Fabrication of large-scale nanomechanical resonators with highest quality factors and first application studies

• Applicant: Dr. Christoph Reinhardt
• Subject Area: Microsystems ; Measurement Systems ; Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 556301065

Mechanical membrane resonators are patterned thin films that are freely suspended from a silicon chip. They feature oscillation modes with an exceptionally-low level of intrinsic loss, or, equivalently, an ultra-high mechanical quality factor Q~10^9. As a result, these devices have attracted widespread interest for applications such as force sensing or quantum motion experiments. Here, the ultra-high Q suppresses thermal force noise (∝Q^(-1/2)) and decoherence rate (∝1/Q), respectively. Due to their outstanding sensitivity, membranes are now being explored for searches of new fundamental physics, such as dark matter and gravitational waves at high frequencies above the established observation window. However, reaching sensitivities required for detecting predicted signals still necessitates major developments, compared to existing membranes. The proposed research, for the first time, aims at overcoming these limitations with regard to high-frequency gravitational wave detection. To this end, the central objective is to demonstrate membranes with a mechanical quality factor that is a hundredfold higher than the current record, thereby realizing Q~10^11. The underlying strategy is to exploit the demonstrated linear scaling of Q with membrane area. Specifically, a dramatic increase from few square millimeters, current state of the art, to few square centimeters is targeted. Furthermore, instead of the commonly-used silicon nitride, which is amorphous, the membranes will be fabricated out of crystalline silicon. This enables further increasing their Q, due to silicon’s lower density of structural defects compared to silicon nitride. An additional advantage of silicon is its higher thermal conductivity, which is particularly useful for the considered membrane-based gravitational wave detector. Here, the membrane’s motion is controlled via the radiation pressure exerted by high-power laser light. The high thermal conductivity enables an effective dissipation of heat resulting from optical absorption inside the membrane. This shall be demonstrated as part of the proposed project. Pushing the performance of membrane resonators to the unprecedented levels targeted here could also enable major steps towards membrane-based quantum motion experiments at room temperature, dark matter searches, and a range of sensing applications. As part of the proposed research, gas pressure sensing will be investigated. Specifically, Q of the membranes targeted here is expected to depend on the gas pressure in its surrounding, from 10^-10 mbar (ultra-high vacuum) to 10^3 mbar (ambient pressure). This shall be demonstrated, thereby realizing the first gas pressure sensor covering this wide and technologically-relevant pressure range. Also, it is planned to demonstrate membrane-based sensing of helium pressure at a temperature of 10 K, for which no commercial sensor exists.

• DFG Programme: Research Grants
• International Connection: Denmark
• Co-Investigator: Professor Dr. Benno Willke
• Cooperation Partner: Professor Dr. Albert Schliesser