M.M. Hasan, N. Austin-Bingamon, D.C. Binod, Y. Miyahara
Texas State University,
United States
Keywords: atomic force microscope, FM-AFM, mechanical resonator, quality factor
Summary:
Mechanical resonators with micro/nanometer dimensions play a significant role in sensing the weak force via the shift of resonance frequency. In a frequency modulation atomic force microscope (FM-AFM) [1], the cantilever serves as a mechanical resonator that can measure the high-resolution surface topography by tracking the resonance frequency shift and the damping simultaneously. However, the signal quality often reduces due to many noise sources. For example, the thermal and displacement sensor noises can affect the total frequency noise depending on the cantilever's quality factor (Q) at different mediums [2]. This demands the additional Q-dependent noise analysis of frequency shift and dissipation to improve the signal quality through the optimization of the frequency noises. The optical excitation to control the Q is effective because it removes the spurious mechanical resonances, thereby allowing accurate measurements of frequency shift and dissipation [3,4]. Inside an optical cavity made of fiber and cantilever, the cantilever excitation is performed by the optical forces, which are also the cantilever’s position-dependent components. As a result, the position-sensitive optical force acts as a spring whose spring constant depends on the slope of the interferogram pattern. Any time delay between the optical forces and the cantilever’s position causes damping and, thereof, the effective Q achieved. We deployed this theory by utilizing a low-temperature FM-AFM unit with two laser excitation/detection techniques. The 1310 nm laser was used for excitation, whereas the 1550 nm was for interferometric detection. We achieved our effective Q by adjusting the average intensity of the 1310 nm laser without changing the Fabry-Perot cavity. We carefully observed the frequency shift and dissipation noise density spectrum for the effective Q, and our results showed these noises are independent of the Q modification. In this report, we will present the effect of modified Q on the measurement bandwidth and noises of frequency shift and dissipation measurements. We sincerely acknowledge funding from NSF (DMR-2122041, DMR-2044920, DMR-2117438) and Texas State University. References: 1) T. R. Albrecht, P. Grütter, D. Horne, and D. Rugar, J. Appl. Phys. 69, 668 (1991). 2) K. Kobayashi, H. Yamada, and K. Matsushige, Rev. Sci. Instrum. 80, 043708 (2009). 3) Y. Miyahara, H. Griffin, A. Roy-Gobeil, R. Belyansky, H. Bergeron, J. Bustamante, and P. Grutter, EPJ Techn. Instrum. 7, 2 (2020). 4) N. Austin-Bingamon, D.C. Binod, and Y. Miyahara, Jpn. J. Appl. Phys. 63, 04SP84 (2024).