Anomalous scaling of flexural phonon damping in nanoresonators with confined fluid.

Various one and two-dimensional (1D and 2D) nanomaterials and their combinations are emerging as next-generation sensors because of their unique opto-electro-mechanical properties accompanied by large surface-to-volume ratio and high quality factor. Though numerous studies have demonstrated an unparalleled sensitivity of these materials as resonant nanomechanical sensors under vacuum isolation, an assessment of their performance in the presence of an interacting medium like fluid environment is scarce. Here, we report the mechanical damping behavior of a 1D single-walled carbon nanotube (SWCNT) resonator operating in the fundamental flexural mode and interacting with a fluid environment, where the fluid is placed either inside or outside of the SWCNT. A scaling study of dissipation shows an anomalous behavior in case of interior fluid where the dissipation is found to be extremely low and scaling inversely with the fluid density. Analyzing the sources of dissipation reveals that (i) the phonon dissipation remains unaltered with fluid density and (ii) the anomalous dissipation scaling in the fluid interior case is solely a characteristic of the fluid response under confinement. Using linear response theory, we construct a fluid damping kernel which characterizes the hydrodynamic force response due to the resonant motion. The damping kernel-based analysis shows that the unexpected behavior stems from time dependence of the hydrodynamic response under nanoconfinement. Our systematic dissipation analysis helps us to infer the origin of the intrinsic dissipation. We also emphasize on the difference in dissipative response of the fluid under nanoconfinement when compared to a fluid exterior case. Our finding highlights a unique feature of confined fluid-structure interaction and evaluates its effect on the performance of high-frequency nanoresonators.

Energy Dissipation in Fluid Coupled Nanoresonators: The Effect of Phonon-Fluid Coupling.

Resonant nanomechanical systems find numerous sensing applications both in the vacuum and in the fluid environment but their performance is degraded by different dissipation mechanisms. In this work, we study dissipation mechanisms associated with high frequency axial excitation of a single-walled carbon nanotube (CNT) filled with argon, which is a representative fluid coupled resonator system. By performing molecular dynamics simulations, we identify two dissipative processes associated with the axial excitation of the resonator: (i) perturbation of the resonator phonons and their relaxation and (ii) oscillatory fluid flow developed by the resonator motion. Dissipation due to the first process, a form of "intrinsic" dissipation, is found to be governed by the Akhiezer mechanism and is verified for an empty CNT in vacuum. To estimate the dissipation due to the second process, which is the conventional "fluid" dissipation, we formulate an approach based on the response of the hydrodynamic force on the resonator. Our analysis of the coupled system reveals that phonon relaxations associated with the Akhiezer dissipation are significantly modified in the presence of fluidic interactions, which have been ignored in all previous dissipation studies of fluid-resonator systems. We show that an important consequence of this phonon-fluid interaction is inverse scaling of dissipation with density at low excitation frequencies.

Optimally focused cold atom systems obtained using density-density correlations.

Resonant absorption imaging is a common technique for detecting the two-dimensional column density of ultracold atom systems. In many cases, the system's thickness along the imaging direction greatly exceeds the imaging system's depth of field, making the identification of the optimally focused configuration difficult. Here we describe a systematic technique for bringing Bose-Einstein condensates (BEC) and other cold-atom systems into an optimal focus even when the ratio of the thickness to the depth of field is large: a factor of 8 in this demonstration with a BEC. This technique relies on defocus-induced artifacts in the Fourier-transformed density-density correlation function (the power spectral density, PSD). The spatial frequency at which these artifacts first appear in the PSD is maximized on focus; the focusing process therefore both identifies and maximizes the range of spatial frequencies over which the PSD is uncontaminated by finite-thickness effects.