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We report on the design of photonic crystal nanobeam cavity fully encapsulated in silica. The proposed design, based on the principle of gentle confinement of the electromagnetic field, is mostly analytical and emphasizes on the most realistic options for fabricating nanocavities, in particular in III-V semiconductor materials. After determining the field decay inside the photonic bandgap of a nanobeam photonic crystal, we engineer the envelope of the cavity mode into a Gaussian shape by shifting only progressively the lattice constant. We discuss the various implementations of such shifts and give a simple algorithm to position each hole. The resonant wavelengths are found to depend linearly on the central lattice constant and on the radius of the holes. High Q factors above 10 6 and modal volume V close to ( λ/n) 3 are obtained. In particular, Q factors remain high for a wide range of values of the central lattice constant and of holes radii, hence showing exceptional tunability properties as well as robustness with respect to common fabrication defects.