In the mid-20th century, Charlie Weir et al. invented the first diamond anvil cell (DAC), in which samples were observed directly through diamond anvils at pressures of over 3 GPa [6]; this revolutionised high pressure materials research, making it possible to monitor phase changes and chemical reactions as pressure was applied [6]. Soon thereafter, laser beams were introduced into DACs for the first time (to process materials) by Takahashi and Bassetta [1].

An apparatus that focuses a laser beam between diamond anvils to heat, shock, or induce chemical reactions within a sample, is known as a laser diamond anvil cell (L-DAC) [7]. L–DAC systems have opened a door to entirely new research fields, such as the measurement of material properties at high pressures and high temperatures (HPHT) ^{[8][9][10][11][12][13][14]}[8,9,10,11,12,13,14] or the study of extremophile biological organisms at pressures similar to those extant at hydrothermal vents ^[3][15][3,15]. Figure 1A illustrates how the number of journal articles involving L–DAC experiments has risen steadily since the early 1990′s—and is now approaching 60–70 articles per year (blue curve).

The three parts of this review are based on the primary modes of material modification presented in Figure 1A,B. Part I of this series summarises the more common laser-heated diamond anvil cell (LH–DAC) mode of experimentation (71% of papers) [7]. This paper (Part II) focuses on dynamic material compression within DACs using intense pulsed lasers, rather than laser heating samples. ResWearchers refer to this as laser-driven dynamic-compression diamond anvil cell (LDC–DAC) experimentation (9% of papers). The LDC–DAC mode is distinguished from other L–DAC modes through the presence of a pressure-wave or shockwave that considerably modifies a sample material’s structure/composition, and where heating is a secondary contributor, if present at all. Part III reviews the application of lasers to induce chemical reactions within diamond anvil cells (20% of papers); this latter method is dubbed laser reactive synthesis diamond anvil cell (LRS-DAC) experimentation. Although noteworthy reviews have previously outlined the development of DACs and high pressure research methods ^{[1][5][6][14][16][17][18]}[1,5,6,14,16,17,18], this review speaks to laser materials processing—where all three modes of material modification/synthesis are described ^[19][20][19,20]. While the LH–DAC mode has been a principal driver throughout early L–DAC research [1], applications have now diversified, and interest is shifting toward the LDC–DAC and LRS-DAC modes—as evidenced by the Red/Green plots of Figure 1A.

This article (Part II), provides the key physics, historical events, and recent developments of LDC–DAC experimentation. Tables of materials modified/synthesised via LDC–DAC systems are given along with their corresponding process conditions of static pressure, shock pressures, temperature rises, and laser source wavelengths/energies (where available). TheOur intent is for the article to act as a field guide for others in setting-up and conducting their own high-pressure LDC–DAC research endeavours. Note that only laser-induced dynamic compression within DACs is considered here; neither light-gas gun experiments (without driving lasers) nor laser-induced shock compression (without transparent anvils) will be discussed in this review.

As further evidenced in this article, LDC–DAC experimentation provides researchers with a singular route to attaining extreme pressures across a large cross-sectional area (with pressures up to 1 TPa, thus far); the method is especially interesting as a multiplicative effect between the DAC pre-compression and the laser dynamic compression (LDC) has been documented [21]. This is well beyond the pressures attainable with single-stage hydrostatic or gas-membrane-driven diamond anvil cells ^[20][22][20,22] where otherwise multiple stages and/or nano-scale surface areas are required to attain such pressures [23].

LDC–DAC experimentation has enabled condensed matter physicists to determine material densities, melting curves, Hugoniot curves, equations of state (EOS), and transport properties for many common elements and compounds over a wide variety of conditions ^{[19][20][24][25][26][27]}[19,20,24,25,26,27]. Furthermore, LDC–DAC methods have enabled geophysicists and planetary physicists to determine the properties of dense matter at conditions akin to those deep within planetary interiors, providing insight into the structure and dynamics present within the terrestrial planets ^[19][28][19,28]. The technique has allowed researchers to better understand how lasers interact with matter—and how shock-/pressure-waves travel through materials at high pressures ^[27][29][27,29]. Although multi-stage DACs can achieve pressures greater than 380 GPa, when sample sizes greater than 20 μm across are to be compressed, the LDC–DAC mode is presently the primary means to achieve pressures >> 380 GPa ^{[19][30][31][32]}[19,30,31,32].

Alternative methods similar to, but distinguished from LDC–DAC processing are confined laser shock compression (e.g., explosive compression inside crystals) ^[33][34][33,34], open-atmosphere laser-driven dynamic compression (OA–LDC) ^[32][35][32,35], and high-power, multi-beam, laser-driven dynamic compression (HP–MB–LDC) ^[36][37][36,37]; all of these methods are generally conducted without the use of pre-compression within a diamond anvil cell, so they are excluded from this review. However, it is important to note that materials processing has indeed been carried out using these methods. For example, in the latter case (HP–MB–DC), the world’s highest-ever recorded pressures while synthesising novel materials was achieved (≈1.5 and 5 TPa) and high-pressure forms of Si–C and diamond were realised ^[37][38][37,38]. Even greater pressures have been achieved by HP–MB–DC methods (to 100 TPa) during plasma fusion experiments, but without synthesising any intended material ^[39][40][39,40]. However, OA–LDC and HP–MB–LDC methods typically require large facilities with multiple laser sources, e.g., the National Ignition Facility (NIF) or OMEGA Laser Facility [30], which have traditionally been outside the scope of many researchers. That said, the availability of high-peak-power lasers is growing rapidly, with new capabilities in short-laser pulse lasers becoming widely accessible at a reasonable cost ^{[41][42][43][44]}[41,42,43,44].

2. Overview of Laser Dynamic Compression in Diamond Anvil Cells (LDC–DACs)

A typical laser-driven dynamic compression diamond anvil cell (LDC–DAC) experiment is illustrated in Figure 2A–D. LDC–DAC systems include a pulsed laser system ((i), not shown), a focused laser beam (ii), a material sample to be compressed (iii), at least one diamond anvil (iv), a chamber gasket (v), and an optional laser target (xvii) placed at the laser focus. Diamond is commonly used for the transparent anvils (iv) in LDC–DAC experiments due to its high shock impedance, wide transmission bandwidth, and dielectric constant [22], although other materials, such as sapphire (Al₂O₃) and quartz (SiO₂), have been utilised ^{[32][45][46][47]}[32,46,47,48]. In some experimental setups, the sample itself is the laser target, as displayed in Figure 2A ^[22][30][48][22,30,49]. In most configurations, however, a target comes before the sample to be compressed (See Figure 2B–D). The laser target’s purpose is to absorb the laser light before it arrives at the sample; this target can be placed in several locations, such as at the inside surface of the incident anvil (Figure 2B) ^{[24][25][49][50][51][52]}[24,25,50,51,52,53], or on the exterior surface of the same anvil (Figure 2C) ^[53][54][55][54,55,56]—or on both surfaces of the incident anvil (Figure 2D) ^{[29][45][46][56][57][58][59][60]}[29,46,47,57,58,59,60,61]. Note that in the references for Figure 2C, the examples provided are not strictly L-DACs but are arranged similarly to that of an L–DAC (with a fully enclosed pressurised cell). Many variations on these configurations have been attempted in the past ^{[45][61][62][63]}[46,62,63,64]. In LDC–DACs, the laser system typically includes a high-peak-power laser, with pulse widths ranging from nanoseconds down to picoseconds. As each focused laser pulse arrives, a portion of the target is ablated, a plasma plume emerges, and a shock- or pressure-wave (xx) results which passes through the sample (iii); these pressure waves or shockwaves momentarily act as a “second stage” DAC to locally compress the sample to higher pressures, while maintaining the DAC’s background pre-compression pressure [20]. Often rarefaction waves follow the shock- or pressure-waves ^[64][65], and all of these waves reflect (somewhat) at interfaces between materials, e.g., where the target contacts the sample ^[60][61]. These waves can subsequently interfere with each other, which often degrades (or enhances) the compression ^[57][58]. Pressure waves are broadly defined as a disturbance that propagates through a medium at sonic velocities ^[65][66][67][66,67,68]. Shockwaves typically propagate faster than the speed of sound and there is an abrupt change in pressure as the wave arrives (approaching that of a step function) ^[67][68]. In LDC–DAC systems, pressure waves and shockwaves are generated from a combination of the instant (thermal) pressure induced by the expanding plasma and momentum transferred to the target as material is ablated away ^[56][57]. As the applied driving energy increases, i.e., the intensity of the laser, the velocity and amplitude of the wave typically increases proportionally ^{[59][65][67][68]}[60,66,68,69]. Using nanosecond and shorter laser pulse widths, researchers can drive samples without a significant temperature rise during compression. This is beneficial for several reasons: first, it permits materials to be compressed in an isentropic manner, so that their density/structure can be more easily determined [31]. Second, large-scale diffusion is delayed or prevented—which otherwise leads to contamination—as is known to occur during LH–DAC experiments ^[69][70]. Third, samples may be retained in a solid phase rather than melting immediately, so that potential new solid phases can be identified [31]. Fourth, thermal stresses on the anvil’s materials are often reduced, extending the pressure range of the devices [25]. And finally, short-pulse widths provide access to extreme pressures, but at low-to-moderate temperature ranges, which are not otherwise readily accessible (refer to the lower portion of the red boxed region in Figure 3) ^{[22][24][32][46][48]}[22,24,32,47,49]. For instance, samples possessing cryogenic boiling points, such as H, He, Ar, etc. are much easier to compress from cooled samples when laser heating does not occur ^[70][71]. Note that the low-temperature, high-pressure region (<500 K, 100–600 GPa) is also similar to conditions anticipated within many ice moons or gas giant planets [22], (for reference, estimated P–T conditions within several planetary bodies are provided in Figure 3). Many investigators attempting to clarify the internal structure of planetary bodies are conducting LDC–DAC experiments. For example Kimura et al. studied many of the primary constituents in theour solar system, including the low-Z compounds: H₂, He, H₂O, NH₃, and CH₄, and determined their EOS to better model planetary dynamics [22]. Another study by Eggert et al. attained Hugoniot data of fluid He at moderate temperatures and within the hundred GPa regime using LDC–DAC ^[46][47]. From this, Eggert et al. measured material properties that had previously been predicted by Path-Integral Monte Carlo (PIMC) and Activity Expansion (ACTEX) calculations ^[46][47]. Similarly, Rygg et al. measured the crystal structures of C, MgO, Fe, Cu, Zr, Sn, Ta, and Pb samples at up to 900 GPa and close to 300 K ^[48][49]. LDC–DAC experimentation has also been used to examine materials within high-pressure, high-temperature (HPHT) regimes that are difficult to access otherwise (>5000 K, >250 GPa) [22]. For instance, hydrogen’s phase diagram was explored at conditions at a depth of ~7000 km within Jupiter’s atmosphere (~5000 K and ~50 GPa) ^[21][201][21,72], and researchers were able to investigate the potential for metallisation of water at up to ≈19,000 K and 250 GPa with 4 ns laser pulses [30]. Similarly, Coppari et al. obtained phase transition data and the EOS of magnesium oxide over the range of 4000–9000 K and 600–900 GPa utilising ≈4.5 ns laser pulses [30]. And finally, Loubeyre et al. extended the known properties of hydrogen and deuterium over the range of 297–40,900 K and 0.3–175 GPa using 1-ns laser pulses [28]. In all these cases, high fluences and extended pulse widths (≥

$$ ≥ 1 ns) were employed to ensure both strong shock loading and heating of the samples [26]. A primary advantage of the LDC–DAC experimental mode is the ability to control initial-, peak-, and final-conditions (ρ-P-T $$ ρ - $$ P - $$ T ) of the sample material. For example, initial densities of a sample are controlled via (isothermal) compression within a DAC prior to delivering a laser shock, while the maximum (peak) compression is obtained through the laser fluence, pulse width, pulse shape, and target/sample geometries. By conducting the experiment within a DAC, the material arrives at a final, elevated pressure, making it possible to preserve metastable states that would otherwise deteriorate at lower pressures (which sometimes happens when using LDC alone) [22]. Nevertheless, LDC–DACs do have some limitations, and as shown in Figure 1B, comprise only a minor fraction of all L–DAC publications. This may in part be due to the complexity of the experiments and/or lack of access to the necessary high-energy laser sources [19]. In addition, there is a limited selection of material characterisation probes that can be used during LDC–DAC experiments, where real-time, ultrafast characterisation is required. One important complication is the requirement for custom anvil shapes—often thick on the opposing (diagnostic) anvil (iv), and thinned on the incident (drive beam) anvil (iv)—and specialty shapes are often needed along the laser beam path [25]. The opposing anvil material must also be designed to have high impedance matching with the sample [31]. Of course, a significant limitation is the strength of the anvil materials in their required geometries. Diamond is the most commonly-used anvil material, yet it is often thinned to <400 micron thicknesses on the incident anvil ^[56][57]. Diamond anvils are subject to failure during compression experiments, which raises the experimental cost, and requires significant effort to reset the LDC–DAC system for subsequent runs. Although sapphire has lower compressive strength (~2 GPa static, ~21 GPa dynamic) than diamond (~35 static, ~98 GPa dynamic), it has also been used as an anvil material during LDC–DAC experiments ^{[32][45][202][203][204]}[32,46,203,204,205]. Similarly, quartz has also been used as an anvil or window (~1 GPa static, ~9 GPa dynamic) ^[205][206][206,207]. Further research is required to maximise the peak pressures attained during shock loading and minimise the cost/effort of LDC–DAC experimental runs ^[48][49].