Hi Lorenzo,
There is no way to restrict the camera to rotate left and right only. You could add a custom render loop where you could do this:
// perform built in camera updates
scene.initializeFrame();
var camera = scene.getCamera();
// set the camera position to be in the plane of the equator
camera.position.z = 0.0;
// reset the direction to point towards the center of the earth
Cartesian3.negate(camera.position, camera.direction);
Cartesian3.normalize(camera.direction, camera.direction);
// orient so that ‘up’ is pointing north
Cartesian3.clone(Cartesian3.UNIT_Z, camera.up);
Cartesian3.cross(camera.direction, camera.up, camera.right);
// render scene
scene.render();
This code would only work in 3D. If you need help with 2D or Columbus view modes, let us know.