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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 13 — Jun. 21, 2010
  • pp: 14232–14237
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Ultrafast spatiotemporal relaxation dynamics of excited electrons in a metal nanostructure detected by femtosecond-SNOM

Zhi Li, Song Yue, Jianjun Chen, and Qihuang Gong  »View Author Affiliations


Optics Express, Vol. 18, Issue 13, pp. 14232-14237 (2010)
http://dx.doi.org/10.1364/OE.18.014232


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Abstract

Ultrahigh spatiotemporal resolved pump-probe signal near a gold nano-slit is detected by femtosecond-SNOM. By employing two-color pump-probe configuration and probing at the interband transition wavelength of the gold, signal contributed by surface plasmon polariton is avoided and spatiotemporal evolvement of excited electrons is successfully observed. From the contrast decaying of the periodical distribution of the pump-probe signal, ultrafast diffusion of excited electrons with a time scale of a few hundred femtoseconds is clearly identified. For comparison, such phenomenon cannot be observed by the one-color pump-probe configuration.

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Future technology for information process needs higher density of integration and higher speed. Surface plasmon polaritons (SPPs) [1

1. H. Raether, Surface plasmons on Smooth and Rough Surfaces and on Gratings, (Springer-Verlag, Berlin, 1988).

], propagating bound oscillations of free electrons and light at a metal surface, are promising candidates for next-generation highly integrated nanophotonic devices and are widely investigated in recent years [2

2. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

5

5. A. Kubo, N. Pontius, and H. Petek, “Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface,” Nano Lett. 7(2), 470–475 (2007). [CrossRef] [PubMed]

]. However active plasmonics with short response time are not quite easy to realize. One possible route is to engage the ultrafast electron relaxation process in the metal, since the existence of excited electrons will change the dielectric properties of the metal film which can result in an active action [6

6. K. F. MacDonald, Z. L. Samson, M. I. Stockman, and N. I. Zheludev, “Ultrafast active plasmonics,” Nat. Photonics 3(1), 55–58 (2009). [CrossRef]

]. Thus, ultrafast relaxation dynamics of excited electrons in metal nanostructures must be well investigated, which requires a technique with both spatial and temporal high resolutions. Using femtosecond-SNOM, Imura K et al. have tried to investigate the ultrafast excited electron dynamics in gold nanorod [7

7. K. Imura, T. Nagahara, and H. Okamoto, “Imaging of surface plasmon and ultrafast dynamics in gold nanorods by near-field microscopy,” J. Phys. Chem. B 108(42), 16344–16347 (2004). [CrossRef]

]. However, due to the one-color pump-probe configuration and both the pump and probe exciting the sample at the plasmon resonance wavelength of the nanorod, the resulting pump-probe signal is dominated by the plasmon contribution and does not correspond to the distribution of excited electrons [8

8. K. Imura and H. Okamoto, “Ultrafast photoinduced changes of eigenfunctions of localized plasmon modes in gold nanorods,” Phys. Rev. B 77(4), 041401 (2008). [CrossRef]

]. We propose that, to detect the spatial distribution of excited electrons, the probe light must ensure local detection and keep away from plasmon resonance which represents a collective contribution of electrons. In the letter, by employing a two-color pump-probe femtosecond-SNOM and choosing the probe wavelength to locate at the interband transition wavelength of the metal which has no evident SPP effect, we successfully detect ultrahigh spatiotemporal resolved excited electron dynamics near a gold nano-slit and directly observe the ultrafast spatial diffusion process of excited electrons.

By combining femtosecond ultrafast optical spectroscopy and scanning near-field optical microscope (SNOM), the femtosecond-SNOM can obtain ultrahigh spatial and temporal optical resolutions simultaneously [9

9. A. Lewis, U. Ben-Ami, N. Kuck, G. Fish, D. Diamant, L. Lubovsky, K. Lieberman, S. Katz, A. Saar, and M. Roth, “NSOM the fourth dimension: integrating nanometric spatial and femtosecond time resolution,” Scanning 17, 3–13 (1995). [CrossRef]

,10

10. B. A. Nechay, U. Siegner, M. Achermann, H. Bielefeldt, and U. Keller, “Femtosecond pump-probe near-field optical microscopy,” Rev. Sci. Instrum. 70(6), 2758–2764 (1999). [CrossRef]

]. And it has been successfully applied to study the ultrafast carrier relaxation dynamics in semiconductor nanostructures [11

11. B. A. Nechay, U. Siegner, F. Morier-Genoud, A. Schertel, and U. Keller, “Femtosecond near-field optical spectroscopy of implantation patterned semiconductors,” Appl. Phys. Lett. 74(1), 61–63 (1999). [CrossRef]

], quantum dots [12

12. T. Guenther, C. Lienau, T. Elsaesser, M. Glanemann, V. M. Axt, T. Kuhn, S. Eshlaghi, and A. D. Wieck, “Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy,” Phys. Rev. Lett. 89(5), 057401 (2002). [CrossRef] [PubMed]

] and so on. Figure 1
Fig. 1 Schematic of the two-color pump-probe femtosecond-SNOM system.
schematically shows the experimental setup of our two-color pump-probe femtosecond-SNOM (commercial SNOM, Omicron TwinSNOM). Laser pulses from a femtosecond Ti:Al2O3 laser (120 fs, 1000 nm, 76 MHz, Mira 900F, Coherent) are split into two parts. One passes through the optical delay line and is used as the pump light, and the other is frequency doubled to 500nm to match the interband transition wavelength of the gold and used as the probe light. The pump and probe beams are recombined at a dichroic mirror and then focused onto the sample by a 4 × objective. The transmitted signal is collected in the near field above the sample by a gold-coated fiber tip (chemical etched) with aperture diameter of about 200 nm. The output from the fiber then pass through a color filter to eliminate the 1000 nm pump light and only the 500 nm probe light is detected by a PMT. A mechanical light chopper is engaged to modulate the pump light at 1.2 kHz and the transient transmission of the probe light (that is the pump-probe signal) is retrieved by a lock-in amplifier at the modulating frequency.

For sample preparation, a 20nm-thick gold film is first evaporated on a glass substrate by electron-beam evaporator. AFM measurement (by NTEGRA Spectra AFM, NT-MDT) gives a rms roughness of about 1nm, which means a quite flat sample surface. A 200 nm wide and 30μm long nano-slit is then fabricated on the gold film by focused-ion-beam (FIB). When the 1000 nm pump pulse with electric field perpendicular to the nano-slit illuminates the sample, SPPs are excited by the nano-slit and propagate along the two surfaces of the gold film. The spot size of the pump light is about 15 micron, which is bigger than our scan range (10 micron), so SPPs will interfere with the direct transmitted pump light (schematically shown in Fig. 2(a)
Fig. 2 (a) Schematic view of SPP generation and subsequent interference with the direct transmitted pump light. (b) FEM simulation results of the electric field intensity in the middle of the gold film (with the nano-slit locates at sample position of 0 nm).
). Numerical simulations by finite element method (FEM) using Comsol Multiphysics show that the SPP on the gold/substrate interface is much stronger than the SPP on the gold/air interface (not shown here). So the electric field in the gold film is dominated by the interference between the SPP on the gold/substrate interface and the direct transmitted pump light, and the interference period should be equal to the SPP wavelength on the gold/substrate interface. This is well demonstrated by Fig. 2(b) which displays FEM simulation results of the electric field intensity in the middle of the gold film, since the electric field mainly possesses an interference period of about 620nm which corresponds well to the SPP wavelength on the gold/substrate interface. This periodically distributed electric field then excites electrons via intraband absorption in the pump pulse duration, and the resulting initial distribution of excited electrons corresponds to the distribution of the excitation electric field. After that, the excited electrons undergo relaxation processes in both space and time. Since the existence of the excited electrons changes the dielectric property of the gold film, this will lead to the modulation on the transmittance of the probe light [13

13. C. K. Sun, F. Vallee, L. H. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond-tunable measurement of electron thermalization in gold,” Phys. Rev. B 50(20), 15337–15348 (1994). [CrossRef]

]. Because the probe light locates at the interband transition wavelength of the gold without evident SPP effect, the probe light collected by the near-field tip only locally interacts with excited electrons just below the tip. So the spatiotemporal resolved transient transmission of the probe light detected by femtosecond-SNOM gives a direct measurement on the spatiotemporal evolution of the excited electrons.

The ultrafast electron diffusion process can also be identified from the different relaxation times at different sample positions. Figures 4
Fig. 4 Green symbols and red symbols show pump-probe curves acquired at sample positions of 2000 nm and 2300 nm in Fig. 3, respectively. (a) Original data and (b) normalized results with respect to the peak intensities. Lines correspond to double exponential fitting results.
displays typical time-domain measured pump-probe curves by scanning the time delay while fixing the near-field fiber tip at specific sample positions. The green symbols and the red symbols are acquired at sample positions of 2000 nm and 2300 nm in Fig. 3, respectively, corresponding to two adjacent peak and valley of the pump-probe signals. Figure 4(a) shows the original data and Fig. 4(b) shows the normalized results with respect to the peak intensities. The relaxation dynamics are obviously different and double exponential fit shows that the pump-probe signal at position 2000 nm possesses both a faster rise time and a faster decay time (rise time of 266 fs and decay time of 1446 fs) compared with those at position 2300 nm (rise time of 413 fs and decay time of 1563 fs). Considering that the nonthermal electron density after excitation at position 2000 nm is larger than that at position 2300 nm, they will diffuse from position 2000 nm to position 2300 nm. So that the relaxation process at position 2000 nm is speeded up (in contrary to the pump power dependent behavior in reference [14

14. C. K. Sun, F. Vallee, L. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond investigation of electron thermalization in gold,” Phys. Rev. B 48(16), 12365–12368 (1993). [CrossRef]

], where higher pump intensity gives slower dynamics), which gives faster rise and decay times. On the contrary, due to the additional nonthermal electrons coming from its neighborhood, the relaxation process at position 2300 nm is slowed down. Thus the different relaxation processes indicated by the pump-probe curves at different sample positions also give indications of the ultrafast electron diffusion process in the metal nanostructure.

In conclusion, we have successfully observed the spatiotemporal evolvement of excited electrons in a gold nanostructure by employing two-color pump-probe femtosecond-SNOM and probing at the interband transition wavelength of the gold. Ultrafast nonthermal electron diffusion process is successfully identified on a time scale of about 700 fs, while such phenomenon is not observed in the one-color pump-probe configuration in the comparison experiment. Due to the simultaneously obtained ultrahigh spatial and temporal resolution, the two-color pump-probe femtosecond-SNOM can reveal more detailed information on the ultrafast electron relaxation processes in metal nanostructures and even in ultrafast active plasmonic devices.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 10804004, 10821062 and 90921008), the National Basic Research Program of China (Grant Nos. 2007CB307001 and 2009CB930504), and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 200800011023).

References and links

1.

H. Raether, Surface plasmons on Smooth and Rough Surfaces and on Gratings, (Springer-Verlag, Berlin, 1988).

2.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

3.

E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

4.

R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]

5.

A. Kubo, N. Pontius, and H. Petek, “Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface,” Nano Lett. 7(2), 470–475 (2007). [CrossRef] [PubMed]

6.

K. F. MacDonald, Z. L. Samson, M. I. Stockman, and N. I. Zheludev, “Ultrafast active plasmonics,” Nat. Photonics 3(1), 55–58 (2009). [CrossRef]

7.

K. Imura, T. Nagahara, and H. Okamoto, “Imaging of surface plasmon and ultrafast dynamics in gold nanorods by near-field microscopy,” J. Phys. Chem. B 108(42), 16344–16347 (2004). [CrossRef]

8.

K. Imura and H. Okamoto, “Ultrafast photoinduced changes of eigenfunctions of localized plasmon modes in gold nanorods,” Phys. Rev. B 77(4), 041401 (2008). [CrossRef]

9.

A. Lewis, U. Ben-Ami, N. Kuck, G. Fish, D. Diamant, L. Lubovsky, K. Lieberman, S. Katz, A. Saar, and M. Roth, “NSOM the fourth dimension: integrating nanometric spatial and femtosecond time resolution,” Scanning 17, 3–13 (1995). [CrossRef]

10.

B. A. Nechay, U. Siegner, M. Achermann, H. Bielefeldt, and U. Keller, “Femtosecond pump-probe near-field optical microscopy,” Rev. Sci. Instrum. 70(6), 2758–2764 (1999). [CrossRef]

11.

B. A. Nechay, U. Siegner, F. Morier-Genoud, A. Schertel, and U. Keller, “Femtosecond near-field optical spectroscopy of implantation patterned semiconductors,” Appl. Phys. Lett. 74(1), 61–63 (1999). [CrossRef]

12.

T. Guenther, C. Lienau, T. Elsaesser, M. Glanemann, V. M. Axt, T. Kuhn, S. Eshlaghi, and A. D. Wieck, “Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy,” Phys. Rev. Lett. 89(5), 057401 (2002). [CrossRef] [PubMed]

13.

C. K. Sun, F. Vallee, L. H. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond-tunable measurement of electron thermalization in gold,” Phys. Rev. B 50(20), 15337–15348 (1994). [CrossRef]

14.

C. K. Sun, F. Vallee, L. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond investigation of electron thermalization in gold,” Phys. Rev. B 48(16), 12365–12368 (1993). [CrossRef]

15.

S. D. Brorson, J. G. Fujimoto, and E. P. Ippen, “Femtosecond electronic heat-transport dynamics in thin gold films,” Phys. Rev. Lett. 59(17), 1962–1965 (1987). [CrossRef] [PubMed]

16.

S. Smith, N. C. R. Holme, B. Orr, R. Kopelman, and T. Norris, “Ultrafast measurement in GaAs thin films using NSOM,” Ultramicroscopy 71(1-4), 213–223 (1998). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors
(180.4243) Microscopy : Near-field microscopy

ToC Category:
Ultrafast Optics

History
Original Manuscript: May 5, 2010
Revised Manuscript: June 8, 2010
Manuscript Accepted: June 8, 2010
Published: June 17, 2010

Citation
Zhi Li, Song Yue, Jianjun Chen, and Qihuang Gong, "Ultrafast spatiotemporal relaxation dynamics of excited electrons in a metal nanostructure detected by femtosecond-SNOM," Opt. Express 18, 14232-14237 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-13-14232


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References

  1. H. Raether, Surface plasmons on Smooth and Rough Surfaces and on Gratings, (Springer-Verlag, Berlin, 1988).
  2. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
  3. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]
  4. R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]
  5. A. Kubo, N. Pontius, and H. Petek, “Femtosecond microscopy of surface plasmon polariton wave packet evolution at the silver/vacuum interface,” Nano Lett. 7(2), 470–475 (2007). [CrossRef] [PubMed]
  6. K. F. MacDonald, Z. L. Samson, M. I. Stockman, and N. I. Zheludev, “Ultrafast active plasmonics,” Nat. Photonics 3(1), 55–58 (2009). [CrossRef]
  7. K. Imura, T. Nagahara, and H. Okamoto, “Imaging of surface plasmon and ultrafast dynamics in gold nanorods by near-field microscopy,” J. Phys. Chem. B 108(42), 16344–16347 (2004). [CrossRef]
  8. K. Imura and H. Okamoto, “Ultrafast photoinduced changes of eigenfunctions of localized plasmon modes in gold nanorods,” Phys. Rev. B 77(4), 041401 (2008). [CrossRef]
  9. A. Lewis, U. Ben-Ami, N. Kuck, G. Fish, D. Diamant, L. Lubovsky, K. Lieberman, S. Katz, A. Saar, and M. Roth, “NSOM the fourth dimension: integrating nanometric spatial and femtosecond time resolution,” Scanning 17, 3–13 (1995). [CrossRef]
  10. B. A. Nechay, U. Siegner, M. Achermann, H. Bielefeldt, and U. Keller, “Femtosecond pump-probe near-field optical microscopy,” Rev. Sci. Instrum. 70(6), 2758–2764 (1999). [CrossRef]
  11. B. A. Nechay, U. Siegner, F. Morier-Genoud, A. Schertel, and U. Keller, “Femtosecond near-field optical spectroscopy of implantation patterned semiconductors,” Appl. Phys. Lett. 74(1), 61–63 (1999). [CrossRef]
  12. T. Guenther, C. Lienau, T. Elsaesser, M. Glanemann, V. M. Axt, T. Kuhn, S. Eshlaghi, and A. D. Wieck, “Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy,” Phys. Rev. Lett. 89(5), 057401 (2002). [CrossRef] [PubMed]
  13. C. K. Sun, F. Vallee, L. H. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond-tunable measurement of electron thermalization in gold,” Phys. Rev. B 50(20), 15337–15348 (1994). [CrossRef]
  14. C. K. Sun, F. Vallee, L. Acioli, E. P. Ippen, and J. G. Fujimoto, “Femtosecond investigation of electron thermalization in gold,” Phys. Rev. B 48(16), 12365–12368 (1993). [CrossRef]
  15. S. D. Brorson, J. G. Fujimoto, and E. P. Ippen, “Femtosecond electronic heat-transport dynamics in thin gold films,” Phys. Rev. Lett. 59(17), 1962–1965 (1987). [CrossRef] [PubMed]
  16. S. Smith, N. C. R. Holme, B. Orr, R. Kopelman, and T. Norris, “Ultrafast measurement in GaAs thin films using NSOM,” Ultramicroscopy 71(1-4), 213–223 (1998). [CrossRef]

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