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Schematic of waveguide comprising asymmetric resonators

3.4 Numerically simulated waveguide transmission . . . 50 3.5 Theory . . . 53 3.6 Modulating the waveguide transmission . . . 56 3.7 Discussions . . . 59

This chapter describes the near-field coupling of surface plasmon waves from asym- metric resonators in a THz plasmonic waveguides. Here the asymmetric shape res- onators are placed in close proximity and the waveguide transmission as well as elec- tric field profiles are examined to understand the near field coupling behaviour.

3.1 Introduction

The plasmonic waveguide has the ability to support highly confined surface modes in a corrugated metal surfaces. This can be important in a variety of applications, from engineering to the medical field. At terahertz frequencies, one can build a highly ef- ficient refractive index sensor by patterning the metal surface with periodic corruga- tions, as described in Chapter 2. In order to use plasmonic guided wave devices in real-world applications, it is required to explore more optimal designs including inter- nal corrugations. Plasmonic waveguide resonators in near field configurations have a variety of effects such as mode hybridization, induced absorption, broadband mod- ulation, and electromagnetically induced transparency (EIT). When two corrugated structures are close enough to each other, they interact via magnetic and electric field lines, giving rise to novel phenomena and exciting applications that may not exist in a traditional plasmonic waveguide. Further, near field coupling between asymmet- ric resonators can provide additional degrees of freedom for adjusting the dispersion characteristics of the surface plasmon. A strong near-field coupling between the reso- nances of an asymmetric resonator can lead to support mode interference. As a result, various applications are possible depending on the effects mentioned above. By con- trolling the near-field coupling of the resonances, such effects can be observed without changing the physical parameters. In the design of plasmonic waveguides for on-chip communication devices, near-field coupling of resonances and their manipulation in planar configurations can be important. Therefore, there is a strong need to investigate near-field coupling between asymmetric resonators and proceed with research to un- derstand the underlying coupling mechanism.

In this work, we examine near field coupling of a surface plasma wave in a pla- nar terahertz plasmonic waveguide comprising of a one-dimensional array of period- ically arranged asymmetric rectangular apertures placed along the transverse direc- tion. Through the excitation of both the apertures simultaneously, we study near field coupling of surface plasmons from the asymmetric size resonances. The chapter is or- ganized as follows: First, we describe the proposed waveguide geometry and examine the dispersion properties of a rectangular aperture structure to ensure a plasmonic re- sponse. After that, we numerically investigate the transmission spectra for different types of apertures to explore near field coupling of surface plasmons in the proposed waveguide along with the electric field profiles. Further, we employ a theoretical model based on a three level plasmonic system to understand the coupling mecha- nism in conjunction with the proposed geometry. The coupling strength is examined by varying the gap between the asymmetric resonators and the resultant absorption window is modulated by changing the aperture dimensions. Finally, we summarize the results in the discussion.

3.2 Schematic of waveguide comprising asymmetric resonators

The schematic illustration of the proposed terahertz plasmonic waveguide is shown in Fig. 3.1. The waveguide is made up of periodically patterned rectangular apertures in a thin sheet of metal. The unit cell of the waveguide structure is considered to be comprising of two sub-wavelength scale rectangular apertures with different lengths placed adjacent to each other along the transverse direction. The different structure parameters of the waveguide geometry are shown in the magnified view of Fig. 1. The l1 and l2 are lengths of the aperture 1 (AP1) and aperture 2 (AP2), respectively. The w andh represent the width and depth of the apertures. The periodicity and gap be- tween the apertures are represented by ‘p’ and ‘g’, respectively. The periodicity, width, and depth remain fixed in our entire analysis. The waveguide is excited with terahertz broadband signal at the one end and the terahertz surface plasmon polaritons prop-

Figure 3.1: Schematic of planar plasmonic waveguide geometry: 3-D view of proposed waveg- uide design comprising of a one-dimensional array of periodically arranged perforated rectangular apertures. Each unit cell is comprised of two rectangular apertures placed adjacent to each other along the transverse direction. The geometrical parameters of the rectangular apertures are as follows: w=150µm,p=250µm,h=500µm,g=50 µm,l1 =500µm andl2 =550µm.

agating along the corrugated pattern are detected at the other end of the waveguide.

The proposed terahertz planar plasmonic waveguide can be fabricated as discussed in previous Chapter 2. The fabricated samples can be characterized using the technique of terahertz time-domain spectroscopy. In this setup, a femtosecond laser with a repe- tition rate of 80 MHz, and temporal pulse duration of 100 fs may be used as the optical source in the experiment. The optical radiation is divided into 80:20 for use as the optical pump and probe beams, respectively. The optical pump beam is used to gener- ate terahertz waveforms via a Lt-GaAs based photoconductive emitter. The terahertz waveforms produced by the emitter are collimated and collected via the parabolic mir- rors and is normally incident onto the coupler. The coupler couples the incident broad- band terahertz pulses to the waveguide pattern. The information-carrying signals i.e.

THz surface plasmon polaritons (SPPs) are detected by the terahertz receiver i.e. a ZnTe followed by a differential detection scheme [125]. It helps to minimize the back- ground noise signal. The transient current through photodetector is amplified and measured using a lock-in amplifier. The data of the lock-in amplifier can be recorded on the computer via a Labview program. The efficient coupling of free-space terahertz waveform to the corrugated pattern is very crucial in experiments. For coupling, one

may use a semi-circular or few micron dip rectangular groove. These techniques have been previously used by Kumar et. al. [118, 119]] in experimental realization. The metal behaves as perfect conductors at terahertz frequencies, and therefore material losses can be considered to be negligible. However, the terahertz surface plasmons experience scattering and diffraction losses as they propagate along the waveguide.