How to narrow the frequency band of a wavelet
Center frequency and bandwidth are linked in a single "scale" parameter for wavelets. That means if you change the bandwidth you also change the center frequency and vice versa.

For the discrete wavelet transform on a dyadic lattice the relative bandwidth is also fixed at approximately one octave.

If you want a continuous wavelet transform frame with a smaller relative bandwidth you need to construct a different wavelet. Intuitively, it will come with more oscillatory cycles, i.e. more zero crossings.

The Ricker wavelet you have coincides with the 2nd so called Hermite function. Higher order Hermite functions also have an increasing number of zero crossings. Unfortunately they're one of the examples of where the intuition of adding more cycles will not work. The Hermite functions grow in both frequency as well as time support with increasing order. So that's not the way to go. I'm only making this clear because you might find a reference to them in relation to the Ricker Wavelet.

So instead I would try to adapt the construction of the wavelet to your specific needs, which you will have to outline in your question to receive a more detailed answer.