## 1 Introduction  
It is well known that Raman scattering is an effective tool in detecting the strain state in crystalline solids [1; 2] and binary semiconductors, such as GaAs, under stress [3; 4]. Besides mechanical stress, other fields may induce strain in a crystal, which in turn can modify the frequency of lattice modes. This is the case of Fe-doped Lithium Niobate. In this crystal, the photorefractive electric field--that can be produced by the Raman probe itself--can induce such a strain via the piezoelectric effect. As a result, a shift in Raman modes can be detected [5]. The phonon deformation potentials (PDP) are the rates of change of long-wavelength optical phonon frequencies with strain. The sensitivity of phonon frequencies to the strain state can be exploited via PDP not only in bulk materials, but also in layers and even heterostructures. An example of special interest is that of the GaAs\({}_{1\text{-}}\)N\({}_{\text{x}}\) alloy [6; 7] grown as a pseudomorphic layer on top of GaAs buffer, where the reduced lattice parameter of the alloy results in the development of a biaxial tensile strain in the layer [6; 7; 8; 9]. The incorporation of hydrogen via irradiation makes the lattice parameter increase up to the value of GaAs, or even beyond, eventually leading to the buildup of compressive strain [8; 9; 10]. In fact, it is now well established that hydrogen almost completely neutralizes the effect of nitrogen on the band structure of the GaAs host matrix via the formation of nitrogen-hydrogen complexes N-nH (with n = 2, 3) [8; 9; 10; 11; 12; 13; 14]. The redshifted band gap E- blueshifts toward the typical energy position in GaAs materials. On the other hand the E+ band disappears after hydrogen irradiation [8; 9; 10] and the photoreflectance (PR) spectra of the H-treated GaAsN layers closely resemble (broad) GaAs spectra [8; 15]. At the same time, after hydrogenation, the GaAs-like Raman features near 292 cm\({}^{-1}\) persist. In contrast, the Ga-N local vibrational mode (LVM), near 475 cm\({}^{-1}\), disappears [8]. The GaAs-like Raman modes are only weakly perturbed by hydrogenation. Nevertheless, the longitudinal optical (LO) phonon frequency exhibits a blueshift (of the order of 1 cm\({}^{-1}\) or less) which is indicative of a reduction of the native biaxial tensile strain of the untreated material [8]. In fact, the frequency shift of the GaAs-like LO mode can be usefully applied for monitoring the strain state of GaAsN layers in planar heterostructures, as reported for GaAsN wires separated by GaAsN:H barriers [16]. This approach may be effective in improving the design of planar heterostructures for light polarization control obtained by selective H-irradiation of dilute nitride semiconductors [11]. In fact, spatially selective hydrogenation [17] can be used to engineer the strain profile and, hence, the mixing of heavy hole and light hole valence-band states. This allows tuning of the optical properties of the material. In particular, by realizing arrays of GaAsN/GaAsN:H wires at different orientations [18], a high degree of linear polarization was achieved for the light emitted by the sample. On the other hand, the same approach can be used to get a deeper insight (on a microscale) on the strain modulation ensuing the formation of N-nH complexes in the patterned GaAsN/GaAsN:H structures. In particular, it was shown that the formation of N-2H complexes (typical of early stage of the hydrogenation process) produces, besides N passivation, near-perfect lattice-matching with the GaAs buffer layer. On the other hand, N-3H centers are related to a sizeable \"overshooting\" of the lattice parameter of the host material, and consequently induce strain inversion in the H irradiated GaAsN layers [17; 18; 19; 20; 21]. If detailed and accurate knowledge of the strain profile derived from Raman map analysis is available, it will reliably compare with results from Finite-Elements calculations, thus serving as a method to check the strain profile in the heterostructure.