Riad Haïdar

Shaping the spatial and spectral emissivity at the diffraction limit

Metasurfaces have attracted a growing interest for their ability to artificially tailor an electromagnetic response on various spectral ranges. In particular, thermal sources with unprecedented abilities, such as directionality or monochromaticity, have been achieved. However, these metasurfaces exhibit homogeneous optical properties whereas the spatial modulation of the emissivity up to the wavelength scale is at the crux of the design of original emitters. In this letter, we study an inhomogeneous metasurface made of a nonperiodic set of optical nano-antennas that spatially and spectrally control the emitted light up to the diffraction limit. Each antenna acts as an independent deep subwavelength emitter for given polarization and wavelength. Their juxtaposition at the subwavelength scale encodes far field multispectral and polarized images. This opens up promising breakthroughs for applications such as optical storage, anti-counterfeit devices, and multispectral emitters for biochemical sensing.

Funneling of light in combinations of metal-insulator-metal resonators

Abstract. Plasmonic metal-insulator-metal resonators can totally absorb incident light. However, it is necessary to know where the incident energy is headed and which mechanism contributes to the funneling of energy. Numerical simulations have shown the concentration of light for simple resonators but other designs may allow a sharper tuning of the absorption properties. An energetic analysis was conducted with different combinations of grooves and horizontal metal-insulator-metal resonators. It showed where the energy was preferentially funneled, which highlights the possibility of photons sorting and displays the localization of the energy dissipation in such plasmonic nanoresonator. These results are giving promising design rules for multi-spectral absorbing materials.

MULTICAM: a miniature cryogenic camera for infrared detection

There is an emerging demand for compact infrared instruments, imagers and/or spectrometers, integrated on ground or air vehicles for spatial and spectral data collection. To reach this goal, technological barriers have already been overcome, leading to the development of infrared focal plane arrays (IRFPAs) for high-performance applications (megapixel format, bispectral technology) but also for low-cost and high-volume manufacturing (technology of uncooled micro-bolometers). The next step is to reduce the optics and make it compatible with the successful IRFPAs fabrication technology. This paper presents MULTICAM, a small cryogenic infrared camera. This optical system is composed of multi-level arrays of microlenses integrated in the cryostat and inspired from invertebrate compound eyes. First experimental results will be presented.

Theory of Resonant Cavity-Enhanced Detection Applied to Thermal Infrared Light

The performances of thermal infrared light detector based on a model system of resonant semiconductor microcavities are theoretically investigated. An original transfer matrix formalism of cavity enhanced absorption is presented which makes use of the small thickness of the absorbing layer compared to the light wavelengths. This formalism yields exact expressions which take standing wave effects into account in a built-in way. Approximations lead to tractable expressions which allow deriving asymptotic behaviors and general trends. The tradeoff between large cavity absorption enhancement and reduction of the detector bandwidth is particularly studied, leading to a gain-bandwidth product analysis. Approximated expressions for detectors based on resonant (i.e type I quantum dots) and nonresonant (bulk or type II quantum wells) optical transitions are also derived, which are physically meaningful and may be conveniently used for engineering purposes. It is found that the limitations due to the gain-bandwidth product conservation can be overcome. However, these cavity enhancement effects are only important for very small quantum efficiency for which the finesse of the microcavity is not seriously deteriorated.

Investigating the optical properties of nanogap optical antennas

We investigate the optical properties of nanogap MIM nanoantennas with a 2.1 nm thick gap. These plasmonic nanoantennas consist of a gold film, an insulating layer deposited by Atomic Layer Deposition, and a periodically structured gold layer. MIM nanoantennas are characterized by a tunable spectral response and a high field confinement within the nanogap. Nanometric gaps enable high coupling between the plasmons of the top and bottom metal surfaces. We demonstrated an excellent agreement between optical measurements and classical electromagnetic simulations. In particular, we observed a fluctuation in the gap thickness of 0.2 nm.

Experimental demonstration of the optical Helmholtz resonance

Optical nanoantennas are studied to manipulate light and enhance light matter interactions. Here, we experimentally demonstrate the optical Helmholtz resonance in a metallic slit-box structure, which is predicted to be harmonic and to enhance the electric field intensity by several orders of magnitude. It is fabricated thanks to a two step electron beam lithography process, between which the box was filled with benzocyclobutene (BCB). Up to 80% of the light is absorbed at a λu2009=u20092.84u2009μm wavelength under a beam focused by a Cassegrain objective (NAu2009=u20090.4), even if the dimensions of this resonator are deeply subwavelength for both the slit (width λ/55 and height λ/77) and the box (width λ/7 and height λ/37). As expected from the inductance nature of the box, the optical properties of the BCB filling the box have no influence on the resonance behavior.

Nanostructured diode for infrared photodetection through non degenerate two-photon absorption

Two-photon absorption (TPA) is a third order non-linear process that relies on the quasi-simultaneous absorption of two photons. Therefore, it has been proved to be an interesting tool to measure ultra-fast correlations1 or to design all-optical switches.2 Yet, due to the intrinsically low efficiency of the non-linear processes, these applications rest upon high peak power light sources such as femtosecond and picosecond pulsed laser. However TPA has also been noticed as an appealing new scheme for quantum infrared detection.3, 4 Indeed, typical quantum detection of IR radiation is based on small gap semiconductors that need to be cooled down to cryogenic temperature to achieve sufficient detectivity. TPA enables the absorption of IR photons by wide gap semiconductors when pump photons are provided to complete optical transitions across the gap. Still, the low efficiency of TPA represents a difficulty to detect usual infrared photon fluxes. To tackle this issue, we combined three strategies to improve the detection efficiency. First, it has been proved theoretically and experimentally that using different pump and signal photon energies which is known as non degenerate TPA (NDTPA) help increasing the TPA efficiency by several orders of magnitude.5 Thus we decided to work with different pump and signal wavelength. Secondly, since TPA is a local quasi-instantaneous process, both pump and signal photons must be temporarily and spatially co-localized inside the active medium. We made sure to maximize the overlap of the fields inside our device. Finally, it is well known that TPA has a quadratic dependence with the signal electric fields modulus, so we designed a specific nanostructure to enhance the signal field inside the active medium of the detector.

Limited-size guided-mode resonance filters under focused beams

We investigate the impact of focused beams on metal-dielectric guided-mode-resonance filters. Under a planewave illumination, these filters show a resonant transmission at an easily tunable wavelength and a good angular acceptance. Although guided-mode resonance is involved, calculations show that under a focused beam the lateral extension of the electromagnetic field inside the waveguide is limited to the width of the beam. We investigate evolution of this lateral extension and the resonant transmission with the size of the beam, as well as the impact of a tilted beam. Guided-mode-resonance filtering under a focused beam is illustrated through the example of a Cassegrain microscope lens. A process for the fabrication of simple metal-dielectric filters is also presented.

Controlling the emissivity with plasmonic nano-antennas

We experimentally demonstrate the spatial and spectral control of the thermal emission of a gold mirror in the infrared thanks to plasmonic nano-antennas made of Metal-Insulator-Metal patches. Six juxtaposed arrays of antennas with various geometries were realized on a sample in the same technological process. Their emissivity was characterized thanks to a dedicated bench, based on the combination of a Fourier transform infrared spectrometer and a high resolution infrared camera. We show that these arrays are infrared emitters that exhibit a near unity monochromatic and omnidirectional emissivity in the [3 - 5] μm spectral band.

Multispectral inhomogeneous metasurface for emissivity control

We experimentally demonstrate a multispectral metasurface that exhibits controlled inhomogeneous optical properties leading to a spatial modulation of the emissivity up to the wavelength scale in the infrared. A metasurface made of a non-periodic set of 100 million optical nano-antennas that spatially and spectrally control the emitted light up to the diffraction limit has been realized and studied. Each antenna acts as an independent deep subwavelength emitter for a given polarization and wavelength, and their juxtaposition at the wavelength scale can encode far field multispectral and polarized images.