NCAFM2023 Programme Booklet

Monday 1640 - 1700

FREQUENCY AND DAMPING NOISE OF ATOMIC FORCE MICROSCOPY CANTILEVERS WITH OPTOMECHANICALLY MODIFIED QUALITY FACTOR AT LOW TEMPERATURE

Noah Austin-Bingamon 1 , Binod DC 2 , and Yoichi Miyahara 1,2*

1 Department of Physics, Texas State University, San Marcos, Texas 78666, USA 2 Materials Science, Engineering and Commercialization program, Texas State University Email: yoichi.miyahara@txstate.edu

Noise in the frequency and the damping signals are key parameters in determining microscope performance in frequency modulation atomic force microscopy (FM-AFM). We present a study of noise in the frequency and damping signals of AFM cantilevers used in a low temperature FM-AFM system with fiber-optic interferometric sensing of the cantilever deflection. Due to an optomechanical coupling between cantilever oscillation and the optical field, the quality (Q) factor and resonant frequency are both dependent on the optical cavity length (fiber-cantilever gap distance) [1,2]. We use an optical setup with two infrared lasers (1550nm and 1310nm), each of which is used for detection and excitation of the cantilever oscillation. While the intensity of the 1310nm laser (excitation laser) is modulated to excite the cantilever oscillation via optical force [3], varying the mean intensity of the excitation laser can change the effective Q factor, allowing us to compare the frequency and damping noises with the same detection sensitivity. An automated protocol was developed to scan the fiber-cantilever distance and measure the effective Q factor, resonant frequency, and frequency noise at each distance. A digital phase-locked loop was used to drive the cantilever oscillation and measure the frequency and Q. The fiber position was scanned using a computer-controlled piezoelectric stick-slip motor. Figure 1 shows (a) thermal noise spectra of a Pt coated cantilever (f0 = 145 kHz, k = 20 N/m), (b) time trace of frequency shift with four different effective Q factors ranging from 588k to 61k. The measurement temperature is 3.5 K. This result shows that the frequency shift noise decreases with increasing effective Q factor that is modified by optomechanical effect. Similarly, we have observed the decreasing damping noise with increase effective Q factor. We will present the details of the experiment and discuss the mechanism of the noise reduction and the implication of the results on FM-AFM measurements. We gratefully acknowledge funding from NSF-PREM (DMR-2122041), NSF-CAREER (DMR-2044920) and NSF-MRI (DMR-2117438). This work is also supported by Texas State University.

Fig. 1 (a) Thermal spectra of the cantilever. Each curve is shifted vertically by 10-2 for clarity. (b) Time trace of frequency shift noise. Each curve is shifted vertically by 1 Hz for clarity. In (a) and (b), the effective quality factor of green, blue, pink and black traces are 588k, 120k, 77k, and 62k, respectively.

References [1] C. H. Metzger and Kh Karrai, Nature 2004, 432 , 1002. [1] H. Hoelscher, et al., Appl. Phys. Lett. 2009, 94 , 223514. [2] Y. Miyahara, EPJ Tech. Instrum. 2020, 7 , 2.

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