Silvan Schmid

Cantilever-like micromechanical sensors

The field of cantilever-based sensing emerged in the mid-1990s and is today a well-known technology for label-free sensing which holds promise as a technique for cheap, portable, sensitive and highly parallel analysis systems. The research in sensor realization as well as sensor applications has increased significantly over the past 10 years. In this review we will present the basic modes of operation in cantilever-like micromechanical sensors and discuss optical and electrical means for signal transduction. The fundamental processes for realizing miniaturized cantilevers are described with focus on silicon- and polymer-based technologies. Examples of recent sensor applications are given covering such diverse fields as drug discovery, food diagnostics, material characterizations and explosives detection.

Optical detection of radio waves through a nanomechanical transducer

Low-loss transmission and sensitive recovery of weak radio-frequency and microwave signals is a ubiquitous challenge, crucial in radio astronomy, medical imaging, navigation, and classical and quantum communication. Efficient up-conversion of radio-frequency signals to an optical carrier would enable their transmission through optical fibres instead of through copper wires, drastically reducing losses, and would give access to the set of established quantum optical techniques that are routinely used in quantum-limited signal detection. Research in cavity optomechanics has shown that nanomechanical oscillators can couple strongly to either microwave or optical fields. Here we demonstrate a room-temperature optoelectromechanical transducer with both these functionalities, following a recent proposal using a high-quality nanomembrane. A voltage bias of less than 10 V is sufficient to induce strong coupling between the voltage fluctuations in a radio-frequency resonance circuit and the membrane’s displacement, which is simultaneously coupled to light reflected off its surface. The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude less than that of standard optical modulators. The noise of the transducer—beyond the measured Johnson noise of the resonant circuit—consists of the quantum noise of light and thermal fluctuations of the membrane, dominating the noise floor in potential applications in radio astronomy and nuclear magnetic imaging. Each of these contributions is inferred to be when balanced by choosing an electromechanical cooperativity of with an optical power of 1 mW. The noise temperature of the membrane is divided by the cooperativity. For the highest observed cooperativity of , this leads to a projected noise temperature of 40 mK and a sensitivity limit of . Our approach to all-optical, ultralow-noise detection of classical electronic signals sets the stage for coherent up-conversion of low-frequency quantum signals to the optical domain.

Real-time particle mass spectrometry based on resonant micro strings.

Micro- and nanomechanical resonators are widely being used as mass sensors due to their unprecedented mass sensitivity. We present a simple closed-form expression which allows a fast and quantitative calculation of the position and mass of individual particles placed on a micro or nano string by measuring the resonant frequency shifts of the first two bending modes. The method has been tested by detecting the mass spectrum of micro particles placed on a micro string. This method enables real-time mass spectrometry necessary for applications such as personal monitoring devices for the assessment of the exposure dose of airborne nanoparticles.

Position and mass determination of multiple particles using cantilever based mass sensors

Resonant microcantilevers are highly sensitive to added masses and have the potential to be used as mass-spectrometers. However, making the detection of individual added masses quantitative requires the position determination for each added mass. We derive expressions relating the position and mass of several added particles to the resonant frequencies of a cantilever, and an identification procedure valid for particles with different masses is proposed. The identification procedure is tested by calculating positions and mass of multiple microparticles with similar mass positioned on individual microcantilevers. Excellent agreement is observed between calculated and measured positions and calculated and theoretical masses.

Damping mechanisms of single-clamped and prestressed double-clamped resonant polymer microbeams

In this article, an investigation of the damping mechanisms of resonant single- and double-clamped polymer microbeams for a frequency range from 10 kHz to 5 MHz is presented. The suspended structures are made of SU-8, an epoxy-type photoresist, by means of a sacrificial layer technique. The vibration was measured with a laser-Doppler vibrometer in high vacuum at different temperatures and at atmospheric pressure. The influence of air damping in rarefied air was investigated and the intrinsic damping mechanisms were determined in high vacuum (p<0.05 Pa). After excluding a variety of possible damping factors, the dominant intrinsic dissipation mechanism of the single-clamped microbeams was understood to be the material damping with maximum quality factors (Q) of around 70 at 20 °C. Quality factors of up to 720 at 20 °C were measured for stringlike double-clamped microbeams, which suggest a different intrinsic damping mechanism than material loss. It is shown that internal damping mechanisms due to flexure a...

Nonconductive polymer microresonators actuated by the Kelvin polarization force

The authors present a method, based on the Kelvin polarization force, to actuate nonconductive polymer microstructures. A proof of principle was conducted by finite element simulations. Microresonators made of SU-8 were fabricated and characterized under resonant conditions at applied ac voltage of 5Vpp. A quality factor of Q=87 in vacuum and a square dependence of the force on the applied voltage were obtained. The presented actuator design and fabrication do not require additional electrodes on the movable structure for actuation and thus allow for the full exploration of the exceptional variety of polymer materials for microscaled actuators and sensors.

Real-time single airborne nanoparticle detection with nanomechanical resonant filter-fiber

Nanomechanical resonators have an unprecedented mass sensitivity sufficient to detect single molecules, viruses or nanoparticles. The challenge with nanomechanical mass sensors is the direction of nano-sized samples onto the resonator. In this work we present an efficient inertial sampling technique and gravimetric detection of airborne nanoparticles with a nanomechanical resonant filter-fiber. By increasing the nanoparticle momentum the dominant collection mechanism changes from diffusion to more efficient inertial impaction. In doing so we reach a single filter-fiber collection efficiency of 65 ± 31% for 28 nm silica nanoparticles. Finally, we show the detection of single 100 nm silver nanoparticles. The presented method is suitable for environmental or security applications where low-cost and portable monitors are demanded. It also constitutes a unique technique for the fundamental study of single filter-fiber behavior. We present the direct measurement of diffusive nanoparticle collection on a single filter-fiber qualitatively confirming Langmuirs model from 1942.

Influence of air humidity on polymeric microresonators

The influence of air humidity on polymeric microresonators is investigated by means of three different resonator types. SU-8 microbeams, SU-8 microstrings and a silicon micromirror with SU-8 hinges are exposed to relative humidities between 3% and 60%. The shifts of the resonant frequencies as a function of the relative humidity (RH) are explained based on mechanical models which are extended with water absorption models in polymer materials. The dominant effect causing the resonant frequency change is evaluated for each structure type. The eigenfrequency of the microstrings and the micromirror in the out-of-plane mode, which both mainly are defined by the pre-stress of the polymeric structures, are found to be highly sensitive to changes of air humidity. The humidity-induced (hygrometric) volume expansion reversibly reduces the pre-stress which results in relative frequency changes of up to 0.78%/%RH for the microstrings. A maximum coefficient of humidity-induced volume expansion for SU-8 of ?hyg = 52.3 ppm/%RH is evaluated by fitting the data with the analytical model. It was found that microstrings that were stored at 150 ?C over 150 h are more moisture sensitive compared to structures that were stored at room temperature. For the SU-8 microbeams and the micromirror in the tilt mode, the eigenfrequency is mainly defined by the modulus of the polymer material. The measured relative resonant frequency changes were below 1% for the given RH range. For low RH values, antiplasticization is observed (the modulus increases) followed by a plasticization for increasing RH values.

Superparamagnetic photocurable nanocomposite for the fabrication of microcantilevers

We present a photocurable polymer composite with superparamagnetic characteristics for the fabrication of microcantilevers. Uniform distribution and low particle agglomeration (<50 nm) in the photocurable polymer matrix SU-8 are achieved by using superparamagnetic nanoparticles with a surfactant. Particles and composite are characterized by a transmission electron microscope, UV-VIS spectrometer and magnetic measurements. The composite contains 5 vol.% (18 wt.%) of Fe3O4 nanoparticles with diameters of 12.1 ± 3.5 nm. The composite exhibits a magnetization saturation of 13.2 kA m −1 . Superparamagnetic composite microcantilevers with typical dimension of 2 μm × 14 μm × 80‐300 μm are successfully fabricated by two conventional photolithography steps and a sacrificial layer etch. Exposure doses of 10000 mJ cm −2 must be applied for microcantilever thicknesses of 1.8 μm due to the high UV absorption of the particles in the composite. The magnetic polymer cantilevers are successfully actuated in resonance in air with an amplitude of 29 nm. An off-chip coil is used to generate a magnetic field to actuate the cantilevers. (Some figures in this article are in colour only in the electronic version)

Ultrasensitive string-based temperature sensors

Resonant strings are a promising concept for ultra sensitive temperature detection. We present an analytical model for the sensitivity with which we optimize the temperature response of resonant strings by varying geometry and material. The temperature sensitivity of silicon nitride and aluminum microstrings was measured. The relative change in resonant frequency per temperature change of −1.74±0.04%/°C of the aluminum strings is more than one order of magnitude higher than of the silicon nitride strings and of comparable state-of-the-art AuPd strings.