resonant enhancement of coherent phonons in carbon nanotubes observed with sub 10fs time resolution

JTh2A.46.pdf CLEO:2013 Technical Digest © OSA 2013 Resonant Enhancement of Coherent Higher-Order Phonons in Single-Walled Carbon Nanotubes Kenji Sato∗ , Ikufumi Katayama∗∗ , Keisuke Tahara, Yasuo Minami, Jun Takeda, Kazuhiro YanagiA , and Masahiro KitajimaB Department of Physics, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan A Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan B Department of Applied Physics, National Defence Academy, Yokosuka 239-8686, Japan ∗ sato-kenji-dm@ynu.ac.jp ∗∗ katayama@ynu.ac.jp Abstract: Coherent phonon dynamics in metallic carbon nanotubes has been investigated by pump-probe spectroscopy using a 7.5-fs laser High-frequency coherent phonons up to 100 THz have been observed including second-order modes due to the electronic resonances © 2013 Optical Society of America OCIS codes: 320.7120 300.6500 300.6250 Introduction Single-walled carbon nanotubes (SWCNTs) are π-conjugated, quasi-one-dimensional structures of a rolled-up graphene sheet that become either semiconductors or metals depending on their chirality Their electronic states have divergences of density of states due to one-dimensional van-Hove singularities and behave as Dirac electrons, both of which bring groundbreaking applications in nanoelectronics and nanophotonics [1] Around these singularities, both electronic and atomic motions are expected to be controlled through strong resonant electron-phonon interaction around these singularities [2] In this paper, we show that the resonant excitation with ultrashort 7.5-fs laser pulses, coherently drives high-frequency and higher-order phonons, which might be related to phonon quantum states such the as squeezed states [3, 4] Although the normal Raman spectroscopy reveals fundamentals of the high-frequency mode such as 2D and 2G modes, the ultrafast dynamics andthe strong coupling with electronic states are still unclear Coherent phonon spectroscopy using a sub-10-fs laser is therefore applied for elucidating these properties in SWCNTs [5] Experiment SWCNTs with diameters of 1.4 nm were prepared from synthesized carbon nanotubes (Meijo nanocarbon Co Ltd.) using the density-gradient purification processing [6] This method separates metallic and semiconductor carbon nanotubes, thereby we could obtain high-purity SWCNTs samples with narrow chirality distribution and low residual species content To reveal the coherent lattice vibrations in metallic SWCNTs, we used a Ti:sapphire laser with 7.5 fs pulse duration to excite and detect high-frequency coherent phonons The output of the laser is separated into two beams; one is used as a pump beam to excite the coherent phonons and the other is used as a probe beam The transient transmission was measured with electro-optic sampling that can detect an anisotropic transmission change induced by pump pulses The delay between pump and probe beams are controlled using an optical shaker with 15 ps scanning range at 20 Hz repetition rate The signal is amplified and then analyzed by a personal computer with an analog-to-digital converter Results and Discussion Figure 1(a) shows the transient transmission change in the metallic SWCNTs The strong electronic response near t = and subsequent phonon oscillations are observed As shown in Fig 1(b), the Fourier transformed spectrum after subtracting the electronic response clearly shows coherent D mode (40 THz), G mode (47.4 THz), 2D mode (78.3 THz), and 2G mode (96 THz) phonons, whose frequencies correspond to the normal Raman spectrum The observed dephasing times of the G and 2D modes are 0.78 ps and 0.23 ps, respectively The 2D mode has faster dephasing time because the second-order Raman process allows the excitation of not only the Γ point phonons, but also those 978-1-55752-973-2/13/$31.00 ©2013 Optical Society of America JTh2A.46.pdf 1.5 CLEO:2013 Technical Digest © OSA 2013 1000 (a) (b) 0.5 0.1 0.2 FT Intensity 1.0 ΔΤ/Τ G 800 0.3 0.0 600 400 2D -0.5 200 -0.1 0.0 0.1 Time(ps) 0.2 0.3 D RBM -1.0 2G 20 40 60 Frequency[THz] 80 100 Fig (a) The transient transmission change in metallic SWCNTs (b) The Fourier transformed spectrum of the oscillatory components observed in (a) with large wavevectors Note that the amplitude of the 2D mode is relatively large compared with those observed in graphite, which might come from the resonant enhancement at the van-Hove singularity of the M2 transition in our metallic SWCNTs whose energy is close to the center wavelength of laser Summary In summary, we have successfully measured the high-frequency coherent phonons in metallic SWCNTs In addition to the first-order coherent phonons such as the D, G, and RBM modes, we could clearly observed the second-order 2D and 2G modes The amplitude of the 2D mode observed is resonantly enhanced by the van-Hove singularity of the M2 transition As the two phonons are simultaneously emitted in the 2D mode by an impulsive excitation, the phonon quantum states might be the squeezed states, which could interaction with Dirac electrons References S J Tans et al., “Room-temperature transistor based on a single carbon nanotube,” Nature 393, 49 (1998) J.-H Kim et al., “Chirality-Selective Excitation of Coherent Phonons in Carbon Nanotubes by Femtosecond Optical Pulses,” Phys Rev Lett 102, 037402 (2009) G Garrett et al., “Ultrafast optical excitation of a combined coherent-squeezed phonon field in SrTiO3 ,” Opt Express 1, 385 (1997) G Garrett et al., “Vacuum Squeezing of Solids:Macroscopic Quantum States Driven by Light Pulse,” Science 275, 1638 (1997) W T Pollard, S.-Y Lee, and R A Mathies, “Wave packet theory of dynamic absorption spectra in femtosecond pump–probe experiments,” J Chem Phys 92, 4012 (1990) K Yanagi et al., “Optical and Conductive Characteristics of Metallic Single-Wall Carbon Nanotubes with Three Basic Colors; Cyan, Magenta, and Yellow,” Appl Phys Express 1, 034003 (2008)