Pump-probe Spectroscopy

We study ultrafast carrier dynamics in nanostructured materials using nonlinear optical techniques. Most of the studies are carried out using the available femtosecond laser system that includes a 100fs Ti: Sapphire based oscillator-amplifier and an optical parametric amplifier. The experimental techniques used are different variations of pump-probe nonlinear spectroscopy including transient grating, differential reflection and differential transmission spectroscopy, propagation of ultrafast pulses in nonlinear materials and single beam measurements of nonlinearities. The recovery time of the photo-excited carriers is measured as a function of experimental parameters like the excitation wavelength, excitation fluence, sample temperature and orientation to develop a better understanding of the potential of these materials as opto-electronic devices.

Currently we are working with metal nanoparticles, semiconductor quantum wells and optical fibres. We welcome collaboration with groups making novel nanostructured materials, including structures designed for the THz region as the available spectral region is expected to be extended to THz in the near future.

Some recent results

1. Metal Nanoparticles

   Many applications of nanoparticles are based on their remarkable linear and nonlinear optical properties. We are studying the basic physical processes in metal nanoparticles, including the effect of shape and size on optical properties. This is the key information required for building devices based on nanoparticles.
   
   

   1. Effect of shape on linear optical properties of metal nanoparticles

      Several applications need tuning of localized surface plasmon resonance (LSPR) of metal particles to any given wavelength. We have tuned the LSPR resonance of silver nanocolloids by preparing of different shaped nanoparticles, by changing its distribution, and irradiating the nanocolloids with femtosecond laser.
      [J. Phys:Condens. Matter, 20, 445222 (2008)]

      1. Tuning LSPR by changing the shape of nanoparticles

         	Silver nanocolloidal solutions of nanoplatelets with different aspect ratios. The nanoparticles in the yellow sample are spherical in shape, while the particles in the order magenta, orange, green, light blue, and dark blue colored samples are nanoplatelets of increasing aspect ratios.
         	Extinction spectrum of colloids having nanoplatelets of different aspect ratios. The arrows indicate the in-plane dipole SPR peak positions of the silver nanoplatelets. The in-plane dipole peak wavelengths increase with increase in aspect ratio of the nanoplatelets.
         	TEM pictures Left -  silver nanospheres Middle – silver nanoplatelets LSPR - 506 nm Right – silver nanoplatelets LSPR - 660 nm

         
         
         
      2. Changing the distribution of the nanoparticles (aggregating)

         	Changes in the extinction spectrum due to aggregation of nanospheres. Before aggregation (solid line) After aggregation (dashed line) TEM picture of the aggregated nanospheres.
         	Changes in the extinction spectrum due to aggregation of platelets. Before aggregation (solid line) After aggregation (dashed line) TEM picture of the aggregated platelets.

         
         
         
      3. Irradiating the nanocolloids with femtosecond laser

         Changes in the LSPR peak during exposure to femtosecond laser. Left – At fluence of 20 Jcm-2 the nanoplatelets breaks and forms spheres. Right – At fluence of 20 mJcm-2 the nanoplatelets reshapes.

         
         
         
   2. Effect of shape on third-order nonlinear optical properties of metal nanoparticles

      In order to compare, which shape can give large local field enhancement, using transient grating geometry, we have measured the third-order nonlinearity of silver nanoplatelets and silver nanospheres at their respective surface plasmon resonances.
      [Nanotechnology, 18, 315704 (2007)]

      Two-beam forward four-wave mixing setup (Transient grating) used for measuring the third-order optical nonlinearity.		Photograph of bottles containing silver colloids having: Left – nanospheres, Right – nanodisks having prepared by photo-induced conversion. Middle – a mixture of nanospheres and nanodisks.		The third-order nonlinearity at the LSPR of silver nanodisk is higher than that of silver nanospheres.

      
      
   3. Higher-order optical nonlinearities and carrier dynamics in metal nanoparticles

      With a new two-dimensional way of analyzing conventional transient absorption measurement data the magnitude and decay time of the imaginary part of the third as well as the fifth and seventh order optical nonlinearities were extracted. Compared to spheres the measured values are 28, 5000, and 700000 times higher for the third, fifth and seventh orders respectively, at 800 nm.
      [App. Phys. Lett., 94, 181902 (2009)]

      The measured change in absorption of the silver nanoplatelet colloid with respect to pump-probe delay time and pump peak intensities.		Variation of (a) α2, (b) α4 and (c) α6 , which are related to the decay of third, fifth and seventh order nonlinearities, with delay between pump-probe pulses. The corresponding response times are 0.9 ps, 0.41 ps and 0.27 ps showing that the higher order nonlinearities decay much faster.

      
      
   4. Theoretical studies on linear and second-order polarizabilities of metal particles having different shapes.

      We are also looking at finding out the optimum shape and size for the largest nonlinear response. The localized surface plasmon resonance of a nanoparticle is not necessarily at the same wavelength as the resonance of nonlinearity. Since both the LSPR position and the resonance of nonlinearity depend on shape it might be possible to obtain a shape where both coincide. Such a shape will have much higher nonlinearity at the resonance position. Therefore we have calculated the linear polarizability and first order hyperpolarizability for two different classes of shapes: sectors and wedges. The scaling properties of linear and second-order polarizabilities are related to the electron cloud length and “asymmetric volume” of the nanoparticles.
      
      [Phys. Rev. B., 77, 045421 (2008) & Phys. Rev. B., 68, 075319 (2003)]

      		Two classes of the nanoparticle shape, sectors (left) and wedges, for which optical properties have been calculated.
      		Variation of the real part of second-order βxxx, at zero frequency for wedge-shaped particles having 240 electrons as a function of X. The wedge angle of the particle is π/X.  The inset shows the variation of real part of |βxxx| for some small values of X.

      
      
2. Quantum wells

   Understanding the carrier dynamics in semiconductor nanostructures on picosecond and sub-ps time scales is important for fast optoelectronic device design. Quantum wells are an important class of such nanostructures. We are looking into the effect of quantum well structure on the ultrafast recovery time of carriers excited near the band-edge.  Many times the substrate on which the quantum well is grown is opaque to the excitation wavelength. In such case the reflection geometry is used.

   GaAsP/AlGaAs quantum wells are utilized extensively in diode lasers, detectors and modulators.  Single quantum wells of this material have been grown by the Semiconductor Laser Section, RRCAT with varying thickness of the top barrier layer.  Transient pump-probe reflectivity (TPPR)  measurements in these samples show a fast (< 1ps) component followed by an unexpected slow rise and subsequent decay. By varying the excitation and probe wavelength around the band-edge we have determined that the fast component is due to transition between the quantum well states but the slower component arises from defect states arising from the interfaces.

   

   Recent publications

   Journals

   1. Transient absorption and higher-order nonlinearities in silver nanoplatelets, J. Jayabalan, Asha Singh, Rama Chari, Salahuddin Khan, Himanshu Srivastava, and S. M. Oak, App. Phys. Lett., 94, 181902 (2009).
   2. Aggregated nanoplatelets: optical properties and optically induced deaggregation, J. Jayabalan, Asha Singh, Rama Chari, Himanshu Srivastava, P. K. Mukhopadhyaya, A. K. Srivastava, and S. M. Oak, J. Phys.: Cond. Matt., 20, 445222 (2008).
   3. Linear and nonlinear second-order polarizabilities of hemispherical and sector-shaped metal nanoparticles, J. Jayabalan, M. P. Singh, A. Banerjee and K. C. Rustagi, Phys. Rev. B., 77, 045421 (2008).
   4. Ultrafast third order nonlinearity of silver nanospheres and nanodiscs, J Jayabalan, Asha Singh, Rama Chari and S. M. Oak, Nanotechnology, 18, 315704 (2007).
   5. Experimental demonstration of fiber differential spectral technique for monitoring small fluctuations in ultrashort laser pulses, Fozia Aziz, Rama Chari, S. M. Oak, Opt Engg, 46, 073601 (2007).
   6. Stability monitoring of ultrashort laser pulses using single mode fibres, Rama Chari and S. M. Oak, Optical Engineering, 45, 094202 (2006).
   7. Single-Shot Measurement of Nonlinear Absorption and Nonlinear Refraction, J. Jayabalan, Asha Singh and S. M. Oak, App. Opt., 45, 3852-3858 (2006).
   8. Achieving high Signal to Noise Ratio in transient reflectivity measurements, A. R. Vijayaragavan, R. Chari, S. M. Oak, Ind J Pure & Appl Phys, 44, 330-333(2006).

   International Conferences

   1. Ultrafast Resonant Higher-Order Optical Nonlinearities of Silver Nanoplatelet Colloids, J. Jayabalan, Asha Singh, and Rama Chari, Proceedings of the International Conference on Materials for Advanced Technologies (ICMAT-2009), Symposium G: Plasmonics and Applications, pp 13, 28th June - 4th July 2009.
   2. Stimulated Raman Scattering in One-dimensional Mott-Hubbard Insulators: A Novel THz source, Haranath Ghosh and Rama Chari, International conference on Emerging Trends in Electronic and Photonic Devices and Systems (ELECTRO-2009) Dec 22-24, pp155 2009 BHU.
   3. Shape Induced Nonlinearities in Metal Nanoparticles, J. Jayabalan, Manoranjan P. Singh, and K. C. Rustagi, Proc. Eighth International Conference on Optoelectronics, Fiber Optics and Photonics, University of Hyderabad, Hyderabad, Dec (2006).
   4. Ultrashort pulse laser stability monitoring using fibre nonlinearity, R. Chari, V. S. Suvith and S. M. Oak, paper no NLO23, Photonics 2006, Hyderabad, Dec 13-16, (2006).
      
      
   National Conferences
   
   
   1. Extinction spectrum of metal nanocubes with smoothened tips, J. Jayabalan, Topical conference on interaction of EM radiation with atoms, molecules & clusters (TC-2010), Raja Ramanna Centre for Advanced Technology, Indore, 3-6 March 2010.
   2. Improving the stability of silver nanoplatelet colloid against femtosecond laser irradiation, Asha Singh, J. Jayabalan, Rama Chari, and S. M. Oak, National Laser Symposium NLS-9, Jan 13-16, 2010 BARC, Mumbai.
   3. Some issues in analysis of time resolved pump probe reflectivity data, Salahuddin Khan, J. Jayabalan and R. Chari, National Laser Symposium NLS-9, Jan 13-16, 2010 BARC, Mumbai.
   4. Enhanced high-order harmonic generation from nanoparticle targets H.Singhal, R.A.Ganeev, P.A.Naik, J.A.Chakera, A.K.Srivastava, J.Jayabalan, A.Singh, R.Chari, S.R.Kumbhare, and P.D.Gupta, National Laser Symposium NLS-9, Jan 13-16, 2010 BARC, Mumbai.
   5. A comparative study of scanning-pixel and scanning-knife-edge techniques for spot-size measurements of narrow laser beams, S. K. Tiwari, J. Jayabalan, S. P. Ram, S. R. Mishra, S. C. Mehendale,  Eighth DAE-BRNS National Laser Symopsium-2008, Laser Science and Technology Centre (LASTEC), Delhi.
   6. Simulation tool for dispersion compensation assembly design, Fozia Aziz, R. Chari and S.M. Oak, PM 38 Photonics 2008, IIT Delhi.
   7. Cryocooler vibrations in ultrafast reflection spectroscopy, S. Khan, R. Chari, S. M. Oak, M. S. Ansari, A. K. Sagar, L. Chowdhary, R. S. Doohan, P. K. Kush, National Conference on recent trends in Optoelectronics and laser technology (NCOLS 2007), April 9-11, Thiruvananthapuram.
   8. Nanosecond Laser Induced De-aggregation of Silver Nanoprism Aggregates, J. Jayabalan, Asha Singh, Rama Chari, P. K. Mukhopadhaya, Himanshu Srivastava and S. M. Oak, National Laser Symposium - 2007 (NLS-07), Vadodara, India, Dec,  301-302 (2007),
   9. Measurement of a narrow spot-size by scanning of single pixel of a CCD camera, S. P. Ram, S. K. Tiwari, J. Jayabalan, S. R. Mishra, National Laser Symposium - 2007 (NLS-07), Vadodara, India, Dec, 519-520 (2007).
   10. Transient reflectivity in GaAsP/AlGaAs quantum wells, S. Khan, J. Jayabalan, R. Chari, S. Pal, S. Porwal, T. K. Sharma, S. M. Oak, National Laser Symposium - 2007 (NLS-07) , Vadodara, India, Dec, 263-264 (2007).
   11. Silver Nanocluster –A promising material for nonlinear optics, J.Jayabalan, Asha Singh, Rama Chari and S M Oak., Himanshu Srivastava, A.K.Srivastava,  National Conference on recent trends in Optoelectronics and laser technology (NCOLS 2007), April 9-11, Thiruvananthapuram.
   12. Ultra-fast Nonlinear Optical Properties of Metal Nanoparticles, J. Jayabalan, Asha Singh, Rama Chari and S. M. Oak, Workshop on Materials Science and Atomic/Molecular Physics Experiments, IUAC, New Delhi, 21-22 February (2007).
   13. Transformation of silver nanodiscs by laser photolysis, A. Singh, J. Jayabalan, P. K. Mukhopadhyaya, A. K. Nath, R. Chari and S. M. Oak, Proc. of Fifth DAE-BRNS - National Laser Symposium-2006, RRCAT, Indore, Dec (2006).
   14. Optimization of white light continuum in the spectral range 500-700 nm, J. Jayabalan, A. Singh, R. Chari and S. M. Oak, Proc. of Fifth DAE-BRNS - National Laser Symposium-2006, RRCAT, Indore, Dec5-8,(2006).