We report observations of the rotation of the plane of polarization of photon echoes by a weak axial magnetic field. The echoes are generated on the
6S21 to
7P21 transition in cesium (4593 \AA{}) by a sequence of two collinear laser pulses polarized in the same direction. When a magnetic field is applied along the propagation axis the polarization vector of the echo is rotated from that of the exciting pulses. We measure the intensity of the photon echo signal polarized orthogonal to the excitation pulse polarization as a function of magnetic field and pulse separation. The theoretical analysis decribes cesium atoms as a nondegenerate multilevel system, consisting of ground and excited Zeeman-split hyperfine-structure levels. The theory presented predicts the observed dependence on magnetic field strength, pulse separation, and excited- and ground-state
g factors. A novel method of observing photon echoes is described which uses the polarization rotation effect to replace electro-optical shutters often employed in photon echo experiments.
Non-Faraday rotation of photon-echo polarization was investigated at the $J=1\ensuremath{\leftrightarrow}J=0$ transition in a wide range of longitudinal magnetic field strength. The echo was generated at the intercombination transition $(6s6p)^{3}P_{1}\ensuremath{\rightarrow}(6{s}^{2})^{1}S_{0}$ of
174Yb by two resonant laser pulses of linear (parallel or mutually orthogonal) polarization. A detailed analysis of the echo polarization performed by an angled echo technique has shown quite different behavior in the weak and strong magnetic field limits. The photon echo has polarization close to linear at a magnetic field strength $\mathcal{B}\ensuremath{\leqslant}5\phantom{\rule{0.3em}{0ex}}\mathrm{G}$; its polarization plane rotates around the magnetic field vector clockwise or counterclockwise depending on the magnetic field orientation relative to the wave vector. The photon-echo polarization components and echo power oscillate as functions of
B. As the magnetic field increases, the oscillations become smaller and almost disappear at a magnetic field $\mathcal{B}\ensuremath{\approx}40\phantom{\rule{0.3em}{0ex}}\mathrm{G}$; the photon echo does not disappear at this magnetic field, but its polarization vector no longer has a preferred orientation. Numerical calculations of the photon echo generated at a spectral wide line agree with the experimental behavior of the photon-echo polarization for an arbitrary magnetic field.
Detailed experimental analysis of photon echo polarization in a longitudinal magnetic field is performed for the first time at the simplest quantum transition J = 1 ↔ J = 0 for which the non-Faraday rotation of photon echo polarization plane was predicted. The echo was generated at the intercombination transition (6s6p) 3P1 → (6s2) 1S0 of 174Yb by two resonant laser pulses of linear (parallel or mutually orthogonal) polarization, and the angled echo optical scheme was applied for the detection. At the magnetic field strength ℬ ⩽ 5 G the photon echo has polarization very close to linear one; its polarization plane rotates around magnetic field vector, and photon echo polarization components and integral echo power oscillate versus ℬ from its maximum value to zero, in agreement with theory. At a stronger magnetic field ℬ ∼ 40 G photon echo power oscillations disappear, no preferable orientation of polarization vector is observed; the fluctuations of exciting radiation spectrum are supposed to be responsible for this "non-polarized" echo.