Thomas Jefferson used a version of the phrase "empire of liberty" on multiple occasions, including the following three examples:
... we shall divert through our own Country a branch of commerce which the European States have thought worthy of the most important struggles and sacrifices, and in the event of peace4 on terms which have been contemplated by some powers we shall form to the American union a barrier against the dangerous extension of the British Province of Canada and add to the Empire of liberty an extensive and fertile Country thereby converting dangerous Enemies into valuable friends.1
The addition of a country so extensive, so fertile, as Louisiana, to the great republican family of this hemisphere, while it substitutes, for our neighbors, brethren & children in the place of strangers, has secured the blessings of civil & religious freedom to millions yet unborn. by enlarging the empire of liberty, we multiply it's auxiliaries, & provide new sources of renovation, should it's principles at any time, degenerate; in those portions of our country which gave them birth. the securing for you the peace & friendship of the various Indian tribes is among the highly valued advantages of this acquisition.2
[W]e should then have only to include the North in our confederacy, which would be of course in the first war, and we should have such an empire for liberty as she has never surveyed since the creation: & I am persuaded no constitution was ever before so well calculated as ours for extensive empire & self government.3
1. Jefferson to George Rogers Clark, December 25, 1780, in PTJ, 4:237-38. Transcription available at Founders Online.