At the crossroads between condensed matter and plasmas lies warm dense matter, for which the thermal energy is comparable to the electrons’ Fermi energy and the ionic cores are strongly correlated. Difficult to study either theoretically or experimentally, warm dense matter is a largely uncharted frontier. Now, physicists at Lawrence Livermore National Laboratory have used intense femtosecond laser pulses to turn a thin film of gold into a warm dense state and have measured a key parameter of the sample: the frequency-dependent dielectric function, which contains information about the density of states and band structure. After an initial femtosecond laser pulse heated the gold to energy densities as high as 107 J/kg, the team sent in a second pulse, spanning the energy range from 1.5 to 2.8 eV, to probe the gold’s dielectric response. The results show an initial transient response that settles after about a picosecond into a...

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