Nanometric layer thickness imaging is crucial for fundamental research and characterization of micro fabricated devices. Here, we assess the potential of a non-contact opto-acoustic frequency domain laser ultrasound (FreDomLUS) microscopy technique for imaging nanometric thickness variations via GHz zero-group velocity (ZGV) elastic plate resonances. The method exploits the ZGV's lateral energy confinement that leads to sharp resonance peaks which can be effectively probed with the FreDomLUS technique at GHz acoustic frequencies. For demonstration purposes we introduced sub-10 nm height variation patterns in the topmost layer of solidly mounted bulk-acoustic wave resonators with a design frequency of around 1.7 GHz. They are raster-scanned to retrieve ZGV-frequencies from local acoustic spectra as a contrast quantity for imaging. We show how to retrieve quantitative height information by numerically calibrating the factor which inversely relates ZGV frequency change with the layer thickness change. Height variations in stacks with nominal thickness changes of 8 nm, 4 nm, and 1 nm can be resolved and indicate sub-nanometer depth resolution capabilities. The lateral resolution is studied by measuring the method's step edge function and it is found to be in the micrometer range. Atomic force microscopy imaging is used to validate the results.

A method for the determination of longitudinal and transverse bulk acoustic wave attenuation from measurements of the decay-rate of two independent zero-group-velocity resonances in a couple of matched plates is presented. A linear relation is derived, which links the bulk-wave attenuation coefficients to the decay-rate of plate-resonances. The relation is used to determine the acoustic loss of tungsten at GHz frequencies from noncontact laser-ultrasonic measurements in plates with thicknesses of about 1 µm. The longitudinal and transverse attenuation was found to amount to 1918 m-1 and 7828 m-1 at 2.16 GHz and 3265 m-1 and 12181 m-1 at 2.46 GHz. The presented approach is validated with calculated responses to a thermoelastic source, and the accuracy of the obtained attenuation values is estimated to be in the range of 10%.

New Generation of BAW Technology has been developed. It supports the full frequency range from 1 GHz to above 8 GHz including the 5G NR and WiFi6E frequency bands. Furthermore, it provides significant improvements in resonator performance: up to 100% higher Q, over 50 % TCF improvement and improved modes behavior. It also makes Sc doping or higher Sc doping applicable to more frequency bands. Moreover, the resonator size could be decreased significantly at the same time. Thus, filters have better power handling, IL and rejection in pair with device shrinkage of up to 50 %.

Photothermal radiometry with an infrared camera allows the contactless temperature measurement of multiple surface pixels simultaneously. A short light pulse heats the sample. The heat propagates through the sample by diffusion and the corresponding temperature evolution is measured at the surface of the sample by an infrared camera. The main drawback in radiometric imaging is the loss of the spatial resolution with increasing depth due to heat diffusion, which results in blurred images for deeper lying structures. We circumvent this information loss due to the diffusion process by using blind structured illumination, combined with a nonlinear joint-sparsity reconstruction algorithm. The structured illumination is realised by parallel laser lines from a vertical-cavity surface-emitting laser (VCSEL) array controlled by a random binary pattern generator. By using 150 different patterns of structured illumination and our iterative joint-sparsity algorithm, it was possible to resolve 1 mm thick lines at a distance down to 0.5 mm, which results in a resolution enhancement of approximately a factor of four compared to the resolution of 5.9 mm for homogeneous illuminated thermographic reconstruction.

We present an evaluation of attenuation of elastic waves in the GHz range, based on the decay of thickness-stretch resonances of plates. We measured the elastic response of micro-scale aluminum plates, using a laser-ultrasound technique. The thermo-elastic laser excitation provides significant coupling into thickness-stretch modes in the frequency range above 1.5 GHz. To suppress interference from other resonant and transient modes, we used an excitation spot size well above the plate thickness and applied signal processing in the time and frequency domain. We adapted existing theory on the decay of thickness-shear modes to apply for thickness-stretch modes, validated the derived theory with simulations, and applied it to experiments. A set of samples with different thicknesses in the range of 1.1–2.15 μm allowed us to obtain damping values in the corresponding frequency range of 1.5–3 GHz.

In conventional photoacoustic tomography, several effects contribute to the loss of resolution, such as the limited bandwidth and the finite size of the transducer, or the space-dependent speed of sound. They can all be compensated (in principle) technically or numerically. Frequency-dependent acoustic attenuation also limits spatial resolution by reducing the bandwidth of the photoacoustic signal, which can be numerically compensated only up to a theoretical limit given by thermodynamics. The entropy production, which is the dissipated energy of the acoustic wave divided by the temperature, turns out to be equal to the information loss, which cannot be compensated for by any reconstruction method. This is demonstrated for the propagation of planar acoustic waves in water, which are induced by short laser pulses and measured by piezoelectric acoustical transducers. It turns out that for water, where the acoustic attenuation is proportional to the squared frequency, the resolution limit is proportional to the square root of the distance and inversely proportional to the square root of the logarithm of the signal-to-noise ratio. The proposed method could be used in future work for media other than water, such as biological tissue, where acoustic attenuation has a different power-law frequency dependence.

The International Photoacoustic Standardisation Consortium (IPASC) emerged from SPIE 2018, established to drive consensus on photoacoustic system testing. As photoacoustic imaging (PAI) matures from research laboratories into clinical trials, it is essential to establish best-practice guidelines for photoacoustic image acquisition, analysis and reporting, and a standardised approach for technical system validation. The primary goal of the IPASC is to create widely accepted phantoms for testing preclinical and clinical PAI systems. To achieve this, the IPASC has formed five working groups (WGs). The first and second WGs have defined optical and acoustic properties, suitable materials, and configurations of photoacoustic image quality phantoms. These phantoms consist of a bulk material embedded with targets to enable quantitative assessment of image quality characteristics including resolution and sensitivity across depth. The third WG has recorded details such as illumination and detection configurations of PAI instruments available within the consortium, leading to proposals for system-specific phantom geometries. This PAI system inventory was also used by WG4 in identifying approaches to data collection and sharing. Finally, WG5 investigated means for phantom fabrication, material characterisation and PAI of phantoms. Following a pilot multi-centre phantom imaging study within the consortium, the IPASC settled on an internationally agreed set of standardised recommendations and imaging procedures. This leads to advances in: (1) quantitative comparison of PAI data acquired with different data acquisition and analysis methods; (2) provision of a publicly available reference data set for testing new algorithms; and (3) technical validation of new and existing PAI devices across multiple centres.

In this work, we theoretically and experimentally deal with photoacoustic resolution enhancement by means of saturated modulation quenching. It is shown that experimental systems for resolution enhancement with saturated modulation quenching in fluorescence microscopy are not necessarily suited for photoacoustic modulation quenching. Here, we show that modulation quenching is not limited to fluorescent dyes but can be also applied to metallic nanoparticles. For modulation quenching in photoacoustic microscopy it is sufficient that the signal saturates with increasing excitation intensity.

In this work, we theoretically describe photoacoustic signal generation of fluorophores, for which triplet relaxation can be neglected. In the theoretical model, the excited state lifetimes, the fluorescence quantum yield, and fast vibrational relaxation are considered. Based on our findings, we propose that for fluorophores the photoacoustic signal together with knowledge about the absorption and emission spectrum allows to directly determine the fluorescence quantum yield without comparative measurements.

In this work, we demonstrate a new technique which has the potential for super-resolution fluorescence imaging. In this technique, similar to STED microscopy, a tightly focused intensity-modulated excitation beam and a donut shaped cw beam are confocally raster-scanned over the sample. In contrast to STED microscopy, both beams need to be absorbed by the fluorophore. A lock-in amplifier is used to measure only the modulated fluorescence. Sufficiently high cw donut beam intensities lead to saturation of the fluorophores in the outer rim of the modulated point spread function which enables resolution enhancement. Theoretically, sub-diffraction resolution can be achieved.

In this work we are using spatially localized acoustic resonances, so-called zero group velocity (ZGV) Lamb waves, to assess the attenuation parameters from the temporally decaying acoustic waves in thin plates. As these waves are not propagating, the temporal decay of these waves is directly linked to the intrinsic material damping. Guided waves and ZGV Lamb waves are excited and detected fully optically using frequency domain laser excitation and optical vibrometry. The measured responses are fitted with results obtained from finite element simulations. Samples of tungsten with thicknesses in the micrometer range, featuring ZGV frequencies around 2 GHz, were produced. As the damping of the ZGV resonance is influenced by both, the shear and longitudinal attenuation, two ZGV resonances are evaluated in order to assess the overall attenuation.

We investigate the numerical predictions of the Finite Element Method for bulk acoustic wave resonators. A conventional 2D model is used to calculate major performance indicators of the piezoelectric resonators, such as energy confinement (Q-value) and effective piezoelectric coupling K-eff. The results are compared to experimentally evaluated values. We show that these indicators can be accurately predicted by using a 2D model in combination with appropriate boundary conditions.

In photoacoustic imaging, ultrasound waves generated by a temperature rise after illumination of light absorbing structures are measured on the sample surface. These measurements are then used to reconstruct the optical absorption. We develop a method for reconstructing the absorption inside the sample based on a discrete linear state space reformulation of a partial differential equation that describes the propagation of the ultrasound waves. Fundamental properties of the corresponding state space model such as stability, observability and controllability are also analyzed. By using Stokes' equation, the frequency dependent attenuation of the ultrasound waves is incorporated into our model, therefore the proposed method is of general nature. This approach allows for inhomogeneous probes with arbitrary absorption profiles and it accounts for the decrease in laser intensity due to absorption. Furthermore, it provides a method for optimizing the laser modulation signal such that the accuracy of the estimated absorption profile is maximized. Utilizing the optimized laser modulation signal yields an increase in reconstruction accuracy compared to short laser pulses as well as chirp modulation in many scenarios.

In this paper, multimodal optical-resolution frequency-domain photoacoustic and fluorescence scanning microscopy is presented on labeled and unlabeled cells. In many molecules, excited electrons relax radiatively and non-radiatively, leading to fluorescence and photoacoustic signals, respectively. Both signals can then be detected simultaneously. There also exist molecules, e.g. hemoglobin, which do not exhibit fluorescence, but provide photoacoustic signals solely. Other molecules, especially fluorescent dyes, preferentially exhibit fluorescence. The fluorescence quantum yield of a molecule and with it the strength of photoacoustic and fluorescence signals depends on the local environment, e.g. on the pH. Therefore, the local distribution of the simultaneously recorded photoacoustic and fluorescence signals may be used in order to obtain information about the local chemistry.

In this Letter, we theoretically describe photoacoustic signal generation of molecules, for which triplet relaxation can be neglected, by considering the excited state lifetime, the fluorescence quantum yield, and the fast vibrational relaxation. We show that the phase response of the photoacoustic signal can be exploited to determine the excited state lifetime of dark molecules. For fluorescent molecules, the phase response can be used to determine the fluorescence quantum yield directly without the need of reference samples.

We present a prototype for all-optical photoacoustic projection imaging. By generating projection images, photoacoustic information of large volumes can be retrieved with less effort compared to common photoacoustic computed tomography where many detectors and/ or multiple measurements are required. In our approach, an array of 60 integrating line detectors is used to acquire photoacoustic waves. The line detector array consists of fiber-optic MachZehnder interferometers, distributed on a cylindrical surface. From the measured variation of the optical path lengths of the interferometers, induced by photoacoustic waves, a photoacoustic projection image can be reconstructed. The resulting images represent the projection of the three-dimensional spatial light absorbance within the imaged object onto a two-dimensional plane, perpendicular to the line detector array. The fiber-optic detectors achieve a noise-equivalent pressure of 24 Pascal at a 10 MHz bandwidth. We present the operational principle, the structure of the array, and resulting images. The system can acquire high-resolution projection images of large volumes within a short period of time. Imaging large volumes at high frame rates facilitates monitoring of dynamic processes.

In photoacoustic imaging, optically generated acoustic waves transport the information about embedded structures to the sample surface. Usually, short laser pulses are used for the acoustic excitation. Acoustic attenuation increases for higher frequencies, which reduces the bandwidth and limits the spatial resolution. One could think of more efficient waveforms than single short pulses, such as pseudo noise codes, chirped, or harmonic excitation, which could enable a higher information-transfer from the samples interior to its surface by acoustic waves. We used a linear state space model to discretize the wave equation, such as the Stoke's equation, but this method could be used for any other linear wave equation. Linear estimators and a non-linear function inversion were applied to the measured surface data, for one-dimensional image reconstruction. The proposed estimation method allows optimizing the temporal modulation of the excitation laser such that the accuracy and spatial resolution of the reconstructed image is maximized We have restricted ourselves to one-dimensional models, as for higher dimensions the one-dimensional reconstruction, which corresponds to the acoustic wave without attenuation, can be used as input for any ultrasound imaging method, such as back-projection or time-reversal method.

Chemical mapping was demonstrated with a mid-infrared (MIR) microspectroscopy setup based on a supercontinuum source (SC) emitting in the spectral range from 1.55 to 4.5 µm and a MEMS-based Fabry-Pérot filter spectrometer. Diffraction limited spatial resolution in reflection geometry was achieved. A multilayer film consisting of different polymers and mixtures thereof was measured and results were compared to those gained with a conventional FTIR microscope equipped with a thermal MIR source. Results show that compared to thermal sources, the application of the SC source results in higher signal-to-noise ratios together with better spatial resolution and faster scanning. Furthermore, diffraction limited imaging of red blood cells was demonstrated for the first time in the MIR spectral region in reflection mode. The distinctive characteristics of the MIR spectral region in conjunction with the high brightness, spatial coherence and broadband nature of supercontinuum radiation show the potential for improving infrared microscopy significantly.