High Order Harmonic Generation
High order harmonics of an ultrashort laser pulse is the shortest light bursts on the earth. The duration of such pulses can be as small as 100 attoseconds (10-18 s) this property of high order harmonics combined with their coherent nature and short wavelength make them an ideal tool for ultrafast high resolution imaging. Using such source one can probe the dynamics of electron motion in an atom.
At Ti:sapphire Laser Laboratory, RRCAT extensive study on harmonic generation of ultra-short Ti:sapphire laser pulses in underdense plasma plume produced by part of uncompressed Ti:sapphire laser pulse (~200 ps) has been studied using various atomic number targets.
High order harmonic generation of laser: attoscience
The high order harmonics of the ultrashort laser pulses inherited various properties of the lasers such as their short pulse duration, high degree of coherence and directionality. On top of this the interaction of the laser with underdense media generate not one but a comb of harmonics usually at the odd multiple of laser frequency of the laser. These harmonics are inherently phase locked and hence a band of such harmonics can generate a train of ultrashort pulses where duration of each pulse in this train equals to tens of attosecond. Attosecond is a time scale over which atomic transitions take place and hence the use of attosecond pulses have given a quantum leap in possible temporal resolution of pump-probe experiments. Therefore increasing the efficiency of harmonic radiation, decrease in the duration of attosecond pulses and their use in various practical applications are the hot research topics in this area.
High order harmonics are generated by the interaction of the laser pulses with underdense plasma plumes. A schematic of the experimental set up is shown below.
The generated harmonics are then dispersed and recorded by the xuv Flat Field Grating Spectrograph (FFGS), consists of focussing gold mirror which collects the harmonic radiation in the vertical direction, flat field grating which disperses the harmonics in horizontal direction and focus it on a micro channel plate detector the spectra is finally recorded on a CCD camera. Following figure shows spectra of higher harmonics of silver recorded by FFGS without and with focussing gold mirror.
(a) Exposure time 20 sec, (b) Exposure time 2 sec
Higher harmonic spectra of silver plasma plume (a) Without focusing mirror (b) With focusing mirror
During our studies, it was observed that in certain plasma plume viz. Indium plume, one of the harmonic order (frequency of the 13th harmonic order) closely matches with an atomic transition of the Indium resulting into to enhancement of that particular harmonic intensity. A more than two orders of intensity enhancement is observed in case of 13th indium harmonic order compared to corresponding harmonic of silver plasma. Similar enhancement is also observed in case of InSb and chromium plasma plumes as shown in figure below.
High harmonic spectrum from (a) indium plasma, (b) InSb plasma (c) Cr plasma
Further, generation of higher harmonics is extended in composite material such as fullerene, nanoparticle containing plasma plumes, carbon aerogel, and carbon containing plasma plumes. Higher harmonic yield similar to indium 13th harmonic order is demonstrated in all the lower harmonics (up to 15th order) in composite material as shown in figure xxx.
Coherence measurement of HHG:
Study of coherence measurements of HHG have been performed using double slit experiment. The fringe visibility of the double slit interference fringes recorded for each harmonic order gives spatial coherence of the harmonic orders. A typical interference fringes recorded for the 11th order is shown in figure xxx. One can easily see the high contrast or visibility of the fringes recorded. The calculated coherence area is ~4.5×10-2 mm2.