I want to know the physical meaning of photon confinement in such particles.
I assume you are referring to plasmonics. Basically the free carriers (electrons/holes) within the metal can respond to incident electromagnetic fields, such as light. Provided the frequency of the light is less than the plasma freq. of that metal (this is specific to each material under a given carrier density, etc...) the carriers can oscillate coherently to oppose that incident field. This causes the field (light) to be reflected. This is why metals are shiny.
Now make a nanoparticle of this metal. You cause the same oscillations of the carriers, but now in the new sub-diffractional shaped particle, this causes oscillations between the opposite sides, effectively setting up a dipole resonance. By doing so this creates a quasi particle between the electrons and photons called a surface plasmon. The plasmon retains the properties of the original photon and can be reemitted into free-space in the form of that photon again, so the light was confined to nanoscale, sub-diffractional dimensions in the form of a surface plasmon.
These surface plasmons can also be stimulated at noble metal surfaces in the form of propagating modes (desired for waveguides for instance). All of these modes can also be observed within highly doped semiconductors or in polar dielectric materials. In the case of the latter, the oscillating charge is the lattice ions (for instance silicon and carbon in silicon carbide) in the form of optic phonons. These modes are referred to as Surface Phonon Polaritons and are obsrved in the mid-infrared to terahertz spectral range, but otherwise are very similar in the physics and mathematical constructs as plasmons.
Nevertheless the velocity of the photon has remained the c, hasn't it? P.S. Such a small photon could be!
To be clear, you're not talking about a mechanism of photon absorption in such particles that is responsible for excitation of collective oscillations of electrons leading to the effect known as the localized surface plasmon resonance (LSPR)? If not, then what is it?
P.S. I'd stronly suggest you to reformulate the question since we're all in the dark guessing about what exactly did you mean:) Electron can be confined in 1D, 2D, 3d structures (like, quantum dots etc), while photon can be confined in so called photonic bandgap crystals or materials (also, in 1D, 3D, 3dD. Phonons can also acquire unusual properties on a small scale. And you meant....?
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