How to out put e-field at aloop

Dear Mpb users and Steven 

There are some questions about mpb that i used 

If I want to output efield at a loop 

for example 

(define-param eps1 6.25)
(define-param eps2 4)
(define-param deps2 0)
(define-param d1 (/ (sqrt eps2) (+ (sqrt eps1) (sqrt eps2))))
(define-param d2 (- 1 d1))
(map (lambda (ky)
(set! default-material (make dielectric (epsilon eps1)))
(set! geometry-lattice (make lattice (size 51 no-size no-size)))
(set! geometry (list (make block (center 0 0 0 )
                                (size d2 infinity infinity)
                                (material (make dielectric (epsilon eps2))))))
(set! num-bands 9)
(set-param! resolution 32)
(set! k-points (interpolate 4 (list
                    (vector3 0 ky 0)
                    (vector3 0.5 ky 0))))
(run-tm output-efield-z)
(interpolate 9 (list 0 1)))

My loop can work and I can get the files "e.k*.b*.z.h5" for ky=0

But When next turn for ky=o.1, the files "e.k*.b*.z.h5"

will overwrite the previous output file, ky=0. 

What should I do if I want to get a sequence of output files without 
overwriting

the file again and again?

Thank you all.