arXiv subjects

R. H. Hauge

Publications and source records attributed to R. H. Hauge.

13 recordsLinked to original sources

arXiv2013

Midinfrared Third Harmonic Generation from Macroscopically Aligned Ultralong Single-Wall Carbon Nanotubes

We report the observation of strong third harmonic generation from a macroscopic array of aligned ultralong single-wall carbon nanotubes (SWCNTs) with intense midinfrared radiation. Through power-dependent experiments, we determined the absolute value of the third-order nonlinear optical susceptibility, $χ^{(3)}$, of our SWCNT film to be 5.53 $\times$ 10$^{-12}$ esu, three orders of magnitude larger than that of the fused silica reference we used. Taking account of the filling factor of 8.75% for our SWCNT film, we estimate a $χ^{(3)}$ of 6.32 $\times$ 10$^{-11}$ esu for a fully dense film. Furthermore, through polarization-dependent experiments, we extracted all the nonzero elements of the $χ^{(3)}$ tensor, determining the magnitude of the weaker tensor elements to be $\sim$1/6 of that of the dominant $χ^{(3)}_{zzzz}$ component.

cond-mat.mes-hall↗

arXiv2013

Collective Antenna Effects in the Terahertz and Infrared Response of Highly Aligned Carbon Nanotube Arrays

We study macroscopically-aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than $\sim$3 THz with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 10 THz in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at $\sim$10 THz in agreement with the experiment.

cond-mat.mes-hall↗

arXiv2012

Fundamental Optical Processes in Armchair Carbon Nanotubes

We have used post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes for spectroscopic studies. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the near-infrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G^{-}) feature is a result of resonance with non-armchair "metallic" nanotubes. These findings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.

cond-mat.mes-hall↗

arXiv2011

The Unique Origin of Colors of Armchair Carbon Nanotubes

The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals.

cond-mat.mes-hall↗

arXiv2010

Polarization dependence of coherent phonon generation and detection in highly-aligned single-walled carbon nanotubes

We have investigated the polarization dependence of the generation and detection of radial breathing mode (RBM) coherent phonons (CP) in highly-aligned single-walled carbon nanotubes. Using polarization-dependent pump-probe differential-transmission spectroscopy, we measured RBM CPs as a function of angle for two different geometries. In Type I geometry, the pump and probe polarizations were fixed, and the sample orientation was rotated, whereas, in Type II geometry, the probe polarization and sample orientation were fixed, and the pump polarization was rotated. In both geometries, we observed a very nearly complete quenching of the RBM CPs when the pump polarization was perpendicular to the nanotubes. For both Type I and II geometries, we have developed a microscopic theoretical model to simulate CP generation and detection as a function of polarization angle and found that the CP signal decreases as the angle goes from 0 degrees (parallel to the tube) to 90 degrees (perpendicular to the tube). We compare theory with experiment in detail for RBM CPs created by pumping at the E44 optical transition in an ensemble of single-walled carbon nanotubes with a diameter distribution centered around 3 nm, taking into account realistic band structure and imperfect nanotube alignment in the sample.

cond-mat.mes-hall↗

arXiv2009

Enrichment of Armchair Carbon Nanotubes via Density Gradient Ultracentrifugation: Raman Spectroscopy Evidence

We have used resonant Raman scattering spectroscopy to fully analyze the relative abundances of different (n,m) species in single-walled carbon nanotube samples that are metallically enriched by density gradient ultracentrifugation. Strikingly, the data clearly show that our density gradient ultracentrifugation process enriches the metallic fractions in armchair and near-armchair species. We observe that armchair carbon nanotubes constitute more than 50% of each (2n + m) family.

cond-mat.mes-hall↗

arXiv2009

Carbon Nanotube Terahertz Polarizer

We describe a film of highly-aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data we calculated the reduced linear dichrosim to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

cond-mat.mtrl-sci↗

arXiv2008

Alignment Dynamics of Single-Walled Carbon Nanotubes in Pulsed Ultrahigh Magnetic Fields

We have measured the dynamic alignment properties of single-walled carbon nanotube (SWNT) suspensions in pulsed high magnetic fields through linear dichroism spectroscopy. Millisecond-duration pulsed high magnetic fields up to 56 T as well as microsecond-duration pulsed ultrahigh magnetic fields up to 166 T were used. Due to their anisotropic magnetic properties, SWNTs align in an applied magnetic field, and because of their anisotropic optical properties, aligned SWNTs show linear dichroism. The characteristics of their overall alignment depend on several factors, including the viscosity and temperature of the suspending solvent, the degree of anisotropy of nanotube magnetic susceptibilities, the nanotube length distribution, the degree of nanotube bundling, and the strength and duration of the applied magnetic field. In order to explain our data, we have developed a theoretical model based on the Smoluchowski equation for rigid rods that accurately reproduces the salient features of the experimental data.

cond-mat.mes-hall↗

arXiv2006

Coherent Lattice Vibrations in Carbon Nanotubes

We have generated and detected the radial-breathing mode of coherent lattice vibrations in single-walled carbon nanotubes using ultrashort laser pulses. Because the band gap is a function of diameter, these diameter oscillations cause ultrafast band gap oscillations, modulating interband excitonic resonances at the phonon frequencies (3-9 THz). Excitation spectra show a large number of pronounced peaks, mapping out chirality distributions in great detail.

cond-mat.mes-hall↗

arXiv2005

Excitons in Carbon Nanotubes with Broken Time-Reversal Symmetry

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields ($>$ 55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or ``dark'').

cond-mat.mes-hall↗

arXiv2004

Stability of 1-D Excitons in Carbon Nanotubes under High Laser Excitations

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.

cond-mat.mes-hall↗

arXiv2004

Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes

We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks, and consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe sub-picosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultrafast carrier relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to radiative carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H$^+$ ions surrounding the nanotubes.

cond-mat.mes-hall↗

arXiv2004

Interband Recombination Dynamics in Resonantly-Excited Single-Walled Carbon Nanotubes

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in non-resonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H$^+$ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1-D manifestation of the Burstein-Moss effect.

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