تاثیر جهت هدایت لیزر بر شفافیت الکترومغناطیسی

The influence of laser propagating direction on electromagnetically induced transparency (EIT) in a ladder type three-level thermal atomic system is presented. It is found that when the coupling laser beam co-propagates with the probe one, the EIT window at probe resonant frequency will disappear due to Doppler effect. The broadened two peaks beside of the transparent window overlap together and form the maximum value at the resonant point. While as the two laser beams counter-propagate, the probe absorption presents an interesting property, especially when the two lasers are with the same wavelength, we can get a broadened Doppler absorptive profile and a narrowed EIT window. The width of the EIT window decreases with the increase of temperature. But if the wavelengths of the two lasers are different, the EIT window at resonant point will be instead by a broadened dip. This dip will disappear gradually with the increase of the wavelength difference. Thus at thermal environment, when we perform the investigation associated with EIT, we ought to not only let two laser beams counter-propagate, but also select an atomic system owns a cascade equal energy interval as far as possible.