Laser Plasma Section

Laser Energy Absorption and X-ray Emission Studies from Nano-Structured Targets Irradiated by Laser Pulses

Nanoparticle formation experiments:

Pulsed laser deposition is an efficient nano-particle formation method and we have demonstrated metal nanoparticles can be produced both by fs and sub-ns pulses. Ti:sapphire laser pulses (λ=795 nm, E =30 mJ, 10 Hz pulse repetition rate) focussed on planar Ag, Cu, In, Cr targets yield nanoparticles efficiently. The deposition collected on glass slides, silicon wafer, and Al foil is analysed for the presence of nano-particles. Inferences are based on analysis of the spatial characteristics using AFM, SEM and TEM microscopy and the spectral analysis of the deposited material using a spectrophotometer. Similarly small nanoparticles like fullerenes can be generated from graphite by sub-ns pulses irradiation. Results of silver nanoparticle generated by laser is summarised in Fig. 1

Fig.1:  Ag nano-particles formed by 45 fs pulses with size of 25 nm (Top) and with sub-ns pulses (Bottom) with size of 40 nm.
Fig.1: Ag nano-particles formed by 45 fs pulses with size of 25 nm (Top) and with sub-ns pulses (Bottom) with size of 40 nm.

Nano-ripple formation in semiconductors:

Grating-like surface structure may be produced by ultra-short laser pulses on interaction with a solid surface and they are called nano-ripples. We have explored the possibility of forming nano-ripples on semiconductors with different band gaps and identified conditions which control the width of nano-ripples by varying the laser parameters as seen from Fig 2. Wide band gap material, shorter wavelength, and a dense ambient forms form smaller ripple width. The critical role of the surface plasmons and surface plasma electron density in deciding ripple period is identified to help in controlling the ripple period. The surface plasmon interferes with incident laser light leading to the nano-ripple formation.

Fig.2 : Nano-ripple formation in wide band gap semiconductors : SiC a) and b); GaP  e) and f); and in narrow band gap semiconductor InP  c) and d).
Fig.2 : Nano-ripple formation in wide band gap semiconductors : SiC a) and b); GaP e) and f); and in narrow band gap semiconductor InP c) and d).

Experimental and theoretical studies of absorption and x-ray emission in nano-structured targets:

A high absorption of the intense 45 fs laser pulses and efficient x-ray conversion in both soft (water window) and hard x-ray region (> 1 keV) is observed from in situ formed clusters. We have also carried out experiments for finding the laser parameters and other factors for finding conditions for maximization of x-ray yield. Absorption and x-ray emission was studied as a function of pre-pulse intensity, main pulse intensity and main pulse duration and the delay between the pulses.
Nano-structures are also used to study their interaction with intense ultra-short laser pulses and studying the dynamics of laser nanostructure interaction. The laser energy absorption in nano-structures has a profound dependence on shape, surface morphology, and other geometric factors. Nano-structures in various shapes like elongated and pointed nano-structures have shown enhanced x-ray and energetic particle emission and this has been attributed to the enhanced electric field at the tip, also referred to as the “Lightning rod effect” and linear and nonlinear resonances. It is desirable to design a nano-target with a geometry which will enable even higher field enhancement when irradiated by ultra-short laser pulses for efficient x-ray and energetic particle generation. Recently, we studied different kinds of nano-structures for laser matter interaction studies, especially the ones which have a hollow structure for example fullerene, nano-hole alumina, carbon nano-tubes etc. They show efficient x-ray and hot electron generation. Their resonance densities have also been explored. In order to understand the reason for the x-ray enhancement from nano-tubes, we investigated both theoretically and experimentally the effect of the hollow structure on the field enhancement as shown in Fig.3.

Fig 3
FIG. 3: Left a) Absorption of 45 fs laser pulse in planar Mo (triangles) and Mo coated with CNTs (circles); b) X-ray spectrum from planar Mo and Mo coated with CNTs, (intensity ~8x1016 W/cm2). Right a) The variation of high and low resonance density with degree of hollowness. The dashed vertical line is for the class of CNTs used in our experiments with ao/b0 =0.9; b) The r.m.s. intensity enhancement factor inside the nano-tube, compared to the incident intensity of the laser (the inset shows a top view of CNT with a hollowness factor of 0.9).

Publications in journals

  1. Pulsed laser deposition of metal films and nano-particles in vacuum using sub nanosecond laser pulses
    R. A. Ganeev, U. Chakravarty, P. A. Naik, H. Srivastava, C. Mukharjee, M. K. Tiwari, R. V. Nandedkar, and P. D. Gupta
    Applied Optics 46, 1205, 2007

  2. Efficient keV X-ray generation from irradiation of in-situ produced silver clusters by Ti:sapphire laser pulses
    U. Chakravarty, P.A. Naik, S.R. Kumbhare, and P.D. Gupta,
    J. Opt. Soc. Korea 13, 80, 2009

  3. Formation of metal nano-particles of various sizes in plasma plumes produced by Ti:sapphire laser pulses,
    U. Chakravarty, P. A. Naik, C. Mukherjee, S.R. Kumbhare, and P. D. Gupta.
    J. Appl. Phys. 108, 053107, 2010

  4. Enhanced soft X-ray emission from carbon nano-fibers irradiated with ultra-short laser pulses
    U. Chakravarty, P. A. Naik, B. S. Rao, V. Arora, H. Singhal, G. M. Bhalerao, A. K. Sinha, P. Tiwari and P. D. Gupta
    Applied Physics B: Lasers and Optics , 103, 571, 2011

  5. X-ray enhancement in a nano-hole target irradiated by intense ultra-short laser pulses
    U. Chakravarty, V. Arora, J. A. Chakera, P. A. Naik, H. Srivastava, P. Tiwari, A.Srivastava, and P. D Gupta.
    J. Appl. Phys. 109, 053301, 2011

  6. Nano-ripple formation on different band-gap semiconductor surfaces using femtosecond pulses
    U. Chakravarty, R. A. Ganeev, P. A. Naik, J. A. Chakera, M. Babu, and P. D. Gupta.
    J. Appl. Phys. 109, 084347, 2011

  7. Electric field enhancement at multiple densities in laser irradiated nano-tube plasma
    U. Chakravarty, P.A. Naik, and P.D. Gupta
    Pramana J. Phys. 79, 443, 2012

  8. Enhancement of Kα emission through efficient hot electron generation in carbon nano-tubes on intense laser pulse irradiation
    U. Chakravarty, V. Arora, P. A. Naik, J. A. Chakera, H. Srivastava, A. Srivastava, G. D. Varma, S. R Kumbhare, and P.D Gupta
    J. Appl. Phys. 112, 053301, 2012

  9. Enhanced water window x-ray emission from in situ formed carbon clusters irradiated by intense ultra-short laser pulses
    U. Chakravarty, B. S. Rao, V. Arora, A. Upadhyay, H. Singhal, P. A. Naik, J. A. Chakera, C. Mukharjee, and P.D. Gupta
    Appl. Phys. Lett. 103, 054107 (2013)

  10. Estimation of electron density and temperature of semiconductor surfaces excited by ultra-short laser pulses.
    U. Chakravarty, P. A. Naik, J. A. Chakera, A. Upadhyay, and P. D. Gupta
    Applied Physics A 115, 1457 (2014)

Conference Paper:
  1. Oscillator model for nano-tube plasma interacting with few cycle laser pulse
    U. Chakravarty, P.A Naik, J.A. Chakera, and P.D. Gupta
    28th National Symposium on Plasma Science & Technology, Bhubaneshwar, Dec. 2013
    (Plasma Science Society of India's Best Poster Award)

  2. Hole size effect in hard x-ray emission from intense laser irradiated nano-holes.
    U. Chakravarty, V Arora, J. A. Chakera, P. A. Naik, H. Srivastava, P.Tiwari, A. Srivastava, and P.D Gupta
    26th National Symposium on Plasma Science & Technology, Patna, Dec.2011.
    (Plasma Science Society of India's Best Poster Award)


  3. High order harmonic generation in plasma plume of in situ laser produced silver nanoparticles, H. Singhal , R. A. Ganeev, P.A. Naik, J.A. Chakera, U. Chakravarty, H.S. Vora, A.K. Srivastava, C. Mukharjee , C.P. Navathe, S.K. Deb, P.D. Gupta.
    25th National Symposium on Plasma Science & Technology, Guwahati, Dec. 2010.
    (Plasma Science Society of India's Best Poster Award)

  4. Dependence of high order harmonic intensity on the medium length in laser produced plasma plumes,
    H.Singhal , V.Arora, B.S.Rao, U.Chakravarty, P.A.Naik, R.A. Khan, and P. D. Gupta
    23rd National Symposium on Plasma Science & Technology, Mumbai, Dec.2008.
    (Plasma Science Society of India's Best Poster Award)

  5. A novel method of intense keV x-ray generation from in-situ produced silver clusters using Ti:sapphire laser pulses.
    U Chakravarty, P.A Naik, R.A Khan and P.D. Gupta.
    DAE-BRNS National Laser Symposium-2007, Vadodara, Dec. 2007
    (Indian Laser Association's Best Poster Award)

  6. Enhancement and extinction of single harmonic intensity of high order harmonics
    H.Singhal, V.Arora, P.A.Naik, U.Chakravarty, J.A.Chakera, M.Raghuramaiah, S.R.Kumbhare, R.A.Khan, P.D.Gupta and R. A. Ganeev
    21st National Symposium on Plasma Science & Technology, Jaipur, Dec.2006.
    (Plasma Science Society of India's Best Poster Award)

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