Comprehensive imaging of terahertz surface plasmon polaritons Sen Wang,1,2 Feng Zhao,3 Xinke Wang,2 Shiliang Qu,1,4 and Yan Zhang1,2,5,* 1 Department of Physics, Harbin Institute of Technology, Harbin 150001, China Department of Physics, Capital Normal University, Beijing Key Lab for Terahertz Spectroscopy and Imaging, and Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Beijing 100048, China 3 School of Physics and Telecommunication Engineering, Shaanxi University of Technology, Hanzhong 723000, China 4 Optoelectronics Department, Harbin Institute of Technology at Weihai, Weihai 264209, China 5 Beijing Center for Mathematics and Information Interdisciplinary Sciences, Beijing 100048, China * [email protected] 2

Abstract: A comprehensive system with a high speed is built for imaging the terahertz (THz) surface plasmon polaritons (SPPs). Both the amplitude and the phase information of the focusing THz-SPPs excited by a semicircular plasmonic lens are achieved by using this system. The amplitude images present the focusing profiles of the THz-SPPs with different frequencies and the phase images reveal the Gouy phase shift as the THz-SPPs evolving through the focus. The simulations are also performed and a good agreement between the experimental and simulated results has been found. ©2014 Optical Society of America OCIS codes: (170.6795) Terahertz imaging; (240.6680) Surface plasmons; (050.5080) Phase shift.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

S. A. Maier, Plasmonics: Fundamentals and Applications (Springer, 2007). W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824– 830 (2003). J. B. Lassiter, H. Sobhani, J. A. Fan, J. Kundu, F. Capasso, P. Nordlander, and N. J. Halas, “Fano resonances in plasmonic nanoclusters: geometrical and chemical tunability,” Nano Lett. 10(8), 3184–3189 (2010). S. Vedantam, H. Lee, J. Tang, J. Conway, M. Staffaroni, and E. Yablonovitch, “A plasmonic dimple lens for nanoscale focusing of light,” Nano Lett. 9(10), 3447–3452 (2009). N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308(5721), 534–537 (2005). V. S. Volkov, S. I. Bozhevolnyi, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Wavelength selective nanophotonic components utilizing channel plasmon polaritons,” Nano Lett. 7(4), 880–884 (2007). Z. Fang, Q. Peng, W. Song, F. Hao, J. Wang, P. Nordlander, and X. Zhu, “Plasmonic focusing in symmetry broken nanocorrals,” Nano Lett. 11(2), 893–897 (2011). H. Ditlbacher, J. R. Krenn, G. Schider, A. Leitner, and F. R. Aussenegg, “Two-dimensional optics with surface plasmon polaritons,” Appl. Phys. Lett. 81(10), 1762 (2002). H. Kim, J. Park, S. W. Cho, S. Y. Lee, M. Kang, and B. Lee, “Synthesis and dynamic switching of surface plasmon vortices with plasmonic vortex lens,” Nano Lett. 10(2), 529–536 (2010). F. Bleckmann, A. Minovich, J. Frohnhaus, D. N. Neshev, and S. Linden, “Manipulation of Airy surface plasmon beams,” Opt. Lett. 38(9), 1443–1445 (2013). M. A. Bavil, Z. P. Zhou, and Q. Z. Deng, “Active unidirectional propagation of surface plasmons at subwavelength slits,” Opt. Express 21(14), 17066–17076 (2013). P. S. Tan, G. H. Yuan, Q. Wang, N. Zhang, D. H. Zhang, and X. C. Yuan, “Phase singularity of surface plasmon polaritons generated by optical vortices,” Opt. Lett. 36(16), 3287–3289 (2011). T. Zentgraf, Y. M. Liu, M. H. Mikkelsen, J. Valentine, and X. Zhang, “Plasmonic Luneburg and Eaton lenses,” Nat. Nanotechnol. 6(3), 151–155 (2011). C. Zhao, Y. Liu, Y. Zhao, N. Fang, and T. J. Huang, “A reconfigurable plasmofluidic lens,” Nat. Commun. 4, 2305 (2013). C. R. Williams, S. R. Andrews, S. A. Maier, A. I. Fernández-Domínguez, L. Martín-Moreno, and F. J. GarcíaVidal, “Highly confined guiding of terahertz surface plasmon polaritons on structured metal surfaces,” Nat. Photon. 2(3), 175–179 (2008). T.-I. Jeon and D. Grischkowsky, “THz Zenneck surface wave (THz surface plasmon) propagation on a metal sheet,” Appl. Phys. Lett. 88(6), 061113 (2006).

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16916

17. S. S. Li, M. M. Jadidi, T. E. Murphy, and G. Kumar, “Terahertz surface plasmon polaritons on a semiconductor surface structured with periodic V-grooves,” Opt. Express 21(6), 7041–7049 (2013). 18. W. Q. Zhu, A. Agrawal, and A. Nahata, “Direct measurement of the Gouy phase shift for surface plasmonpolaritons,” Opt. Express 15(16), 9995–10001 (2007). 19. M. van Exter and D. Grischkowsky, “Characterization of an optoelectronic terahertz beam system,” IEEE Trans. Microw. Theory Tech. 38(11), 1684–1691 (1990). 20. X. K. Wang, Y. Cui, W. Sun, J. S. Ye, and Y. Zhang, “Terahertz real-time imaging with balanced electro-optic detection,” Opt. Commun. 283(23), 4626–4632 (2010). 21. J. He, X. Wang, D. Hu, J. Ye, S. Feng, Q. Kan, and Y. Zhang, “Generation and evolution of the terahertz vortex beam,” Opt. Express 21(17), 20230–20239 (2013). 22. H. T. Chen, R. Kersting, and G. C. Cho, “Terahertz imaging with nanometer resolution,” Appl. Phys. Lett. 83(15), 3009–3011 (2003). 23. A. Nahata and W. Zhu, “Electric field vector characterization of terahertz surface plasmons,” Opt. Express 15(9), 5616–5624 (2007). 24. M. A. Seo, A. J. L. Adam, J. H. Kang, J. W. Lee, S. C. Jeoung, Q. H. Park, P. C. M. Planken, and D. S. Kim, “Fourier-transform terahertz near-field imaging of one-dimensional slit arrays: mapping of electric-field-, magnetic-field-, and Poynting vectors,” Opt. Express 15(19), 11781–11789 (2007). 25. A. J. L. Adam, J. M. Brok, M. A. Seo, K. J. Ahn, D. S. Kim, J. H. Kang, Q. H. Park, M. Nagel, and P. C. M. Planken, “Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures,” Opt. Express 16(10), 7407–7417 (2008). 26. X. K. Wang, W. F. Sun, Y. Cui, J. S. Ye, S. F. Feng, and Y. Zhang, “Complete presentation of the Gouy phase shift with the THz digital holography,” Opt. Express 21(2), 2337–2346 (2013). 27. C. A. Werley, Q. Wu, K.-H. Lin, C. R. Tait, A. Dorn, and K. A. Nelson, “Comparison of phase-sensitive imaging techniques for studying terahertz waves in structured LiNbO3,” J. Opt. Soc. Am. B 27(11), 2350–2359 (2010).

1. Introduction Surface plasmon polaritons (SPPs) are collective electromagnetic excitations propagating along the metal/dielectric interfaces, evanescently confined in the perpendicular direction [1, 2]. With the ability of sub-wavelength scale manipulation, the SPPs are attractive for a variety of application areas such as near-field sensing, super-resolution imaging, and nanolithography [3–5]. Fundamental devices for exciting, guiding, and focusing the SPPs have been extensively investigated [6–8]. Devices for generating the SPPs vortex, airy SPPs, and unidirectional SPPs have also been fabricated [9–11]. In the visible spectral range, the primary ways used to characterize the functionality of those devices include scanning nearfield optical microscopy, fluorescence imaging, and leakage radiation imaging [12–14]. However, these techniques can obtain only the intensity of the SPPs and the phase information is lost completely, which is unfavorable for the design and characterization of the SPPs devices. In the terahertz (THz) spectral range, metals are no longer described by the Drude model but are considered as the perfect electrical conductors (PEC) [15]. The SPPs excited by the THz field, which is known as the Sommerfeld or Zenneck waves [16, 17], are weakly confined in the dielectric side. The 1/e field extension of THz-SPPs from the metal surface can reach several centimeters into the air [18]. The weak confinement and the scale of the wavelength enable the detection of the THz-SPPs with techniques which are available in the THz spectral range. The THz time domain spectroscopy (THz-TDS) system and THz holographic imaging system are primary techniques to detect the THz radiation [19–21]. Their main advantage is the ability to simultaneously obtain the amplitude and phase of the THz radiation. The THz-TDS system has already been successfully used to detect the THzSPPs by raster scanning [18, 22–25]. Compared with the THz-TDS system, the THz holographic imaging system can achieve the imaging measurement with a higher speed and enough signal to noise ratio (SNR) [26]. To our best knowledge, there is no report about the application of THz holographic imaging on the measurement of the THz-SPPs. In this paper, we propose a THz-SPPs imaging system based on the principle of THz holographic imaging to obtain both the amplitude and phase images of the THz-SPPs. A CCD camera is used as the detector and the line probe beam is expanded for achieving more information, which could significantly reduce the imaging time and enlarge the imaging area. A ZnTe crystal, which is used as the sensor, is scanned along the propagating direction of the THz-SPPs for coherently measuring the two-dimensional distribution of the THz-SPPs. #212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16917

In this way, the linear phase shift can be effectively avoided to clearly reveal the phase evolution of the THz-SPPs. The feasibility of this system is demonstrated by imaging the focusing THz-SPPs generated by a semicircular slit. The amplitude images of the THz-SPPs for different frequencies are presented and analyzed. The Gouy phase shift during the focusing process is clearly observed from the phase images. The finite-difference-timedomain (FDTD) algorithm is adopted to replicate the experimental THz-SPPs focusing images. The simulated results verify the reliability of the imaging system. This paper is arranged as follows: The first section gives a brief introduction. The second section describes the experimental setup. The third section presents the experimental and simulated results and gives the corresponding discussions, and at last, a brief summary is drawn. 2. Experimental setup

Fig. 1. Schematic diagram of the plasmonic lens (a) and the experimental setup (b) for imaging the THz-SPPs. The generated THz-SPPs propagate along the x-axis in the vicinity of the foil. A quarter wave plate (QWP), a Wollaston prism (WP), and a CCD camera are used for the differential detection. The ZnTe crystal is mounted on a one-dimensional positioning stage and auto-scanned along the x-axis.

The structure of the plasmonic lens is a semicircular slit fabricated on a 150 μm-thick stainless steel foil by the precision laser cutting, which is schematically shown in Fig. 1(a). The inner radius of the semicircular is r = 6 mm and the width of the slit is w = 120 μm. Figure 1(b) shows the schematic diagram of the THz-SPPs imaging system. The THz radiation generated by a ZnTe crystal with a 1/e beam diameter of 20 mm is incident on the semicircular slit perpendicularly from one side of the foil and the THz-SPPs is generated on both sides of the foil. The incident THz beam is modulated by a mechanical chopper with a frequency of 50 Hz. Due to the curve nature of the slit, the generated THzSPPs will be focused in the centre of the semicircular. Another 10 mm × 10 mm × 3 mm ZnTe crystal and an expanded line probe beam are used to measure the THz-SPPs generated on the other side of the foil (opposite to the incident side of the THz beam), which can effectively avoid the influence of the incident THz beam. The expanded line probe beam is generated by using an inverse telescope system and a slit. Its length and width are 10 mm and 1 mm, respectively. The distance between the foil and the detection crystal is about 500 μm and the probe beam is about 1 mm away from the foil. The distances are well within the 1/e field extension of the THz-SPPs [18]. The probe beam containing the information of THzSPPs is incident into the imaging module composed of a lens, a quarter wave plate (QWP), a Wollaston prism (WP), and a CCD camera with a 4 Hz frame rate. The THz and probe beams

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16918

are linearly polarized along the x-axis and y-axis, respectively, which are indicated by the blue and red arrows in Fig. 1(b). The probe beam is imparted a π / 2 optical bias by the QWP and is split into two orthogonal linearly polarized beams by the WP. The two probe components are projected onto the CCD camera which acts as the double-eye detector in the THz-TDS system for the differential detection. The THz-SPPs information can be extracted from the subtraction of the two probe components. To improve their subtraction accuracy, an affine transformation is applied to match the sizes of these two probe images [27]. The CCD camera is synchronously controlled with the mechanical chopper and the dynamic subtraction technique is used to improve the SNR of the imaging system. The principle of the detection technique has been detailedly discussed in Ref [20]. In order to get the amplitude and phase images of the focusing THz-SPPs in the x-y plane, the ZnTe crystal is mounted on a one-dimensional motor positioning stage and is automatically scanned along the x-axis. The scan range is from −3 mm to 4 mm with a step of 0.4 mm, considering the centre of the semicircular as the original point. At each scan point, 50 frames are averaged to further enhance the SNR. 3. Results and discussions

Fig. 2. The simulated amplitude images of the THz-SPPs. (a)-(c) are the amplitude images of Ex, Ey, and Ez, respectively. (d) is the amplitude distribution along the cut line x = 0 of (a)-(c).

First of all, the focusing properties of the plasmonic lens are simulated with a commercial software FDTD Solutions. The incident THz beam is linearly polarized along the symmetry axis of the semicircular slit and its frequency is 0.73 THz which is around the central frequency of the THz wave generated in the experiment. The parameters of the semicircular slit are the same as those of the sample used in the experiment and the metal foil is set as the PEC in the simulation. Figures 2(a)–2(c) show the simulated amplitude image of the three vector components of the THz-SPPs, including two in-plane components Ex, Ey and an outof-plane component Ez, respectively. The excited Ex components along the slit are in-phase and will interfere constructively in the centre of the semicircular slit after experiencing the same optical path length. A weak focal spot is formed in the centre. The Ey component excited by the upper quarter slit is out of phase with the one excited by the lower quarter slit.

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16919

Thus these two parts will interfere destructively in the centre and form two weak focal spots. The excited Ez components along the slit are in-phase and form a strong focal spot in the centre. It is clearly seen that the focusing effect of Ez is much stronger than those of Ex and Ey. This is because that the Ez component excited by the slit is much larger than the Ex and Ey components, which has already been experimentally verified in the Ref [23]. To quantitatively compare the focusing strengths of the three components, cut lines along the x = 0 are shown in Fig. 2(d). The amplitude of Ez in the centre is ten times larger than those of Ex and Ey, which means the intensity of Ez is one hundred times stronger than those of Ex and Ey. Namely, the Ez component contains the major energy and information of the THz-SPPs. Thus it is more convenient and meaningful to measure the dominant Ez component of the THz-SPPs in the experiment.

Fig. 3. (a) The maximum amplitude image of the THz-SPPs. (b) The temporal signals of the THz-SPPs measured at y = −1 mm, −0.5 mm, 0 mm, 0.5 mm, and 1 mm along the x = 0 direction which is indicated by the white arrow in (a).

Based on the discussion above, only the Ez component is measured in the experiment. To enhance the accuracy of the measurement, only the line data on the central line of the probe beam at each scan point is extracted to build the whole complex field distribution of the THzSPPs. There are 300 pixels on each column from y = −2 mm to y = 2 mm. The maximum amplitude value of the THz-SPPs temporal signal at each pixel is extracted and shown in Fig. 3(a). In the imaging module of the THz system, the focal length of the lens is 15 cm and the image distance between the lens and the CCD is about 19 cm, so the spatial resolution in the y direction is evaluated as 18 μm, which is the approximately same as that of the reported nearfield imaging systems based on the THz-TDS techniques [24, 25]. The resolution in the x direction is limited due to the 3 mm-thick detection crystal and can be further improved with a thinner crystal. Even so, Fig. 3(a) clearly shows the focusing effect of the THz-SPPs. It takes about 30 minutes to obtain the 300 temporal signals at each column. To detect the same amount of signals, the time consumption is estimated to be about 150 minutes for the previous near-field imaging systems. Thus, the proposed imaging system can achieve the imaging measurement with a higher speed and a larger imaging area. For the whole imaging area of 7 mm × 4 mm, the imaging time of the proposed method is about 9 hours (limited by the speed of CCD camera) and that of the previous near-field imaging systems is about two days. The five temporal signals of the THz-SPPs obtained at y = −1 mm, −0.5 mm, 0 mm, 0.5 mm, and 1 mm along the x = 0 direction are extracted, as shown in Fig. 3(b). It can be clearly seen that the temporal signal in the focal point is obviously stronger than others. Its SNR reaches to 100, which is comparable to the SNR of the THz holographic imaging system [20]. The temporal signal at each pixel is Fourier transformed to obtain the amplitude and phase distributions of the THz-SPPs with different frequencies. Besides, it should be noted that the complex fields of the Ex and Ey components can be also detected by using the THz-SPPs imaging system. The Ex components can be detected by replacing the ZnTe crystal with a ZnTe crystal and the Ey components can be measured by rotating the ZnTe crystal by 90°.

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16920

The measured amplitude image of the focusing THz-SPPs with 0.73 THz is shown in Fig. 4(a). It can be seen that the THz-SPPs are focused to a spot in the centre of the semicircular slit but with a larger size than the simulated result in Fig. 2(c). This is caused by the integral effect of the 3 mm-thick ZnTe crystal used in the experiment. So the simulated Ez component in Fig. 2(c) is integrated every 3 mm along the x-axis and the integrated image is shown in Fig. 4(b), in which the size of the focal spot is significantly enlarged. The focusing profiles obtained from the experiment and simulation are compared quantitatively in Figs. 4(c) and 4(d), which show the normalized transverse and longitudinal amplitude profiles through the focal spot, respectively. The experimental transverse and longitudinal full widths at half maximum (FWHM) are 493 μm and 4 mm, which is slightly different from the simulated values of 540 μm and 3.6 mm. The difference is caused by the non-uniformity of the detection crystal and the expanded probe beam.

Fig. 4. Experimental (a) and simulated (b) amplitude images for the 0.73 THz THz-SPPs, respectively. (c) and (d) are the normalized transverse and longitudinal amplitude profiles.

Taking advantage of the broad bandwidth of the generated THz radiation, the dependence of the focusing profile of THz-SPPs on the frequency is investigated by using the imaging system. Figures 5(a) and 5(b) show the experimental amplitude images of the THz-SPPs with the frequencies of 0.3 THz and 0.2 THz, corresponding to wavelengths of 1000 μm and 1500 μm, respectively. The simulated results are shown in Figs. 5(c) and 5(d), which are also integrated every 3 mm along the x-axis. The focal spots of different frequencies are all in the centre of the semicircular slit. That is because all the THz-SPPs with different frequencies interfere constructively in the centre. But the size of the focal spot is larger for the THz-SPPs with a longer wavelength, which is the same as the focusing property of the optical wave in the free space. Quantitatively, the cut lines along the y-axis through the focal spot for different frequencies are shown in Figs. 5(e) and 5(f). The simulated results are in good agreement with the experimental results. The transverse FWHMs for the THz-SPPs with the frequencies of 0.3 THz and 0.2 THz are 786 μm and 998 μm, respectively.

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16921

Fig. 5. Experimental amplitude images for the 0.3 THz (a) and 0.2 THz (b) THz-SPPs and simulated amplitude images for the 0.3 THz (c) and 0.2 THz (d) THz-SPPs, respectively. (e) and (f) are the comparison of the normalized transverse amplitude profiles of the 0.3 THz and 0.2 THz THz-SPPs.

In the experiment, the optical paths of the THz and probe beams change synchronously during the scanning of the ZnTe crystal, so the linear phase shift of the THz-SPPs is effectively avoided. The experimental and simulated phase images of the 0.73 THz THz-SPPs are shown in Figs. 6(a) and 6(b), respectively. For the pixels where the THz-SPPs amplitude is less than 0.3, the phase of the pixel is set as zero to filter the uncertain phase noise and the corresponding color is set as gray. It is can be seen that the THz-SPPs undergo a π / 2 Gouy phase shift through the focal spot. The cut lines along the y = 0 of Figs. 6(a) and 6(b) are shown in Figs. 6(g) and 6(h) to reveal the Gouy phase shift more clearly. Unlike the π phase shift of the converging spherical optical wave in the three dimensional space, the focusing THz-SPPs takes on a phase change of π / 2 which confirms the surface nature (two dimension) of the excited THz-SPPs. The dependence of the Gouy phase shift of THz-SPPs on the frequency is also investigated and the measured phase images for the 0.3 THz and 0.2 THz THz-SPPs are shown in Figs. 6(c) and 6(e), respectively. The cut lines of the experimental phase images along the y = 0 are shown in Fig. 6(g) to observe the differences of the Gouy phase shift for different frequencies. It is seen that the Gouy phase shifts for all frequencies take on a same value of π / 2 , but the higher frequency shows a shaper variation and the phase shift reaches to π / 2 rapidly. The phase shift for a lower frequency tends gradually to π / 2 . This is because the THz-SPPs with a higher frequency have a shorter Rayleigh range and larger transverse wave vectors around the focal spot [26]. The simulated phase images for the 0.3 THz and the 0.2 THz components are shown in Figs. 6(d) and 6(f), respectively. The cut lines of the simulated phase images along the y = 0 are shown in Fig. 6(h), which show the same tendencies as the experiment results. It should be pointed out that the phase fluctuation of the #212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16922

0.73 THz THz-SPPs shown in Fig. 6(h) is caused by the THz-SPPs generated at the ends of the slit, which can be weakened by reducing the width of slit or increasing the wavelength. In the experiment, the phase fluctuation is flattened by the integral effect of the 3 mm-thick detector crystal. For the 0.3 THz and 0.2 THz THz-SPPs, corresponding to longer wavelengths of 1000 μm and 1500 μm, the phase fluctuation is also suppressed because the THz-SPPs generated at the ends of the slit are weak.

Fig. 6. Experimental phase images of the THz-SPPs with frequencies of 0.73 THz (a), 0.3THz (c) and 0.2THz (e). The simulated phase images for the 0.73 THz (b), 0.3THz (d) and 0.2THz (f) THz-SPPs. The experimental (g) and simulated (h) longitudinal phase distributions for THz-SPPs with different frequencies.

4. Conclusions In conclusions, we build up a THz-SPPs imaging system with a higher speed and use it to obtain both the amplitude and phase images of the focusing THz-SPPs excited by a metallic semicircular slit. The amplitude and phase images present the focusing profile and the Gouy phase shift of the focusing THz-SPPs with different frequencies, respectively. Concretely, the

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16923

THz-SPPs with a higher frequency shows a smaller FWHM and a sharper phase shift. The experimental results are also compared with the FDTD simulations and a good agreement is achieved, which confirms the reliability of the imaging system. The proposed THz-SPPs imaging system provides a powerful tool to fully characterize the functionality of the SPPs devices. Acknowledgments This work was supported by the 973 Program of China (No. 2013CBA01702), the National Natural Science Foundation of China (Nos. 11204188, 61205097, 91233202, 11374216, and 11174211), the National High Technology Research and Development Program of China (No. 2012AA101608-6), the Beijing Natural Science Foundation (No. 1132011), the Program for New Century Excellent Talents in University (NCET-12-0607), the Scientific Research Base Development Program of the Beijing Municipal Commission of Education, and the CAEP THz Science and Technology Foundation (CAEPTHZ201306).

#212369 - $15.00 USD (C) 2014 OSA

Received 19 May 2014; revised 21 Jun 2014; accepted 23 Jun 2014; published 2 Jul 2014 14 July 2014 | Vol. 22, No. 14 | DOI:10.1364/OE.22.016916 | OPTICS EXPRESS 16924

Comprehensive imaging of terahertz surface plasmon polaritons.

A comprehensive system with a high speed is built for imaging the terahertz (THz) surface plasmon polaritons (SPPs). Both the amplitude and the phase ...
3MB Sizes 2 Downloads 4 Views