Terahertz (THz) technologies for THz devices and THz imaging systems have developed considerably.1 Many researchers have extensively investigated for developing THz devices such as THz filters, switches, mirrors, and modulators.188.8.131.52.7.8.–9 In particular, active THz modulation over a broad THz range became an important function for versatile THz devices.89.10.11.–12 For active THz modulation, quantum-well or plasmonic structures were employed,3,7 and metamaterials based on hybrid structures of metals and semiconductors were also used to obtain a relatively high modulation efficiency.5,13,14 Very recently, a new method for active THz modulation has been reported by Yoo et al., who demonstrated highly efficient THz modulations over a wide spectral range by using organic copper phthalocyanine (CuPc) thin films deposited on a silicon (Si) substrate.15,16 The novelty of this paper is to understand the characteristics of THz modulation efficiency under the condition of different polarization of the photoexciting beam, which is important for wide applications of the new method for active THz wave modulation.
In this article, we present the characteristics of THz modulation on transmission through organic-inorganic hybrid structures which are composed of a 500-nm-thick CuPc thin film and a 500-μm-thick Si substrate. We report that the THz modulation efficiency is related to the carrier concentration injected from the Si substrate to the CuPc thin film, which depends on the incidence polarization of the photoexciting beam. The modulation efficiency of THz transmission is relatively high when the photocarriers are excited by a transverse-magnetic (TM)-polarized incident light, compared with the case of TE-polarized incident light. This phenomenon is explained by the enhancement of carrier injection which is expected since the TM-polarized light is more transmitted through the surface of organic thin films than the transverse-electric (TE)-polarization light.
Basic Properties and Fabrication of CuPc Thin Films
In this study, the CuPc powder -form (Sigma-Aldrich) was used to fabricate CuPc films. Figure 1(a) shows the chemical structure of a CuPc molecule which consists of a core metal ion and a chelate organic ring with four nitrogen atoms. CuPc films of 500-nm thickness were deposited on a Si substrate with thickness of 500 μm, using thermal evaporation method. Three different types of samples were prepared at the annealing temperature, 27°C (room temperature), 150°C, and 250°C, respectively. The sample fabricated at 27°C shows randomly stacked CuPc molecules, as depicted in Fig. 1(b). As the annealing temperature increases, the column-like structures formed by stacking CuPc molecules together are increasingly crystallized, as shown in Figs. 1(c) and 1(d). The well-ordered - and -phase CuPc films are fabricated under the conditions of temperatures of 150°C and 250°C, respectively.1718.–19 A main difference of two phases is the angle between b-axes and orthogonal axes to molecular planes,20 which of necessity causes the change of the direction of molecular stacking and its stacking density. Moreover, the structural change may be associated with the characteristics of carrier injection between the CuPc thin films and the Si substrate and charge transport in the CuPc films, thus altering the absorption properties of the incident electromagnetic waves.2184.108.40.206.–26
THz Time-Domain Spectroscopy and Transmission Measurements
To measure the characteristics of THz transmission through the hybrid structures of the CuPc thin films and the Si substrate, we used a standard THz time-domain spectroscopy system.27,28 THz pulses are generated by using a p-type InAs wafer with (100) orientation and detected by using a photoconductive antenna method. The generated THz pulses are collimated by using two parabolic mirrors. The THz pulses are normally incident to the CuPc/Si hybrid structures, and a continuous optical beam is obliquely incident with the incidence angle of 75 deg at the same time. We used the laser beam with the wavelength of 785 nm at which the carriers are strongly excited on the Si substrate and the absorption on the CuPc thin film can be strongly suppressed.29 The polarization of the incident optical beam is changed to be TM- and TE-polarized light, as shown in Fig. 2, by rotating a polarizer. In the case of the TM-polarized light, the electric fields are parallel to an incidence plane, whereas in the case of the TE-polarized light, the electric fields are normal to the incidence plane.
Polarization-Dependent Transmission Modulation of THz Waves in CuPc Thin Films
Figure 3 shows the peak values of normalized THz transmission of four different samples, a bare Si substrate and CuPc/Si hybrid structures prepared at different temperatures, 27°C (room temperature), 150°C, and 250°C, on the incident polarization and power of the optical beams for photoexcitation. The peak values of transmission amplitudes measured under the condition of photoexcitation were normalized by ones measured without photoexcitation. As we easily expect, the transmission peaks may decrease with increasing the power of the optical beams as well as with improving the ordering properties of the crystallizing CuPc molecules.15 At the photoexciting power of 80 mW and the well-ordered structures of CuPc molecules, the peak value is . Note that the peak values observed under the condition of TM-polarized excitation are relatively higher than those obtained under the condition of TE-polarized excitation.
To quantitatively compare the effects of differently polarized waves on THz modulation, we used the term modulation efficiency, , where and are the peak values of transmission amplitudes measured without and with photoexcitation, respectively.15 Figure 4 shows the ratio between the modulation efficiencies, and , in cases of TE- and TM-polarized optical beams. Only the values obtained at well-ordered - and -phase CuPc films with relatively high modulation efficiencies are compared. At higher laser powers, the ratio can be reliably extracted since the values of modulation efficiency are relatively high. On the other hand, at lower laser powers, both the values of modulation efficiency in TM- and TE-polarized cases are too small. The ratio between two small numbers is therefore likely to be inaccurate. Nevertheless, the results at lower laser powers are quite consistent with those at higher laser powers. The modulation efficiencies for the TM polarization are as large as those for the TE polarization. The reflectivity in the case when the electric field is perpendicular to the plane of incidence is larger than that in the case when the electric field is parallel to the plane of incidence on the surface of a CuPc thin film. This means that much more power of the incident optical beam for photoexcitation reaches the Si substrate and therefore the concentration of the photoexcited carriers will be increased. The enhancement of carrier injection from a Si substrate to a CuPc thin film and the amount of carrier transport within the CuPc film will be followed. The density of photoexcited carriers may increase and the incident THz waves are strongly modulated consequently. This may improve the modulation efficiency of THz transmission.
In conclusion, we have demonstrated that the carrier concentration injected from a silicon substrate to a CuPc thin film depends on the incidence polarization of the photoexciting beam. The modulation efficiency of THz transmission due to TM-polarized excitation is distinctly higher than that due to TE-polarized excitation. We have found that this phenomenon is due to the increase of the concentration of the photoexcited carriers, which is expected when the incident optical beams are more transmitted through the organic thin film.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0021181; NRF-2012R1A6A3A01018114). This study was financially supported by Chonnam National University in 2013 and Research Program (Applications of Ultrashort Quantum Beam Facility) through a grant provided by the GIST in 2013. Hyung Keun Yoo made the samples and obtained the transmission signals, Chul Kang helped to obtain the transmission signals from THz time-domain spectroscopy system, Chul-Sik Kee and In-Wook Hwang analyzed the signals, and Joong Wook Lee directed this study, analyzed the results, and wrote this manuscript.
Hyung Keun Yoo is a postdoctoral researcher of the Advanced Photonics Research Institute at Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea. He has investigated terahertz waves spectroscopy and application based on carrier dynamics in optically excited states of organic semiconductors. He graduated from Sogang University with a BS in 2004 and an MS in 2006 in physics and received a PhD from Sogang University in 2012.
Chul-Sik Kee is a principal researcher of the Advanced Photonics Research Institute (APRI) at Gwangju Institute of Science and Technology (GIST), Republic of Korea. He received his PhD degree in physics at Korea Advanced Institute of Science and Technology (KAIST), Korea (2000). He worked as a postdoctoral fellow in the Department of Electrical Engineering at the University of California Los Angeles (UCLA), USA (2000 to 2001), a research professor in Ajou University, Korea (2001 to 2003), and a senior researcher in Electronics and Telecommunications Research Institute (ETRI), Republic of Korea (2003 to 2004). He has been with the Nanophotonics Laboratory of APRI since 2004. His research has included photonic crystals, metamaterials, and terahertz pulse generation from nanostructures.
Chul Kang is a senior research scientist of Advanced Photonics Research Institute at Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea, where he develops terahertz generation and spectroscopy for semiconductors and nanomaterials. He received his BS degree in physics in 2001 and his PhD degree in physics in 2006 at University of Seoul.
In-Wook Hwang is a senior research scientist of Advanced Photonics Research Institute at Gwangju Institute of Science and Technology (GIST), Republic of Korea. His research fields are ultrafast laser spectroscopy and organic solar cells. He received his PhD degree in chemistry at Yonsei University 2002. In the period of 2002 to 2009, he worked as a researcher in Yonsei University, UCSB, and GIST, characterizing ultrafast energy and electron dynamics in various organic systems (porphyrin arrays, liquid crystals, and semiconducting polymers) and fabricating high-performance solar cells.
Joong Wook Lee is an assistant professor in Department of Physics at Chonnam National University (CNU), Gwangju, Korea, where he develops terahertz sensing & imaging systems for biological applications and terahertz devices for communications. He received his BS degree in earth & environments science in 2001 and his PhD degree in physics in 2006 at Seoul National University. He worked as a postdoc in Department of Chemistry at Seoul National University, developing nanoparticle-based biosensors and supported lipid bilayer systems, for the following one-year period. After that, he worked as a postdoc at Electrical & Computer Engineering, Rice University, Houston, Texas, to 2010 and as a senior research scientist of Advanced Photonics Research Institute at Gwangju Institute of Science and Technology (GIST) to 2013.