[Python] How do I change the axis tick font in a matplotlib plot when rendering using Latex?

The font of the axis tick mark labels produced from the following code isn't Helvetica, but is still the default serif Computer Modern. Any suggestions are greatly appreciated.

from matplotlib import rc, font_manager
from numpy import arange, cos, pi
from matplotlib.pyplot import figure, axes, plot, xlabel, ylabel, title, \
grid, savefig, show

sizeOfFont = 12
fontProperties = {'family':'sans-serif','sans-serif':['Helvetica'],
'weight' : 'normal', 'size' : sizeOfFont}
ticks_font = font_manager.FontProperties(family='Helvetica', style='normal',
size=sizeOfFont, weight='normal', stretch='normal')
rc('text', usetex=True)
rc('font',**fontProperties)
figure(1, figsize=(6,4))
ax = axes([0.1, 0.1, 0.8, 0.7])
t = arange(0.0, 1.0+0.01, 0.01)
s = cos(2*2*pi*t)+2
plot(t, s)

for label in ax.get_xticklabels():
label.set_fontproperties(ticks_font)

for label in ax.get_yticklabels():
label.set_fontproperties(ticks_font)

xlabel(r'\textbf{time (s)}')
ylabel(r'\textit{voltage (mV)}',fontsize=16,family='Helvetica')
title(r"\TeX\ is Number $\displaystyle\sum_{n=1}^\infty\frac{-e^{i\pi}}{2^n}$!",
fontsize=16, color='r')
grid(True)
savefig('tex_demo.pdf')

show()

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