Abstracts Track 2022

Area 1 - Lasers
Area 2 - Optics
Area 3 - Photonics

Area 1 - Lasers

Nr: 50
Title:

Development of a Depth-controlled Laser Ablation via Picosecond Pulsed Laser for Applications in the Monolithic Interconnection of Organic Photovoltaic (OPV)
Authors:

Nastaran Hayatiroodbari, Frank Reil and Roman Trattnig
Abstract: In recent years, selective Laser micromachining as a cost-effective and scalable method has been used for monolithic interconnection of organic photovoltaics (OPV) [1,2]. Due to their low costs, picosecond pulsed lasers are more often operated in the industry than femtosecond pulsed lasers. However, as the picosecond laser has a longer pulse duration, finding the process window for layer ablation via laser without harming the underlying layer is more challenging [3]. For monolithic interconnection via laser ablation, three different layers must be structured as defined below: P1: ablation of the bottom electrode for galvanic isolation P2: ablation of isolation of the absorber layer P3: separation of the front contact One of the processes in fabricating an OPV module is forming a conductive connection via the top and the bottom electrodes of neighbouring cells. During the ablation of the P2 and the P3 lines, damage to the bottom electrode has to be avoided; otherwise, the module's functionality may be negatively affected. This work has developed a laser process that allows the monolithic interconnection with a picosecond laser (pulse length 10 ps, 532 nm wavelength) without harming underlying layers. Laser parameters such as fluence, number of overscans, pulse frequency, etc. have been optimized to do the required isolation without harming different cell layers. This way, the electrical resistance is small and does not affect device efficiency. OPVs with a layer sequence glass/ITO/PEDOT:PSS/absorber layer/PNDIT-F3NBr/Ag have been used. For P1 in the ITO layer (back electrode), increasing the laser power leads to more effective removal of ITO. However, too much laser energy leads to the ablation of glass. The optimal laser fluence per pulse is 0.08 J/cm2 at a beam diameter of 6.25 µm, a scanning speed of 1mm/s and a pulse frequency of 100 kHz. For P2, which separates the active organic layer, the ablation requires lower laser power as the organic layer decomposes faster than the inorganic layers (P1 and P3). However, laser power should be high enough to make an electrically isolating area but sufficiently low to prevent ablation of the ITO layer. Best results were obtained for a fluence per pulse of 0.00075 J/cm2, a scanning speed of 100 mm/s and a pulse frequency of 100 kHz. In P3, the silver top electrode is removed without harming underlying layers. With increasing laser power, metal removal becomes more thorough. Yet, more heat might diffuse to and harm the layers beneath. Laser scan speeds reduce local heating but require more overscans. At the same time, the pulse frequency needs to be optimized, because when it is too high: (1) plasma and/or particle shielding limit laser ablation efficiency, (2) lower precision results due to beam absorption/deflection, (3) heat accumulation leads to magma-like or messy ejection. For P3, best results were obtained via a fluence per pulse of 7.68 J/cm² at a beam diameter of 6.25 µm, a scanning speed of 1000 mm/s, and a pulse frequency of 100 kHz. The quality and effectiveness of the applied laser settings are examined by SEM, confocal laser microscopic images and electrical conductivity measurements. 1. Kubis P, Lucera L, Machui F, et al. Org Electron. 15(10):2256–63. 2. Lucera L, Machui F, Kubis P, et al. Energy Environ Sci. 9(1):89–94. 3. Kubis P, Winter J, Gavrilova A, et al. Prog Photovoltaics Res Appl. 27(6):479–90.

Area 2 - Optics

Nr: 46
Title:

Six-wave Mixing of Optical and Microwave Fields using Rydberg Atoms in Thermal Atomic Vapor
Authors:

Tanim Firdoshi, Sujit Garain, Suman Mondal and Ashok K. Mohapatra
Abstract: Today’s communication technology with the motto of ‘radio over fiber’ aims to achieve a wireless network structure with a faster data communication rate. Hence, researchers prefer using atomic systems over traditional field sensing techniques as atomic properties are directly traceable to fundamental constants and international standards, have high sensitivity, larger dynamic range, and are not needed to be calibrated over time. Rydberg atoms, due to their large orbital radius and huge dipole moment, can sense extremely small electric fields with an operational frequency range from MHz to THz. Conversion of microwave field to the optical domain is a requirement for the transfer of quantum data. Rydberg electromagnetically induced transparency(EIT) proves to be an efficient technique for free space conversion of microwave to optical fields. The Rydberg EIT medium allows data transfer from microwave to the optical domain with a rate of the order of a few Mbit/s fundamentally limited by the optical pumping rate to the dark state. In our work, we perform the parametric six-wave mixing of optical and microwave fields in thermal atomic vapor using Rydberg atoms. The system consists of a probe, coupling, and two microwave fields in a four-level system. The probe and coupling field satisfy the two-photon resonance to the Rydberg state. A strong resonant microwave field couples the Rydberg state to another nearby Rydberg state and a second microwave field, which is weak as compared to the strong field, is also applied to the same state with a frequency difference known as the beat frequency. Inside the atomic medium, a six-wave mixing phenomenon occurs leading to the emission of a newly generated field. The frequency and direction of the generated field are found out from the energy conservation principle and phase-matching condition. The frequency difference between the probe and the newly generated field is the same as the beat frequency. We observe the bandwidth of the generated beat spectrum to be nearly 17 MHz with our experimental parameters. We also study the beat spectrum at a fixed beat frequency by changing the detuning of both the microwaves from the atomic resonance with FWHM of nearly 400 MHz. We also find the beat amplitude to depend linearly on weak microwave field strength. We also model the system of the four-level system with the perturbative expansion of the density matrix and study the optical Bloch equations to find out the first-order ground to excited state coherence. The system is Doppler averaged for the thermal atoms using Maxwell-Boltzmannian velocity distribution. The parametric process is expected to be faster than the EIT-based techniques. We amplitude modulate the weak microwave field to observe the sidebands in the beat signal. We find out the modulation index for the upper and lower sidebands of the modulated signal as a function of the modulation frequency. The modulation spectrum is compared with the beat spectrum where the bandwidth matches well. This bandwidth is limited by the available coupling rabi frequency which is nearly 5 MHz for our system. We study theoretically that in order to achieve a bandwidth of 100 MHz, we need a coupling rabi frequency of 120 MHz which requires 58 Watt power. Hence, we propose that the thermal vapor system can be used to perform data transfer at a rate of more than 100 Mbit/s which is of great technological advantage.

Area 3 - Photonics

Nr: 13
Title:

Pixel Engineering for Suppression of Spurious Image Duplicates in Reconstructions of Fourier Holograms
Authors:

Joanna Starobrat, Antoni Frej, Szymon Fiderkiewicz and Michał Makowski
Abstract: Ever since the birth of holography, its applications are growing: from beam shaping to optical tweezers, to 3-dimensional holographic displays. The presented research explores the potential of the latter. Current widely available 3D displays are burdened with disadvantages, such as accommodation-convergence conflict or limited capabilities of projecting images at dynamically adjusted distances. Computer-generated holography (CGH) technology solves these issues, creating wavefronts corresponding to an unlimited range of 3D objects. Unfortunately, spatial light modulators (SLMs) used in CGH introduce limitations of their own. Liquid crystal on silicon (LCoS) SLMs, while appreciated for the ease of pixel addressing and good diffraction efficiency, cause spurious image duplicates to appear in the reconstruction plane. The state-of-the-art approach employs spatial filtering in the Fourier plane, increasing the volume of the set-up. Alternatively, correlation systems can be used at the cost of a heightened susceptibility to noise. In our research we explore the root causes for the appearance of image duplicates. Theoretical analysis shows that their source lies in the regular pattern of rectangular pixels – an additional component to the Fourier hologram displayed on the modulator. It is responsible for the presence of the higher number of images, and it impacts the shape of a signal envelope, boosting their visibility. To counteract this effect, we have explored two methods of pixel engineering that can lead to decreased intensity in the image duplicates: pixel randomization and pixel apodization. Pixel randomization targets the regular array of pixels of LcOS SLMs. A non-uniform pixel pattern reduces the visibility of additional images, directing intensity into noise. Especially for applications such as holographic animations, static noise is an acceptable compromise. We have proven experimentally that pixel randomization can reduce visibility of image duplicates. However, an alternative medium to an LCoS SLM is necessary to create an irregular pixel array. For this purpose we have explored iron garnets which exhibit spontaneous randomization of written patterns. This class of magneto-optical materials is characterized by high transparency, fast writing times and non-visible writing wavelengths, which makes them potentially applicable in 3D displays. However, pixel randomization increases pixel pitch and thus reduces available diffraction angles of the display. For this reason, an alternative, more promising method has been researched. Pixel apodization targets the rectangular shape of an SLM pixel. By applying amplitude masks, it is possible to modify signal envelopes of image reconstructions, dampening image duplicates while also redirecting light into desired areas. Gaussian and sine masks were considered for their simplicity and effectiveness, proven in both simulation and experiment. For experimental proof, amplitude masks were fabricated with the use of electron beam lithography. The obtained results confirm theoretical predictions and are especially promising for off-axis projection, favorable in holographic displays for the ease of filtering out the zeroth diffraction order. The cost of this method lies in the intensity loss at the amplitude apodization mask in the set-up. However, its undeniable strength is in the simplicity of a set-up, and possibility to combine amplitude and phase apodization in the future stages of research.
Nr: 49
Title:

Giant Enhancement of the Transversal Magneto Optical Kerr Effect through Hyperbolic Modes in Magneto Optical Hyperbolic Metamaterials
Authors:

Brayan F. Diaz
Abstract: Hyperbolic metamaterials (HMM) are artificial photonic structures fabricated as one- or two-dimensional systems, where the sizes of the components are very small compared to the operational wavelength. Therefore, hyperbolic metamaterials can be considered as an effective media. Particularly, this type of structures can be made of a stack of metal/dielectric layers which support electromagnetic modes with hyperbolic dispersion relation, which have been exploited in spontaneous emission enhancement, super lens, etc. The hyperbolic electromagnetic modes also called bulk plasmon polariton (BPP) modes excited in HMM are highly confined into the structure.[1] On the other hand, materials with magneto optical activity have been implemented in studies about the known Kerr effect, particularly in the transversal configuration, where an external magnetic field is applied parallel to the interface of the magneto optical material and perpendicular to the plane of incidence. In this case, it is the transversal magneto optical Kerr effect (TMOKE) the principal feature of study, and which can be quantified by means of the difference of the reflectance when the external magnetic field is reversed. The TMOKE has been used in applications such as magnetic storage, optical isolators, biosensing [2] etc. It is important to mention that this magneto optical effect is very small, therefore applications are restricted to this limitation. In this work we present the enhancement of TMOKE through the excitation of BPP modes in a hyperbolic metamaterial. The metamaterial is composed by four metal/dielectric bilayers, where the dielectric material presents magneto optical activity.[3] The enhancement was obtained due to the high confinement of light, as a consequence of the excitation of the bulk plasmon polariton modes into the HMM. B. F. Diaz-Valencia acknowledges the financial support from the Colombian agency COLCIENCIAS (Postdoctoral stays - No. 848). [1] I. Avrutsky et. al., “Highly confined optical modes in nanoscale metal-dielectric multilayers”, Phys. Rev. B 75, 241402(R) (2007). [2] E. Moncada-Villa et. al., “Uniaxial epsilon-near-zero metamaterials for giant enhancement of the transverse magneto-optical Kerr effect,” Phys. Rev. B 102, 165304 (2020). [3] B. F. Diaz-Valencia, "Extraordinary transverse magneto-optical Kerr effect through excitation of bulk plasmon polariton modes in type II magneto-optical hyperbolic metamaterials," Opt. Lett. 46, 4863-4866 (2021).
Nr: 9
Title:

Boosting Nonlinear Processes with Stacked Metasurfaces
Authors:

Maria Antonietta Vincenti, Jiannan Gao, Domenico de Ceglia, Jesse A. Frantz, Michael Scalora and Natalia Litchinister
Abstract: Chalcogenide glasses are commonly used for advanced photonics applications in the infrared regime. On the other hand, their use in the visible and ultraviolet parts of the spectrum has been limited because of their high absorption. Here we demonstrate that third harmonic generation in the UV part of the spectrum is not only possible [1], but can be tailored by properly stacking metasurfaces. A nanowire-based As2S3 metasurface shows a Mie resonance that strongly localizes both magnetic and electric fields inside the nanostructure [1]. Here we first design an As2S3 metasurface having nanowire height and width w = 300 nm, and periodicity p = 600 nm, so that a Mie resonance appears at 900 nm. We then stack several identical metasurfaces with identical spacing and periodicity p and evaluate the linear transmittance of one, two- and three-layers configurations as functions of the wavelength and found that the Mie resonance that usually manifests itself as a transmittance dip quickly transforms into a band gap with a mere two layers configuration. As the layers increase the band edges become very sharp as one might expect from photonic crystal structures. The formation of the band gap and sharpening of band edges results in a significant increase of the third harmonic efficiency, notwithstanding the fact that the harmonic is tuned deep in the absorption range. Moreover, our theoretical investigation performed by increasing the total number of layers in the metasurface stack also reveals that nanostructures with only a few layers might indeed perform better than stacks with a larger number of layers, thanks to broader resonant features and saturation effects that occur under higher pump intensity conditions.
Nr: 11
Title:

Electron Beam Diagnosis Utilizing Fiber Bragg Gratings
Authors:

Gabriel P. Bleotu, Andrei Stancalie, Laura Mihai, Razvan Mihalcea, Daniel Ighigeanu, Antreas Theodosiou and Kyriacos Kalli
Abstract: The ionizing radiation fields such as gamma, electrons, protons, X-rays can significantly affect the proper functioning of electronic devices designed for space applications or nuclear reactors. The reports on the effects of electron radiation on Fiber Bragg Gratings (FBG) are limited compared to the database of gamma or proton radiation effects. Thus, two types of FBGs, radiation resistant and commercial, were irradiated in the electron field up to an absorbed dose of 120 kGy at the energy of 5.5 MeV. The fibers were mounted in a 3D configuration that allows to diagnose the electron beam profile/energy distribution with respect to the absorbed dose and the distance to a “traveling-wave” linear accelerator exit. A total number of 11 commercially and custom FBGs, fabricated by fs-laser inscription were investigated in real time for the Proof-of concept and compared from the point of view of Bragg Wavelength Shift (BWS), absorbed dose, and radiation tolerance. The obtained information will lead to a better understanding of the electron beam profile characteristics, the centroid location, the beam shape and flatness, and their functionality in critical installations or in laser-accelerated electrons beams.
Nr: 47
Title:

Tailoring Magnetic Dipole Emission of Eu(3+) by Broken Symmetry TiO2 Metasurface
Authors:

Ayesheh Bashiri, Aleksandr Vaskin, Katsuya Tanaka, Thomas Pertsch and Isabelle Staude
Abstract: High index dielectric metasurfaces supporting Mie type resonances are a versatile platform for both spectral and spatial tailoring of spontaneous emission from coupled quantum emitters as they provide low absorption loss, high radiation efficiency, and substantial radiative decay rate enhancement [1,2]. In a symmetry-broken metasurface geometry, coupling between bright and dark modes of the nanostructure can occur, leading to the formation of high-quality factor hybrid modes that can provide strong photoluminescence (PL) enhancement and emission directionality [3]. Here, we designed and fabricated a broken symmetry titanium dioxide (TiO2 ) metasurface supporting an out-of-plane magnetic dipole (MD) resonance at 590 nm wavelength, corresponding to the MD transition of trivalent Europium ions (Eu(3+)). We spin coated the fabricated metasurface with a polymethyl methacrylate (PMMA) resist film containing Europium(III) thenoyltrifluoroacetone and performed PL spectroscopy to investigate PL emission enhancement provided by the metasurface. Using a 4=0:1NA collective objective in PL spectroscopy, we achieved a brightness enhancement of up to 15.5 for the MD transition at 590 nm. The enhancement ratio of the MD transition at 590 nm to the electric dipole (ED) transition at 610 nm is 8.7. Furthermore, by investigating the spatial emission characteristics of the coated metasurface using back focal plane imaging of emission, we were able to show that the obtained selective MD enhancement is mainly due to an improved collection efficiency for that transition. REFERENCES: [1] Baranov DG. et al., “Modifying magnetic dipole spontaneous emission with nanophotonic structures”, Laser Photon Rev, 1600268 (2017). [2] Vaskin, A. et al., “Directional and Spectral Shaping of Light Emission with Mie-Resonant Silicon Nanoantenna Arrays”, ACS Photonics, 1359-1364 (2018). [3] Sheng Liu. et al., “Light-Emitting Metasurfaces: Simultaneous Control of Spontaneous Emission and Far-Field Radiation”, Nano Letters, 6906-6914 (2018).