Position and Orientation Control of Complex-Shaped Samples in Acoustic Levitation

Controlled levitation has been limited to sub-wavelength samples with few exceptions of spheres and cubes. We demonstrate controlled airborne levitation of 2λ-sized complex-shaped objects (3D letters, X, U, T). The pressure field was tailored to a particular sample shape by arranging multiple small asymmetric acoustic traps. This allows stable locking of the object and control of its orientation and position in mid-air. The pressure field was produced with a 450-element phased array (f = 40kHz) featuring element-wise amplitude and phase control.Peer reviewe

4D Scanning Acoustic Microscopy

Scanning acoustic microscopy (SAM) can map mechanical surface properties and topography of materials with high spatial resolution. Coded-excitation scanning acoustic microscopy (CESAM) enables high SNR while maintaining fast imaging. The high scan speed of our CESAM allows us to study how surface mechanical properties change almost in real time. The 4D scanning capability was demonstrated with a UV-curable adhesive coating. The 4D data was constructed from three spatial dimensions and one time dimension. We set the C-scan scan rate to 30 s and measured a 400 μm x 400 μm area with 175 MHz and step size of 5 μm. With 400 MHz and step size of 1.5 μm we measured one 150 μm x 150 μm plane every 30 s.Scanning acoustic microscopy (SAM) can map mechanical surface properties and topography of materials with high spatial resolution. Coded-excitation scanning acoustic microscopy (CESAM) enables high SNR while maintaining fast imaging. The high scan speed of our CESAM allows us to study how surface mechanical properties change almost in real time. The 4D scanning capability was demonstrated with a UV -curable adhesive coating. The 4D data was constructed from three spatial dimensions and one time dimension. We set the C-scan scan rate to 30 s and measured a 400 µm x 400 µm area with 175 MHz and step size of 5 µm. With 400 MHz and step size of 1.5 µm we measured one 150 µm x 150 µm plane every 30 s.Peer reviewe

Tailored Acoustic Holograms with Phased Arrays

Advances in phased array of transducers (PATs) allow for precise control of each element's phase and amplitude and can consist of hundreds of transducers. The widely adopted Gerchberg-Saxton type of algorithms do not provide sufficient accuracy for complex control tasks like formation of detailed holograms as they perform well only for a small number of control points with respect to the number of transducers. Here we present an efficient PATs amplitude and phase solver based on the iterative first-order gradient optimization of the user defined loss function (APGO). We demonstrate high resolution hologram reconstruction when the number of control points is 25 times the number of transducers.Peer reviewe

Analog Cancellation of Unwanted Reflections for Enhanced Ultrasound Microscopy

Our scanning acoustic microscope uses coded excitation. The excitation signal consists of linear chirps of 1 μs duration. Parts of this signal are reflected from the internal structures of the lens, which causes high signal amplitudes in the recorded signal. The signals from the lens reflections can be 5 to 10 times larger than the signal from the sample. As coded excitation utilizes long modulated signals, the signals originating from the lens reflection will overlap with signals originating from the sample. The frequency encoding combined with the pulse compression method enables the reconstruction of the time resolution and allows the separation of the reflections. Pulse compression works properly only if the full duration and the full amplitude of the signal is captured. Therefore, the amplifier and the gain of the analog-to-digital converter (ADC) resolution needs to be adjusted to the amplitude of the strongest reflection present, i.e. the lens echoes. This results in quantization of the small-amplitude echoes from the sample. To overcome these limitations, we implemented an analog signal cancellation method. The signal of the lens reflections was first recorded. During measurements, the signal containing lens reflections was inverted, then reproduced by an arbitrary waveform generator, and finally combined with an analog signal combiner to the measurement signal from the microscope. The maximum signal amplitude was reduced by 80 %, which made it possible to change from the 1000 mV-range to the 200 mV-range of the ADC, thus improving the dynamic range. This reduced the quantisation noise and led to a 3 times lower noise-level in the image.Peer reviewe