def fn_k2_TE(h_11,c_11,beta_11):
    """EM coupling factor k^2 for thickness expander mode, as function of:
    -- piezoelectric tensor component h_11
    -- elastic stiffness tensor component c_11
    -- impermittivity tensor component beta_11.
    """
    return h_11**2 / (c_11 * beta_11)