When Barack Obama became the first African-American president in 2009, he also impressed the 1.3 billion Chinese by appointing Stephen Chu and Gary Locke, two Chinese Americans, to his cabinet.

With Locke departing his commerce secretary post in August 2011 to serve as US ambassador to China, the time has now come for Chu, the energy secretary.

In a letter to his department employees on Friday, Chu announced that he is leaving his post after the ARPA-E Summit, a conference to promote leading energy technologies, at the end of this month.

Chu, 64, is the first person appointed to the US cabinet after having won a Nobel Prize. He shared the Nobel Prize for Physics in 1997 with two others for the development of methods to cool and trap atoms with laser light.

Chu, born in St Louis, Missouri, and with ancestry from Taicang in East China's Jiangsu province, is the second Chinese American to head a US government department after Elaine Chao, the labor secretary under George W. Bush.

"I came with dreams, and am leaving with a set of accomplishments that we should all be proud of," Chu told his colleagues in the letter.

He cited Martin Luther King as his inspiration by saying that "his Dream of an America where people are judged not by skin color but 'by the content of their character.'"

"In the scientific world, people are judged by the content of their ideas. Advances are made with new insights, but the final arbitrator of any point of view are experiments that seek the unbiased truth, not information cherry- picked to support a particular point of view. The power of our work is derived from this foundation," said Chu, a professor at University of California of Berkeley and director of the Lawrence Berkeley National Laboratory before he joined the Obama cabinet in January 2009.

After Chu's announcement, Obama praised him for helping his administration "move America towards real energy independence".

"Over the past four years, we have doubled the use of renewable energy, dramatically reduced our dependence on foreign oil, and put our country on a path to win the global race for clean energy jobs," Obama said.

However, it is widely believed that Chu's resignation is linked to the harsh criticism he has received from the Republican side for his advocacy of renewable energy, especially the $535 million Energy Department loan guarantee to solar-panel maker Solyndra, which went bankrupt.

Although this is just a small part of the US government funding for renewable energy, it has been aggressively used by Republicans to attack Obama, especially during last year's presidential campaign.

Chu's resignation has been expected for a while. Spending much of his career in research and academia, Chu has never seemed to like the sharply divided Washington politics.

In his letter, Chu said that he informed Obama of his decision a few days after the election, and that his and his wife, Jean Fetter, also a physicist, were eager to return to California.

"I would like to return to an academic life of teaching and research, but will still work to advance the missions that we have been working on together for the last four years," Chu said.

Chu has defended his clean energy policy and criticized climate change deniers in the US in his letter. His long list of accomplishments also includes fostering cooperative agreements that resulted in three US-China Clean Energy Research Centers announced by Obama and President Hu Jintao.

"Virtually all of the other OECD countries, and most developing countries including China, India, Mexico, and Brazil, have accepted the judgment of climate scientists," Chu said.

China now exceeds the US in internal development of clean energy and in government investments to further develop the technologies, according to Chu.

Contact the writer at chenweihua@chinadailyusa.com