The incredible progress in ultrafast laser technology and Ti:sapphire lasers have lead to many important applications, one of them being high-order harmonic generation (HHG). HHG is a source of coherent extreme ultraviolet (XUV) radiation, which has opened new frontiers in science by extending nonlinear optics and time-resolved spectroscopy to the XUV region, and pushing ultrafast science to the attosecond domain. Progress in attosecond science has revealed many new phenomena that have not been seen with femtosecond pulses. Clearly, the next frontier is to study nonlinear effects at the attosecond timescale and in the XUV. However, a problem with present-day attosecond pulses is that they are just too weak to induce measurable nonlinearities, which severely limits the application of this source. While HHG from solid targets has shown promise for higher conversion efficiency, there is no experiment so far that demonstrates isolated attosecond pulse generation. The generation of isolated, several 100-as pulses with few-µJ energy will enable us to enter a completely new phase in attoscience.
In past works, we have demonstrated that high-order harmonics from lowly ionized plasma is a highly efficient method to generate coherent XUV pulses. For example, indium plasma has been shown to generate intense 13th harmonic of the Ti:sapphire laser, with conversion efficiency of 10-4. However, the quasi-monochromatic nature of indium harmonics would make it difficult to generate attosecond pulses. We have also demonstrated that one could increase the harmonic yield by using nanoparticle targets. Specifically, we showed that by using indium oxide nanoparticles or C60 film, we could obtain intense harmonics between wavelengths of 50 to 90 nm. The energy in each of these harmonic orders was measured to be a few µJ, which is sufficient for many applications. However, the problem of using nanoparticle or film targets is the rapid decrease in the harmonic intensity, due to the rapid change in the morphology of these targets. This prevents us from using this intense harmonic source in applications, or to perform attosecond measurements, which require stable harmonics.
In this paper, I will describe our recent results on the generation of high-order harmonics from graphitic carbon plasma, whose efficiency is measured to be higher than that from indium (> 10-4), and at the same time spans over five harmonic orders, from the 11th to the 19th harmonic. The energy per pulse of each of the 11th to the 19th harmonic is greater than 1 µJ, and the broad bandwidth over which such intense harmonics are generated is ideal for producing intense single-cycle attosecond pulses. Furthermore, we have started to explore the generation of intense, isolated attosecond pulses using the Double Optical Gating  (DOG) method. Experiments show that we can generate continuum high-order harmonic spectra from carbon plasma, spanning an energy range from 17 to 25 eV. The conversion efficiency of these continuum harmonics are found to be 10 times more energetic than those generated from gas, which is the first step towards the generation of intense isolated attosecond pulses.
 H. Mashiko, S. Gilbertson, C. Li, S. Khan, M. Shakya, E. Moon and Z. Chang, Phys. Rev. Lett. 100, 103906 (2008).