Abstract

We study the ultrafast insulator-to-metal transition in nanoparticles of VO2, obtained by ion implantation and self-assembly in silica. The nonmagnetic, strongly correlated compound VO2 undergoes a reversible phase transition, which can be photoinduced on an ultrafast time scale. In the nanoparticles, prompt formation of the metallic state results in the appearance of surface-plasmon resonance. We achieve large, ultrafast enhancement of optical absorption in the near-infrared spectral region that encompasses the wavelength range for optical-fiber communications. One can further tailor the response of the nanoparticles by controlling their shape.

© 2005 Optical Society of America

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References

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  1. K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
    [CrossRef]
  2. T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
    [CrossRef] [PubMed]
  3. S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
    [CrossRef]
  4. F. J. Morin, Phys. Rev. Lett. 3, 34 (1959).
    [CrossRef]
  5. H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
    [CrossRef]
  6. M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
    [CrossRef]
  7. A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
    [CrossRef]
  8. A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
    [CrossRef]
  9. Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
    [CrossRef]
  10. R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
    [CrossRef]
  11. H. C. van de Hulst, Light Scattering by Small Particles (Dover, New York, 1981).
  12. The experimental setup is based on a 1-kHz Ti:sapphire laser. Pump pulses at 800 nm have 1–3?µJ energy and durations below 50 fs. Near-infrared pulses are obtained from an optical para-metric amplifier (signal, 1.1–1.6 µm; idler, 1.6–2.6 µm). Mid-infrared 2.8–6 µm pulses are generated by difference-frequency mixing in GaSe of signal and idler pulses. The time resolution varies from 100 to 150 fs, with increasing values from high to low frequencies. As the phase transition is fully reversible, the sample is not moved from shot to shot.
  13. The particles are considered perfect ellipsoids placed in a homogeneous electromagnetic field with their major axes perpendicular to the direction of light propagation and a random in-plane orientation. Inhomogeneous effects that are due to a size and aspect ratio distribution of the nanorods are neglected, as are interparticle coupling mechanisms. The complex dielectric function of the high-temperature metallic VO2 is employed.8
  14. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
    [CrossRef]

2004

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

2003

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

2002

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

2001

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

2000

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

1997

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

1994

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

1968

H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
[CrossRef]

1959

F. J. Morin, Phys. Rev. Lett. 3, 34 (1959).
[CrossRef]

Ashida, M.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Atwater, H. A.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Barker, A. S.

H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
[CrossRef]

Becker, M. F.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Berglund, C. N.

H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
[CrossRef]

Boatner, L. A.

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

Brun, A.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Buckman, A. B.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Cavalleri, A.

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Chong, H. H.

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

Dekorsy, Th.

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

Eisaki, H.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Feldman, L. C.

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

Forget, P.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Georges, P.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Ghosh, H.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Haglund, R. F.

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

Harel, E.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Haynes, T. E.

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

Iwai, S.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Kieffer, J. C.

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Kik, P. G.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Kishida, H.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Koel, B. E.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Kuwata-Gonokami, M.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Lépine, T.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Lopez, R.

R. Lopez, T. E. Haynes, L. A. Boatner, L. C. Feldman, and R. F. Haglund, Opt. Lett. 27, 1327 (2002).
[CrossRef]

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

Maeda, A.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Maier, S. A.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Matsuzaki, H.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Mazumdar, S.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Meltzer, S.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Miyano, K.

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

Morin, F. J.

F. J. Morin, Phys. Rev. Lett. 3, 34 (1959).
[CrossRef]

Motoyama, N.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Ogasawara, T.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Okamoto, H.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Ono, M.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

Ráksi, F.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Requicha, A. A. G.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Schoenlein, R. W.

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

Shukla, A.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

Siders, C. W.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Squier, J. A.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Tanaka, T.

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

Tokura, Y.

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

Tomioka, Y.

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

Tóth, Cs.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

Uchida, S.

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

van de Hulst, H. C.

H. C. van de Hulst, Light Scattering by Small Particles (Dover, New York, 1981).

Verleur, H. W.

H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
[CrossRef]

Walser, R. M.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

J. Appl. Phys.

M. F. Becker, A. B. Buckman, R. M. Walser, T. Lépine, P. Georges, and A. Brun, J. Appl. Phys. 79, 2404 (1994).
[CrossRef]

Samples are prepared by implantation of vanadium ions (1.5×1017 V ions/cm2 at 150 keV) and oxygen ions (3.0×1017 O ions/cm2 at 55 keV) at equal depths of 120 nm into an amorphous SiO2 substrate and then annealing in an argon atmosphere at 1000 °C. Depending on the annealing time, the mean radius of the nanoparticles varies from 40 to 80 nm. See R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund, J. Appl. Phys. 92, 4031 (2002).
[CrossRef]

Nature Mater.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, Nature Mater. 2, 229 (2003).
[CrossRef]

Opt. Lett.

Phys. Rev.

H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172, 788 (1968).
[CrossRef]

Phys. Rev. B

A. Cavalleri, Th. Dekorsy, H. H. Chong, J. C. Kieffer, and R. W. Schoenlein, Phys. Rev. B 70, 161102 (2004).
[CrossRef]

Phys. Rev. Lett.

A. Cavalleri, Cs. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 (2001).
[CrossRef]

K. Miyano, T. Tanaka, Y. Tomioka, and Y. Tokura, Phys. Rev. Lett. 78, 4257 (1997).
[CrossRef]

T. Ogasawara, M. Ashida, N. Motoyama, H. Eisaki, S. Uchida, Y. Tokura, H. Ghosh, A. Shukla, S. Mazumdar, and M. Kuwata-Gonokami, Phys. Rev. Lett. 85, 2204 (2000).
[CrossRef] [PubMed]

S. Iwai, M. Ono, A. Maeda, H. Matsuzaki, H. Kishida, H. Okamoto, and Y. Tokura, Phys. Rev. Lett. 91, 057401 (2003).
[CrossRef]

F. J. Morin, Phys. Rev. Lett. 3, 34 (1959).
[CrossRef]

Other

H. C. van de Hulst, Light Scattering by Small Particles (Dover, New York, 1981).

The experimental setup is based on a 1-kHz Ti:sapphire laser. Pump pulses at 800 nm have 1–3?µJ energy and durations below 50 fs. Near-infrared pulses are obtained from an optical para-metric amplifier (signal, 1.1–1.6 µm; idler, 1.6–2.6 µm). Mid-infrared 2.8–6 µm pulses are generated by difference-frequency mixing in GaSe of signal and idler pulses. The time resolution varies from 100 to 150 fs, with increasing values from high to low frequencies. As the phase transition is fully reversible, the sample is not moved from shot to shot.

The particles are considered perfect ellipsoids placed in a homogeneous electromagnetic field with their major axes perpendicular to the direction of light propagation and a random in-plane orientation. Inhomogeneous effects that are due to a size and aspect ratio distribution of the nanorods are neglected, as are interparticle coupling mechanisms. The complex dielectric function of the high-temperature metallic VO2 is employed.8

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Figures (3)

Fig. 1
Fig. 1

Change of absorbance in VO2 across the temperature-driven phase transition for thin-film and spherical nanoparticles as derived from steady-state absorption measurement in the low- and high-temperature phases. The VO2 volume filling for nanospheres is approximately 10% in a single 100-nm layer.

Fig. 2
Fig. 2

(a) Relative change of transmission across the phase transition for the VO2 nanospheres. (ΔT, change of transmission in the presence of the pump; T, sample transmission). The thermally induced curve was derived from spectra measured at room temperature (VO2 in the insulating phase) and at 373 K (VO2 in the metallic phase). The photoinduced wave was measured at a delay of 200 fs after photoexcitation. (b) Pump–probe signal as a function of pulse delay measured at 1.55 and 2.5 µm.

Fig. 3
Fig. 3

Relative change of transmission as a function of wavelength for several aspect ratios (top to bottom: 1, 1.5, 2, 3.5) as obtained by varying the annealing time (3, 7, 10, and 45 min, respectively). Squares, measured at a delay of 200 fs after photoexcitation; solid curves, calculations based on Mie scattering theory. As a comparison, dashed curves show the prediction of Mie theory for spherical particles (aspect ratio, 1; the same as in the topmost figure). Representative TEM pictures of the corresponding samples are shown at the right.

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