Suraj Bramhavar

Motion Transduction in Nanoelectromechanical Systems (NEMS) Arrays Using Near-field Optomechanical Coupling

Development of efficient and sensitive motion transducers for arrays of nanoelectromechanical systems (NEMS) is important for fundamental research as well as for technological applications. Here, we report a single-wire nanomechanical transducer interface, which relies upon near-field optomechanical interactions. This multiplexed transducer interface comes in the form of a single-mode fiber taper on a fiber-optic cable. When the fiber taper is positioned sufficiently close to the NEMS array such that it can attain evanescent optical coupling with the array, individual NEMS resonances can be actuated using optical dipole forces. In addition, sensitive detection of nanomechanical motion can be realized when the evanescent waves confined around the taper are scattered by the motion. We have measured resonances from an array of 63 NEMS resonators with a displacement sensitivity of 2-8 pm·Hz(-1/2) at a detection power of ~100 μW (incident on the entire array).

Monolithic integration of a nanomechanical resonator to an optical microdisk cavity

We report a Silicon nano-opto-mechanical device in which a nanomechanical doubly-clamped beam resonator is integrated to an optical microdisk cavity. Small flexural oscillations of the beam cause intensity modulations in the circulating optical field in the nearby microdisk cavity. By monitoring the corresponding fluctuations in the cavity transmission via a fiber-taper, one can detect these oscillations with a displacement sensitivity approaching 10 fm·Hz-1/2 at an input power level of 50 μW. Both the in-plane and out-of-plane fundamental flexural resonances of the beam can be read out by this approach - the latter being detectable due to broken planar symmetry in the system. Access to multiple mechanical modes of the same resonator may be useful in some applications and may enable interesting fundamental studies.

Sensitive micromechanical displacement detection by scattering evanescent optical waves

We describe a simple approach to detect small mechanical displacements by scattering evanescent optical waves confined around an optical waveguide. Our experimental setup consists of a microcantilever brought into the proximity of a tapered optical fiber. The scattering of evanescent waves and hence the optical transmission through the tapered fiber is strongly dependent on the separation between the fiber and the microcantilever, allowing for sensitive detection of the small oscillations of the microcantilever. Our approach does not require a coherent laser source, yet it provides a displacement sensitivity of ~260fmHz(-1/2) at a small power level of 38microW. It is suitable for scanning probe microscopy and could eventually be adapted to nanomechanical resonators.

Superheterodyne detection of laser generated acoustic waves

A superheterodyne approach to the detection of laser generated acoustic waves is presented. An amplitude modulated laser source is used to generate high frequency, narrow bandwidth acoustic waves, and the resulting surface displacement is detected using a stabilized Michelson interferometer. The detection laser used in the interferometer is amplitude modulated at a frequency that is offset from the generation laser modulation frequency by a fixed amount, allowing for the optical down-conversion of the high frequency intensity modulation associated with acoustic wave propagation to a low and fixed intermediate frequency, thereby obviating the need for high frequency detection electronics. Results are presented demonstrating the approach for the detection of bulk and surface acoustic waves at frequencies of up to 1 GHz.

Near-field optical transducer for nanomechanical resonators

We demonstrate a near-field interface in the form of a tapered-fiber waveguide for transduction of nanomechanical motion. Small oscillations of a nanomechanical resonator are actuated by dipole forces and detected by scattering of evanescent waves.

Superheterodyne techniques in laser ultrasonics.

Laser‐based ultrasonics is a powerful tool for the mechanical characterization of thin films and nanostructures. In this paper, superheterodyne approaches to single‐point and full‐field detection of laser generated ultrasonic signals will be discussed. In these techniques, an amplitude modulated laser source is used to excite a narrow‐bandwidth signal. The detection laser, incorporated into an interferometric detection scheme, is also amplitude or phase modulated at frequency that is offset from the generation laser modulation frequency by a fixed amount, serving as the local oscillator for superheterodyne detection. Introduction of the local oscillator allows for the optical down‐conversion of the high‐frequency intensity modulation associated with sample motion to a low and fixed intermediate frequency given by the difference between excitation and detection laser modulation frequencies. The primary benefit of using this approach is that the upper frequency bound is not dictated by the speed of the dete...

Theory and applications of frequency domain photoacoustic microscopy

In frequency domain photoacoustic microscopy, the pulsed laser source used for ultrasound excitation in conventional photoacoustic imaging is replaced by a low power, amplitude modulated laser source. The acoustic signals are detected using an interferometer or contact transducer, coupled to a RF lock-in amplifier or vector network analyzer. The detection bandwidth reduction aorded by this technique allows for a significant improvement in signal-to-noise ratio (SNR) over systems using pulsed laser excitation. In this paper, the method of frequency domain photoacoustic microscopy is reviewed and compared to the pulsed-laser based approach. Methods for processing the frequency domain data to extract the information of interest are discussed, along with the eects of measurement bandwidth and frequency resolution. A new technique to optically downshift acoustic signals detected using an optical interferometer to a fixed intermediate frequency is presented, which allows for the detection of high frequency (100’s of MHz- GHz) acoustic signals using low frequency, low cost detection electronics. Several applications of frequency domain photoacoustic microscopy are presented including the inspection of thin films and environmental barrier coatings and the photothermal operation of nano-electro-mechanical systems. Finally, potential applications of this technique for the characterization of biological media are discussed.