High rejection bandpass optical filters based on sub-wavelength metal patch arrays

Hybrid SWM-IR narrow bandpass filters with high optical density

Narrow bandpass filters (NBFs), which are designed to accept a narrow wavelength range and simultaneously reject a much wider range, show great potential in applications such as spectral imaging, lidar detection, fluorescence microscopy, and others. In this paper, we propose and numerically simulate NBF technology for infrared (IR) optical applications. The filter is a combination of plasmonic nanostructures and improved induced transmission layers. The operating wavelength range is from 1360 to 5000 nm [short wave mid-infrared radiation(SWM-IR)], with a FWHM of less than 10 nm and maximum optical density of around 10. Therefore, our SWM-IR hybrid filter can distinguish much smaller differences in terms of spectrum information and reduce the background noise level even if using an optical amplifier.

Selective terahertz absorber for angle and polarization-independent spectral sensing

Polarization- and incident-angle-independent narrow-band terahertz (THz) absorbers were developed to enable THz imaging, radar, and spectroscopy applications. The design comprises a transparent fused silica (SiOx) substrate backed by an optically thick metal layer and topped by a periodic array of metal cross patterns. Finite element analysis (FEA) simulations optimized the geometry of devices fabricated by contact photolithography. Resonances were characterized by Fourier-transform reflectance spectroscopy. The design tunable absorption bands appeared in the range 50-200 cm^-1 (1.5-6 THz) with full widths at half maximum of 20-56 cm^-1 (0.6-1.68 THz). Maximum absorption was -8.5 to -16.8 dB. The absorption bands are independent of incidence angle and polarization in agreement with simulation.

Improved selectivity of optical transmission through cascaded waveguide-metal-grating filters

We analyze the resonant transmission of two near-field coupled, cascaded bandpass filters, based on metallic stripe or patch gratings. The response, in terms of both maximum efficiency and light rejection out of the resonance, overpasses the simple convolution of the responses of two isolated filters, while keeping good angular tolerance. Illustrations are given in the infrared and visible ranges where the technological integration of such compact structures is particularly relevant for detection applications.

On the giant enhancement of light in plasmonic or all-dielectric gratings containing nano-voids

We report the possibility of generating tremendous light-field enhancements within shallow nano-trenches made in a high-index dielectric material because of resonant behaviors that are reminiscent of what we get with sub-wavelength plasmonic cavities. High quality factors are explained through a modal analysis which gives explicit design rules. The thin dielectric void gratings could be a relevant alternative to plasmon-based devices for chemical sensing, or could inspire novel efficient wavelength-selective photo-absorbers by taking poorly absorbing materials.

Highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator

Mid-infrared of the 3 - 5 μm spectral region has attracted considerable attention in various fields. We report a highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator. Two hybrid etalon resonators are serially stacked to make a single peak resonance with a well-suppressed sideband. Each etalon resonator, which exploits a pair of cavities, includes not only a cavity featuring a resonance but also a functional cavity lowering the sideband of transmission spectrum. By taking advantage of double cavities, each etalon features a resonant characteristic with suppressed sideband. Consequently, the filter shows a peak transmission of 92% at resonance and well-suppressed sideband within the 3 - 5 μm spectral region. Transparent dielectric materials make the device highly efficient for both transmission and reflection. Furthermore, the filter utilizing a high-index cavity leads to an enhanced angular tolerance regardless of incident light polarization. The transmission of the filter exhibits center wavelength shifts of around 0.1 μm at an angle of 50° for both transverse electric and transverse magnetic modes, compared to normal incidence.

Fluid-controlled tunable infrared filtering in hollow plasmonic nanofin cavities

Subwavelength structures sustaining surface plasmons have been employed in numerous fields due to their small size and ability to manipulate light beyond the diffraction limit. Light filtering using small-size plasmonic devices is a promising means of portable spectroscopy for purposes such as on-site chemical analyses. However, most plasmonic filters can only tune the resonance band by modifying the geometry of the structure or changing the incident light angle. Here, we present a plasmonic nanofin-cavity structure having a narrow band with its resonance wavelength controlled by varying the fluid in the hollow cavities of the filter. Control of the narrow-band resonance is realized over a wide range because of the coupling between the stationary surface plasmons generated from the nanofin-cavity mode and the propagating surface plasmons. The hollow cavity design enables fluid to be easily injected and removed, so that the filtered band can be controlled without the need for a complex and bulky structure or application of an external voltage.

Two-mode model for metal-dielectric guided-mode resonance filters

Symmetric metal-dielectric guided-mode resonators (GMR) can operate as infrared band-pass filters, thanks to high-transmission resonant peaks and good rejection ratio. Starting from matrix formalism, we show that the behavior of the system can be described by a two-mode model. This model reduces to a scalar formula and the GMR is described as the combination of two independent Fabry-Perot resonators. The formalism has then been applied to the case of asymmetric GMR, in order to restore the properties of the symmetric system. This result allows designing GMR-on-substrate as efficient as free-standing systems, the same high transmission maximum value and high quality factor being conserved.

Extraordinary transmission in optical Helmholtz resonators

Optical Helmholtz resonators (OHRs) have been adapted from acoustics designs for light absorbing structures, exhibiting extreme light confinement. Here, extraordinary transmission of light is theoretically demonstrated through symmetric OHRs, comprising a cavity with two λ/500 narrow slits on either side. This device has appealing features to act as a spectral bandpass filter in the context of multispectral imaging, in particular its high angular tolerance because of the localized nature of the resonance. Besides, the cavity can be modeled as an inductor and the two slits can be modeled as capacitors, the whole design acting as a LC circuit thus preventing any harmonic features.

Temporal coupled mode theory of standing wave resonant cavities for infrared photodetection

Standing wave resonating cavities have been proposed in the past to increase the performance of infrared detectors by minimizing the volume of photogeneration, hence the noise, while maintaining the same quantum efficiency. We present an approach based on the temporal coupled mode theory to explain their behavior and limitations. If the ratio of the imaginary part of the absorber's dielectric function to the index of the incident medium ε″(d)/n₀ is larger than 1.4, then the absorption cross section σ(a) can attain its maximum value, which for an isolated cavity is approximately 2λ/π. Besides, for σ(a) to exceed the cavity width, the incident medium refractive index must be close to unity. Metallic loss is negligible in the infrared, making those resonators suitable for integration in infrared photodetectors.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Free-standing guided-mode resonance band-pass filters: from 1D to 2D structures

We study experimentally and theoretically band-pass filters based on guided-mode resonances in free-standing metal-dielectric structures with subwavelength gratings. A variety of filters are obtained: polarizing filters with 1D gratings, and unpolarized or selective filters with 2D gratings, which are shown to behave as two crossed-1D structures. In either case, a high transmission (up to ≈ 79 %) is demonstrated, which represents an eight-fold enhancement compared to the geometrical transmission of the grating. We also show that the angular sensitivity strongly depends on the rotation axis of the sample. This behavior is explained with a detailed description of the guided-mode transmission mechanism.

Loss mechanisms of surface plasmon polaritons propagating on a smooth polycrystalline Cu surface

We study the propagation properties of surface plasmon polaritons on a Cu surface by means of photoemission electron microscopy. Use of a CMOS process to fabricate the Cu thin film is shown to enable very high propagation distances (up to 65 μm at 750 nm wavelength), provided that the copper native oxide is removed. A critical review of the optical loss mechanisms is undertaken and shed light on the effect of single grain boundaries in increasing the propagation losses of the plasmon. A microscopic interpretation is provided, relying on groove induced electromagnetic hot spots.