What is Photon Echo?

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- GoPhotonics

Jan 16, 2024

Photon echo is a phenomenon in physics where a pulse of light is spontaneously emitted from a system of atoms previously irradiated by two coherent resonant light pulses. This emission is observed at a time shortly after the second pulse, and this timing closely aligns with the time between the two initial excitation pulses.

In 1964, photon echo was first observed by Kurnit et al. at Columbia University in the optical regime. This phenomenon was first documented in a dilute ruby crystal through the application of a pulsed ruby laser. This phenomenon has important applications in various areas of physics, chemistry, and information processing, allowing scientists to study ultrafast processes and create advanced technologies such as quantum memory devices and optical communication systems. Usually, short and ultrashort multiphoton laser pulses are used in photon echo generations.

Working of Photon Echo

The process begins with the exposure of a material or quantum system to a series of precisely timed laser pulses. The first pulse, often called the excitation pulse or π/2 pulse, is designed to excite the quantum system to a super-radiant state, characterized by an oscillating macroscopic electric dipole moment. Following the excitation, the macroscopic electric dipole moment quickly undergoes dephasing due to inhomogeneous crystal-field strains or other factors. As a result of dephasing, the coherence among the excited particles within the material is lost, leading to a loss of synchronization.

The dephased atoms within the material then undergo spontaneous emission at the normal rate, releasing light. The second pulse, known as the π pulse, is introduced to reverse the dephasing process. This pulse interacts with the dephased atoms, effectively restoring coherence and rephasing the system. When the rephasing process is complete, the macroscopic electric dipole moment is momentarily reformed. This results in the emission of an intense burst of light, known as the photon echo, which is a coherent reproduction of the initial pulse but delayed in time.

Photon echoes are standard tools of nonlinear spectroscopy for determining the dephasing time, T2. A short laser pulse (π/2 pulse) initially excites the population in a sample at a frequency corresponding to a quantum transition, followed by another pulse (a π pulse) from the same source. If the time interval, Δt = t′, between the two pulses is less than the dephasing time, a photodetector will detect an optical signal at a time Δt after the second pulse. The photon echo rapidly diminishes as Δt becomes larger than the dephasing time T2.

Photon echo is a result of the rephasing of the excited states in the material, leading to the emission of light waves that echo the original excitation pulses. Echo is a delayed and coherent reproduction of an initial pulse of light.

Factors Responsible for Photon Echo

  • Temporal Control: Photon echo generation relies on precise timing between the laser pulses. Short and ultra-short pulses offer extremely precise temporal control, allowing for manipulation on the order of femtoseconds or picoseconds. This precision is essential to interact with the material's polarization decay and induce the necessary coherence for photon echo.
  • Coherence Preservation: These pulses generate coherent excitations within the material that preserve coherence long enough for the second pulse to interact before the induced polarization fully decays. Maintaining coherence is crucial for observing the time-reversed re-emission characteristic of photon echoes.
  • Interference Effects: Short pulses allow for the creation of interference effects within the material. When the second pulse interacts with the remaining coherence from the first pulse, it can induce interference that leads to the coherent re-emission of light.
  • High Spectral Resolution: Ultra-short pulses offer high spectral resolution that ensures detailed studies of the material's response to the laser pulses. This resolution aids in observing and analyzing the resulting photon echo signal.
  • Controlled Excitation: Short pulses provide controlled excitation to allow for precise manipulation of the material's response to the laser pulses. This control is crucial for inducing and studying the conditions necessary for photon echo.

The generation of photon echo relies on several specific conditions that need to be met to observe this phenomenon:

  • Pulse Timing: It requires precise timing between the laser pulses. The second pulse must interact with the material before the induced polarization from the first pulse fully decays. This typically involves timing on the order of picoseconds or femtoseconds, depending on the material and experimental setup.
  • Material Characteristics: The material being studied should possess properties that allow for the induced polarization to persist long enough for the second pulse to interact with it. Some materials, particularly those with specific quantum properties or homogeneous broadening, are more suitable for observing photon echo effects.
  • Experimental Setup: Proper experimental conditions, such as precise control over the laser pulses, environmental factors like temperature, and the arrangement of optical elements, are crucial for observing and studying photon echo phenomena.

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