Luminescence Upconversion Nanoscopy - LUNA

• Applicant: Dr. Niclas Müller
• Subject Area: Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551280726

Throughout my scientific career I have been fascinated by the manipulation and confinement of light on the nanoscale, spanning across both the visible (VIS) and infrared (IR) spectral ranges. However, IR and VIS nanooptics currently exist as two almost independent research fields, such that the tremendous potential of IR+VIS dual-band applications combining the unique functionalities of both fields is currently not exploited. This disconnect arises from a fundamental challenge: there is no efficient coupling channel between the respective materials excitations, such as excitons in the VIS and phonon polaritons in the IR. The aim of the LUNA project is to implement nanoscopic approaches for coupling phonon polaritons with VIS optical excitations and thereby bridge the fields of IR and VIS nanooptics. We will break conventional selection rules of electron-phonon coupling using specifically designed nanostructures to activate new IR+VIS coupling channels. This will involve color centers in polar semiconductors and excitons in 2D materials and heterostructures. We will use the extreme confinement of IR phonon polaritons to overcome the size mismatch to the VIS. The key method to study this coupling will be Luminescence Upconversion NAnoscopy (LUNA): A visible laser illuminates a material below its electronic transition energy, and the missing energy is provided by the IR excitation of a phonon polariton. The coupling of phonon polaritons to excitons and color centers is thus encoded in their upconverted photoluminescence. We will implement this in a near-field optical microscope where IR and VIS laser fields are tightly confined by a metallic tip that acts as a dual antenna. This unique setup will enable correlative IR+VIS nanoscopy and tip-enhanced upconversion. We will use excitons as sensors for the peculiar propagation patterns of phonon polaritons. The coupling will be further enhanced and tailored with dual cavities and waveguides to realize frequency converters and multiplexers for future nano-optical circuits. Our work towards these major goals will provide a detailed understanding of the interaction of VIS excitations with phonon polaritons and more generally set the basis for combined IR+VIS nanooptics with solid-state materials.

• DFG Programme: Emmy Noether Independent Junior Research Groups
• Major Instrumentation: Tunable IR+VIS laser ; VIS spectrometer + CCD camera ; near-field optical microscope
• Instrumentation Group: 1800 Spektralphotometer (UV, VIS), Spektrographen (außer Monochromatoren 565) ; 5091 Rasterkraft-Mikroskope ; 5700 Festkörper-Laser