In the word "Ultramodern"; the prefix "ultra" refers to -

• In the word "Ultramodern"; the prefix "ultra" refers to - very.

• Ultra- (prefix)
English Meaning: to an extreme degree; very, beyond; on the other side of.
Bangla Meaning: ছাড়িয়ে; অতি-; একান্ত-।

- Examples : 
• to an extreme degree; very
- ultra-expensive
- ultra-modern architecture
- ultra-sensitive
- an ultra-short haircut

• beyond; on the other side of.
- ultramontane.

Source: Cambridge Dictionary, Oxford Learner's Dictionary and Accessible Dictionary by Bangla Academy.