Radiation pressure measurement in the piconewton range using a micro-opto-electro-mechanical system (MOEMS)

Quantitative and in-line determination of the power of a laser plays an important role in industry at laser manufacturing. Because of its straightforward principle and ease of application, the method of measuring the laser power by means of measuring the radiation pressure of a laser beam has gained ever more interest. This requires, in return, a force measurement method with high resolution down to sub-piconewton corresponding to laser powers in the mW range.

It is worthwhile to mention that weak and ultra-weak forces down to the sub-piconewton (pN) range are also of great importance in various research fields including atomic force microscopy (AFM), ultra-shallow nanoindentation, nondestructive optical tweezing and manipulating of micro- and nanoparticles and light-material interaction among different meta-materials.

The micro-opto-electro-mechanical system (MOEMS) can be used in two modes. In the normal measurement mode, where the built-in capacitive sensor is used, the MEMS force sensor prototype has the following characteristics:

• a stiffness of 5 N/m, a displacement range of 10 μm,
• a force measuring range of 50 μN and
• a quasi-static force resolution of 1 nN.

In the high-resolution measurement mode, an integrated Fabry-Perot interferometer is used to increase the force resolution. For this purpose, a new pico-force sensor based on a MOEMS) was developed in cooperation with the Center for Microtechnologies of the TU Chemnitz.

As shown in Fig. 1a, the MOEMS sensor consists of (1) an electrostatic comb drive actuator, (2) an embedded Fabry-Perot resonator with integrated Bragg mirrors, and (3) a passive gripper for fixing different reflectors with highly reflective coatings to minimize the power loss during the radiation pressure measurement.

With the integrated embedded micro-resonator, high-resolution force measurements with a sub-pN force resolution are possible in the future. As shown in Fig. 1b, the MOEMS force transducer can measure the laser power of a diode laser with a noise floor ~ 1 pN.



Figure. 1a: Schematic diagram of the MOEMS radiation pressure sensor with clamped AFM cantilever as reflector


  

Figure. 1b: Comparison between the measured laser radiation pressure of the MOEMS force sensor and the nominal laser force

Literature:

[1] Z. Li, S. Gao, U. Brand, K. Hiller, S. Hahn, H. Wolff, L. Koenders, "Towards quantitative determination of weak forces using a micro-machined optomechanical force sensor", Poster presentation in European Optical Society Biennial Meeting (EOSAM) 2018, Delft, Netherlands, 8-12 October 2018.