Large-area polycrystalline α-MoO3 thin films for IR photonics

In recent years, the excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide (α-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10–20 µm). Here, we report on the fabrication, structural, morphological, and optical IR characterization of large-area (over 1 cm2 size) α-MoO3 polycrystalline film deposited on fused silica substrates by pulsed laser deposition. Due to the random grain distribution, the thin film does not display any optical anisotropy at normal incidence. However, the proposed fabrication method allows us to achieve a single α-phase, preserving the typical strong dispersion related to the phononic response of α-MoO3 flakes. Remarkable spectral properties of interest for IR photonics applications are reported. For instance, a polarization-tunable reflection peak at 1006 cm−1 with a dynamic range of ∆R = 0.3 and a resonance Q-factor as high as 53 is observed at 45◦ angle of incidence. Additionally, we report the fulfillment of an impedance matching condition with the SiO2 substrate leading to a polarization-independent almost perfect absorption condition (R < 0.01) at 972 cm−1 which is maintained for a broad angle of incidence. In this framework our findings appear extremely promising for the further development of mid-IR lithography-free, scalable films, for efficient and large-scale sensors, filters, thermal emitters, and label-free biochemical sensing devices operating in the free space, using far-field detection setups

Large-area polycrystalline α\alpha-MoO3 thin films for IR photonics

In recent years, excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide ($\alpha$-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10-20 $\mu$m). Here, we report on the fabrication and IR characterization of large-area (over 1 cm$^2$ size) $\alpha$-MoO3 polycrystalline films deposited on fused silica substrates by pulsed laser deposition. Single alpha-phase MoO3 films exhibiting a polarization-dependent reflection peak at 1006 cm$^{-1}$ with a resonance Q-factor as high as 53 were achieved. Reflection can be tuned via changing incident polarization with a dynamic range of $\Delta$R=0.3 at 45 deg. incidence angle. We also report a polarization-independent almost perfect absorption condition (R<0.01) at 972 cm$^{-1}$ which is preserved for a broad angle of incidence. The development of a low-cost polaritonic platform with high-Q resonances in the mid-infrared (mid-IR) range is crucial for a wide number of functionalities including sensors, filters, thermal emitters, and label-free biochemical sensing devices. In this framework our findings appear extremely promising for the further development of lithography-free, scalable films, for efficient and large-scale devices operating in the free space, using far-field detection setups.Comment: 17 pages, 12 figure

VO2 Tungsten Doped Film IR Perfect Absorber

We investigated infrared reflectivity of undoped and Tungsten (W) doped Vanadium dioxide (VO2) films at varying temperatures. Undoped VO2 exhibited a clear phase transition at 100°C, achieving near 0% reflectivity, or perfect light absorption. As W doping concentration increased, the phase-transition temperature decreased, maintaining the zero-reflectivity condition. Only a 0.75% W doping enabled room temperature perfect absorption without heating the film

Emilim ve foto algılama için metal-yarı iletken çoklu istifleri

Cataloged from PDF version of article.Thesis (M.S.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2017.Includes bibliographical references (leaves 82-90).Metal-insulator (MI) stacks are one of the most studied nanoscale devices of the recent decade. These structures have opened a new door to endless photonic applications ranging from solar cells to waveguides and polarizers. The main attribute of metal-insulator stacks is possibility of scaling down device dimensions with them that is the main trend in photonic and electronic technology nowadays. The conventional photonic structures require very high thicknesses where novel photonic devices can show many arti cial properties by tailoring speci cally designed metal-insulator cells also known as metamaterials. In this thesis, we will investigate some metal-insulator absorber stacks with capability of highly con ning light speci cally for photodetection. The near-infrared part of the electromagnetic spectrum is problematic in photocurrent generation due to the fact that conventional narrow band gap PN photodiodes fail to function in room temperature. Adding to this predicament is their large dimensions. Some of these problems are addressed in this thesis. First a plasmonic MIM structure is studied with random nanoparticles obtained by dewetting in the top layer which con nes the incident light in the plasmonic MIM cavity and gives rise to high absorption through surface plasmon polariton excitation in the bottom lossy metal. Several materials are investigated in order to engineer best absorbers with the focus on absorption in the bottom metal which is critical for photodetection. Our simulations and experimental results demonstrate over 90 percent absorption for most of the visible and near-infrared region. The absorption in the bottom metal in a structure comprised of chromium-aluminum oxide-silver nanoparticles (bottom to top) reaches 82 percent at 850 nm. After obtaining appropriate NIR absorption, an MIMIM photodetector is designed and fabricated where another insulator-metal layer is added to the bottom of the previous absorber. The formerly reported plasmonic photodetectors put the burden of absorption and photocurrent path on the same MIM structure putting restrictions on device design. In our proposed structure, however, tunneling MIM photocurrent junction is used which shares only its top metal with the top absorbing MIM. The main advantage of this structure is that it separates the absorption and photocurrent parts of the photodetector, making separate optimization of each MIM possible. The best structure which is silver-hafnium oxide-chromium-aluminum oxide-silver nanoparticles (top to bottom) demonstrates a peak photoresponsivity (from nonradiative decay of surface plasmon polaritons) of 0.962 mA/W at 1000 nm and a dark current of only 7 nA in a bias of 50 mV. Our results demonstrate approximately two orders of magnitude enhancement in photoresponsivity compared to previously reported MIMIM photodetectors. In another attempt to obtain perfect absorbers for visible and near-infrared regions, we put forth an MIMI absorber. In this work, the contribution of metal layers is studied in detail and material choice is discussed. Our optimization process suggests a versatile method for designing perfect absorbers. Transfer matrix method as well as FDTD simulations are used to optimize thicknesses. Furthermore, in order to shed light on material selection, impedance matching of the waves in the multilayer media to free space is proposed for the extraction of ideal metal permittivity values and comparing them to existing metals. Our experimental result of a tungsten-aluminum oxide-titanium-aluminum oxide (bottom to top) structure illustrates over 90 percent absorption for wavelength range of 400 nm to 1642 nm which is the highest perfect absorption bandwidth reported in similar MIMI structures to the best of our knowledge.by Sina Abedini Dereshgi.M.S

Highly Efficient Light Absorption of Monolayer Graphene by Quasi-Bound State in the Continuum

Graphene is an ideal ultrathin material for various optoelectronic devices, but poor light–graphene interaction limits its further applications particularly in the visible (Vis) to near-infrared (NIR) region. Despite tremendous efforts to improve light absorption in graphene, achieving highly efficient light absorption of monolayer graphene within a comparatively simple architecture is still urgently needed. Here, we demonstrate the interesting attribute of bound state in the continuum (BIC) for highly efficient light absorption of graphene by using a simple Si-based photonic crystal slab (PCS) with a slit. Near-perfect absorption of monolayer graphene can be realized due to high confinement of light and near-field enhancement in the Si-based PCS, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Theoretical analysis based on the coupled mode theory (CMT) is proposed to evaluate the absorption performances of monolayer graphene integrated with the symmetry-broken PCS, which indicates that high absorption of graphene is feasible at critical coupling based on the destructive interference of transmission light. Moreover, the absorption spectra of the monolayer graphene are stable to the variations of the structural parameters, and the angular tolerances of classical incidence can be effectively improved via full conical incidence. By using the full conical incidence, the angular bandwidths for the peak absorptivity and for the central wavelength of graphene absorption can be enhanced more than five times and 2.92 times, respectively. When the Si-based PCS with graphene is used in refractive index sensors, excellent sensing performances with sensitivity of 604 nm/RIU and figure of merit (FoM) of 151 can be achieved