Materials for Advanced Detectors 2025

6 Oct 2025, 09:00 → 7 Oct 2025, 22:00 Europe/Berlin

Leibniz-Insitut für Kristallzüchtung (IKZ)

Materials for Advanced Detectors 2025 (MAD25)

Workshop: Oct. 6-7, 2025

MAD Workshop is an excellent opportunity to connect with collegues involved in the development of advanced materials for gravitational wave detectors as well as with researchers from crystal growth community and industry who share a deep interest in the topic. Together we will discuss recent achievements and challenges in material research and outline prospects for future cooperation. We are confident that face-to-face workshops with a lot of space for personal interaction and individual meetings will contribute to achieving our common goals.

Where:

The event takes place on the premises of Leibniz-Institut für Kristallzüchtung (IKZ) and hosted by Forschungsverbund Berlin e.V. 

IKZ is located in the capital region Berlin-Branderburg in the middle of Germany's largest Science and Technology Park Adlershof.

 

General Info:

This is a hybrid event. Participants can join all sessions either in person or online via Zoom. Personal attendance is highly appreciated.

Zoom link will be provided to all registered participants. Minutes for the meeting will be recored in the shared document.

All participants are welcome to contribute to the note taking in the meeting and the creation of a report after the meeting.

 

Beyond the sessions:

Crystal growth lab tour featuring a variety of materials and techniques

Social event

 

Presentations and Posters:

It is possible to submit an abstract for either a talk or a poster. Note that, depending on the topic and on the available time slot, we might ask you to convert your contribution to either a talk or a poster.

 

Sponsoring:

We have opportunities for sponsoring that will highlight your support of the community. If you are interested to present your company or institute, meet the attendees and/or support us, please contact mad2025@ikz-berlin.de. We have the right bundle for your successful participation!

 

Registration:

Registration will open soon. Note that the registration is considered completed only when the payment has been fulfilled. In addition, if no payment has been received after the deadline, you will be automatically deregistered from in-person participation.

No payment fee is required for remote participation.

 

Important Deadlines:

·         Registration Open: 14th July

·         Early Registration Close: 31st August

·         Final Registration Close: 7th September

 

·         Abstract Submission Open: 4th July

·         Abstract Submission Close: 10th August

·         Abstract Review Completed: 24th August

 

Local Organizing Committee:  

Iryna Buchovska, Robert Menzel

 

Scientific Committee:

Alex Amato, Iryna Buchovska, Elisabetta Cesarini, Margot Henning, Robert Menzel, Luca Naticchioni, Andrew Spencer, Flavio Travasso.

 

We look forward to welcoming all participants, both in person and online, for a productive and engaging meeting!

 

Monday 6 October

10:00 → 12:00 Lab Tour: Lab Tours of IKZ. !!!Please note, the Lab Tour starts at IKZ Lobby!!!

12:00 → 12:30 Coffee Break

12:30 → 12:45 Welcome and opening

Convener: Dr Iryna Buchovska (Leibniz-Institut für Kristallzüchtung)

12:45 → 13:45 MAD25: Session 1. Substrates

MAD: Substrates

12:45 Introduction on Substrate Research

Speaker: Jessica Steinlechner

13:00 Polishing and characterisation of sapphire substrates for third-generation gravitational wave detectors in Lyon.

In Lyon, we are investigating the potential of using sapphire mirrors for the Einstein Telescope Low-Frequency (ET-LF) detector. Compared to silicon, sapphire offers several advantages in cryogenic environments. These include superior thermal conductivity, higher density and optical transparency across the visible and the near infrared spectrum. Although KAGRA currently uses sapphire test masses, there is still limited R&D activity focused on this material. At the University of Lyon campus, we grow, polish and characterise sapphire substrates. This presentation will focus on our progress in the polishing process, in which we have achieved a flatness of $\lambda/60$ in the central region. The presentation will also include our plan for future developments.

Speaker: Severin Nadji (Laboratoire des Matériaux Avancés (LMA))

13:15 Large diameter Float-Zone crystals for gravitational wave detectors

The Float-Zone (FZ) method is the only industrially established technique for producing large silicon crystals of ultrahigh purity. The resulting low absorption at 1550 nm makes FZ silicon a strong candidate for cryogenic mirror substrates in next-generation gravitational wave detectors such as the Einstein Telescope (ET). Absorption levels below 10 ppm/cm have been measured in crystals grown by the FZ method.
A major barrier for the application of FZ-Si as mirror substrate is the achievable crystal diameter, currently limited to 200 mm, whereas the ET requires mirror substrates of at least 450 mm in diameter. Although larger FZ-Si crystals also offer cost advantages for the production of electronic power devices, technological limitations and market constraints have so far hindered the introduction of larger-diameter FZ crystals.
This talk provides insight into the FZ-Si wafer production chain, challenges in scaling FZ-Si crystal diameter and semiconductor market dynamics. A research project for the development of 300 mm FZ-Si crystals is outlined. Potential synergies between semiconductor industry requirements and third-generation gravitational wave detectors are considered.

Speaker: Dr Sune Duun (Topsil GlobalWafers A/S)

13:30 Growth method dependent purity of silicon crystals for ET-mirrors

The Einstein Telescope (ET) is a future 3rd generation gravitational wave observatory in Europe and crystalline silicon is under investigation to be used for mirrors. For the semi-transparent ET interferometer mirrors, large Si crystals [1] with ultrahigh purity are needed to minimize thermal noise (low laser light absorption and mechanical loss).
The defect structure and purity of the volume crystals are determined by the growth method. The crucible-free floating zone (FZ) method allows to grow high-purity single crystals with limited diameter, while crystals grown by the Czochralski (Cz) method exhibit in general more impurities due to the contact between the melt and the crucible, but they can be grown with larger diameter.
These differences in the growth method dependent purity level will be discussed and ideas for reducing crucible erosion by using travelling magnetic fields during the Cz-growth will be presented.

References:
[1] Frank M. Kießling et al.: “Quasimonocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors”, Phys. Rev. Research 4 (2022), 043043

Speaker: Frank Kiessling (Leibniz-Institut für Kristallzüchtung)

13:45 → 14:45 Lunch break and poster session

14:45 → 16:00 MAD25: Session 1. Substrates

MAD: Substrates

14:45 Low oxygen, high purity, proven crystal growth method for 450mm + silicon optics

The NeoGrowth method, originally developed for high-efficiency photovoltaics, is a crucible-free crystal growth technique with promising potential for producing cryogenic mirror substrates for next-generation gravitational wave detectors such as the Einstein Telescope (ET). Crystal diameters of 450 mm—within the required range for ET—have already been demonstrated. The method enables oxygen levels and metallic impurity concentrations comparable to or lower than those achieved with magnetic Czochralski (MCz) silicon, along with low defect densities. This talk presents the current development status of the NeoGrowth method and assesses its potential to meet the stringent crystal purity and structural requirements necessary for achieving minimal optical absorption and mechanical losses.

Speaker: Nathan Stoddard (Lehigh University)

15:00 Towards high-purity, large-diameter silicon mirror substrates: A self-crucible growth concept

Third-generation gravitational wave detectors operating at cryogenic temperatures, such as the Einstein Telescope, require silicon mirror substrates with both exceptional purity and large diameter (>450 mm). The float-zone (FZ) method enables the growth of ultra-high purity silicon crystals but is limited in diameter beyond 200 mm. The Czochralski (Cz) method allows larger diameters but introduces impurities, e.g., from the quartz crucible. In this talk, a self-crucible growth concept is presented that aims to combine the advantages of both methods while avoiding their drawbacks. Experimental results from the growth of crystals with diameters up to 4 inches in the developed setup are presented. The method’s potential for the production of mirror substrates for next-generation detectors is discussed.

Speaker: Dr Robert Menzel

15:15 Composite silicon optics for next generation ground based GW detectors

The success of the next generation of ground based GW detectors will require large diameter test mass optics that can be operated at cryogenic temperatures. Silicon test masses of >45cm diameter, and 100-200kg are the leading contenders for mirror substrate material, however currently there is no one who can provide silicon of the required high quality and large diameter that is needed. Researchers at the IKZ (Leibniz Institut für Kristallzüchtung), the DZA (Deutsches Zentrum für Astrophysik) and at the IGR (Institute for Gravitational Research) are working together to come up with a design for a composite test mass, which will bond together high quality float zone silicon to attain a composite test mass for installation into 3G detectors. This talk will discuss models of possible composite mass geometries, bonding approaches taken to attain a composite mass, and the optical characterisation measurements that are most important for this project.

Speaker: Dr Margot Hennig

15:30 Risks and Opportunities with Composite Substrates and Alternative optical modes

The use of composite silicon substrates — assembled from multiple bonded components — offers a practical path to realising large test masses for the Einstein Telescope Low-Frequency (ET-LF) interferometer, while relaxing constraints on boule diameter and availability. However, these designs introduce bond lines across the optical aperture, which are expected to exhibit excess absorption and scattering. To mitigate this, we propose the use of Hermite-Gaussian (HG) optical modes, specifically HG$_{33}$, whose transverse intensity nulls can be aligned with the bond lines, suppressing their optical impact.

In this talk, I will assess the compatibility of the HG$_{33}$ mode with ET-LF infrastructure, focusing on constraints imposed by the vacuum envelope and mirror apertures. Based on my previous work, I will quantify the impact of optical mode mismatch on interferometer performance, showing that HG$_{33}$ is approximately twenty times more sensitive to mismatch than the fundamental Gaussian. However, this increased sensitivity is accompanied by significantly stronger spatial error signals — as shown in the work of Fulda and collaborators — which may enable effective control. I will reinterpret their results in the context of ET-LF to clarify the transverse control requirements should composite substrates be adopted.

Finally, I will present recent technical progress from the ET-OPT platform, including the assembly of a laser system and mode converter capable of generating high-purity HG modes, as well as the conceptual design of a full-scale test facility. This facility will demonstrate the use of HG modes at scale, validating mode control strategies and confirming their suitability for ET-LF. By enabling the use of large-diameter boules with lower absorption, this approach reopens a promising design path previously abandoned due to technical risk.

Speaker: Aaron Goodwin-Jones (UCLouvain)

15:45 Surface absorption of crystalline silicon

Due to a very low mechanical loss, resulting in low thermal noise, crystalline silicon is a very interesting mirror-substrate material for future cryogenic gravitational-wave detectors.
To maintain the low detector operation temperature, low optical absorption of the mirrors is required. This requirement includes all components of the mirrors, i.e. the mirror substrates as well as the coatings involved (highly-reflective as well as anti-reflective coatings). In the past, additional optical absorption of varying magnitude at, or close to, the mirror surfaces has been observed after polishing of crystalline silicon. The origin of this absorption is unknown.
In this presentation, we will give an overview of the issue, and of the efforts currently undertaken to identify and eliminate this absorption source with the aim to minimize the overall mirror absorption.

Speakers: Jessica Steinlechner, Laura Silenzi

16:00 → 16:30 Coffee Break

16:30 → 17:00 MAD25: Session 1. Substrates: Discussion

MAD: Substrates

19:00 → 22:00 Social event

Tuesday 7 October

09:00 → 10:15 MAD25: Session 2. Suspensions

MAD: Suspensions

09:00 Introduction on Suspension Research

09:15 Float zone silicon fibers for suspension application in Einstein Telescope

Monocrystalline silicon fibers are a promising candidate for suspending silicon test masses in gravitational-wave detectors. The excellent thermal and mechanical properties of crystalline silicon enable stable support of heavy mirrors and efficient extraction of laser-induced heat. Moreover, silicon's exceptional material behavior at cryogenic temperatures aligns well with the operational requirements of the Einstein Telescope (ET), which will function under such conditions. As-grown monocrystalline silicon fibers are particularly attractive for suspension applications, as they are both in bulk and surface free from cracks and surface defects (e.g., dislocations), ensuring high tensile strength.
In our research, we explore crucible-free crystal growth methods for fabricating fibers suitable for mirror suspensions in the ET. We present recent results using conventional float zone (FZ) and pedestal techniques to produce thin, monocrystalline silicon fibers with circular cross-sections. Special focus is given to reducing the fiber diameter from 3 mm to smaller dimensions. These crucible-free techniques are also successfully applied to fabricate fibers with customized shapes to facilitate attachment to silicon mirrors and mounts. Our latest developments in this area will be also discussed.

Speaker: Dr Iryna Buchovska (Leibniz-Institut für Kristallzüchtung)

09:30 ET-FIBER: Monocrystalline Silicon Fibre Development for Cryogenic Test Mass Suspensions

The ET-FIBER project is a collaborative R&D initiative, focused on developing monocrystalline silicon fibres for suspending a 100 kg test mass in the E-TEST prototype. E-TEST serves as a critical cryogenic prototype for the Einstein Telescope , operates a large monocrystalline silicon mirror cooled radiatively to 20–25 K, while achieving low seismic noise below 10 Hz through advanced vibration isolation. In this context, ET-FIBER aims to design, manufacture, and characterise silicon fibres capable of supporting these large test masses under cryogenic conditions, while meeting the stringent mechanical and thermal noise requirements of next-generation detectors.

While second-generation detectors like Advanced LIGO and Virgo employ fused silica fibres for suspension, their mechanical and thermal properties degrade at cryogenic temperatures. Silicon, in contrast, offers low mechanical loss and excellent thermal conductivity at low temperatures, making it a strong candidate for third-generation cryogenic suspensions. However, working with crystalline materials presents unique challenges. The mechanical loss in such materials is anisotropic and depends on crystal orientation, defect density, and impurity levels. Furthermore, welding or bonding techniques required to attach fibres to the test mass can disrupt the crystal structure, introducing additional dissipation mechanisms and compromising performance. Unlike amorphous materials such as fused silica, monocrystalline silicon lacks isotropy, making it more sensitive to geometrical and fabrication-induced imperfections.

ET-FIBER addresses several unresolved questions critical to silicon fibre technology. These include determining the breaking strength and yield thresholds of monocrystalline silicon fibres under cryogenic conditions, identifying optimal manufacturing techniques for producing ultra-pure, defect-free fibres, and understanding the mechanical and thermal behaviour of different fibre geometries. Additionally, the project explores how internal friction and surface losses contribute to overall thermal noise, and how these losses scale with diameter, surface finish, and bonding interfaces. Thermal conductivity is a key parameter in fibre design, as fibre diameter directly influences heat extraction from the test mass. Therefore, careful optimisation of fibre geometry is necessary to balance thermal transport with mechanical stability. This includes a detailed examination of variations in mechanical loss with temperature and crystal orientation, further highlighting the influence of crystalline anisotropy.

Through a combination of theoretical modelling, material characterisation, and experimental testing, we aim to provide a comprehensive understanding of silicon fibre performance under cryogenic conditions. By enabling the systematic development and evaluation of monocrystalline silicon fibres, ET-FIBER supports the Einstein Telescope’s technical roadmap. The outcomes of this work will help define suspension design parameters, manufacturing protocols, and mechanical loss budgets for cryogenic test masses. By integrating fibre R&D into the E-TEST platform, ET-FIBER ensures that critical performance characteristics can be tested under realistic operational conditions, paving the way for reliable, low-noise silicon suspensions in future gravitational wave observatories.

Speakers: Prof. Christophe Collette (University of Liege), Mr Gilles Magain (University of Liege)

09:45 Cautions for a correct evaluation of breaking strength measurements

In the last few months, tests have been carried out between Perugia/Camerino and Glasgow to evaluate how much a correct assembly technique and a specifically sized set-up affected the evaluation of the breaking load and the loss angle measurements.

Speaker: Flavio Travasso

10:00 Precision Mechanical Loss Measurement & Noise Characterisation: E-TEST Cryogenic Prototype

The ETEST prototype, designed for the Einstein Telescope, a next-generation gravitational-wave observatory, employs a 100-kg test mass cooled to 20–25 K via radiative cooling to minimise thermal noise while maintaining effective seismic isolation below 10 Hz. The system integrates active isolation to suppress low-frequency seismic disturbances and incorporates cryogenic sensors and electronics for precise monitoring of vibrational dynamics within the penultimate cryogenic stage.

As a critical R&D platform, ETEST advances suspension technologies vital to the Einstein Telescope’s technical design. This study also assesses the performance of electrostatic actuators and examines the influence of air damping under cryogenic conditions. It addresses challenges  associated with the use of huge mono-crystalline silicon test mass, including anisotropic mechanical losses influenced by crystalline orientation, manufacturing processes, and impurity levels, and loss due to suspension. Additionally, an adapted method for determining the mechanical quality factor, closely linked to thermal noise is experimentally explored. This approach involves driving the resonator at resonance with constant amplitude to measure the required drive amplitude, enabling continuous, real-time quality factor assessment with improved signal stability compared to conventional free-decay methods. These developments underscore E-TEST’s significant role in enhancing the performance of future gravitational-wave detectors.

Speaker: Hemendra Singh

10:15 → 10:45 Coffee Break

10:45 → 11:30 MAD25: Session 2. Suspensions

MAD: Suspensions

10:45 Mechanical Dissipation in Maraging Steel at Room Temperature and Very Low Frequency

Seismic attenuation in gravitational wave detectors relies on materials that show very small creep. Maraging steel has now been used since almost thirty years but systematic data about dissipation at low frequency haven’t been collected yet. We present measurements at room temperature of mechanical oscillation damping in a purpose-built maraging steel cantilever spring at low frequency under stress conditions usually found in gravitational wave detectors. Results obtained could be relevant in the assessment of mirror suspension thermal noise.

Speaker: Matteo Baratti (University of Pisa, INFN Pisa)

11:00 Sapphire-Based Suspension at ARC-ETCRYO Laboratory: Ongoing Test Campaigns

At the ARC-ETCRYO laboratory in Rome, a full-scale (1:1) cryogenic payload is being developed to investigate conductive cooling techniques for the Einstein Telescope. Materials with outstanding mechanical and thermal specifications are required to meet both suspension and test mass substrate requirements.

Sapphire is a very promising optimal candidate at least for the suspension elements, primarily due to its excellent thermal conductivity at cryogenic temperatures (10–20 K), which facilitates effective heat extraction during cooldown and operation, and its high quality factor.
Multiple test campaigns are currently underway and continuously evolving to characterize the sapphire components considered for integration into the payload. These include:
•⁠ ⁠Sapphire Rods with Half-Cone Locking: designed to suspend the marionette to the platform;
•⁠ ⁠Sapphire Ribbon Mirror Suspension: designed to suspend the mirror from the marionette.
Mechanical characterization includes tensile strength tests, conducted at room temperature, and Q-factor measurements, conducted both ar room and cryogenic temperature. Material analysis (optical measurements, spectrometry, laser ablation) and fracture analysis on broken samples are also planned to be done-
Parallel efforts are also focused on overcoming manufacturing challenges. Producing sapphire samples both long and with relatively large cross-sections remains not-trivial, due to limitation in current fabrication processes.
These ongoing developments aim to define a scalable solution for the sapphire suspension systems, laying the groundwork for future implementation for the Einstein Telescope.

Speakers: Dr Emanuele Tofani (INFN Roma1-Università degli studi della Campania "Luigi Vanvitelli"), Eugenio Benedetti (INFN Roma1), Van Long Hoang (INFN-Roma1)

11:15 Sapphire fibre suspensions for KAGRA and ET with low loss jointing techniques

The KAGRA suspension system has succesfully demonstrated the use of Sapphire as a suspension material for cryogenic gravitational wave detectors, however for this to be fully exploited in KAGRA and in the Einstein Telesopce new methods of fibre treatment and jointing are need to reduce the thermal noise cotnribution to detector sensitivity while maintaining heat extraction and mechanical performance. We present a technique or laser polishing and laser welding that has been demonstrated to be able to produce high strength, high conductivity, low loss suspensions systems. We present the outlook for how this technology could be applied as a possible upgrade of the KAGRA suspensions system in future and how this can be used as a solution for the Einstein Telescope Low Frequency detector.

Speaker: Andrew Spencer

11:30 → 12:00 MAD25: Session 2. Suspensions: Discussion

MAD: Suspensions

12:00 → 12:50 Lunch break and poster session

12:50 → 14:15 MAD25: Session 3. Coatings

MAD: Coatings

12:50 Introduction on Coating Research

Speaker: alex amato (Maastricht University - Nikhef)

13:00 Development of mixed oxide coatings for future detectors

Future gravitational-wave observatories will require larger area mirror coatings with reduced thermal noise, absorption loss and scattering loss. The development of germanium dioxide mixed with titanium dioxide (TiO$_2$:GeO$_2$) for A+ LIGO has highlighted the need for additional R&D to address specific challenges of mixed oxide coatings.

We will present an overview of the development of mixed oxide coatings by ion beam sputtering. While cation ratio affects the material properties like refractive index and mechanical loss, other factors such as process base pressure and chamber geometry also play a significant role. We will discuss inclusions and blisters, and strategies for mitigation. Annealing protocols designed to reduce thermal noise will also be reviewed based on results from TiO$_2$:GeO$_2$ / SiO$_2$ stacks. Finally, we will describe the progress and plans for deposition of large area coatings (up to 62 cm diameter) at the Extreme Performance in Optical Coatings laboratory.

Speaker: Mariana Fazio

13:15 Evolution Optical Coating Thin Films Following Thermal Annealing: IR Spectroscopic Ellipsometry and Raman Investigation

The development of future gravitational wave observatories such as the Einstein Telescope requires the development of multilayer mirror coatings with low absorption and constant refractive indices [1]. These coatings, which are usually amorphous mixed oxides, are deposited by ion beam sputtering. Thermal annealing after deposition is used to enhance the performance of the coatings. However, the specific atomic-scale structural changes that control this tuning are not well understood, especially for promising germania-based systems. This study investigates the effects of thermal annealing at 600° on amorphous oxide coatings based on germanium oxide. We used a complementary approach using infrared spectroscopic ellipsometry (IRSE) to measure the complex dielectric function (ε = ε₁ + iε₂) and Raman spectroscopy to investigate the bond angle distribution and medium-range order in the oxide coatings [2, 3]. Our analysis shows a direct correlation between the annealing-induced structural relaxation [4] and the evolution of vibrational signatures in both the IR and Raman spectra. This study shows the effects of thermal processing to optimize the atomic structure and optical performance of germanium-based coatings for use in next-generation GWDs.

Acknowledgments:
We gratefully acknowledge the Project Einstein Telescope Infrastructure Consortium (ETIC)(IR0000004)-MUR call n. 3264 PNRR, Miss. 4-Comp. 2, Line 3.1.

Keywords: Optical coatings, thermal annealing, Infrared spectroscopic ellipsometry, Raman, GWD mirrors

References:
1. Amato, Alex, Michele Magnozzi, and Janis Wöhler. "Mirror Coating Research and Developments for Current and Future Gravitational‐Wave Detectors." Adv. Photon. Res. (2025): 2400117.
2. Yin, Shenwei, et al. "Preventing overfitting in infrared ellipsometry using temperature dependence: fused silica as a case study." Opt. Mater. Expr. 15.8 (2025): 1939-1948.
3. Micoulaut, Matthieu, L. Cormier, and G. S. Henderson. "The structure of amorphous, crystalline andliquid GeO2." J. Phys.: Cond. Matt.18.45 (2006): R753.
4. Suzuki, K., M. Misawa, and Y. Kobayashi. "What difference exists in the structure of SiO2 and GeO2 between melt-quenched bulk glass and sputter-deposited amorphous film." Le Journal de Physique Colloques 46.C8 (1985): C8-617.

Speaker: Shima Samandari (OptMatLab, Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy)

13:30 Development of Silicon Nitride by RF Magnetron Sputtering for GW Coatings

Gravitational wave (GW) interferometers necessitate the use of specialized mirrors designed to minimize mechanical loss and absorption, thereby ensuring the required sensitivity for effective gravitational wave detection.
Currently, a stack of alternating layers of tantalum oxide (Ta$_2$O$_5$) combined with titanium oxide (TiO$_2$) and silicon oxide (SiO$_2$) is considered the state of the art. Nonetheless, these materials, despite having satisfactory optical absorption, exhibit relatively elevated mechanical losses, thus limiting the interferometer sensitivity. Silicon Nitride (SiN$_x$) is considered a promising candidate for replacing Ti:Ta$_2$O$_5$ in view of its superior mechanical properties. However optical absorption in this material is still too high for GW applications, for reasons that are not well understood yet.
The prevailing method for depositing current-generation gravitational wave coatings is ion beam sputtering (IBS). However, with this technique an optimal control of the film stoichiometry may be difficult.
Since the deposition technique employed significantly influences coating properties, in this study radiofrequency magnetron sputtering is being explored as an alternative approach for depositing silicon nitride, due to its versatility. Initially, the deposition geometry was optimized to achieve film uniformity over a maximal area, considering available deposition system. Subsequently, efforts have been directed towards achieving Si$_3$N$_4$ stoichiometry, supposing the condition under which the material exhibits its optimal properties, utilizing Rutherford Back Scattering (RBS) for atomic composition control.
Thereafter, the optical properties of the films have been investigated through ellipsometry and photothermal deflection, also examining the influence of oxygen content in the film and its resultant optical properties.

Speaker: Dr Simone Marchetti (University of Padova - Department of Physics and Astronomy, Istituto Nazionale di Fisica Nucleare - Sezione di Padova)

13:45 Advancing Gravitational-Wave Detection Through Ion Implantation for Low Thermal Noise Coatings

Coating thermal noise limits the sensitivity of gravitational-wave detectors in the most sensitive frequency band. As third-generation detectors, like the Einstein Telescope or LIGO Voyager, advance toward cryogenic interferometers to reduce thermal noise, current coating materials such as Ta$_{2}$O$_{5}$ and SiO$_{2}$ become inadequate due to their high mechanical loss at low temperatures. Alternative materials, like amorphous silicon (a-Si) and silicon nitride (SiN), are promising due to low thermal noise, but the high optical absorption of a-Si is a limiting factor. We propose a paradigm-shifting approach: forming highly reflective structures directly inside the crystalline silicon (c-Si) mirror substrates via ion implantation. This technique, widely adopted in the semiconductor industry, is unexplored in this context. Ion implantation has the potential to achieve significantly lower optical absorption compared to a-Si by preserving the optical properties of high-purity c-Si layers. Using a dedicated ion implanter, we created buried SiO$_{2}$ and SiN layers at controlled depths inside c-Si substrates. We report the first successful fabrication of a multilayer structure exhibiting no visible surface damage. Simulations of the implantation schedule are compared with Rutherford Backscattering Spectrometry (RBS) measurements. Furthermore, preliminary optical analysis and mechanical loss results are presented. Ongoing development of SiN-implanted structures may be a way to meet—and exceed—the demanding requirements of third-generation observatories. While the presentation by Ismail El Ouedghiri concentrates on the optical properties, this presentation focuses on the implantation schedules and mechanical properties of the layers.

Speaker: Luca Massaro

14:00 Optical Simulations of Ion Implanted Layers for Advanced Gravitational-Wave Detection

Thermal noise originating from mirror coatings remains a key limitation to the sensitivity of gravitational-wave detectors, particularly within their most sensitive frequency range. As next-generation detectors, such as the Einstein Telescope and LIGO Voyager, move toward cryogenic temperatures to mitigate this noise source, conventional coating materials like Ta\textsubscript{2}O\textsubscript{5} and SiO\textsubscript{2} prove insufficient due to their high mechanical losses at low temperatures. Promising alternatives, including amorphous silicon (a-Si) and silicon nitride (SiN), offer improved thermal noise performance; however, the substantial optical absorption of a-Si hinders its practical application. To overcome this, we introduce a novel strategy: fabricating highly reflective structures directly within crystalline silicon (c-Si) mirror substrates using ion implantation, a well-established technique in the semiconductor industry that has not been explored in this context. This method enables the integration of SiO\textsubscript{2} and SiN layers at precisely controlled depths, while maintaining the advantageous optical properties of high-purity c-Si. We present the first successful demonstration of such a multilayer structure, exhibiting no visible surface degradation. While the presentation by Luca Massaro concentrates on the implantation schedules and mechanical properties of the layers, the present work reports the optical analysis of single and double silica (SiO\textsubscript{2}) layers implanted in a c-Si substrate, as well as single and double silicon nitride layers (SiN) implanted in a similar substrate.

Speaker: Dr Ismail El Ouedghiri-Idrissi (Maastricht University)

14:15 → 14:45 Coffee Break

14:45 → 15:15 MAD25: Session 3. Coatings

MAD: Coatings

14:45 Measuring the mechanical and optical losses of coatings in cryogenic conditions using an optomechanical cavity

One of the most critical components of gravitational wave interferometers are their mirror test masses which are coated with multilayer dielectric films to achieve the required reflectivity. However, these coatings introduce coating thermal noise (CTN) that limits the sensitivity of the detectors, particularly in the crucial frequency band from 20 to 2000 Hz. To reduce CTN, one of the two nested interferometers composing Einstein Telescope (ET-LF) is designed to operate under cryogenic conditions. However, characterizing both mechanical and optical losses in mirror coatings under realistic conditions is challenging, especially at cryogenic temperatures, where thermo-elastic interactions between the coating and the substrate makes very hard to use standard characterization methods based on the measurements of mechanical Q factors of substrates coated with the materials to be characterized.
To address this issue, we propose an innovative experimental setup to measure both optical losses and mechanical dissipation in freestanding coating membranes over a broad temperature range. By suspending the coating in the form of a thin membrane, the thermo-elastic interactions between the coating and the substrates are minimized, which enables precise measurement of the coating's properties in the whole range between room temperature and few Kelvins.
Our experimental apparatus features a low-vibration cryostat housing a high finesse optical cavity and piezo actuators for precise membrane positioning and alignment. The measurement consists in placing the membrane inside the resonator so that it couples with the stationary electromagnetic field circulating inside the cavity. The optical losses are determined by monitoring the finesse of the Fabry-Perot cavity as a function of membrane position along the optical axis. The mechanical dissipations are instead measured using the cavity as a sensitive transducer of the membrane vibration spectrum, with dissipation values extracted through a dedicated data analysis procedure.
Preliminary tests on low-stress silicon nitride (SiN) membranes confirm the functionality of the setup. We are now optimizing a reliable fabrication process to enable the routine preparation of membranes starting from arbitrary coatings. These developments will help in expand the study to other materials and optimizing the apparatus.

Speaker: Nicole Busdon

15:00 Vacuum issues and possible mitigations in cryogenic gravitational wave detectors and cold mirrors test benches

In the upcoming third generation of gravitational wave (GW) detectors, the push toward low-frequency detection necessitates the use of cryogenic optics at temperatures as low as a few Kelvin. This unprecedented technological challenge has spurred a series of R&D efforts aimed at validating the use of cryogenic optics in GW detectors. The performance of cryo-mirrors must be carefully measured and optimized to determine the best operational solution, and several promising R&D initiatives have been launched to address these challenges.

Here, we examine a simple yet critical experimental phenomenon that, if not properly accounted for, could compromise some of these observations: the unavoidable accumulation of a frost layer on cryogenically cooled mirrors. A cold surface effectively acts as a pump, allowing contaminants to build up depending on residual vacuum conditions, the cooling process, and other factors. If overlooked, this phenomenon can severely hinder the detailed optical characterization of cryogenic optics, as the unwanted and unspecified contaminant layer may reach thicknesses of several microns or more.

In this work, we present a straightforward method to estimate the thickness of this frost layer and propose a potential mitigation strategy for its removal, applicable both in test benches and actual detectors. The approach involves irradiating the optical elements with low-energy electrons (up to few hundreds eV).
We report on the experimental activities conducted at LNF-INFN and outline the necessary R&D efforts to transition this validated concept into a practical solution, potentially integrating it into the complex design of low-frequency detection systems.

Speaker: Roberto Cimino

15:15 → 15:45 MAD25: Session 3. Coatings: Discussion

MAD: Coatings

15:45 → 16:00 Closing Remarks