Publications
Dissertation
Optical programming of infrared phase-change material metasurfaces
Dissertation, RWTH Aachen (2022)
Andreas Heßler
Infrared light is tightly interwoven with modern technology, from telecommunications over thermal imaging to medical diagnostics. Infrared optical instruments, like mirror objectives, are bulky and only have one functionality while compact optical components with multiple integrated, reconfigurable functionalities are needed, for example, for mobile phone technology or autonomous driving. Nanometer-thick active metasurfaces based on phase-change materials (PCMs) have emerged as a promising way for realizing such devices. They consist of periodically arranged subwavelength-sized antennas (“meta-atoms”). The PCM in or around the antennas can be switched non-volatilely between its amorphous and crystalline phases. The corresponding drastic change in the refractive index of the PCM tunes the antennas’ resonance. Commonly, multiple operation states are achieved by switching the whole PCM on the metasurface equally. For programmable metasurfaces with freely reconfigurable functionality, however, the ability to selectively change the light amplitude and phase of each individual meta-atom is required. In this thesis, therefore, concepts for the programming of infrared PCM metasurfaces by local optical switching with a focused laser are developed and realized experimentally. First, the local optical addressing of individual meta-atoms is introduced and demonstrated at aluminum nanorod antenna arrays covered with a 75 nm-thick layer of the PCM Ge3Sb2Te 6 (GST). Simultaneous control of size, position and crystallization depth of the switched PCM volume is used to tune the resonances of the antennas by more than one resonance width (FWHM) from about 5 µm to 6 µm. Next, this technology is applied to dielectric infrared Huygens’ metasurfaces where each meta-atom consists of a disk with a germanium core sandwiched by two 70 nm-thick GST layers. The antenna resonances of individual disks are tuned by up to 360 nm (1.8 FWHM), leading to a change in the light phase of up to 0.8*2π at an average transmittance of 50%. Different spatial light phase distributions are optically encoded onto the metasurface. Finally, the next-generation plasmonic PCM In3SbTe2 (IST) is introduced. Its ability to switch between dielectric and metallic optical properties in the infrared spectral range enables completely new resonance tuning mechanisms. Direct optical writing, erasing and reconfiguring of plasmonic antennas in a 50 nm-thick IST film are demonstrated. Electric dipole resonances of rod antennas are thus tuned by more than 4 µm and magnetic dipole resonances of split-ring resonators are shifted by more than 1.6 µm. Finally, a tunable mid-infrared absorber with nearly 90% absorptance as well as nanoscale selective screening of Al-slit antennas and soldering of Au-dimers are presented. The developed technology of optical programming of PCM metasurfaces may pave the way towards the ideal of a “universal” metasurface which can freely manipulate incident light. This could lead to highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.
Papers
Reconfigurable and Polarization‐Dependent Grating Absorber for Large‐Area Emissivity Control Based on the Plasmonic Phase‐Change Material In3SbTe2
Advanced Optical Materials 11, 2202696 (2023)
Metasurfaces with perfect infrared absorption promise integrated filters and compact detector elements with narrowband thermal emission. Phase-change materials (PCMs) are prime candidates for active, non-volatile absorption tuning. Commonly, the response of the entire metasurface is tuned, while local adaptions remain elusive. In this work, flexible encoding of different absorption/emission properties within a metasurface is shown. The plasmonic PCM In3SbTe2 (IST) is employed to obtain control over the emissivity by patterning an adaptable grating absorber metasurface. Using a commercial direct laser writing setup, the IST is locally switched from an amorphous dielectric into a crystalline metallic state, and cm-sized stripe gratings are written above a reflecting mirror. Modification of already written patterns is demonstrated by changing the laser power and thus the IST stripe width to encode different polarization-sensitive patterns with nearly perfect absorption into the same metasurface. Finally, an apparent local temperature pattern due to the large-area emissivity shaping metasurface is measured with a conventional thermal camera. The results pave the way towards low-cost, large-area, and adaptable patterning of metasurfaces with wavelength and polarization-selective perfect absorption, enabling applications like enhanced thermal detection, infrared camouflage, or encoding anti-counterfeiting symbols.
Infrared Resonance Tailoring of Individual Split‐Ring Resonators with Phase‐Change Materials by Locally Changing the Dielectric Surrounding of the Antenna Hotspots
Advanced Optical Materials 11, 2300499 (2023)
For miniaturized active metasurfaces, resonance tuning of nanoantennas is a key ingredient. Phase-change materials (PCMs) have been established as prime candidates for non-volatile resonance tuning enabled by a change in the refractive index around nanoantennas. Conventionally, this tuning is induced by annealing the entire sample equally and does not allow changes on a meta-atom level. Recently, it is demonstrated that individual rodantenna resonances can be adjusted by addressing each meta-atom locally with precise laser pulses and switching the PCM there. However, simultaneously controlling several different modes remains elusive. In this work, PCM-covered aluminum split-ring resonators (SRRs) are switched locally to tune both the electric dipole resonances as well as the magnetic dipole resonances. By selectively switching the PCM at different hotspots of the SRRs, both resonances can be tuned individually. Finally, the field enhancement in the magnetic resonance allows continuous tuning of surface-enhanced infrared absorption of native SiO2. This work serves as a proof of principle for sophisticated resonance tuning via changes in the refractive index at the hotspots of the selected antennas enabling fine-tuning functionalities on a meta-atom level and allows for post-fabrication adjustments of metasurfaces.
Investigation of low-confinement surface phonon polariton launching on SiC and SrTiO3 using scanning near-field optical microscopy
Applied Physics Letters 120, 211107 (2022)
Surface phonon polaritons (SPhPs) are important building blocks of nanophotonics, as they enable strong light–matter interaction on the nanoscale, are well-suited for applications in the mid- to far-infrared regime, and can show low losses. SrTiO3 is an interesting material for SPhPs, because it allows for reversible, nonvolatile doping with free charge carriers via oxygen vacancies and for local switching with conductive AFM tips. As a result, SrTiO3 could enable programmable nanophotonics with tunable SPhPs and direct writing of metasurfaces. Surface polariton properties can be determined by mapping their real-space propagation using scattering-type scanning near-field optical microscopy (s-SNOM), which is sensitive to the high local electric fields with nanoscale lateral resolution. Low-confinement (LC) SPhPs with wavevectors close to that of free-space radiation, such as in SrTiO3 and the model polar dielectric SiC, can be difficult to investigate in s-SNOM due to interference effects with the incident illumination and fringe spacings exceeding the scan range or the size of the focus spot. Here, we present s-SNOM measurements of LC-SPhPs on SiC and SrTiO3 launched at gold stripes, retrieve physical quantities such as launching amplitude and phase, and show that they are influenced strongly by gold stripe geometry as well as illumination angle. Using two complementary measurements, we show a convenient way to determine the out-of-plane angle of the s-SNOM setup. Finally, we predict how control over the free charge carrier concentration in SrTiO3 could enable tunable LC-SPhPs, showing the potential of SrTiO3 for programmable nanophotonics.
Reconfiguring Magnetic Infrared Resonances with the Plasmonic Phase-Change Material In3SbTe2
ACS Photonics 9 (2022)
For miniaturized active nanophotonic components like adjustable lenses, resonance tuning of nanoantennas is essential. Phase-change materials (PCMs) have been established as prime candidates for nonvolatile resonance tuning based on a change in the refractive index. Currently, a novel material class of switchable infrared plasmonic PCMs, like In3SbTe2 (IST), is emerging. Because IST can be locally optically switched between dielectric (amorphous) and metallic (crystalline) states, it becomes possible to directly change the geometry and shape of nanoantennas to tune their infrared resonances. Here, crystalline IST split-ring resonators (SRRs) are directly optically written and reconfigured to continuously tune their magnetic dipole resonance wavelengths from 10.6 to 8.2 μm without changing their electric dipole (ED) resonances. The SRRs are further modified into crescents and J-antennas, displaying electric quadrupole and rotated ED modes, respectively. Our concepts may be well suited for rapid prototyping, speeding up workflows for engineering ultrathin, tunable, plasmonic devices for infrared nanophotonics.
Nanostructured In3SbTe2 antennas enable switching from scharp dielectric to broad plasmonic resonances
Nanophotonics 11 (2022)
Phase-change materials (PCMs) allow for non-volatile resonance tuning of nanophotonic components. Upon switching, they offer a large dielectric contrast between their amorphous and crystalline phases. The recently introduced “plasmonic PCM” In3SbTe2 (IST) additionally features in its crystalline phase a sign change of its permittivity over a broad infrared spectral range. While optical resonance switching in unpatterned IST thin films has been investigated before, nanostructured IST antennas have not been studied, yet. Here, we present numerical and experimental investigations of nanostructured IST rod and disk antennas. By crystallizing the IST with microsecond laser pulses, we switched individual antennas from narrow dielectric to broad plasmonic resonances. For the rod antennas, we demonstrated a resonance shift of up to 1.2 µm (twice the resonance width), allowing on/off switching of plasmonic resonances with a contrast ratio of 2.7. With the disk antennas, we realized an increase of the resonance width by more than 800% from 0.24 µm to 1.98 µm while keeping the resonance wavelength constant. Further, we demonstrated intermediate switching states by tuning the crystallization depth within the resonators. Our work empowers future design concepts for nanophotonic applications like active spectral filters, tunable absorbers, and switchable flat optics.
Observing 0D subwavelength-localized modes at ~100 THz protected by weak topology
Science Advances 7, eabl3903 (2021)
Topological photonic crystals (TPhCs) provide robust manipulation of light with built-in immunity to fabrication tolerances and disorder. Recently, it was shown that TPhCs based on weak topology with a dislocation inherit this robustness and further host topologically protected lower-dimensional localized modes. However, TPhCs with weak topology at optical frequencies have not been demonstrated so far. Here, we use scattering-type scanning near-field optical microscopy to verify mid-bandgap zero-dimensional light localization close to 100 THz in a TPhC with nontrivial Zak phase and an edge dislocation. We show that because of the weak topology, differently extended dislocation centers induce similarly strong light localization. The experimental results are supported by full-field simulations. Along with the underlying fundamental physics, our results lay a foundation for the application of TPhCs based on weak topology in active topological nanophotonics, and nonlinear and quantum optic integrated devices because of their strong and robust light localization.
Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6
Nano Letters 21, 9012-9020 (2021)
Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge3Sb2Te6 (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.
Ultra-Thin Switchable Absorbers Based on Lossy Phase-Change Materials
Advanced Optical Materials 9, 2101118 (2021)
Absorbers for infrared light are important optical components in key areas like biosensing, infrared imaging, and (thermal) light emission, with special need for thin and reconfigurable devices. Here, the authors demonstrate ultra-thin, switchable infrared absorbers based on thin layers of chalcogenide phase-change materials (PCMs) with high optical contrast between a lossless amorphous and an exceptionally lossy crystalline phase (Ge3Sb2Te6, Ge2Sb2Te4, Ge2SbTe4, Ag4In3Sb67Te26, and GeTe) on top of polar substrates (SiC, Al2O3, and SiO2). It is found that light is mainly absorbed in the substrate for amorphous PCMs, and in the thin layer for crystalline PCMs. Using the concept of admittance matching, the authors demonstrate dramatic layer thickness reduction by a factor f = πκ of up to 14 for high PCM extinction coefficients κ compared to classic λ/4 anti-reflection coatings. The authors show continuous tuning of the maximum absorption wavelength by up to 2.5 µm in the epsilon-near-zero ranges of the substrates via annealing on a hot plate and optical switching. By selecting a suitable PCM-substrate combination, the tuning range and its size can be shifted through the whole infrared range. The results demonstrate that exceptionally lossy PCMs show great potential for ultra-thin, reconfigurable nanophotonic devices.
Combining Switchable Phase-Change Materials and Phase-Transition Materials for Thermally Regulated Smart Mid-Infrared Modulators
Advanced Optical Materials 9, 2100417 (2021)
In3SbTe2 as a programmable nanophotonics material platform for the infrared
Nature Communications 12, 924 (2021)
The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices. Here, we employ the next-generation PCM In3SbTe2 (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale “soldering” of metallic nanoantennas. Our concepts can empower improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms.
All-Dielectric Programmable Huygens’ Metasurfaces
Advanced Functional Materials 30, 1910259 (2020)
Low-loss nanostructured dielectric metasurfaces have emerged as a breakthrough platform for ultrathin optics and cutting-edge photonic applications, including beam shaping, focusing, and holography. However, the static nature of their constituent materials has traditionally limited them to fixed functionalities. Tunable all-dielectric infrared Huygens’ metasurfaces consisting of multi-layer Ge disk meta-units with strategically incorporated non-volatile phase change material Ge3Sb2Te6 are introduced. Switching the phase-change material between its amorphous and crystalline structural state enables nearly full dynamic light phase control with high transmittance in the mid-IR spectrum. The metasurface is realized experimentally, showing post-fabrication tuning of the light phase within a range of 81% of the full 2π phase shift. Additionally, the versatility of the tunable Huygen’s metasurfaces is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta-unit precision and retrieving high-resolution phase-encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces, paving the way for “universal” metasurfaces and highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.
Surface Polariton-Like s-Polarized Waveguide Modes in Switchable Dielectric Thin Films on Polar Crystals
Advanced Optical Materials 8, 1901056 (2020)
Surface phonon polaritons (SPhPs) and surface plasmon polaritons (SPPs), evanescent modes supported by media with negative permittivity, are a fundamental building block of nanophotonics. These modes are unmatched in terms of field enhancement and spatial confinement, and dynamical all-optical control can be achieved, e.g., by employing phase-change materials. However, the excitation of surface polaritons in planar structures is intrinsically limited to p-polarization. On the contrary, waveguide modes in high-permittivity films can couple to both p- and s-polarized light, and in thin films, their confinement can become comparable to surface polaritons. Here, it is demonstrated that the s-polarized waveguide mode in a thin Ge3Sb2Te6 (GST) film features a similar dispersion, confinement, and electric field enhancement as the SPhP mode of the silicon carbide (SiC) substrate, while even expanding the allowed frequency range. Moreover, it is experimentally shown that switching the GST film grants nonvolatile control over the SPhP and the waveguide mode dispersions. An analytical model is provided for the description of the GST/SiC waveguide mode and it is shown that the concept is applicable to the broad variety of polar crystals throughout the infrared spectral range. As such, complementarily to the polarization-limited surface polaritons, the s-polarized waveguide mode constitutes a promising additional building block for nanophotonic applications.
Advanced Optical Programming of Individual Meta-Atoms Beyond the Effective Medium Approach
Advanced Materials 31, 1901033 (2019)
Nanometer-thick active metasurfaces (MSs) based on phase-change materials (PCMs) enable compact photonic components, offering adjustable functionalities for the manipulation of light, such as polarization filtering, lensing, and beam steering. Commonly, they feature multiple operation states by switching the whole PCM fully between two states of drastically different optical properties. Intermediate states of the PCM are also exploited to obtain gradual resonance shifts, which are usually uniform over the whole MS and described by effective medium response. For programmable MSs, however, the ability to selectively address and switch the PCM in individual meta-atoms is required. Here, simultaneous control of size, position, and crystallization depth of the switched phase-change material (PCM) volume within each meta-atom in a proof-of-principle MS consisting of a PCM-covered Al–nanorod antenna array is demonstrated. By modifying optical properties locally, amplitude and light phase can be programmed at the meta-atom scale. As this goes beyond previous effective medium concepts, it will enable small adaptive corrections to external aberrations and fabrication errors or multiple complex functionalities programmable on the same MS.
Highly Confined and Switchable Mid-Infrared Surface Phonon Polariton Resonances of Planar Circular Cavities with a Phase Change Material
Nano Letters 19, 2549–2554 (2019)
Mid-infrared (MIR) photonics demands highly confined optical fields to obtain efficient interaction between long-wavelength light and nanomaterials. Surface polaritons excited on polar semiconductor and metallic material interfaces exhibit near-fields localized on subwavelength scales. However, realizing a stronger field concentration in a cavity with a high quality (Q) factor and a small mode volume is still challenging in the MIR region. This study reports MIR field concentration of surface phonon polaritons (SPhPs) using planar circular cavities with a high Q factor of ∼150. The cavities are fabricated on a thin film of the phase change material Ge3Sb2Te6 (GST) deposited on a silicon carbide (SiC) substrate. Scattering-type scanning near-field optical microscopy visualizes the near-field distribution on the samples and confirms directly that the SPhP field is strongly concentrated at the center of the centrosymmetric cavities. The smallest concentrated field size is 220 nm in diameter which corresponds to 1/50 of the wavelength of the incident light that is far below the diffraction limit. The thin GST film enhances the SPhP confinement, and it is used to switch the confinement off by tuning the cavity resonance induced by the phase change from the amorphous to the crystalline phase. This subwavelength and switchable field concentration within a high-Q polariton cavity has the potential to greatly enhance the light-matter interaction for molecular sensing and emission enhancement in MIR systems.