Optics Communications 359 (2016) 349–352
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Terahertz modulation using TIPS-pentacene thin films deposited on patterned silicon substrates Jung-Min Park a, Ik-Bu Sohn b, Chul Kang b, Chul-Sik Kee b, In-Wook Hwang b, Hyung Keun Yoo b, Joong Wook Lee a,n a b
Department of Physics and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 500-757, Republic of Korea Advanced Photonics Research Institute, GIST, Gwangju 500-712, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2015 Received in revised form 25 September 2015 Accepted 1 October 2015
We demonstrate the characteristics of terahertz (THz) wave modulation by 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) thin films deposited on silicon substrates patterned with one-dimensional periodic multi-channels. The polarization independence of the THz wave modulation in the hybrid structures with one-dimensional channels indicates that the rapid in-plane diffusion of the carriers injected into the TIPS pentacene thin films plays an important role in the high modulation of THz waves. Understanding the mechanisms of THz modulation is important for optimizing the performance of THz devices based on organic/inorganic hybrid structures. & 2015 Elsevier B.V. All rights reserved.
Keywords: Laser spectroscopy THz spectroscopy Organic materials Modulation
1. Introduction Terahertz (THz) technologies have attracted much attention for various applications, such as high-speed communications, spectroscopy, and sensing/imaging systems [1–5]. The realization of active THz devices such as THz modulators is necessary to improve applications based on THz technologies. Many researchers have demonstrated the possibility of active control of the transmission, reflection, spatial position, polarization, and phase of THz waves, and various materials and structures have been employed to achieve active THz modulation with high modulation efficiency [6–13]. Recently, our group and others reported that active broadband THz wave modulators can be realized based on organic/inorganic hybrid structures, and high modulation efficiency can be achieved by the selection of appropriate organic materials and their structural variations [14–18]. The modulation technique based on organic/inorganic hybrid structures provides several advantages such as high modulation efficiency, extreme broadband modulation, and easy fabrication, comparing with other techniques based on metamaterials [6–10], plasmonic structures [11,12], and semiconductor multilayer structures [13]. In order to take advantage of the technique in practice, the characteristics of THz wave n
Corresponding author. E-mail addresses:
[email protected] (H. Keun Yoo),
[email protected] (J. Wook Lee). http://dx.doi.org/10.1016/j.optcom.2015.10.008 0030-4018/& 2015 Elsevier B.V. All rights reserved.
modulation should be investigated for a variety of structural conditions and the diffusion properties of the photo-excited carriers injected into the organic thin film should be also studied. In this paper, we present the characteristics of THz modulation by organic/inorganic hybrid structures that consist of a 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) film and a silicon (Si) substrate. The TIPS pentacene thin film was deposited on the Si substrate patterned with one-dimensional periodic multi-channels. We carried out polarization-dependent transmission measurements using a THz time-domain spectroscopy method and obtained the modulation efficiencies of THz transmission. The results demonstrate polarization-independent modulation efficiency, which means that the injected carriers from the patterned Si substrate to the TIPS pentacene thin films undergo rapid in-plane diffusion. These results are important for developing versatile THz devices and for optimizing their performance.
2. Experimental setup and sample fabrication Fig. 1(a) shows the schematic diagram of the designed structure. TIPS pentacene molecules were deposited on a high-resistivity silicon (Si) substrate patterned with one-dimensional U-shaped channels. First, three types of patterned Si substrates were fabricated by femtosecond laser machining [19], a flat Si substrate without any pattern (Sample A), a Si substrate with 10μm-deep channels (Sample B), and a Si substrate with 30-μmdeep channels (Sample C), as shown in Fig. 1(b). TIPS pentacene,
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Fig. 1. (a) Three-dimensional schematic of the Si/TIPS pentacene hybrid structures. The dimensions are as follows: p ¼ 150 μm, w¼ 40 μm, and d ¼ 0, 10 and 30 μm. (b) Microscopy images of Si substrates with U-shaped channels of d ¼ 10 μm (Sample B) and d ¼30 μm (Sample C). (c) Molecular structure of TIPS pentacene. (d) Schematic view of THz transmission through the samples. The incident THz pulses and the optical beam for photo-excitation are incident onto the same area of the sample surface normally and obliquely, respectively.
which is a high-performance soluble organic semiconductor used in optical devices and flexible electronics (Fig. 1(c)) was purchased from Sigma-Aldrich (product number: 716006). The molecules were deposited on top of the Si substrates by drop casting. After dropping 50 μl of TIPS pentacene solution (2 mg/ml) onto the Si substrate, the molecules were covered with a glass lid and heated for 5 min on a hotplate at 50 °C. The THz modulation characteristics were measured in a transmission THz time-domain spectroscopy system [20–22]. THz pulses were generated using a (100) p-type InAs crystal, generating the photo-Dember field due to its high electron mobility, collimated by parabolic mirrors, focused with a THz lens and detected
with a photoconductive antenna. The THz pulses were transmitted through a 3-mm-diameter pinhole onto the samples under photoexcitation by a cw diode laser. An optical beam with a wavelength of 785 nm was used as the light source for photo-excitation. It was incident with an oblique angle of 45° and a power of 80 mW and excited the area of the sample surface sectioned by the 3-mmdiameter pinhole. As shown in Fig. 1(d), the angle θ was counted off from the x-axis on the surface plane of the samples and indicates the polarization direction of the incident THz waves. At θ ¼ 0°, the direction of the U-shaped channels in the Si/TIPS pentacene layers was perpendicular to the electric field of the polarized, incident THz waves. Polarization dependent transmission
J.-M. Park et al. / Optics Communications 359 (2016) 349–352
measurements were carried out by rotating the samples in the xy plane.
3. Experimental results and discussion Fig. 2 shows the normalized transmission amplitudes of THz waves through the three samples (Sample A, Sample B, and Sample C). The measurements were performed with (red curves) and without (black curves) photo-excitation. A reduction of THz wave transmission is clearly observed for all samples, which may be associated with carrier injection from the Si substrate into the TIPS pentacene thin film and the subsequent diffusion within the two layers [14,17]. The properties of photo-excited carrier injection are characterized by the relative HOMO–LUMO energy levels between the two interface materials. As shown in the inset of Fig. 2 (d), the energy gap separating the HOMO and the LUMO states is approximately 2.0 eV and the energy of the LUMO state of the TIPS pentacene is higher than that of Si [23,24]. The photo-excited holes on the Si substrate due to the incident optical beam are therefore transferred to the TIPS pentacene thin film. The separated charge carriers then increase the carrier density near the Si/ TIPS pentacene interface. In particular, significant numbers of
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carriers may accumulate in the relatively thin TIPS pentacene layer, making the layer metallic. Thus, the THz wave transmission intensity decreases when the photo-carriers are excited due to the external optical beam. For a quantitative comparison of the effects of patterned Si structures on THz transmission modulation, we obtained the modulation efficiency of the three samples as shown in Fig. 2(d). The modulation efficiency is defined as the difference between the integrals of the THz transmission intensities measured with and without photo-excitation according to the equation [17,25]. 2
Meff =
2
∫ E w / o(ν ) dν − ∫ E w(ν ) dν 2
∫ E w / o(ν ) dν
where Ew / o and Ew indicate the THz electric-field amplitudes measured without and with photo-excitation, respectively. Note that the modulation efficiency decreases with the depth of channels patterned on the Si substrate. The reduction in modulation efficiency seems to occur because the rough Si/TIPS pentacene interface formed on the rough Si surface of the channels fabricated by femtosecond laser machining restrains the transfer of photoexcited carriers.
Fig. 2. THz wave transmission spectra for (a) Sample A, (b) Sample B, and (c) Sample C measured with photo-excitation (red curves) and without photo-excitation (black curves). The insets show microscopy images of the three samples. (d) Modulation efficiencies of THz wave transmission plotted versus the depth d of channels patterned on the Si substrate. The inset shows the HOMO–LUMO energy level diagram. The yellow arrow indicates the charge transfer process following photo-excitation on the Si substrate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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independent modulation efficiency is due to the fast in-plane diffusion of the carriers injected from the Si substrate into the TIPS pentacene layer. Understanding the characteristics of THz wave modulation quantitatively in such structures may enable systematic analysis of the diffusion properties of carriers in organic layers and its mechanisms.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (KRF 2014R1A1A2057920) and the Advanced Photonics Research Institute Research Program (Asian Laser Center) of GIST. This study was also financially supported by Chonnam National University in 2015.
Fig. 3. Modulation efficiency, Meff, of the samples, Sample A (blue triangles), Sample B (black squares), and Sample C (red circles), measured while varying the polarization of the incident THz waves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The effects of one-dimensional arrangements of periodic multichannels were also investigated by polarization-dependent THz transmission measurements. Fig. 3 shows the modulation efficiency measured by varying the polarization of the incident THz waves for the three samples. As shown in Fig. 2(a)–(c), the modulation efficiency decreases as the depth of the channels fabricated by femtosecond laser machining method becomes deeper. This means that the number of photo-excited carriers into the organic thin film is decreased with increasing the depth of the channels. One-dimensional pattern of the distribution of carriers should be then formed as a result of spatially non-uniform carrier injection and transport on the channels and the remaining area except of the channels. Contrary to the expectation that one-dimensional structures have polarization-dependent modulation due to the non-uniform property of photo-excited carriers, the modulation efficiencies of all the samples were polarization-independent. Thus, despite non-uniformity in the injection process, the concentration of carriers injected into the TIPS pentacene thin film rapidly becomes uniform over the whole area of the organic layer near the organic–inorganic interface. This phenomenon may be possible because of the fast in-plane diffusion of the carriers in the TIPS pentacene thin film.
4. Conclusions In conclusion, we investigated the transmission modulation of THz waves in the TIPS pentacene thin films deposited on Si substrates patterned with one-dimensional arrangements of periodic multi-channels. We have shown that the measured polarization-
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