What are the triple realms of Tang poetry?

It can be said that the late Tang poetry, represented by Li Shangyin, created the third realm of Tang poetry after the prosperous and middle Tang dynasties. From the continuous trajectory of these three realms, we can see that, first, with the ups and downs of the times, the poet's psychology has changed from broad-minded and cheerful to implicit, the poetic emotion has changed from clear to obscure, and the momentum has changed from publicity to convergence; Secondly, with the development of poetic techniques, the language has become more refined and refined, and the structure has changed from natural stretching to exquisite and meticulous; Thirdly, with the shrinking of literati's life and disappointment with reality, the content of poetry has changed from mainly absorbing from the outside world to seeking more inner experience. However, although this third realm inevitably has a sense of spiritual decline, from the perspective of poetry development, it has turned around by going deep into the inner world and opened up a new world of poetry. In art, it has its own characteristics, that is, the exquisite language skills of modern poetry, the exquisite emotional expression, the connection of structural layout and the profound and hazy artistic conception.