Scientists and technologists have always coveted a bright hard x-ray source due to its diverse applications in basic sciences - for example, to determine the structure of a crystal or a complex protein - as well as in technology - for example in nano-lithography, x-ray microscopy etc. Till recently, synchrotrons have been the only sources of hard x-rays, their colossal size and exorbitant budgetary requirements severely restricting their commercial viability. Recent advances in intense ultrashort lasers have breathed a new life into this field. Such lasers can create extremely hot, dense plasmas in a solid target, which in turn can radiate out x-rays with energies as high as millions of electron-volts. Furthermore, these x-rays are emitted in femtosecond or picosecond pulses, depending on the laser pulsewidth, mimicking or even improving on synchrotron-based x-ray sources. Currently, these plasma x-ray sources are being actively investigated to comprehend the underlying physics as well as their viability as commercial, compact, inexpensive ultrashort hard x-ray sources.
While the laser-produced plasma x-ray source has a brightness comparable to or even exceeding that of a synchrotron in the moderately hard x-ray regime, the yields in the very hard (tens and hundreds of kilo-electron-volts) regime need to be improved for a more meaningful realization of the aforesaid applications. To achieve this, one needs to couple more and more of the laser energy into the plasma, which poses a significant problem because of the high reflectivity of the incident laser light from the plasma at moderately high intensities. To overcome this problem, our innovative target-engineering techniques have provided additional coupling of the laser energy to the plasma by enhancing the laser-induced `local� electric fields on the surface of the target, thereby simulating a �lightning-rod effect�. This was achieved in a copper nanoparticle-coated target, which exhibited a significant 13-fold enhancement in the hard x-ray bremsstrahlung emission in the range (10-200) kilo-electron-volts at modest laser intensities of ~1015 W/cm2 (P. P. Rajeev et al., Phys. Rev. Lett., 90, 115002 (2003)). We have explored additional avenues for such an enhancement via �sub-lambda� gratings, which demonstrated a near-100% absorption of the laser energy, and a consequent increased x-ray emission, due to a structure-induced surface-plasmon resonance (S. Kahaly et al., Phys. Rev. Lett. 101, 145001 (2008)). Similar enhancement in hard x-ray emission has also been observed from copper-nanorod arrays (S. Mondal et al., Phys. Rev. B 83, 035408 (2011)), in carbon nanotubes (S. Bagchi et al., Phys. Plasmas 18, 014502 (2011)) as well as in silicon nanowires (P. K. Singh et al., App. Phys. Lett. 100, 244104 (2012)). One of our recent experiments has in fact observed a ~10,000 fold enhancement in hard x-ray emission from Ag-doped bacterial (E. coli) cells (M. Krishnamurthy et al., Opt. Express 20, 5754 (2012), International Patent WO 2011/055376 A1).