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Graphene is an attractive material for use in optical detectors because it absorbs light from mid-infrared to ultraviolet wavelengths with nearly equal strength. Graphene is particularly well suited for bolometers-devices that detect temperature-induced changes in electrical conductivity caused by the absorption of light-because its small electron heat capacity and weak electron-phonon coupling lead to large light-induced changes in electron temperature. Here, we demonstrate a hot-electron bolometer made of bilayer graphene that is dual-gated to create a tunable bandgap and electron-temperature-dependent conductivity. The bolometer exhibits a noise-equivalent power (33 fW Hz(-1/2) at 5 K) that is several times lower, and intrinsic speed (>1 GHz at 10 K) three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.

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The resistivity of nearly solid-density Al was measured as a function of temperature over 4 orders of magnitude above ambient by observing the self-reflection of an intense, <0.5 psec, 308-nm light pulse incident on a planar Al target. As an increasing function of electron temperature, the resistivity is observed initially to increase, reach a maximum which is relatively constant over an extended temperature range, and then decrease at the highest temperatures. The broad maximum is interpreted as "resistivity saturation," a condition in which the mean free path of the conduction electrons reaches a minimum value as a function of temperature, regardless of the extent of any further disorder in the material.PACS numbers: 72.15.Cz, 52.25.Fi, 78.47.+p We report the first experimental study of the electrical resistivity of a solid-density material, in this case a simple Drude metal, over an extended range (4 orders of magnitude) of elevated temperature with little or no change in its density. The results show three general regions of the dependence of the resistivity on the temperature: Initially, the resistivity increases with increasing temperatures, reaching a relatively constant value that extends over a wide temperature range, and then it decreases as the temperature is further increased. We argue that these regions reflect differing mechanisms controlling the mean free path of the conduction electrons in different temperature ranges. In particular, the region of maximum resistivity is a result of "resistivity saturation," a condition in which the electron mean free path reaches a minimum value, independent of the degree of material disorder.The ability to study the resistivity of a well characterized solid-density material over a great range of elevated temperatures was made possible here by the use of ultrashort (<0.5 psec), relatively high-energy (0-5 mJ) laser pulses. The self-reflection of a laser pulse, focused onto a smooth target at fixed pulsewidth and spot size, was monitored over 4 orders of magnitude in energy. Frequency shifts of the reflected light were also recorded, and as discussed in detail below, these frequency shifts are shown to arise directly from the expansion velocity of the solid-vacuum interface. From the dependence of the interface velocity upon the laser intensity, we were able to determine the electron temperature and degree of interface expansion for each recorded value of the reflectivity. This information is sufficient to determine the resistivity of solid-density Al as a function of temperature up to 10 6 K.It is important to differentiate between the type of heating-reflectivity experiment reported here and those conducted with high-energy (> 100 mJ), long-duration ( > 50 psec) pulses. In the latter, the majority of the en-ergy is absorbed not by the dense target, but rather by the material expanding away from the interface over a scale of many wavelengths. Detailed hydrodynamic calculations are usually required to analyze the data from this type of experiment, l and no simpl...

Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or 'holes' in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales. We show that high repetition rate filamentation and supercontinuum generation can be strongly affected by these holes, which should also affect all other experiments employing intense high repetition rate laser pulses interacting with gases.