%0 Journal Article %J arXiv preprint %D 2016 %T Large Enhancement of Circular Dichroism Using an Embossed Chiral Metamaterial %A Mousavi, S.; %A Panikkanvalappil, S. R.; %A El-Sayed, M. A.; %A Eftekhar, A. A.; %A Adibi, A. %B arXiv preprint %G eng %R 1604.05244 %0 Journal Article %J ACS Appl Mater Interfaces %D 2015 %T Light-responsive plasmonic arrays consisting of silver nanocubes and a photoisomerizable matrix. %A Ledin, Petr A %A Russell, Michael %A Geldmeier, Jeffrey A %A Tkachenko, Ihor M %A Mahmoud, Mahmoud A. %A Shevchenko, Valery %A El-Sayed, Mostafa A. %A Tsukruk, Vladimir V. %X We report on the synthesis of novel branched organic-inorganic azo-polyhedral oligomeric silsesquioxane (POSS) conjugates (Azo-POSS) and their use as a stable active medium to induce reversible plasmonic modulations of embedded metal nanostructures. A dense monolayer of silver nanocubes was deposited on a quartz substrate using the Langmuir-Blodgett technique and subsequently coated with an ultrathin Azo-POSS layer. The reversible light-induced photoisomerization between the trans and cis states of the azobenzene-terminated branched POSS material results in significant changes in the refractive index (up to 0.17) at a wavelength of 380 nm. We observed that the pronounced and reversible change in the surrounding refractive index results in a corresponding hypsochromic plasmonic shift of 6 nm in the plasmonic band of the embedded silver nanocubes. The reversible tuning of the plasmonic modes of noble-metal nanostructures using a variable-refractive-index medium opens up the possibility of fabricating photoactive, hybrid, ultrathin coatings with robust, real-time, photoinitiated responses for prospective applications in photoactive materials that can be reversibly tuned by light illumination. %B ACS Appl Mater Interfaces %V 7 %P 4902-12 %8 2015 Mar 4 %G eng %N 8 %1 http://www.ncbi.nlm.nih.gov/pubmed/25671557?dopt=Abstract %R 10.1021/am508993z %0 Journal Article %J Small %D 2013 %T The Last Step in Converting the Surface Plasmonic Energy into Heat by Nanocages and Nanocubes on Substrates %A Szymanski, P. %A Mahmoud, M A %A El-Sayed, M. A. %B Small %V 9 %P 3934-3938 %8 Dec %@ 1613-6810 %G eng %M WOS:000331282400005 %R 10.1002/smll.201300233 %0 Journal Article %J Journal of Physical Chemistry B %D 2000 %T Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses %A Link, Stephan %A Burda, Clemens %A Nikoobakht, Babak %A El-Sayed, Mostafa A %X Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system producing pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (similar to 1 J cm(-2)) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the energy of the excitation pulse, only partial melting of the nanorods takes place. Shorter but wider nanorods are observed in the final distribution as well as a higher abundance of particles having odd shapes (bent, twisted, phi-shaped, etc.). The threshold for complete melting of the nanorods with femtosecond laser pulses is about 0.01 J cm(-2). Comparing the results obtained using the two different types of excitation sources (femtosecond vs nanosecond laser), it is found that the energy threshold for a complete melting of the nanorods into nanodots is about 2 orders of magnitude higher when using nanosecond laser pulses than with femtosecond laser pulses. This is explained in terms of the successful competitive cooling process of the nanorods when the nanosecond laser pulses are used. For nanosecond pulse excitation, the absorption of the nanorods decreases during the laser pulse because of the bleaching of the longitudinal plasmon band. In addition, the cooling of the lattice occurring on the 100 ps time scale can effectively compete with the rate of absorption in the case of the nanosecond pulse excitation but not for the femtosecond pulse excitation. When the excitation source is a femtosecond laser pulse, the involved precesses (absorption of the photons by the electrons (100 fs), heat transfer between the hot electrons and the lattice (<10 ps), melting (30 ps), and heat loss to the surrounding solvent (>100 ps) are clearly separated in time. %B Journal of Physical Chemistry B %V 104 %P 6152-6163 %8 Jul %@ 1089-5647 %G eng %M WOS:000088057100012 %R 10.1021/jp000679t %0 Journal Article %J Chemical Physics Letters %D 2000 %T The 'lightning' gold nanorods: fluorescence enhancement of over a million compared to the gold metal %A Mohamed, MB %A Volkov, V. V. %A Link, Stephan %A El-Sayed, Mostafa A %X Gold nanorods capped with micelles and having an aspect ratio of 2.0-5.4 are found to fluoresce with a quantum yield which is over a million times that of the metal. For rods of the same width, the yield is found to increase quadratically while the wavelength maximum increases linearly with the length. We assign this emission to the electron and hole interband recombination. The increase in the emission yield results from the enhancement effect of the incoming and outgoing electric fields via coupling to the surface plasmon resonance in the rods. This is similar to the previously proposed fluorescence and the Raman enhancement on noble metal rough surfaces. (C) 2000 Published by Elsevier Science B.V. All rights reserved. %B Chemical Physics Letters %V 317 %P 517-523 %8 Feb %@ 0009-2614 %G eng %M WOS:000085374100001 %R 10.1016/s0009-2614(99)01414-1 %0 Journal Article %J Journal of Physical Chemistry A %D 1999 %T Laser photothermal melting and fragmentation of gold nanorods: Energy and laser pulse-width dependence %A Link, Stephan %A Burda, Clemens %A Mohamed, MB %A Nikoobakht, Babak %A El-Sayed, Mostafa A %X We studied the shape transformation (by use of TEM and optical absorption spectroscopy) of gold nanorods in micellar solution by exposure to laser pulses of different pulse width (100 fs and 7 ns) and different energies (mu J to mJ) at 800 nm, where the longitudinal surface plasmon oscillation of the nanorods absorb. At moderate energies, the femtosecond irradiation melts the nanorods to near spherical particles of comparable volumes while the nanosecond pulses fragment them to smaller near-spherical particles. At high energies, fragmentation is also observed for the femtosecond irradiation. A mechanism involving the rate of energy deposition as compared to the rate of electron-phonon and phonon-phonon relaxation processes is proposed to determine the final fate of the laser-exposed nanorods, i.e., melting or fragmentation. %B Journal of Physical Chemistry A %V 103 %P 1165-1170 %8 Mar %@ 1089-5639 %G eng %M WOS:000079150000001 %R 10.1021/jp983141k %0 Journal Article %J Journal of Physical Chemistry B %D 1998 %T Low-temperature retinal photoisomerization dynamics in bacteriorhodopsin %A Logunov, Stephan L. %A Masciangioli, Tina M. %A Kamalov, Valey F. %A El-Sayed, Mostafa A %X Retinal photoisomerization dynamics are studied at both room temperature and 20 K in wild-type bacteriorhodopsin using femtosecond pulses. We were able to resolve the decay at 20 K into two components with the dominant component having a similar lifetime to that observed at room temperature. This strongly suggests that the retinal lifetime at physiological temperature is barrierless. The minor, low-temperature long-lived component is discussed in terms of previous results obtained for fluorescence and transient absorption with lower time resolution, and the origin of this component is discussed in terms of low-temperature glass heterogeneity. %B Journal of Physical Chemistry B %V 102 %P 2303-2306 %8 Mar %@ 1089-5647 %G eng %M WOS:000072904200001 %R 10.1021/jp972921a %0 Journal Article %J The Journal of Physical Chemistry A %D 1997 %T Laser Multiphoton Dissociation Ionization of Acrolein Clusters %A Morita, Hiroshi %A Freitas, John E. %A El-Sayed, Mostafa A %X Clusters of acrolein, C3H4O, are formed by supersonic expansion techniques. The acrolein clusters are ionized and decomposed by 193 nm light from an excimer laser and detected by time-of-flight mass spectrometry. The dominant channel of decomposition involves the loss of a carbon atom from all the clusters. The loss of C2, C3, and even up to C6 is also observed from larger clusters. The presence of clusters with more hydrogen atoms than expected for multiples of acrolein molecules suggests that reactions within the cluster and evaporation are important processes. The mechanisms of the dissociation routes are discussed in terms of ladder and ladder-switching processes within individual molecules within the cluster.Clusters of acrolein, C3H4O, are formed by supersonic expansion techniques. The acrolein clusters are ionized and decomposed by 193 nm light from an excimer laser and detected by time-of-flight mass spectrometry. The dominant channel of decomposition involves the loss of a carbon atom from all the clusters. The loss of C2, C3, and even up to C6 is also observed from larger clusters. The presence of clusters with more hydrogen atoms than expected for multiples of acrolein molecules suggests that reactions within the cluster and evaporation are important processes. The mechanisms of the dissociation routes are discussed in terms of ladder and ladder-switching processes within individual molecules within the cluster. %B The Journal of Physical Chemistry A %I American Chemical Society %V 101 %P 3699 - 3701 %8 1997 %@ 1089-5639 %G eng %U http://dx.doi.org/10.1021/jp9605010 %N 20 %! J. Phys. Chem. A %R doi: 10.1021/jp9605010