OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 6 — Mar. 17, 2008
  • pp: 4094–4105
« Show journal navigation

7 GHz resolution waveguide THz spectroscopy of explosives related solids showing new features

N. Laman, S. Sree Harsha, D. Grischkowsky, and Joseph S. Melinger  »View Author Affiliations


Optics Express, Vol. 16, Issue 6, pp. 4094-4105 (2008)
http://dx.doi.org/10.1364/OE.16.004094


View Full Text Article

Acrobat PDF (513 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

The vibrational modes of the explosive related solids 2,4- and 2,6-DNT have been measured with waveguide THz time-domain spectroscopy (THz-TDS) to unprecedented precision. Dramatic sharpening of spectral features upon cooling beyond 80 K to 12 K is observed, illustrating the high quality and alignment of the polycrystalline thin films. Many of the observed features have linewidths below 15 GHz. These linewidths are approximately 8 times narrower than for the corresponding measurements via conventional THz-TDS with cooled pellets. For the 2,4-DNT, the line narrowing resolved 19 absorption features compared to only 7 features with the corresponding pellet measurement.

© 2008 Optical Society of America

1. Introduction

In order to increase the sensitivity and resolution compared to THz-TDS characterization of pellets, our group has developed the technique of waveguide THz-TDS [15

15. J. Zhang and D. Grischkowsky, “Waveguide THz time-domain spectroscopy of nm water layers,” Opt. Lett. 19, 1617–1619 (2004). [CrossRef]

, 16

16. J. S. Melinger, N. Laman, S. S. Harsha, and D. Grischkowsky, “Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide,” Appl. Phys. Lett. 89, 251110 (2006). [CrossRef]

]. Here, a thin film of the material under study is formed on a metal plate. This plate is subsequently incorporated in a single-mode parallel plate waveguide [17

17. R. Mendis and D. Grischkowsky, “Undistorted guided wave propagation of sub-picosecond THz pulses,” Opt. Lett. 26, 846–848 (2001). [CrossRef]

], similar to microwave parallel-plate waveguides, but with the dimensions of the gap scaled down to 50 µm to correspond with the smaller wavelength. The THz radiation is coupled into and out of the gap via high-resistivity Si lenses. The guided THz propagates along, and is partially absorbed by, the sample film. The confinement of the THz radiation to a gap of size b, combined with the long interaction length corresponding to the waveguide length L, results in an improvement of the sensitivity by a factor of Γ=L/(n3b) [18

18. G. Gallot, S. P. Jamsion, R. W. McGowan, and D. Grischkowsky, “THz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

], where n is the refractive index of the film. A typical sensitivity enhancement for our experiment is Γ~100.

However, we have recently found that further cooling below liquid nitrogen temperature has a very large effect when using the ordered films of waveguide THz-TDS. The additional sharpening with further cooling implies that our polycrystalline thin films have very strong planar order (i.e. the crystals are well aligned to the metal surface) and with the individual microcrystals containing a high degree of crystallinity. We can therefore obtain measurements approaching the quality of single crystals, but with the easy to prepare polycrystalline films.

Fig 1. Chemical structures of: a) 2,4-DNT, b) 2,6-DNT, c) 2,4,6-TNT

2. Experimental apparatus

Fig 2. Waveguide THz-TDS. Inset indicates replacement of waveguide with pellet for conventional THz-TDS.

For this application of waveguide THz-TDS, the waveguides were constructed from two Al plates with a length of 30 mm, separated by spacers defining a gap of 50 µm. THz pulses are coupled into and out of these waveguides via high-resistivity Si cylindrical lenses. The amplitude transmission of an empty Al waveguide (50 µm gap) can be as high as 20% at 1 THz, dropping to 5% at 4 THz. This, along with the lack of dispersion, results in a reasonably large signal over a very wide bandwidth. Similarly to the pellets, the waveguides were also cooled to 12 K.

Our instrument resolution is determined by the maximum temporal scan length before encountering a strong reflection, as shown in Fig. 3. For the pellet experiments, the first reflection at 30 ps is due to the pellet itself. However, this reflection is weak due to the low index n~1.5 for polyethylene and the additional double pass through the absorptive pellet, inducing only minor Fabry-Perot oscillations on the amplitude spectrum. The first strong reflection is due to the 10 mm thick Si windows (with the high index of n~3.4) of the vacuum chamber and occurs at 227 ps, corresponding to a frequency resolution of 4.4 GHz (0.15 cm-1). This is not a concern for the relatively broad linewidths observed with the pellet. Typically, scan lengths of 67 ps were used for the pellet measurements. For the waveguide, the first strong reflection is due to the 6.56 mm thick high-resistivity Si coupling lenses, and occurs at 150 ps, corresponding to a spectral resolution of 6.7 GHz (0.22 cm-1). Given the very narrow linewidths seen with the waveguide, this spectral resolution can be a limiting factor. The ringing after the main pulse is pronounced for the waveguide with measurable information continuing to the reflection at 150 ps, while the corresponding ringing for the pellet decays below the noise floor before the end of the scan. For most of the measurements, 4 scans were averaged in order to improve the signal to noise (S/N) ratio. The exception was the 2,4-DNT waveguide measurement at 11 K, where 12 scans were averaged. All scans were zero-padded before taking a numerical Fourier transform.

In the case of both the pellet and the waveguide, a reference Aref is estimated by fitting the amplitude spectra Aspec at points away from any sharp features with a spline. The amplitude absorbance is then calculated via the expression: Absorbance=-ln[Aspec/Aref].

Fig 3. Normalized THz pulses transmitted through 2,4-DNT pellet and waveguide at 11 K. Inset shows pulses trailing edges at times of 50 through 60 ps. THz pulses transmitted through waveguide are offset for clarity. Reflection at 30 ps is due to pellet. Reflections at 150 ps and at 227 ps are due to Si lenses and windows respectively.

3. Results and discussion

3.1 2,4-DNT

The 2,4-DNT pellet is comprised of 32 mg DNT and 330 mg of polyethylene mixed powders and is formed at a pressure of 11 metric tonnes. It has a diameter of 12.5 mm and a thickness of 3 mm. The amplitude spectra of the transmitted THz as a function of temperature is shown in Fig. 4. The features sharpen upon cooling, with only a minor change below 80 K, resulting in 7 observable features at 11 K. The features also blue shift during cooling, possibly due to the anharmonicity of the vibrational modes.

Table 1. Comparison of center line frequencies in THza of 2,4-DNT for this work and references 3, 6, 8 and 9.

table-icon
View This Table
Fig. 4. Amplitude Spectra of THz transmitted through 2,4-DNT pellet as a function of temperature. Spectra at 80 K and 293 K are offset for clarity.
Fig. 5. Amplitude spectra of THz transmitted through 2,4-DNT waveguide as a function of temperature. Spectra at 20 K through 293 K are offset for clarity. Inset shows detail of 11 K spectrum.

The 2,4-DNT waveguide film was formed by drop casting 100 µl of a 2.5 mg/ml acetone solution onto a polished Al plate. The relatively thick edges were removed by a solvent soaked swab, resulting in a visibly uniform film with an estimated mass of less than 100 µg, compared to 32 mg of DNT used for the pellet sample. The amplitude spectra of the transmitted THz through the waveguide as a function of temperature are shown in Fig. 5. Similarly to the pellet, the features both sharpen and blue-shift at lower temperatures. In sharp contrast to the pellet, there is substantial sharpening of the features at lower temperatures, particularly below 80 K. At 11 K, this sharpening reveals 19 observable lines. This is nearly 3 times the number of lines observed with the pellet and corresponds to a substantial increase in both the obtainable information about the vibrational modes of this material as well as the precision of their identification.

The amplitude absorbance of both the 2,4-DNT pellet and waveguide at 11 K is shown in Fig. 6. Using the waveguide, the doublet near 1.1 THz, the doublet near 1.5 THz and the broad feature near 1.9 THz has been resolved into four lines each, corresponding to a narrowing by a factor of 5, 10 and 20, respectively. In addition, the 0.58 and 0.68 THz lines are approximately 4 times narrower than for the pellet. Furthermore, lines at 0.88, 2.22, 2.48, 2.77, 3.02 and 3.46 THz are sufficiently sharp to be observed with the waveguide. Of the 19 lines, 11 have a measured linewidth of less than 15 GHz (0.5 cm-1). Given the instrument resolution of 6.7 GHz (0.22 cm-1), the intrinsic linewidths will be smaller.

Fig. 6. Normalized amplitude absorbance of 2,4-DNT waveguide at 11 K, 2,4-DNT pellet at 11 K and 293 K and 2,4-DNT pellet at 293 K for Ref 3, 6, 8 and 9. The pellet for this work contained 32 mg of 2,4-DNT, whereas the waveguide film contains less than 100 µg of 2,4-DNT. The estimated signal-to-noise ratio for the waveguide measurement is ~10:1 at 0.5 THz, ~70:1 at 1.2 THz and ~8:1 at 2.5 THz. The curves are normalized to unity with the exception of the 11 K waveguide data, which is normalized to two for better visibility.

The increased resolution of the waveguide spectrum compared to the pellet is due to the planar order of the polycrystalline film on the metal waveguide plate. An optical micrograph of the ordered polycrystalline film is shown in Fig 7. X-ray analysis of the film has revealed a strong orientation on the (001) plane. 2,4-DNT has been shown to crystallize in the P21/n space group with eight molecules per unit cell [25

25. W. C. McCrone and S.-M. Tsang, “Crystallographic Data. 88. 2,4-Dinitrotoluene,” Anal. Chem. 26, 1848–1849 (1954). [CrossRef]

]. Density functional theory of an isolated molecule [8

8. Y. Chen, H. Liu, Y. Deng, D. Schauki, M. J. Fitch, R. Osiander, C. Dodson, J. B. Spicer, M. Shur, and X.-C. Zhang, “THz spectroscopic investigation of 2,4-dinitrotoluene,” Chem. Phys. Lett. 400, 357–361 (2004). [CrossRef]

] has attributed the modes in this frequency range to phonons. There are 6B-3 phonon modes predicted for a unit cell containing B molecules, thus for 2,4-DNT 45 phonon modes are anticipated with 21 being infrared active. Furthermore, low frequency intramolecular vibrations (which may differ between a crystal and an isolated molecule) will also occur in the THz region. Given these material properties for 2,4-DNT, it is not surprising the 19 vibrational modes are revealed at low temperature for the waveguide film when inhomogeneous broadening effects are suppressed. It’s possible that high precision THz measurements such as provided here can help refine theoretical modeling [26

26. P. U. Jepsen and S. J. Clark, “Precise ab-initio prediction of terahertz vibrational modes in crystalline systems,” Chem. Phys. Lett. 442, 275–280 (2007). [CrossRef]

, 27

27. D. G. Allis, D. A. Prokhorova, and T. M. Korter, “Solid state modeling of the terahertz spectrum of the high explosive HMX,” J. Phys. Chem. A 110, 1951–199 (2006). [CrossRef] [PubMed]

] of THz vibrational spectra in the crystalline state.

Fig. 7. Optical micrograph of 2,4-DNT film on Al waveguide plate.

3.2 2,6-DNT

The 2,6-DNT pellet is comprised of 62 mg of DNT and 330 mg of polyethylene, while the 2,6-DNT waveguide film was formed by drop casting 100 µl of a 10 mg/ml acetone solution onto a polished Al plate. The estimated mass of the film on the waveguide plate is less than 1 mg. The amplitude spectra of the transmitted THz through the 2,6-DNT pellet as a function of temperature is seen in Fig. 8. Similarly to the 2,4-DNT pellet, there is minor sharpening of features upon cooling with very little change below 80 K.

The amplitude spectra of the transmitted THz through the 2,6-DNT waveguide as a function of temperature is seen in Fig. 9. Similarly to the 2,4-DNT waveguide, there is a large amount of sharpening upon cooling, particularly below 80 K. Unlike the 2,4-DNT waveguide, there is no additional splitting of the features, with a total of six lines observed at 13 K. The broad, temperature independent feature at 1.8 THz is considered to be due to the waveguide itself and not to the 2,6-DNT.

Fig. 8. Amplitude spectra of THz transmitted through 2,6-DNT pellet as a function of temperature. Spectra at 80 K and 293 K are offset for clarity
Fig. 9. Amplitude spectra of THz transmitted through 2,6-DNT waveguide as a function of temperature. Spectra at 40 K through 293 K are offset for clarity.

Fig. 10. Normalized amplitude absorbance of 2,6-DNT waveguide at 12 K, 2,6-DNT pellet at 13 K and 293 K and 2,6-DNT pellet at 293 K for Ref 3, 6 and 7. The estimated signal-to-noise ratio for the waveguide measurement is ~40:1 at 1.18 and 1.68 THz and ~20:1 at 2.10 THz. The curves are normalized to unity with the exception of the 12 K waveguide data, which is normalized to two for better visibility.

The second 2,6-DNT line (1.35 THz at 293 K) has been attributed [7

7. Y. Chen, H. Liu, and X.-C. Zhang, “THz spectra of 4-NT and 2,6-DNT,” Proc. SPIE 6212, 62120P (2006). [CrossRef]

] to the torsion of the NO2 groups, while the other lines have been attributed to phonon modes. The observation of fewer spectral features for 2,6-DNT compared to 2,4-DNT may be partly due to its crystallization in a different space group, P212121 [28

28. J.-J. Nie, D.-J. Xu, Z.-Y. Li, and M. Y. Chaing, “2,6-Dinitrotoluene,” Acta. Crystallogr. E57, o827 (2001).

], with four molecules per unit cell. There are 21 phonon modes predicted for this simpler crystal structure, only 9 of which are infrared active.

4. Conclusions

Fig. 11. Center line frequency and FWHM linewidth for the strongest line in 2,6-DNT pellet and waveguide. The dashed line corresponds to the 6.7 GHz instrument resolution.

The waveguide spectrum of 2,6-DNT also sharpens significantly under cooling, with all six observed lines having a linewidth of less than 15 GHz at 13 K. The pellet spectrum undergoes only minor sharpening under cooling. The median linewidth of the waveguide spectra is approximately 7 times sharper compared to the pellet.

In addition, the technique of waveguide THz-TDS is approximately 100 times more sensitive than conventional THz-TDS with a pellet. This allows one to use considerably less material. This can be useful when measuring dangerous materials such as explosives.

Acknowledgments

References and links

1.

M. C. Kemp, P. F. Taday, B. E. Cole, J. A. Cluff, A. J. Fitzgerald, and W. R. Tribe, “Security applications of terahertz technology,” Proc. SPIE 5070, 44–52 (2003). [CrossRef]

2.

J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security applications - explosives, weapons and drugs,” Semicond. Sci. Technol. 20, S266–S280 (2005). [CrossRef]

3.

H.-B. Liu, H. Zhong, N. Karpowicz, Y. Chen, and X.-C. Zhang, “Terahertz spectroscopy and imaging for defense and security applications,” Proc. IEEE 95, 1514–1527 (2007). [CrossRef]

4.

K. Yamamoto, M. Yamaguchi, F. Miyamaru, M. Tani, M. Hangyo, T. Ikeda, A. Matsushita, K. Koide, M. Tatsuno, and Y. Minami, “Noninvasive inspection of C-4 explosive in mails by terahertz time-domain spectroscopy,” Jpn. J. Appl. Phys. 43, L414–417 (2004). [CrossRef]

5.

T. Lo, I. S. Gregory, C. Baker, P. F. Taday, W. R. Tribe, and M. C. Kemp, “The very far-infrared spectra of energetic materials and possible confusion materials using terahertz pulsed spectroscopy,” Vib. Spectrosc. 42, 243–248 (2006). [CrossRef]

6.

J. Chen, Y. Chen, H. Zhao, G. J. Bastiaans, and X.-C. Zhang, “Absorption coefficients of selected explosives and related compounds in the range of 0.1-2.8 THz,” Opt. Express 15, 12060 (2007). [CrossRef] [PubMed]

7.

Y. Chen, H. Liu, and X.-C. Zhang, “THz spectra of 4-NT and 2,6-DNT,” Proc. SPIE 6212, 62120P (2006). [CrossRef]

8.

Y. Chen, H. Liu, Y. Deng, D. Schauki, M. J. Fitch, R. Osiander, C. Dodson, J. B. Spicer, M. Shur, and X.-C. Zhang, “THz spectroscopic investigation of 2,4-dinitrotoluene,” Chem. Phys. Lett. 400, 357–361 (2004). [CrossRef]

9.

Y. Hu, P. Huang, L. Guo, X. Wang, and C. Zhang, “Terahertz spectroscopic investigations of explosives,” Phys. Lett. A 359, 728–732 (2006). [CrossRef]

10.

M. R. Leahy-Hoppa, M. J. Fitch, X. Zheng, L. M. Hayden, and R. Osiander, “Wideband terahertz spectroscopy of explosives,” Chem. Phys. Lett. 434, 227–230 (2007). [CrossRef]

11.

A.D. Burnett, W.H. Fan, P.C. Upadhya, J.E. Cunningham, H.G.M. Edwards, J. Kendrick, T. Munshi, M. Hargreaves, E.H. Linfield, and A.G. Davies, “Broadband terahertz time-domain and Raman spectroscopy of explosives,” Proc. SPIE 6549, 654905 (2007). [CrossRef]

12.

W. H. Fan, A. Burnett, P. C. Upadhya, J. Cunningham, E. H. Linfield, and A. G. Davies, “Far-infrared spectroscopic characterization of explosives for security applications using broadband terahertz time-domain spectroscopy,” Appl. Spectrosc. 61, 638–643 (2007). [CrossRef] [PubMed]

13.

J. Barber, D. E. Hooks, D. J. Funk, R. D. Averitt, A. J. Taylor, and D. Babikov, “Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy,” J. Phys. Chem. A 109, 3501–3505 (2005). [CrossRef]

14.

R. J. Foltynowicz, R. E. Allman, and E. Zuckerman, “Terahertz absorption measurement for gas-phase 2,4-dinitrotoluene from 0.05 THz to 2.7 THz,” Chem. Phys. Lett. 431, 34–38 (2006). [CrossRef]

15.

J. Zhang and D. Grischkowsky, “Waveguide THz time-domain spectroscopy of nm water layers,” Opt. Lett. 19, 1617–1619 (2004). [CrossRef]

16.

J. S. Melinger, N. Laman, S. S. Harsha, and D. Grischkowsky, “Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide,” Appl. Phys. Lett. 89, 251110 (2006). [CrossRef]

17.

R. Mendis and D. Grischkowsky, “Undistorted guided wave propagation of sub-picosecond THz pulses,” Opt. Lett. 26, 846–848 (2001). [CrossRef]

18.

G. Gallot, S. P. Jamsion, R. W. McGowan, and D. Grischkowsky, “THz Waveguides,” J. Opt. Soc. Am. B 17, 851–863 (2000). [CrossRef]

19.

J. S. Melinger, N. Laman, S. S. Harsha, S. Cheng, and D. Grischkowsky, “High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films,” J. Phys. Chem. A 111, 10977–10987 (2007). [CrossRef] [PubMed]

20.

N. Laman, S. S. Harsha, D. Grischkowsky, and J. S. Melinger, “High resolution waveguide THz spectroscopy of biological molecules,” Biophys. J. 94, 1010–1020 (2008). [CrossRef]

21.

N. Laman, S. S. Harsha, and D. Grischkowsky, “Narrow-line waveguide terahertz time-domain spectroscopy of aspirin and aspirin precursors,” Appl. Spectrosc. 62, 319–326 (2008). [CrossRef] [PubMed]

22.

J.P. Pinan, R. Ouillon, P. Ranson, M. Becucci, and S. Califano, “High resolution Raman study of phonon and vibron bandwidths in isotopically pure and natural benzene crystal,” J. Chem. Phys. 109, 5469–5480 (1998). [CrossRef]

23.

D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006–2015 (1990). [CrossRef]

24.

M. van Exter and D. Grischkowsky, “Characterization of an optoelectronic terahertz beam system,” IEEE Trans. Microwave Theory Tech. 38, 1684–1691 (1990). [CrossRef]

25.

W. C. McCrone and S.-M. Tsang, “Crystallographic Data. 88. 2,4-Dinitrotoluene,” Anal. Chem. 26, 1848–1849 (1954). [CrossRef]

26.

P. U. Jepsen and S. J. Clark, “Precise ab-initio prediction of terahertz vibrational modes in crystalline systems,” Chem. Phys. Lett. 442, 275–280 (2007). [CrossRef]

27.

D. G. Allis, D. A. Prokhorova, and T. M. Korter, “Solid state modeling of the terahertz spectrum of the high explosive HMX,” J. Phys. Chem. A 110, 1951–199 (2006). [CrossRef] [PubMed]

28.

J.-J. Nie, D.-J. Xu, Z.-Y. Li, and M. Y. Chaing, “2,6-Dinitrotoluene,” Acta. Crystallogr. E57, o827 (2001).

OCIS Codes
(130.2790) Integrated optics : Guided waves
(160.4890) Materials : Organic materials
(300.3700) Spectroscopy : Linewidth
(300.6495) Spectroscopy : Spectroscopy, teraherz

ToC Category:
Spectroscopy

History
Original Manuscript: February 6, 2008
Revised Manuscript: March 7, 2008
Manuscript Accepted: March 7, 2008
Published: March 11, 2008

Citation
N. Laman, S. Sree Harsha, D. Grischkowsky, and Joseph S. Melinger, "7 GHz resolution waveguide THz spectroscopy of explosives related solids showing new features," Opt. Express 16, 4094-4105 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-6-4094


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. M. C. Kemp, P. F. Taday, B. E. Cole, J. A. Cluff, A. J. Fitzgerald, and W. R. Tribe, "Security applications of terahertz technology," Proc. SPIE 5070, 44-52 (2003). [CrossRef]
  2. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, "THz imaging and sensing for security applications - explosives, weapons and drugs," Semicond. Sci. Technol. 20, S266-S280 (2005). [CrossRef]
  3. H.-B. Liu, H. Zhong, N. Karpowicz, Y. Chen, X.-C. Zhang, "Terahertz spectroscopy and imaging for defense and security applications," Proc. IEEE 95, 1514-1527 (2007). [CrossRef]
  4. K. Yamamoto, M. Yamaguchi, F. Miyamaru, M. Tani, M. Hangyo, T. Ikeda, A. Matsushita, K. Koide, M. Tatsuno, and Y. Minami, "Noninvasive inspection of C-4 explosive in mails by terahertz time-domain spectroscopy," Jpn. J. Appl. Phys. 43, L414-417 (2004). [CrossRef]
  5. T. Lo, I. S. Gregory, C. Baker, P. F. Taday, W. R. Tribe, and M. C. Kemp, "The very far-infrared spectra of energetic materials and possible confusion materials using terahertz pulsed spectroscopy," Vib. Spectrosc. 42,243-248 (2006). [CrossRef]
  6. J. Chen, Y. Chen, H. Zhao, G. J. Bastiaans, and X.-C. Zhang, "Absorption coefficients of selected explosives and related compounds in the range of 0.1-2.8 THz," Opt. Express 15, 12060 (2007). [CrossRef] [PubMed]
  7. Y. Chen, H. Liu, and X.-C. Zhang, "THz spectra of 4-NT and 2,6-DNT," Proc. SPIE 6212, 62120P (2006). [CrossRef]
  8. Y. Chen, H. Liu, Y. Deng, D. Schauki, M. J. Fitch, R. Osiander, C. Dodson, J. B. Spicer, M. Shur, and X.-C. Zhang, "THz spectroscopic investigation of 2,4-dinitrotoluene," Chem. Phys. Lett. 400, 357-361 (2004). [CrossRef]
  9. Y. Hu, P. Huang, L. Guo, X. Wang, C. Zhang, "Terahertz spectroscopic investigations of explosives," Phys. Lett. A 359, 728-732 (2006). [CrossRef]
  10. M. R. Leahy-Hoppa, M. J. Fitch, X. Zheng, L. M. Hayden,and R. Osiander, "Wideband terahertz spectroscopy of explosives," Chem. Phys. Lett. 434, 227-230 (2007). [CrossRef]
  11. A.D. Burnett, W.H. Fan, P.C. Upadhya, J.E. Cunningham, H.G.M. Edwards, J. Kendrick, T. Munshi, M. Hargreaves, E.H. Linfield, A.G. Davies, "Broadband terahertz time-domain and Raman spectroscopy of explosives," Proc. SPIE 6549, 654905 (2007). [CrossRef]
  12. W. H. Fan, A. Burnett, P. C. Upadhya, J. Cunningham, E. H. Linfield, and A. G. Davies, "Far-infrared spectroscopic characterization of explosives for security applications using broadband terahertz time-domain spectroscopy," Appl. Spectrosc. 61, 638-643 (2007). [CrossRef] [PubMed]
  13. J. Barber, D. E. Hooks, D. J. Funk, R. D. Averitt, A. J. Taylor, D. Babikov, "Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy," J. Phys. Chem. A 109, 3501-3505 (2005). [CrossRef]
  14. R. J. Foltynowicz, R. E. Allman, and E. Zuckerman, "Terahertz absorption measurement for gas-phase 2,4-dinitrotoluene from 0.05 THz to 2.7 THz," Chem. Phys. Lett. 431, 34-38 (2006). [CrossRef]
  15. J. Zhang and D. Grischkowsky, "Waveguide THz time-domain spectroscopy of nm water layers," Opt. Lett. 19, 1617-1619 (2004). [CrossRef]
  16. J. S. Melinger, N. Laman, S. S. Harsha, and D. Grischkowsky, "Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide," Appl. Phys. Lett. 89, 251110 (2006). [CrossRef]
  17. R. Mendis and D. Grischkowsky, "Undistorted guided wave propagation of sub-picosecond THz pulses," Opt. Lett. 26, 846-848 (2001). [CrossRef]
  18. G. Gallot, S. P. Jamsion, R. W. McGowan, and D. Grischkowsky, "THz Waveguides," J. Opt. Soc. Am. B 17, 851-863 (2000). [CrossRef]
  19. J. S. Melinger, N. Laman, S. S. Harsha, S. Cheng, and D. Grischkowsky, "High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films," J. Phys. Chem. A 111, 10977-10987 (2007). [CrossRef] [PubMed]
  20. N. Laman, S. S. Harsha, D. Grischkowsky, and J. S. Melinger, "High resolution waveguide THz spectroscopy of biological molecules," Biophys. J. 94, 1010-1020 (2008). [CrossRef]
  21. N. Laman, S. S. Harsha, and D. Grischkowsky, "Narrow-line waveguide terahertz time-domain spectroscopy of aspirin and aspirin precursors," Appl. Spectrosc. 62, 319-326 (2008). [CrossRef] [PubMed]
  22. J.P. Pinan, R. Ouillon, P. Ranson, M. Becucci, S. Califano, "High resolution Raman study of phonon and vibron bandwidths in isotopically pure and natural benzene crystal," J. Chem. Phys. 109, 5469-5480 (1998). [CrossRef]
  23. D. Grischkowsky, S. Keiding, M. van Exter, Ch. Fattinger, "Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors," J. Opt. Soc. Am. B 7, 2006-2015 (1990). [CrossRef]
  24. M. van Exter and D. Grischkowsky, "Characterization of an optoelectronic terahertz beam system," IEEE Trans. Microwave Theory Tech. 38, 1684-1691 (1990). [CrossRef]
  25. W. C. McCrone and S.-M. Tsang, "Crystallographic Data. 88. 2,4-Dinitrotoluene," Anal. Chem. 26, 1848-1849 (1954). [CrossRef]
  26. P. U. Jepsen and S. J. Clark, "Precise ab-initio prediction of terahertz vibrational modes in crystalline systems," Chem. Phys. Lett. 442, 275-280 (2007). [CrossRef]
  27. D. G. Allis, D. A. Prokhorova, and T. M. Korter, "Solid state modeling of the terahertz spectrum of the high explosive HMX," J. Phys. Chem. A 110, 1951-199 (2006). [CrossRef] [PubMed]
  28. J.-J. Nie, D.-J. Xu, Z.-Y. Li, and M. Y. Chaing, "2,6-Dinitrotoluene," Acta. Crystallogr. E57, o827 (2001).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited