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Electrical and optical properties of polyacetylene film in THz frequency range
Current Applied Physics 5 (2005) 289–292 www.elsevier.com/locate/cap
Electrical and optical properties of polyacetylene film in THz frequency range q Tae-In Jeon a
a,*
, Geun-Ju Kim a, Hyun-Jung Lee b, Ju-Yul Lee b, Yung Woo Park
b
Division of Electrical and Electronics Engineering, Korea Maritime University, Busan 606-791, South Korea b Department of Physics, Seoul National University, Seoul 151-747, South Korea Received 20 November 2003; accepted 30 January 2004 Available online 24 March 2004
Abstract Electrical and optical properties such as power absorption, index of refraction, and complex conductivity of polyacetylene film are determined by transmission measurement using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation and without invoking the Kramers–Kronig relationships. Four types of polyacetylene samples, undoped pristine, FeCl 4 doped pristine, undoped and stretched twice, and FeCl 4 doped and stretched twice, are compared experimentally. The undoped pristine polyacetylene is found to have a resonance of 2.45 THz. The stretched polyacetylene have an angle-dependent curve along the direction of the polarized THz beam. Ó 2004 Elsevier B.V. All rights reserved. PACS: 72.80.Le; 78.66.Qn Keywords: THz time-domain spectroscopy; polyacetylene; FeCl 4 doped
1. Introduction Infrared properties of the conducting polymers have been studied extensively [1,2]. Because of the high absorption in infrared frequency range, the Fourier transform spectroscopy reflection measurement is used to investigate the charge transport properties of the conducting polymers [1,2]. These reflection measurements require the Kramers–Kronig analysis to calculate the absorption and dispersion of the samples. The transmission measurement of the THz time-domain spectroscopy (THz-TDS) has recently been shown as the ideal tool to characterize the carrier dynamics in semiconductors [3,4] and conducting polymers [5,6]. Both the absorption and dispersion of the polyacetylene-conducting polymer films are directly measured in this study. High-density polyacetylene films are synthesized following the modified Shirakawa method [7] as described
q
Original version presented at QTSM&QFS 2003 (Quantum Transport Synthetic Metals & Quantum Functional Semiconductors), Seoul National University, Seoul, Korea, 20–22 November 2003. * Corresponding author. E-mail address: [email protected] (T.-I. Jeon). 1567-1739/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2004.01.014
in previous studies [8,9]. The polyacetylene films are oriented by stretching with l=l0 ¼ 2. FeCl4 -doping is performed up to 1–2 molar % in nitromethane/FeCl 4 solution. The thickness of the films is around a few tenths of microns. Four types of samples are measured: undoped pristine, FeCl 4 doped pristine, undoped and stretched twice, and FeCl 4 doped and stretched twice. These samples are placed within the THz-TDS system. For the stretched sample measurement, the samples are rotated each 30° from perpendicular (90°) to parallel (0°) direction to the THz beam polarization. 2. Experiment THz-TDS was based on the optoelectronic generation and reception of a beam of subpicosecond THz pulses. High-performance optoelectronic source was used to generate and detect the short pulses of THz radiation [10]. A GaAs transmission antenna and a silicon on sapphire (SOS) receiving antenna were both optoelectrically driven by an average power of 10 mW from a mode-locked Ti:sapphire laser. To eliminate the effects of water vapor on the THz beam [10], the THz system
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T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292 100
(a) Amplitude (a.u)
Current (nA)
1.0 0.5 0.0 -0.5 -1.0
(c)
80 60 40 20 0 1.5
(b) Amplitude (a.u)
Current (pA)
3.0
1.5
0.0
-1.5
0
5
10
15
Time (psec)
1.0
0.5
0.0
30
0
1
90
o
60
o
(d)
o
2
3
Frequency (THz)
Fig. 1. (a) Measured reference THz pulse. (b) Measured output THz pulse transmitted through FeCl4 doped two times stretched sample, which is located 30° from THz beam polarization. (c) Amplitude spectrum of reference pulse. (d) Amplitude spectra of output pulses; each angles mean the angle between stretched sample direction and THz beam polarization.
was placed in an airtight dry box. THz-TDS characterizations were performed by first measuring the transmitted pulses with no sample (reference) in place, as shown in Fig. 1(a), which was the average of the four individual measurements that increased the signal-tonoise ratio (S=N ) up to 15000:1. The freely propagating THz electromagnetic pulses were then compared to the measured THz pulses (output) transmitted through the thin polymer film under investigation as shown in Fig. 1(b), which was also the average of the four individual measurements with the S=N of 40:1. In this case, the sample was FeCl 4 doped and stretched twice, the most conducting material in the measurement. The normalized amplitude spectrum of the reference pulse is shown as Fig. 1(c). The output spectrum of Fig. 1(d) was normalized with respect to the reference spectrum of Fig. 1(c). Only 0.25% of the reference spectrum at 1THz was in existence. A very large frequency-dependent absorption of the sample was clearly evident.
3. Measurement The frequency-dependent absorption and dispersion of the sample are obtained using the Fourier analysis of the reference and output pulses without using the Kramers–Kronig relationship. The real index nr is obtained by the phase difference between the reference and output pulses. The frequency-dependent complex dielectric constant e of the sample is equal to the square of the complex index of refraction n ¼ nr þ ini . The imaginary index ni is determined by the power absorption coefficient ni ¼ ak=4p, where k is the free-space wavelength. The complex ratio of the output amplitude spectrum,
Eo ðxÞ, and the reference amplitude spectrum, Er ðxÞ, are simplified by Eo ðxÞ=Er ðxÞ ¼ tðxÞ exp½ibo ðxÞ;
ð1Þ
where the term bo ¼ 2pL=ko accounts for the air space displaced by the sample of length L. For the experiment with a highly absorbing sample such as FeCl 4 doped polymer, the multiple reflection of the THz pulse from the two phases of the sample may be neglected. For a relatively small total absorption and a thin sample such as undoped pristine film, however, the multiple reflection of the THz pulse occurs between the two surfaces of the sample. In this situation, the frequency-dependent complex amplitude transmission tðxÞ is given by [11] tðxÞ ¼ t12 t21 expðaL=2Þ expðibÞ=½1 þ r12 r21 expðaLÞ expði2bÞ;
ð2Þ
t12 and r12 are the complex Fresnel transmission and reflection coefficients from air into sample; t21 and r21 are the transmission and reflection coefficients from sample into air; the propagation vector is b ¼ 2pnr L=ko . Using the Fabry–Perot analysis of Eq. (2), the multiple reflection effects from the measured data of undoped pristine sample can be numerically removed. Therefore, the frequency dependent power absorption aðxÞ from the magnitude ratio and the real part of the index of refraction nr ðxÞ from phase difference of the spectra can be determined. The effective dielectric response for polymer is described by the following general relationship: 2
eðxÞ ¼ e1 poly þ irðxÞ=ðxe0 Þ ¼ ðnr þ ini Þ ;
ð3Þ
where e1 poly is the dielectric constant of the undoped polymer, rðxÞ is the complex conductivity, and eo is the
T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292
free-space permittivity. The real conductivity can be obtained from rReal ðxÞ ¼ 2xe0 nr ni .
4. Discussion The power absorption of undoped pristine sample shown in Fig. 2(a) is from 0.5 to 3.5 THz frequency range. The power absorption is very small in the low frequency range and increases rapidly up to 2.45 THz. The power absorption decreases with increased frequency after 2.45 THz. Therefore, the resonance of power absorption is in the measured frequency. This situation also occurs in the real part of the conductivity because of the small change in the index of refraction in the measured frequency range, as shown in Fig. 2(b) and (c). The real part of the conductivity does not follow the simple Drude model behavior, which has a Lorentzian line shape centered at zero frequency. Usually, metal and semiconductor treat the free carriers in a solid as classical point charges subject to random collisions and
yield the complex conductivity. The resonance absorption is not clear at this moment. Because the power absorption of the FeCl 4 doped polyacetylene pristine sample is extremely high, the high frequency range of the output THz pulse is almost absorbed. Therefore, the spectrum range is useful up to 2.2 THz, as shown in Fig. 3(a)–(c). Because of the high power absorption compared with Fig. 2(a), the resonance from the undoped pristine sample is not shown in Fig. 3(a). Likewise, the index of refraction dramatically decreases with increasing frequency, as shown in Fig. 3(b) which shows almost constant real conductivity around 120 (1=X cm) in the measured frequency range. Unlike undoped pristine sample, this follows the Drude model. These magnitudes are similar to the conducting polypyrrole sample [5] in which the plasma frequency and damping rate is within 70 and 12.6 THz, respectively. The twice-stretched samples along the chain orientation clearly exhibit the angle-dependent characteristic behavior. Because the reference and output THz pulses 6000
100
50
4000 3000 2000 1000 0
2.2
20
2.1
2.0
1.9
(b) 15
10
5
1.8
150
(c) 0.6
0.4
0.2
0
1
2
3
Frequency (THz) Fig. 2. Measurement for undoped pristine. (a) Power absorption coefficient, (b) index of refraction and (c) real conductivity.
Real Conductivity( 1/Ωcm)
0.8
Real Conductivity (1/Ωcm)
(a) 5000
0
(b) Index of Refraction
Power Absorption (1/cm)
(a)
Index of Refraction
Power Absorption (1/cm)
150
0.0
291
(c) 100
50
0 0.0
0.5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 3. Measurement for FeCl4 doped polyacetylene. (a) Power absorption coefficient, (b) index of refraction and (c) real conductivity.
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T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292
have a very small difference, the power absorption of the undoped and twice-stretched sample is also very small. Although inter-chain transport of carriers is superior compared with intra-chain transport, the power absorption itself is very small. Therefore, the power absorption along the sample’s rotation cannot be clearly analyzed. Because the power absorption is below 10 (1/cm) and is very noisy, the resonance is not clearly shown. Nonetheless, the phase difference is clear in each 30° rotation perpendicular from and parallel to the THz beam polarization direction. Fig. 4 shows the measured index of refraction for each 30° rotation. The perpendicular measurement has a big index of refraction around 3 and the parallel measurement has a small index of refraction around 1.7. Because of very noisy power absorption, real conductivity is also very noisy. The reference and output THz pulses and its spectra are shown in Fig. 1(a)–(d) for the twice-stretched FeCl 4 doped polyacetylene sample. Because the parallel mea-
surement absorbs too much THz energy, there is no THz pulse emitted. Even though the output THz pulses are very small compared with the reference THz pulse, the signal to noise ratio is big enough to analyze absorption and dispersion as shown in Fig. 5(a) and (b). Unlike the twice-stretched pristine sample, it has a strong angle-dependent characteristic curve in power absorption and real conductivity. In this case, interchain transport of carriers is superior compared with the intra-chain transport. Nonetheless, the index of refraction has no angle-dependent characteristic curve. It has almost the same index of refraction in each direction. If the sample rotates in a perpendicular direction to the THz beam polarization, power absorption and real conductivity increase. In parallel measurement, the carriers from the stretched direction absorb much THz energy compared with the perpendicular measurement that looks like a wire-grid effect [12].
5. Summary Index of Refraction
3.0
0˚ 2.5
30˚ 60˚
2.0
90˚ 1.5 0.0
0.5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 4. Measured index of refraction for undoped two times stretched pristine.
Power Absorption (1/cm)
8000
The THz frequency-dependent absorption and dispersion for four types of polyacetylene samples–– undoped pristine, FeCl 4 doped pristine, undoped twicestretched, and FeCl doped twice-stretched––are shown 4 in this study. The undoped pristine sample has a resonance of 2.45 THz. It does not follow the simple Drude model, like metal and semiconductor. It has an insulator behavior at dc conductivity. If the sample is doped by FeCl 4 , however, absorption and dispersion dramatically increase. Its behavior follows the Drude model. When the undoped sample is stretched, the dispersion becomes strongly angle-dependent. Nonetheless, the absorption of the stretched and doped sample is strongly angledependent, with the wire-grid effect look.
(a) 6000
4000
2000
30°
References
60° 90°
Real Conductivity (1/Ωcm)
0 200
(b)
30° 150
60° 100
90° 50
0 0.0
0 .5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 5. Measurement for FeCl4 doped two times stretched polyacetylene. (a) Power absorption coefficient and (b) real conductivity.
[1] K. Lee, R. Menon, C.O. Yoon, A.J. Heeger, Phys. Rev. B 52 (1995) 4779. [2] K. Lee, A.J. Heeger, Y. Cao, Phys. Rev. B 48 (1993) 14884. [3] T.-I. Jeon, D. Grischkowsky, Phys. Rev. Lett. 78 (1997) 1106. [4] T.-I. Jeon, D. Grischkowsky, Appl. Phys. Lett. 72 (1998) 2259. [5] T.-I. Jeon, D. Grischkowsky, A.K. Mukherjee, R. Menon, Appl. Phys. Lett. 77 (2000) 2452. [6] T.-I. Jeon, D. Grischkowsky, A.K. Mukherjee, R. Menon, Appl. Phys. Lett. 79 (2001) 4142. [7] K. Akagi, M. Suezaki, H. Shirakawa, H. Kyotani, M. Shimomura, Y. Tanabe, Synth. Met. 27–29 (1989) D1. [8] D.-S. Suh, J.G. Park, J.S. Kim, D.C. Kim, T.J. Kim, A.N. Aleshin, Y.W. Park, Phys. Rev. B 65 (2002) 165210. [9] Y.W. Park, E.S. Choi, D.S. Suh, Synth. Met. 96 (1998) 81. [10] D. Grischkowsky, S. Keiding, M. van Exter, Ch. Fattinger, J. Opt. Soc. Am. B 7 (1990) 2006. [11] M. Born, E. Wolf, Principle of Optics, Pergamon, Oxford, 1987. [12] E. Hecht, Optics, second ed., Addison-Wesley, Reading, MA, 1990.
Electrical and optical properties of polyacetylene film in THz frequency range q Tae-In Jeon a
a,*
, Geun-Ju Kim a, Hyun-Jung Lee b, Ju-Yul Lee b, Yung Woo Park
b
Division of Electrical and Electronics Engineering, Korea Maritime University, Busan 606-791, South Korea b Department of Physics, Seoul National University, Seoul 151-747, South Korea Received 20 November 2003; accepted 30 January 2004 Available online 24 March 2004
Abstract Electrical and optical properties such as power absorption, index of refraction, and complex conductivity of polyacetylene film are determined by transmission measurement using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation and without invoking the Kramers–Kronig relationships. Four types of polyacetylene samples, undoped pristine, FeCl 4 doped pristine, undoped and stretched twice, and FeCl 4 doped and stretched twice, are compared experimentally. The undoped pristine polyacetylene is found to have a resonance of 2.45 THz. The stretched polyacetylene have an angle-dependent curve along the direction of the polarized THz beam. Ó 2004 Elsevier B.V. All rights reserved. PACS: 72.80.Le; 78.66.Qn Keywords: THz time-domain spectroscopy; polyacetylene; FeCl 4 doped
1. Introduction Infrared properties of the conducting polymers have been studied extensively [1,2]. Because of the high absorption in infrared frequency range, the Fourier transform spectroscopy reflection measurement is used to investigate the charge transport properties of the conducting polymers [1,2]. These reflection measurements require the Kramers–Kronig analysis to calculate the absorption and dispersion of the samples. The transmission measurement of the THz time-domain spectroscopy (THz-TDS) has recently been shown as the ideal tool to characterize the carrier dynamics in semiconductors [3,4] and conducting polymers [5,6]. Both the absorption and dispersion of the polyacetylene-conducting polymer films are directly measured in this study. High-density polyacetylene films are synthesized following the modified Shirakawa method [7] as described
q
Original version presented at QTSM&QFS 2003 (Quantum Transport Synthetic Metals & Quantum Functional Semiconductors), Seoul National University, Seoul, Korea, 20–22 November 2003. * Corresponding author. E-mail address: [email protected] (T.-I. Jeon). 1567-1739/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2004.01.014
in previous studies [8,9]. The polyacetylene films are oriented by stretching with l=l0 ¼ 2. FeCl4 -doping is performed up to 1–2 molar % in nitromethane/FeCl 4 solution. The thickness of the films is around a few tenths of microns. Four types of samples are measured: undoped pristine, FeCl 4 doped pristine, undoped and stretched twice, and FeCl 4 doped and stretched twice. These samples are placed within the THz-TDS system. For the stretched sample measurement, the samples are rotated each 30° from perpendicular (90°) to parallel (0°) direction to the THz beam polarization. 2. Experiment THz-TDS was based on the optoelectronic generation and reception of a beam of subpicosecond THz pulses. High-performance optoelectronic source was used to generate and detect the short pulses of THz radiation [10]. A GaAs transmission antenna and a silicon on sapphire (SOS) receiving antenna were both optoelectrically driven by an average power of 10 mW from a mode-locked Ti:sapphire laser. To eliminate the effects of water vapor on the THz beam [10], the THz system
290
T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292 100
(a) Amplitude (a.u)
Current (nA)
1.0 0.5 0.0 -0.5 -1.0
(c)
80 60 40 20 0 1.5
(b) Amplitude (a.u)
Current (pA)
3.0
1.5
0.0
-1.5
0
5
10
15
Time (psec)
1.0
0.5
0.0
30
0
1
90
o
60
o
(d)
o
2
3
Frequency (THz)
Fig. 1. (a) Measured reference THz pulse. (b) Measured output THz pulse transmitted through FeCl4 doped two times stretched sample, which is located 30° from THz beam polarization. (c) Amplitude spectrum of reference pulse. (d) Amplitude spectra of output pulses; each angles mean the angle between stretched sample direction and THz beam polarization.
was placed in an airtight dry box. THz-TDS characterizations were performed by first measuring the transmitted pulses with no sample (reference) in place, as shown in Fig. 1(a), which was the average of the four individual measurements that increased the signal-tonoise ratio (S=N ) up to 15000:1. The freely propagating THz electromagnetic pulses were then compared to the measured THz pulses (output) transmitted through the thin polymer film under investigation as shown in Fig. 1(b), which was also the average of the four individual measurements with the S=N of 40:1. In this case, the sample was FeCl 4 doped and stretched twice, the most conducting material in the measurement. The normalized amplitude spectrum of the reference pulse is shown as Fig. 1(c). The output spectrum of Fig. 1(d) was normalized with respect to the reference spectrum of Fig. 1(c). Only 0.25% of the reference spectrum at 1THz was in existence. A very large frequency-dependent absorption of the sample was clearly evident.
3. Measurement The frequency-dependent absorption and dispersion of the sample are obtained using the Fourier analysis of the reference and output pulses without using the Kramers–Kronig relationship. The real index nr is obtained by the phase difference between the reference and output pulses. The frequency-dependent complex dielectric constant e of the sample is equal to the square of the complex index of refraction n ¼ nr þ ini . The imaginary index ni is determined by the power absorption coefficient ni ¼ ak=4p, where k is the free-space wavelength. The complex ratio of the output amplitude spectrum,
Eo ðxÞ, and the reference amplitude spectrum, Er ðxÞ, are simplified by Eo ðxÞ=Er ðxÞ ¼ tðxÞ exp½ibo ðxÞ;
ð1Þ
where the term bo ¼ 2pL=ko accounts for the air space displaced by the sample of length L. For the experiment with a highly absorbing sample such as FeCl 4 doped polymer, the multiple reflection of the THz pulse from the two phases of the sample may be neglected. For a relatively small total absorption and a thin sample such as undoped pristine film, however, the multiple reflection of the THz pulse occurs between the two surfaces of the sample. In this situation, the frequency-dependent complex amplitude transmission tðxÞ is given by [11] tðxÞ ¼ t12 t21 expðaL=2Þ expðibÞ=½1 þ r12 r21 expðaLÞ expði2bÞ;
ð2Þ
t12 and r12 are the complex Fresnel transmission and reflection coefficients from air into sample; t21 and r21 are the transmission and reflection coefficients from sample into air; the propagation vector is b ¼ 2pnr L=ko . Using the Fabry–Perot analysis of Eq. (2), the multiple reflection effects from the measured data of undoped pristine sample can be numerically removed. Therefore, the frequency dependent power absorption aðxÞ from the magnitude ratio and the real part of the index of refraction nr ðxÞ from phase difference of the spectra can be determined. The effective dielectric response for polymer is described by the following general relationship: 2
eðxÞ ¼ e1 poly þ irðxÞ=ðxe0 Þ ¼ ðnr þ ini Þ ;
ð3Þ
where e1 poly is the dielectric constant of the undoped polymer, rðxÞ is the complex conductivity, and eo is the
T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292
free-space permittivity. The real conductivity can be obtained from rReal ðxÞ ¼ 2xe0 nr ni .
4. Discussion The power absorption of undoped pristine sample shown in Fig. 2(a) is from 0.5 to 3.5 THz frequency range. The power absorption is very small in the low frequency range and increases rapidly up to 2.45 THz. The power absorption decreases with increased frequency after 2.45 THz. Therefore, the resonance of power absorption is in the measured frequency. This situation also occurs in the real part of the conductivity because of the small change in the index of refraction in the measured frequency range, as shown in Fig. 2(b) and (c). The real part of the conductivity does not follow the simple Drude model behavior, which has a Lorentzian line shape centered at zero frequency. Usually, metal and semiconductor treat the free carriers in a solid as classical point charges subject to random collisions and
yield the complex conductivity. The resonance absorption is not clear at this moment. Because the power absorption of the FeCl 4 doped polyacetylene pristine sample is extremely high, the high frequency range of the output THz pulse is almost absorbed. Therefore, the spectrum range is useful up to 2.2 THz, as shown in Fig. 3(a)–(c). Because of the high power absorption compared with Fig. 2(a), the resonance from the undoped pristine sample is not shown in Fig. 3(a). Likewise, the index of refraction dramatically decreases with increasing frequency, as shown in Fig. 3(b) which shows almost constant real conductivity around 120 (1=X cm) in the measured frequency range. Unlike undoped pristine sample, this follows the Drude model. These magnitudes are similar to the conducting polypyrrole sample [5] in which the plasma frequency and damping rate is within 70 and 12.6 THz, respectively. The twice-stretched samples along the chain orientation clearly exhibit the angle-dependent characteristic behavior. Because the reference and output THz pulses 6000
100
50
4000 3000 2000 1000 0
2.2
20
2.1
2.0
1.9
(b) 15
10
5
1.8
150
(c) 0.6
0.4
0.2
0
1
2
3
Frequency (THz) Fig. 2. Measurement for undoped pristine. (a) Power absorption coefficient, (b) index of refraction and (c) real conductivity.
Real Conductivity( 1/Ωcm)
0.8
Real Conductivity (1/Ωcm)
(a) 5000
0
(b) Index of Refraction
Power Absorption (1/cm)
(a)
Index of Refraction
Power Absorption (1/cm)
150
0.0
291
(c) 100
50
0 0.0
0.5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 3. Measurement for FeCl4 doped polyacetylene. (a) Power absorption coefficient, (b) index of refraction and (c) real conductivity.
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T.-I. Jeon et al. / Current Applied Physics 5 (2005) 289–292
have a very small difference, the power absorption of the undoped and twice-stretched sample is also very small. Although inter-chain transport of carriers is superior compared with intra-chain transport, the power absorption itself is very small. Therefore, the power absorption along the sample’s rotation cannot be clearly analyzed. Because the power absorption is below 10 (1/cm) and is very noisy, the resonance is not clearly shown. Nonetheless, the phase difference is clear in each 30° rotation perpendicular from and parallel to the THz beam polarization direction. Fig. 4 shows the measured index of refraction for each 30° rotation. The perpendicular measurement has a big index of refraction around 3 and the parallel measurement has a small index of refraction around 1.7. Because of very noisy power absorption, real conductivity is also very noisy. The reference and output THz pulses and its spectra are shown in Fig. 1(a)–(d) for the twice-stretched FeCl 4 doped polyacetylene sample. Because the parallel mea-
surement absorbs too much THz energy, there is no THz pulse emitted. Even though the output THz pulses are very small compared with the reference THz pulse, the signal to noise ratio is big enough to analyze absorption and dispersion as shown in Fig. 5(a) and (b). Unlike the twice-stretched pristine sample, it has a strong angle-dependent characteristic curve in power absorption and real conductivity. In this case, interchain transport of carriers is superior compared with the intra-chain transport. Nonetheless, the index of refraction has no angle-dependent characteristic curve. It has almost the same index of refraction in each direction. If the sample rotates in a perpendicular direction to the THz beam polarization, power absorption and real conductivity increase. In parallel measurement, the carriers from the stretched direction absorb much THz energy compared with the perpendicular measurement that looks like a wire-grid effect [12].
5. Summary Index of Refraction
3.0
0˚ 2.5
30˚ 60˚
2.0
90˚ 1.5 0.0
0.5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 4. Measured index of refraction for undoped two times stretched pristine.
Power Absorption (1/cm)
8000
The THz frequency-dependent absorption and dispersion for four types of polyacetylene samples–– undoped pristine, FeCl 4 doped pristine, undoped twicestretched, and FeCl doped twice-stretched––are shown 4 in this study. The undoped pristine sample has a resonance of 2.45 THz. It does not follow the simple Drude model, like metal and semiconductor. It has an insulator behavior at dc conductivity. If the sample is doped by FeCl 4 , however, absorption and dispersion dramatically increase. Its behavior follows the Drude model. When the undoped sample is stretched, the dispersion becomes strongly angle-dependent. Nonetheless, the absorption of the stretched and doped sample is strongly angledependent, with the wire-grid effect look.
(a) 6000
4000
2000
30°
References
60° 90°
Real Conductivity (1/Ωcm)
0 200
(b)
30° 150
60° 100
90° 50
0 0.0
0 .5
1.0
1.5
2.0
2.5
Frequency (THz) Fig. 5. Measurement for FeCl4 doped two times stretched polyacetylene. (a) Power absorption coefficient and (b) real conductivity.
[1] K. Lee, R. Menon, C.O. Yoon, A.J. Heeger, Phys. Rev. B 52 (1995) 4779. [2] K. Lee, A.J. Heeger, Y. Cao, Phys. Rev. B 48 (1993) 14884. [3] T.-I. Jeon, D. Grischkowsky, Phys. Rev. Lett. 78 (1997) 1106. [4] T.-I. Jeon, D. Grischkowsky, Appl. Phys. Lett. 72 (1998) 2259. [5] T.-I. Jeon, D. Grischkowsky, A.K. Mukherjee, R. Menon, Appl. Phys. Lett. 77 (2000) 2452. [6] T.-I. Jeon, D. Grischkowsky, A.K. Mukherjee, R. Menon, Appl. Phys. Lett. 79 (2001) 4142. [7] K. Akagi, M. Suezaki, H. Shirakawa, H. Kyotani, M. Shimomura, Y. Tanabe, Synth. Met. 27–29 (1989) D1. [8] D.-S. Suh, J.G. Park, J.S. Kim, D.C. Kim, T.J. Kim, A.N. Aleshin, Y.W. Park, Phys. Rev. B 65 (2002) 165210. [9] Y.W. Park, E.S. Choi, D.S. Suh, Synth. Met. 96 (1998) 81. [10] D. Grischkowsky, S. Keiding, M. van Exter, Ch. Fattinger, J. Opt. Soc. Am. B 7 (1990) 2006. [11] M. Born, E. Wolf, Principle of Optics, Pergamon, Oxford, 1987. [12] E. Hecht, Optics, second ed., Addison-Wesley, Reading, MA, 1990.