  We study the nonlinear optical response of Tomonaga-Luttinger spin liquid in
the context of terahertz (THz) two-dimensional coherent spectroscopy (2DCS).
Using the gapless phase of the XXZ-type spin chain as an example, we show that
its third-order nonlinear magnetic susceptibilities $\chi^{(3)}_{+--+}$ and
$\chi^{(3)}_{-++-}$ exhibit photon echo, where $\pm$ refers to the
left/right-hand circular polarization with respect to the $S^z$ axis. The
photon echo arises from a ``lensing'' phenomenon in which the wave packets of
fractional excitations move apart and then come back toward each other,
amounting to a refocusing of the excitations' world lines. Renormalization
group irrelevant corrections to the fixed point Hamiltonian result in
dispersion and/or damping of the wave packets, which can be sensitively
detected by lensing and consequently the photon echo. Our results thus unveil
the strength of THz-2DCS in probing the dynamical properties of the collective
excitations in a prototypical gapless many-body system.
