Generation of subpicosecond terahertz pulses is observed when graphite surfaces are illuminated with femtosecond near-infrared laser pulses. The nonlinear optical generation of THz pulses from graphite is unexpected since, in principle, the material possesses a centre of inversion symmetry. Experiments with highly oriented pyrolytic graphite crystals suggest that the THz radiation is generated by a transient photocurrent in a direction normal to the graphene planes, along the c-axis of the crystal. This is supported by magnetic-field induced changes in the THz electric-field polarization, and consequently, the direction of the photocurrent. We show that other forms of graphite, such as a pencil drawing on paper, are also capable of emitting THz pulses.
Abstract:We describe a simple terahertz (THz) time domain spectrometer with a bandwidth extending up to 7.5 THz. We show that by keeping the generation and detection crystals close to each other a high signal-to-noise ratio (SNR) can be achieved without using lock-in detection and dry nitrogen flushing. The observed spectra show very good agreement with the spectra calculated based on a simple model which includes phase matching and absorption in the generation and detection crystals. Using this set-up we have measured the absorption lines in D-tartaric acid from 0.5 THz up to 7 THz. We show that the high frequency region > 3 THz is the better choice to measure small changes in the water content of a hygroscopic sample compared to the low frequency region. Lett. 14, 1128-1131 (1989). 2. Y. R. Shen, "Far-infrared generation by optical mixing," Prog. Quantum Electron. 4, 207-232 (1976). 3. L. Xu, X.-C. Zhang, and D. H. Auston, "Terahertz beam generation by femtosecond optical pulses in electro-optic materials," Appl. Phys. Lett. 61, 1784-1786 (1992). 4. C. Kubler, R. Huber, and A. Leitenstorfer, "Ultrabroadband terahertz pulses:generation and field-resolved detection," Semicond. Sci. Technol. 20, S128-S133 (2005). 5. A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. H. Knox, "Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory," Appl. Phys. Lett. 74, 1516Lett. 74, -1518Lett. 74, (1999. 6. Q. Wu and X.-C. Zhang, "Free-space electro-optic sampling of terahertz beams," Appl. Phys. Lett. 67, 3523-3525 (1995). 7. A. Nahata, A. S. Weling, and T. F. Heinz, "A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling," Appl. Phys. Lett. 69, 2321-2323 (1996). 8. D. Mittleman, R. Jacobsen, and M. Nuss, "T-ray imaging," IEEE J. Sel. Top. Quantum Electron. 2, 679-692 (1996). 9. D. M. Mittleman, R. H. Jacobsen, R. Neelamani, R. G. Baraniuk, and M. C. Nuss, "Gas sensing using terahertz time-domain spectroscopy," Appl. Phys. B 67, 379-390 (1998 199-213 (2004). 11. G. C. . Cho, P. Y. Han, X. C. Zhang, and H. J. Bakker, "Optical phonon dynamics of GaAs studied with timeresolved terahertz spectroscopy," Opt. Lett. 25, 1609-1611 (2000). 12. D. Grischkowsky, S. Keiding, M. van Exter, and C. Fattinger, "Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors," J. Opt. Soc. Am. B 7, 2006-2015 (1990 Planken, "Advanced terahertz electric near field measurements at subwavelength diameter metallic apertures," Opt. Express 16, 7407-7417 (2008).
Using near-field, terahertz time-domain spectroscopy ͑THz-TDS͒, we investigate how the addition of a dielectric material into a subwavelength-diameter, cylindrical waveguide affects its transmission properties. The THz electric near-field is imaged with deep subwavelength resolution as it emerges from filled and unfilled waveguides. Spectroscopic data measured for waveguides filled with polycrystalline D-tartaric acid, and with polyethylene and silicon powders, illustrate the feasibility of this approach for obtaining spectroscopic information from a tiny sample volume. © 2010 American Institute of Physics. ͓doi:10.1063/1.3467192͔Terahertz time-domain spectroscopy ͑THz-TDS͒ 1 has emerged as an important spectroscopic tool for investigations of, for example, molecular crystals.2,3 These measurements are commonly performed on macroscopic samples in the far field, meaning that their lateral dimensions are typically much larger than the THz wavelength. The samples must be this large because the classical diffraction limit restricts the lateral sample dimensions to values larger than ϳ / 2, which translates into a value of 150 m at a typical frequency of 1.0 THz. In practice, these samples must be considerably larger than this to avoid diffraction/edge effects. It can be advantageous to perform spectroscopic measurements on a microscopic scale, as one could measure a very small quantity of material and ensure greater sample homogeneity. To do so, however, is not trivial. One must satisfy two conditions: high spatial confinement of the field at the point of measurement and reasonably large signal-to-noise ratio in the detected signal. One way to achieve confinement of the field is by using a waveguide.4-8 For example, a parallel plate waveguide, partially filled with a thin film has been shown to be very effective for thin film far-field measurements of various systems, 9,10 though additional lenses are required for in-and out-coupling of the THz light.In this work, we demonstrate a different approach to waveguide THz-TDS. Free-space THz radiation is focused onto truncated, ͑sub͒wavelength-diameter, cylindrical waveguides, which are completely filled with different dielectric materials. Only THz light that emerges from the guides and thus must have interacted with the sample is measured in the near field. We show how the waveguides can be used to resolve molecular resonances as well as the effective refractive index from a small volume of a sample, such as D-tartaric acid ͑DTA͒. Near-field spectroscopic measurements allow for an extremely high, subwavelength spatial resolution and high sensitivity for measurements of microscopic volumes.Using near-field, terahertz time-domain spectroscopy, [11][12][13][14][15][16] we acquire time-and frequency-domain images, along with corresponding spectral information for filled and unfilled waveguides ͑single, isolated guides, and arrays͒. A schematic drawing that illustrates our near-field measurement technique is shown in Fig. 1. Linearly polarized THz light is focused onto the w...
We present a simple experimental set-up to generate and detect terahertz (THz) radiation with a large bandwidth of 7.5 THz. We show that by keeping the generation and detection crystals close to each other, a high signal-to-noise ratio (SNR) can be achieved without using lock-in detection and dry nitrogen flushing. Using this set-up we have measured the absorption lines in D-tartaric acid from 0.5 THz upto 7 THz
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