Squeezed light at visible wavelength for eye-safe high-spatial-resolution surface microscopy

• Applicants: Professor Dr.-Ing. Christian Rembe; Professor Dr. Roman Schnabel
• Subject Area: Measurement Systems ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431078650

Laser Doppler Vibrometry (LDVy) is a proven industrial measurement technique for vibrations. The measurement beam of a laser-Doppler vibrometer (LDV) can be coupled into the beam path of a confocal scanning microscope allowing vibration measurements in microscopic mechanical structures with diameters that can be a small as 1 µm if the wavelength is visible. A special application of a scanning microscope LDV (SMV) is the measurement of surface topographies. A LDV is an interferometer with a high measurement bandwidth that measures optical path-length variations. Such variations arise from vibrations or from surface height differences if the laser focus of the measurement beam is scanned over an optically smooth surface. The shorter the laser wavelength, the better the spatial lateral resolution limited by diffraction. Eye-safe operation of a LDV requires either optical power below 1 mW at visible wavelengths or appropriate laser safety measures. The first option limits the measurement sensitivity. The second option increases the operating costs and is problematic for sensitive surfaces. Squeezed light provides means to measure with advanced interferometric resolution at eye-safe operation. Unfortunately, efficient generation of squeezed light has so far only been possible at near-infrared wavelengths, but not in the optical range. The reason for this is the sensitivity of non-linear crystals to the previously necessary UV pump light. This project aims (i) to provide proof of principle for a revolutionary new approach to squeezed light generation at visible wavelengths - here at 517 nm – and (ii) to demonstrate its application to eye-safe high-resolution squeezed light-enhanced laser interferometry on the measurement of surface topographies with microscopic lateral resolution. Deconvolution of the topography data with the instrument transfer function (ITF) allows an improvement, however, only until the noise level exceeds the topography deviations. Consequently, a short wavelength, small laser focus and low displacement-noise level of the interferometer are the key parameters. We will proof that laser light with a strongly-squeezed quantum uncertainty (80 MHz squeezing bandwidth is required for a typical LDV with 40 MHz carrier frequency) can boost the sensitivity of interferometric surface topography measurements. Light power is kept eye-safe and the light intensity will remain low enough to prevent influences on the measurement. To achieve this goal, our project combines concepts of quantum optics and engineering sciences. Our result may affect engineering applications of interferometers whenever optical loss can be kept low. Squeezed visible light for the enhancement of interferometry with high spatial resolution may turn out as an example of a quantum technology at the heart of the envisioned second quantum-revolution.

• DFG Programme: Research Grants