光電所所訊

Multi-Band Infrared Thermal Emitter with Selectable Polarization

Professor Si-Chen Lee’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所李嗣涔教授

This study demonstrated a multi-band infrared thermal emitter featuring a narrow bandwidth emission and polarization characteristics, which is quite suitable to be applied to the non-dispersive infrared (NDIR) detection system. NDIR system has been widely used in monitoring the hazardous and harmful substances in the human living environment. The target molecules can be detected by measuring the transmission optical energy at specific wavelengths due to their particular molecular bond vibration, which is inversely proportional to the molecular concentration in the NDIR system. Because the biomolecules usually exhibit multiple absorption peaks in the infrared regions, the multi-wavelength infrared light source plays an indispensable role in the NDIR system. The device is constructed by embedding the metallic grating strips within the resonant cavity of a metal/dielectric/metal (MDM) structure, as shown in Fig. 1(a) and 1(b). This arrangement makes it possible to generate waveguide resonances with mutually orthogonal polarization, thereby providing an additional degree of freedom to vary the resonant wavelengths and polarizations in the medium infrared region. The measured reflection spectra and the finite-difference time-domain simulation indicated that the electric fields of the waveguide modes with two orthogonal polarizations are distributed in different regions of the cavity, as shown in Fig. 1(c) and (d). Resonant wavelengths in different polarizations can be adjusted by altering the period, the metallic line width, or the position of the embedded gold strips. The ratio of the full width at half maximum (FWHM) to the peak wavelength was achieved to be smaller than 0.035. This work has been published in AIP Advances 7, 085122 (2017).

FIG. 1. (a) A photograph of the multi-band infrared thermal emitter with polarized waveguide modes. (b) Schematic diagrams of polarized waveguide thermal emitter. (c) The experimental and FDTD simulated reflection spectra in x- and y-polarizations. (d) The experimental thermal radiation spectra of the device in x- and y-polarizations.

High-resolution Analysis of Leaky Modes in Surface Plasmon Stripe Waveguides

Professor Hung-chun Chang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所張宏鈞教授

The surface plasmon polariton (SPP) existing at an interface between metal and dielectric material is a basic phenomenon in the area of “plasmonics” and has been a well-known wave mode particularly in the visible and infrared wavelength ranges. With the rapid progress of the plasmonics research in recent years, various SPP waveguides have also been proposed and investigated. Here, we report our recent studies of the stripe plasmonic waveguides with a gold stripe having a silica substrate. Such simple structure was one of the earlier studied SPP waveguides. Weeber et al. reported their experimental measurement results of waveguide mode characteristics of such structure in 2003 [Phys. Rev. B 68, 115401 (2003)]. Zia et al. in 2005 [Phys. Rev. B 71, 165431 (2005)] presented finite-difference numerical analysis of the same waveguides, obtained their leaky and bound modes, and concluded that calculated leaky mode characteristics could be consistent with the observed SPP-propagation ones by Weeber et al. Zia et al. calculated the real and imaginary parts of the complex effective refractive indices, n_eff’s, of the leaky, quasi-transverse-magnetic (quasi-TM) SPP modes for different stripe widths, where n_eff is defined as the modal propagation constant divided by the free-space wavenumber. We have recently conducted high-resolution finite-element numerical analysis of the same stripe waveguide using an in-house developed full-vector finite-element imaginary-distance beam propagation method (FV-FE-ID-BPM) and discovered some subtle mode-field characteristics in the leaky modes so that the n_eff versus the stripe width curves appear to be of non-monotonic variation [H. H. Liu and H. C. Chang, J. Lightwave Technol. 34, pp. 2752–2757 (2016)]

Figure 1 Mode-field profiles of the TM₁ mode of the stripe waveguide when the stripe width is 3.7 μm: (a) Re[E_x], (b) Re[H_x], (c) Re[E_y], (d) Re[H_y], (e) Re[E_z], and (f) Re[H_z].

Fig. 1 shows the magnitude profiles of the real parts of the six field-component phasors of the lowest TM₁ mode for the stripe width of 3.7 μm (the gold stripe thickness is 55 nm). In each panel, the horizontal thin black line shows the boundary of the substrate, and the outline of the boundary of the stripe cross-section appears to be the thicker portion of the thin black line. In the analysis we used 638,347 unknowns for the computational domain of 8 μm x 11.5 μm size, which covers the right or left half of the waveguide cross-section making use of the structure symmetry. Such high spatial resolution and large coverage of the substrate region reveals the details of the leaky fields in the substrate including the interference feature. It is interesting to notice that the two stripe edges provide two sources for the propagating leaky fields. Note that H_x (Fig. 1(b)) and E_y (Fig. 1(c)) are the major field components. For further discussions, please refer to [J. Lightwave Technol. 34, pp. 2752–2757 (2016)].