Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been proposed as a sensitive method for electron paramagnetic resonance spectroscopy, enabled by the development of operational single microwave photon detectors at millikelvin temperature. Here, we report the detection of spin echoes in the spin fluorescence signal. The echo manifests itself as a coherent modulation of the number of photons spontaneously emitted after a $\ensuremath{\pi}/{2}_{X}\ensuremath{-}\ensuremath{\tau}\ensuremath{-}{\ensuremath{\pi}}_{Y}\ensuremath{-}\ensuremath{\tau}\ensuremath{-}\ensuremath{\pi}/{2}_{\mathrm{\ensuremath{\Phi}}}$ sequence, dependent on the relative phase $\mathrm{\ensuremath{\Phi}}$. We demonstrate experimentally this detection method using an ensemble of
Er3+ ion spins in a scheelite crystal of
CaWO4. We use fluorescence-detected echoes to measure the erbium spin coherence time, as well as the echo envelope modulation due to the coupling to the
183W nuclear spins surrounding each ion. We finally compare the signal-to-noise ratio of inductively detected and fluorescence-detected echoes, and show that it is larger with the fluorescence method.
The present work is devoted to the description of a nonperturbative treatment of anharmonic effect and mode-mode coupling taking place in molecular vibrational systems. The information is obtained from the four-wave-mixing signal detected in the usual photon-echo phase-matched direction, termed 4WM photon-echo, using heterodyne detection to test separately the real and the imaginary parts of the third-order polarization. This approach allows a complete redistribution of the relaxation, dephasing, and transition constants on account of the magnitude of the coupling strength. In addition, our analytical approach accounts for the contributions provided by the various possible chronological orderings of the field interactions. The high sensitivity of the signal with both the energy shifts induced by the internal couplings and off-resonant energy parameter of the laser excitation is properly analyzed in the framework of our analytical description.