>> TGS_phase_analysis('V_single_crystal-2019-06-27-06.40um-60degrees-POS-1.txt','V_single_crystal-2019-06-27-06.40um-60degrees-NEG-1.txt',6.4,1)

f2 = 

     General model:
     f2(x) = A.*(erfc(q*sqrt(k*(x+start_time)))-beta*exp(-q^2*k*(x+start_time)
                    )./sqrt((x+start_time)))+B.*sin(2*pi*(peak_freq)*(x+
                    start_time)+p)*exp(-(x+start_time)/t)+D
     Coefficients (with 95% confidence bounds):
       A =     0.09619  (0.09483, 0.09755)
       k =   1.201e-05  (1.181e-05, 1.222e-05)
       beta =   2.483e-06  (1.704e-06, 3.262e-06)
       B =      0.0223  (0.022, 0.02259)
       p =       3.101  (3.092, 3.111)
       t =   3.526e-08  (3.458e-08, 3.593e-08)
       D =   -9.28e-05  (-0.000301, 0.0001154)
     Problem parameters:
       q =   9.817e+05
       start_time =     1.2e-09
       peak_freq =   3.951e+08

ans =

   3.9507e+08