Research Report Scherger Benedikt - 京都大学depicted in Fig. 1 (b). The dimensions are chosen...

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Bilateral International Exchange Program (BIEPBilateral International Exchange Program (BIEPBilateral International Exchange Program (BIEPBilateral International Exchange Program (BIEP, invite, invite, invite, invite) ) ) ) reportreportreportreport Send report to: Your responsible Professor in Kyoto UniversitySend report to: Your responsible Professor in Kyoto UniversitySend report to: Your responsible Professor in Kyoto UniversitySend report to: Your responsible Professor in Kyoto University

[email protected]@[email protected]@scphys.kyoto----u.ac.jpu.ac.jpu.ac.jpu.ac.jp , , , , [email protected]@[email protected]@scphys.kyoto----u.ac.jpu.ac.jpu.ac.jpu.ac.jp (Year/Month/Day)_2010/11/09___

Invited StudentInvited StudentInvited StudentInvited Student Name Benedikt SchergerBenedikt SchergerBenedikt SchergerBenedikt Scherger University and Country PhilippsPhilippsPhilippsPhilipps----Universität Marburg, GermanyUniversität Marburg, GermanyUniversität Marburg, GermanyUniversität Marburg, Germany Grade M.Sc., Dipl.M.Sc., Dipl.M.Sc., Dipl.M.Sc., Dipl.----Ing.(FH)Ing.(FH)Ing.(FH)Ing.(FH) Phone and FAX +49(0)6421 28+49(0)6421 28+49(0)6421 28+49(0)6421 28----222222222222----79 or 83 for Fax79 or 83 for Fax79 or 83 for Fax79 or 83 for Fax e-mail address [email protected]@[email protected]@physik.uni----marburg.marburg.marburg.marburg.dededede URL http://www.unihttp://www.unihttp://www.unihttp://www.uni----marburg.de/fb13/forschung/experimentellemarburg.de/fb13/forschung/experimentellemarburg.de/fb13/forschung/experimentellemarburg.de/fb13/forschung/experimentelle----halbleiterphysik/agkochhalbleiterphysik/agkochhalbleiterphysik/agkochhalbleiterphysik/agkoch

Name and Position of Ph.D. advisor Prof. Dr. Martin KochProf. Dr. Martin KochProf. Dr. Martin KochProf. Dr. Martin Koch e-mail address of Ph.D. advisor [email protected]@[email protected]@physik.uni----marburg.demarburg.demarburg.demarburg.de Responsible Researcher in Kyoto UniversityResponsible Researcher in Kyoto UniversityResponsible Researcher in Kyoto UniversityResponsible Researcher in Kyoto University Name Nobuko Naka Group and Faculty Solid State Spectroscopy Group Position Associate Professor e-mail address [email protected] Phone and FAX 075-753-3746 and 075-753-3757 Research ProjectResearch ProjectResearch ProjectResearch Project Title Frequency Selective Surfaces (FSS) in the THz RegimeDuration 2 month2 month2 month2 month

Research Report

Frequency Selective Surfaces (FSS) in the THz Regime Benedikt Scherger, M.Sc., Dipl.-Ing.(FH)

The research on metamaterials has attracted great attention in recent years. The opportunity to get

electromagnetic properties that do not exist in naturally available materials is the driving factor.

Some examples are: Left handed material [1], extraordinary transmission [2], artificial magnetism

and negative refraction [3], cloaking [4] and resonance phenomena in frequency selective surfaces

(FSS) [5]. In combination with the progress in the terahertz (THz) technology the last example will

become an interesting tool as a sensor for chemical and biochemical specimens [6, 7]. In this report

the influence of the incidence angle of an electromagnetic wave, oscillating in the THz regime,

striking to an FSS is described.

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We produced an FSS consisting of an array of asymmetric double split rectangular resonator (ADSRR).

The structure is made from gold on a fused silica glass with a refractive index of n = 1.92 in the THz

regime. The size of the whole structure is 8.5 * 9.5 mm. The schematic layout of the unit cell is

depicted in Fig. 1 (b). The dimensions are chosen such that the structure operates around 1 THz,

resulting in a side length of a = 120 µm, an outer resonator diameter of l = 60 µm, a resonator width

of w = 10 µm, a gap size of g = 8 µm and an asymmetry length of d = 10 µm. In order to simulate a

thin film sensing scenario a film of photoresist is applied to the FSS.

Fig. 1 (a) Microscopic photograph of a sector of the FSS. The dashed box indicates one unit cell. (b) Layout of the ADSRR unit

cell.

The measurements in the THz regime are performed with a standard THz time domain spectroscopy

setup with a photoconductive bowtie shaped emitter antenna and a receiver using the electro optic

sampling technique. The system is driven by a Tsunami©

femtosecond laser. The FSS is measured in

transmission with one movable polarizer in front and one static polarizer after the sample. The

bowtie antenna and the static polarizer are 45° polarized. The movable polarizer is set to 0° (s-

polarization) and 90° (p-polarization) respectively.

In Fig. 2 (a) the transmission results for the loaded and unloaded FSS are depicted. In the inset of

Fig. 2 (a) the simulated reflection magnitude for a complementary FSS with the same dimensions is

plotted. Due to the Babinet principle the results must be similar to the transmission measurement of

the not complementary FSS [8]. Simulation and measurement has resonances at 0.808 THz and

0.909 THz for the loaded and at 1.02 THz for both unloaded cases respectively. The difference in the

resonance frequency for the loaded FSS is due to the slightly difference in the thickness and the

refractive index of the photoresist in comparison with the simulated load material.

(a) (b)

Fig. 2 (a) Transmission data of the loaded (blue) and unloaded FSS (red). Inset: Simulation data for the reflection

measurement of the complementary FSS. (b) Polarization and axis definition. Pos. angles are counted clockwise. The THz

photons are moving in neg. z-direction.

The origin of the strong and narrow spectral response of the ADSRR is traced to the so-called

“trapped modes” [5]. These trapped modes are electromagnetic modes that are weakly coupled to

the free space. In symmetrical split rings these modes are inaccessible but they can be exited if the

metamaterial´s particles have certain structural asymmetry.

We performed two different measurements to analyse the influence of the incidence angle of the

electromagnetic wave to the resonance of the FSS. First we rotated the FSS in the xy-plain around the

z-axis (normal rotation) in 10° steps up to 180°; secondly we rotated the sample in the xz-plain

around the y-axis (tilt rotation) in 5° steps. The axes are defined in Fig. 2 (b). Both rotations are

performed with the THz-focal-spot as the center of the FSS rotation.

The resulting spectra of the normal rotation are plotted in Fig. 3 (a) and (b). Fig. 3 (a) depicts the

spectra for different angels from 0° to 90° in 10° steps. The resonance becomes weaker over the

angle and vanishes completely at 90°. The angle increases in the direction of the arrow. The graph in

the inset is the similar measurement preformed with the unloaded FSS. For both measurements

loaded and unloaded we reserved similar results for the notch depth at different resonant

frequencies. The spectral position of the resonance is stable while rotating the FSS in normal

direction. We were able to simulate the change in the resonance strength as it is shown in Fig. 3 (b).

Here the notch depth is plotted over the rotation angle. As simulated the resonance vanishes at 90°

and we achieve the maximum value at 0° and 180°. The measurement at 180° is missing in the graph

because it is disturbed by any reason and could not be evaluated.

(a) (b)

Fig. 3 (a) Spectra for the normal rotation from 0° to 90° for the loaded and unloaded (inset) FSS in 10° steps. (b) Notch

depth calculated from the spectra of the resonance, simulated (black) and measured (red) plotted over the rotation angle.

In Fig. 4 the results for the rotation in tilt direction are presented. Fig. 4 (a) depicts the spectra for

different tilted angles. To clearly arrange them they are split of into positive angle direction in the

main graph and negative angle direction in the inset. The solid line is in both cases the 0° angle. The

angle increases in the particular direction in 15° steps in the following sequence: dashed, dot-dashed

and dotted lines. The data is plotted in 15° steps but we performed the measurement in 5° steps in

both directions up to 45°. The data from the maximum notch depth and the spectral position of that

notch are extracted and plotted in Fig. 4 (b). The maximum notch depth (black) increases from -45°

to +10° and becomes relatively stable for higher angles. The spectral position where the resonance

occurs rises linearly over the rotation angle from 0.99 THz to 1.05 THz.

Fig. 4 (a) Spectra for the tilt rotation in positive and negative (inset) direction in 15° steps. 0°=solid, 15°=dashed, 30° dot-

dashed and 45°=dotted line. (b) The notch depth (black) and frequency (red) plotted over the angle.

These two effects, the shift in the spectral position and the changing in the notch depth, could not be

simulated by us so far. On that point the cooperative work between the „Solid State Spectroscopy

Group” of Prof. Tanaka in Kyoto and the „Experimentelle-Halbleiterphysik-Arbeitsgruppe” of Prof.

Koch in Marburg will be continued. Hopefully we will be able to publish our results if some additional

measurements and simulations in Kyoto and Marburg are going to be performed.

I would like to express my gratitude and thanks to Professor Tanaka for his supervision during my

stay at his institute in Kyoto. Thanks are due to Professor Naka for her encouragement. I would also

like to acknowledge the financial support of the „Global Center of Excellence” for granting me the

fellowship of Bilateral International Exchange Program (BIEP) that enables my research here.

(a) (b)

(a) (b)

[1] R. Shelby, D. Smith, and S. Schultz, Experimental verification of a negative index of refraction,

Science 292, 77 (2001).

[2] W. Barnes, A. Dereux, and T. Ebbesen, Surface plasmon subwavelength optics, Nature 424,

824–830 (2003).

[3] D. Smith, J. Pendry, and M. Wiltshire, Metamaterials and negative refractive index, Science

305, 788 (2004).

[4] D. Schurig, J. Mock, B. Justice, S. Cummer, J. Pendry, A. Starr, and D. Smith, Metamaterial

electromagnetic cloak at microwave frequencies, Science 314, 977 (2006).

[5] V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I. Zheludev, Sharp Trapped-

Mode Resonances in Planar Metamaterials with a Broken Structural Symmetry, Phys. Rev. Lett. 99,

147401 (2007).

[6] C. Debus and P. Bolivar, Frequency selective surfaces for high sensitivity terahertz sensing,

Applied Physics Letters 91, 184102 (2007).

[7] I. Al-Naib, C. Jansen, and M. Koch, Thin-film sensing with planar asymmetric metamaterial

resonators, Applied Physics Letters 93, 083507 (2008).

[8] I. Al-Naib, C. Jansen, and M. Koch, Applying the Babinet principle to asymmetric resonators,

Electronics Letters 44, 1228 –1229 (2008).

Research Report Scherger Benedikt - 京都大学depicted in Fig. 1 (b). The dimensions are chosen...

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