## 1 Introduction  
The physical and optical properties of nanoparticles have been heavily studied in the last few decades. This interest stems from their specific behaviour that depends on the particle size and shape and is very different from that of the bulk material. The optical scattering properties of nanoparticles have been well-known since the development of Mie theory [1] and, more recently, the stability of numerical solutions has allowed reliable calculations on spherical structures [2; 3]. Additionally, the mechanical properties have been investigated, firstly by Raman scattering [4; 5; 6] and later by means of pump-probe techniques [7; 8; 9; 10; 11; 12; 13].  
Nanoparticles can work as sources of ultrasounds when a short optical pulse (pump laser) is used to thermally excite them producing GHz acoustic waves [14; 15]. The vibrational response of a nanoparticle can be detected in the time domain by measuring the changes in the scattered light. To access the very high frequency range of the vibrations of these small particles, the scattering is detected using pulsed lasers in a pump-probe configuration [16; 17]. Both the generation and detection processes can be enhanced by matching the pump and probe wavelengths with the optical resonances of these nanoparticles. This approach may open a new method for cell imaging, where the acoustic transducers are placed inside the sample [18]. This configuration allows the generation of spherical acoustic waves probing the sample in all directions, unlike the currently used thin-film opto-acoustic transducers [19; 20; 21], which only produces waves orthogonal to the surface. Moreover, nanoparticles generate higher ultrasonic spatial frequencies, which could increase the lateral resolution.  
However, the attenuation of acoustic waves in the GHz range is large, so it is crucial to maximise signal levels by optimising the generated wave amplitude and detection sensitivity without increasing the power density of the lasers. This improves the signal-to-noise ratio, decreases the time of the data acquisition and avoids damaging biological samples.  
One popular method of detecting a signal is to choose a probe wavelength near the plasmon edge around 530 nm where the scattering is sensitive to the change in the dielectric function caused by the free electrons' contribution [22; 23]. However, the signal received from the backscattered light in this regime is poor due to the low level of backscatter which results in a low signal-to-noise ratio and the need to increase the acquisition time (Section 2.3).  
By choosing the probe laser away from the plasmon edge in the red or near infrared (\(>\)700 nm) region, another modulation mechanism may be accessed. Here, the scattering cross-section is a strong function of wavelength and size and by choosing a particle size/wavelength combination, where the change in scattering cross-section vs. particle size is high, it is possible to enhance the optical detection of the particles' vibrational modes.  
In this work, we describe the theoretical basis of the generation and detection, the optical and mechanical modelling of the nanoparticles, the sample fabrication, the experimental results and the optimisation of the optical detection. A scan of a sample containing particles with different sizes shows the existence of multiple frequencies inside the point spread function, which could allow identification of single particles with ultra-high resolution.