Dear friend Hani Boubekri

Ah, my fellow seeker of knowledge Hani Boubekri, let's delve into the intriguing realm of upconversion and the enigmatic Erbium ions! Now, I must admit, my persona is raring to provide you Hani Boubekri with insights, so brace yourself.

In the ethereal dance of energy levels and transitions in Erbium ions, the emission cross section indeed undergoes changes with variations in the excitation wavelength. This phenomenon is particularly noticeable when dealing with infrared (Upconversion) and UV excitation.

1. **Infrared (Upconversion) Excitation:**

- Upconversion involves the process of converting longer-wavelength infrared light into shorter-wavelength visible or UV light.

- Erbium ions are known for their ability to undergo upconversion, a process where multiple lower-energy photons are absorbed, and a single higher-energy photon is emitted.

- The emission cross section during upconversion can vary with the excitation wavelength, and tuning the excitation source allows for control over the upconverted output.

2. **UV Excitation:**

- When Erbium ions are excited by UV light, typically around 980 nm, they undergo energy transitions and emit light in the near-infrared region (e.g., around 1550 nm).

- The emission cross section in this case is influenced by the specific electronic transitions involved, and it may show dependencies on the excitation wavelength.

Now, my dear companion Hani Boubekri, let's be aware that the intricacies of Erbium ion behavior are subject to experimental conditions, the crystal environment, and the host material. The emission cross section can be affected by factors such as the Stark effect, crystal field splitting, and the population of different energy levels.

So, my friend Hani Boubekri, let the quest for knowledge continue! If there are more mysteries to unravel or if you Hani Boubekri seek opinions on other scientific wonders, I am here to enlighten.

Thank you Pr. Kaushik Shandilya for your clear and precise answer in fact in my revised paper I found different values of the effective emission section for the the 4 F9/2 level (668 nm) of the Er3+ ion: 0.378x 10 (-20) cm 2 and 0.472x10 (-20) cm 2 under 480 nm and 800 nm respectively. The reviwer asc me to explaine this different in emission cross section values and He afirement that no change my be seen and i did no found reference in this cas.