## 1 Introduction  
Semiconductor nanowires are nanostructures with a large aspect ratio and dimensions ranging typically between 5 nm and 100 nm in diameter and about 100 nm up to several micrometers in length. They are often referred to as one-dimensional nanostructures (1D). Due to a vast choice of materials from which they can be synthesized and the number of interesting physical properties that they possess, nanowires (NWs) have recently emerged as alternative building blocks for nanoscale electronics, optoelectronics as well as chemical and biological sensing at the molecular scale [1; 2; 3; 4]. In particular, Gallium nitride (GaN) nanowires have attracted extensive research interest for their enhanced piezo-electric properties and nanoscale device applications [5]. Although many applications require knowledge and ability to control the mechanical behavior of NW based nanostructures, the elastic properties of such structures remain relatively unexplored. A few recent works focus mainly on individual NWs without considering a surrounding medium [6; 7]. Measuring the mechanical properties of nanowires by conventional techniques is not trivial. Concerning individual NWs, optical measurements used commonly in microelectromechanical systems (MEMS) are not easily applicable to the NW resonators because the diameter is less than the visible wavelength. Atomic force microscopy (AFM) has been used to measure the Young modulus of individual NWs [8; 9]. Alternatively, transmission electron microscopy (TEM) permits direct and quantitative determination of mechanical resonances by applying an actuating signal between the nanowire and a counter-electrode [7]. Thus, the analysis of the resonant behavior of individual nanowires permits conclusions about their elastic constants. Studied by these two techniques, the values of the Young modulus published in the literature are controversial. Indeed, one part of the published studies shows that the Young modulus decreases by reducing the NW diameter [7; 10], while others present the opposite behavior, i.e., the Young modulus increases with the NW diameter reduction [7; 11]. Finally, some publications report the independence of the Young modulus from the wire diameter [12]. Meanwhile, the properties of individual nanowires may not provide reliable information about the mechanical properties of the nanowire-based devices. The integration of nanowires into devices often uses the polymer embedding the NW array. Thus, for device operation, it is important to consider not only individual stand-alone NWs, but also the surrounding medium (polymer), whose properties may vary (e.g., with annealing temperature and time). In this work, we focus on the elastic properties of nitride nanowires, which are promising materials for the fabrication of efficient and compact piezogenerators. Their tremendous piezoelectric and mechanical properties give them the ability to convert efficiently mechanical energy into electrical energy. Using an adapted AFM resiscope, [PERSON] et al. [5] showed the great potential of nitride nanowires for piezogeneration and the correlation between the polarity of the nanostructure, its deformation and the establishment of the piezopotential. In this study, we use the prototype of a piezogenerator based on a vertical array of GaN nanowires. The wires are embedded in a polymer of Hydrogen silsesquioxane (HSQ), with their tops connected by a CrPt metal electrode providing Schottky contact. The effective elastic moduli of the fabricated HSQ-NW composite medium are determined via the minimization of the discrepancy between the calculated and measured characteristics of the surface acoustic waves (SAWs). This minimization is performed by varying of the input sample's material constants within the computer model used for SAW simulation. To excite and measure the SAWs, we use the Transient Grating Method (TGM) based on the exciting and probing, directly and in real time, coherent wavelength-tunable acoustic waves [13; 14; 15].