THZ TIME DOMAIN SPECTROSCOPY
Background:
Terahertz (THz) spectroscopy has been developed to a versatile research topic in recent years primarily due to the availability of various high performance THz generators and THz detectors thereby almost vanishing the term “THz Gap” and secondarily due to its applicability in a broad area of science ranging from astronomy, communication, security systems to medical imaging, optical characterization of different semiconductors and biomolecules, dielectric relaxation studies of restricted water etc (1-5).
The so-called “THz gap” arises from the nature of the sources and detectors used in spectroscopy both at the optical (high-frequency) side and electronic (low-frequency) side of the gap. One terahertz corresponds to photon energy of 4.1 meV (33.3 cm–1). These energies are much less than the electronic state transitions of atoms and molecules commonly used in lasers or other light sources. Scaling electronics to operate in the THz gap is limited by the frequency response of devices. So, people have combined optics and semiconductor physics gets to produce effective THz generators and detectors.
The so-called “THz gap” arises from the nature of the sources and detectors used in spectroscopy both at the optical (high-frequency) side and electronic (low-frequency) side of the gap. One terahertz corresponds to photon energy of 4.1 meV (33.3 cm–1). These energies are much less than the electronic state transitions of atoms and molecules commonly used in lasers or other light sources. Scaling electronics to operate in the THz gap is limited by the frequency response of devices. So, people have combined optics and semiconductor physics gets to produce effective THz generators and detectors.
Some important characteristics of THz radiation and THz spectroscopy are listed below:
- THz radiation can be transmitted reasonable distances through air, many plastics, cardboard, paper, clothing, and many other materials, with the notable exceptions of metals and water and 1THz corresponds to 4.1 meV energy which is pretty safe for human body. THz radiation is therefore useful for imaging applications.
- In THz spectroscopy we measure both the amplitude and phase of THz pulse simultaneously in a coherent manner. So, both the real and imaginary response function of the systems can be retrieved without using the completed Kramers–Kronig relations (KK) Analysis.
- THz-TDS is a useful technique to measure the complex optical properties (i.e. complex refractive index, complex static and photoexcited a.c. conductivity) of a sample in a non-contact, non-invasive manner, which is extremely advantageous in case of measuring nanostructured samples (5).
Instrument:
The THz measurements are performed in a commercial THz spectrophotometer (TERA K8, Menlo Systems). A 780 nm Er doped fiber laser having pulse width of <100 fs and a repetition rate of 100 MHz is divided into pump and probe beams (of equal intensity; 10 mW) by a polarizing beam splitter. The pump beam passes through a /4 plate and reflects from a mirror mounted on a motorized delay stage and excites the THz emitter antenna producing a THz radiation having a bandwidth up to 2.5 THz (> 60dB). This THz radiation is focused on the sample
guided through polymer lenses. The radiation transmitted through the sample was
then focused on a THz detector antenna which is gated by the probe laser beam.
The THz antennas are gold dipoles with a dipole gap of 5 m deposited on
LT-GaAs substrate. To avoid water vapour absorption, all
the measurements are carried out in dry nitrogen atmosphere with a controlled
humidity of <10% at 293 K.
WORK:
As the importance of THz spectroscopy is continuously increasing with more number of scientists getting involved in this rapidly growing field, the demand of compact, easy to use, reliable THz manipulators i.e. THz polarizers, THz frequency selective devices, THz shielding devices are also continuously rising.
Keeping this in mind we have devoted ourselves in the study of potential THz polarizers and THz shielding devices.
WORK:
As the importance of THz spectroscopy is continuously increasing with more number of scientists getting involved in this rapidly growing field, the demand of compact, easy to use, reliable THz manipulators i.e. THz polarizers, THz frequency selective devices, THz shielding devices are also continuously rising.
Keeping this in mind we have devoted ourselves in the study of potential THz polarizers and THz shielding devices.
THz Polarizer:
People are studying the use of patterned metal wire like structures on top of THz transparent substrates like highly doped silicon, MgO, Quartz as THz polarizers based on their selective reflection properties (6). The polarizibility of these structures depends strongly on its geometry (i.e. width of metal wire, pitch). But, it is quite expensive to prepare such structures in a large area by photolithography or nanoimprinting technique. Another kind of THz polarizers based on the intrinsic absorption properties of aligned carbon nanotubes (CNT’s) have drawn considerable interest in recent past especially after the pioneering work of Kono et. al. (7-9). We have prepared a unique THz polarizer by aligning chemically synthesized Nickel Nanoparticles in external magnetic field inside a polymer matrix. Our sample is basically a metal wire-grid polarizer showing considerable amount of orientational anisotropy when placed in path of a linearly polarized THz beam. We will further try to control the pitch of the structure by manipulating external conditions and thereby improving the result. We will also prepare this kind of samples with magnetic nanowires instead of nanoparticles to observe the effect intrinsic shape anisotropy of nanowires.
People are studying the use of patterned metal wire like structures on top of THz transparent substrates like highly doped silicon, MgO, Quartz as THz polarizers based on their selective reflection properties (6). The polarizibility of these structures depends strongly on its geometry (i.e. width of metal wire, pitch). But, it is quite expensive to prepare such structures in a large area by photolithography or nanoimprinting technique. Another kind of THz polarizers based on the intrinsic absorption properties of aligned carbon nanotubes (CNT’s) have drawn considerable interest in recent past especially after the pioneering work of Kono et. al. (7-9). We have prepared a unique THz polarizer by aligning chemically synthesized Nickel Nanoparticles in external magnetic field inside a polymer matrix. Our sample is basically a metal wire-grid polarizer showing considerable amount of orientational anisotropy when placed in path of a linearly polarized THz beam. We will further try to control the pitch of the structure by manipulating external conditions and thereby improving the result. We will also prepare this kind of samples with magnetic nanowires instead of nanoparticles to observe the effect intrinsic shape anisotropy of nanowires.
THz Shielding Material:
People have studied the effect of CNT’s as a shielding material due to its extremely high aspect ratio which lowers the percolation limit and its high conductivity in MHz and GHz frequency range (10). Recently with the awesome advancement in THz spectroscopy people are trying to extend this work in THz frequency range (11-12).
We have dispersed commercially available CNT in a polymer matrix with different volume fractions and studied their effect as a potential THz shielding device. Transmitted THz pulse through the sample gets continuously decreased with increasing CNT volume fraction and THz transmission effectively goes to zero as shown in figure 4. We will study THz shielding properties by varying the aspect ratio of CNTs inside polymer matrices to observe the effect of percolation prominently.
People have studied the effect of CNT’s as a shielding material due to its extremely high aspect ratio which lowers the percolation limit and its high conductivity in MHz and GHz frequency range (10). Recently with the awesome advancement in THz spectroscopy people are trying to extend this work in THz frequency range (11-12).
We have dispersed commercially available CNT in a polymer matrix with different volume fractions and studied their effect as a potential THz shielding device. Transmitted THz pulse through the sample gets continuously decreased with increasing CNT volume fraction and THz transmission effectively goes to zero as shown in figure 4. We will study THz shielding properties by varying the aspect ratio of CNTs inside polymer matrices to observe the effect of percolation prominently.
References:
1. M. Tonouchi, "Cutting-edge terahertz technology," Nature Photonics 1, 97 - 105 (2007).
2. C. Kulesa, "Terahertz Spectroscopy for Astronomy: From Comets to Cosmology," IEEE Tr. THz. Sci. Technol. 1, 232-240 (2011).
3. D. F. Plusquellic, K. Siegrist, E. J. Heilweil, and O. Esenturk, "Applications of Terahertz Spectroscopy in Biosystems," ChemPhysChem 8, 2412-2431 (2007).
4. B. B. Hu, and M. C. Nuss, "Imaging with terahertz waves," Opt. Lett. 16, 1716-1718 (1995).
5. M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, "Terahertz Spectroscopy," J. Phys. Chem. B 106, 7146-7159 (2002).
6. I. Yamada, K. Takano, M. Hangyo, M. Saito, and W. Watanabe, "Terahertz wire-grid polarizers with micrometer-pitch Al gratings," Optics Letters 34, 274-276 (2009).
7. L. Ren, C. Pint, L. Booshehri, W. Rice, X. Wang, D. Hilton, K. Takeya, I. Kawayama, M. Tonouchi, R. Hauge, and J. Kono, "Carbon nanotube terahertz polarizer," Nano Lett., 2610-2613 (2009).
8. L. Ren, C. L. Pint, T. Arikawa, K. Takeya, I. Kawayama, M. Tonouchi, R. H. Hauge, and J. Kono, "Broadband Terahertz Polarizers with Ideal Performance Based on Aligned Carbon Nanotubes," Nano Lett. 12, 787-790 (2012).
9. J. Kyoung, E. Y. Jang, M. D. Lima, H.-R. Park, R. O. Robles, X. Lepro, Y. H. Kim, R. H. Baughman, and D.-S. Kim, "A reel-wound carbon nanotube polarizer for terahertz frequencies," Nano Lett. 11, 4227-4231 (2011).
10. N. Li, Y. Huang, F. Du, X. He, x. Lin, H. Gao, Y. Ma, F. Li, Y. Chen and P. C. Eklund, “ Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites”, Nano Lett. 6, 1141-1145 (2006).
11. M. A. Seo, J. H. Yim, Y. H. Ahn, F. Rotermund and D. S. Kim, “ Terahertz electromagnetic interference shielding using single walled carbon nanotube flexible films”, Appl. Phys. Lett. 93, 231905_1 -231905_3 (2008).
12. A. Das, C. M. Megaridis, L. Lui, T. Wang, and A. Biswas, “ Design and synthesis of superhydrofobic carbon nanofiber composite coatings for terahertz frequency shielding and attenuation”, Appl. Phys. Lett. 98, 174101_1 – 174101_3 (2011).
1. M. Tonouchi, "Cutting-edge terahertz technology," Nature Photonics 1, 97 - 105 (2007).
2. C. Kulesa, "Terahertz Spectroscopy for Astronomy: From Comets to Cosmology," IEEE Tr. THz. Sci. Technol. 1, 232-240 (2011).
3. D. F. Plusquellic, K. Siegrist, E. J. Heilweil, and O. Esenturk, "Applications of Terahertz Spectroscopy in Biosystems," ChemPhysChem 8, 2412-2431 (2007).
4. B. B. Hu, and M. C. Nuss, "Imaging with terahertz waves," Opt. Lett. 16, 1716-1718 (1995).
5. M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, "Terahertz Spectroscopy," J. Phys. Chem. B 106, 7146-7159 (2002).
6. I. Yamada, K. Takano, M. Hangyo, M. Saito, and W. Watanabe, "Terahertz wire-grid polarizers with micrometer-pitch Al gratings," Optics Letters 34, 274-276 (2009).
7. L. Ren, C. Pint, L. Booshehri, W. Rice, X. Wang, D. Hilton, K. Takeya, I. Kawayama, M. Tonouchi, R. Hauge, and J. Kono, "Carbon nanotube terahertz polarizer," Nano Lett., 2610-2613 (2009).
8. L. Ren, C. L. Pint, T. Arikawa, K. Takeya, I. Kawayama, M. Tonouchi, R. H. Hauge, and J. Kono, "Broadband Terahertz Polarizers with Ideal Performance Based on Aligned Carbon Nanotubes," Nano Lett. 12, 787-790 (2012).
9. J. Kyoung, E. Y. Jang, M. D. Lima, H.-R. Park, R. O. Robles, X. Lepro, Y. H. Kim, R. H. Baughman, and D.-S. Kim, "A reel-wound carbon nanotube polarizer for terahertz frequencies," Nano Lett. 11, 4227-4231 (2011).
10. N. Li, Y. Huang, F. Du, X. He, x. Lin, H. Gao, Y. Ma, F. Li, Y. Chen and P. C. Eklund, “ Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites”, Nano Lett. 6, 1141-1145 (2006).
11. M. A. Seo, J. H. Yim, Y. H. Ahn, F. Rotermund and D. S. Kim, “ Terahertz electromagnetic interference shielding using single walled carbon nanotube flexible films”, Appl. Phys. Lett. 93, 231905_1 -231905_3 (2008).
12. A. Das, C. M. Megaridis, L. Lui, T. Wang, and A. Biswas, “ Design and synthesis of superhydrofobic carbon nanofiber composite coatings for terahertz frequency shielding and attenuation”, Appl. Phys. Lett. 98, 174101_1 – 174101_3 (2011).