[1] Kohler R, Tredicucci A, Beltram F, Beere H, Linfield
E, Davies A, Ritchie D, Iotti R and Rossi F 2002 Nature
417 156

[2] Siegel P H 2002 IEEE Trans. Microwave Theory
Technol. 50 910

[3] Capasso F, Paiella F R, Martini R, Colombelli R, Gmachl C,
Myers T L, Taubman M S, Williams R M, Bethea C G, Unterrainer K,
Hwang H Y, Sivco D L, Cho A Y, Sergent A M, Liu H C and Whittaker E
A 2002 J. Quantum Electron. 38 511

[4] Cao J C 2003 Phys. Rev. Lett. 91 237401

[5] Darmo J, Tamosiunas V, Fasching G, Kroll J, Unterrainer K,
Beck M, Giovannini M, Faist J, Kremser C and Debbage P 2004
Opt. Express 12 1879

[6] Faist J, Capasso F, Sivco D, Sirtori C, Hutchinson A L and
Cho A 1994 Science 264 553

[7] Rochat M, Ajili L, Willenberg H, Faist J, Beere H, Davies
G, Linfield E and Ritchie D 2002 Appl. Phys. Lett. 81
1381

[8] Kohler R, Tredicucci A, Beltram F, Beere H E, Linfield
E H, Davies A G, Ritchie D A, Dhillon S S and Sirtori C 2003
Appl. Phys. Lett. 82 1518

[9] Williams B S, Kumar S, Callebaut H, Hu Q and Reno J L 2003
Appl. Phys. Lett. 83 2124

[10] Williams B S, Kumar S, Callebaut H, Hu Q and Reno J L
2003 Appl. Phys. Lett. 83 5142

[11] Williams B S, Kumar S, Hu Q and Reno J L 2004
Electron. Lett. 40 431

[12] Williams B S, Kumar S, Hu Q and Reno J L 2005 Opt.
Express 13 3331

[13] Lu J T and Cao J C 2006 Appl. Phys. Lett.
88 061119

[14] Beere H E, Fowler J C, Alton J et al 2005 J. Crystal
Growth 278 756

[15] Huang G S, Liu T C, Yao H H, Kuo H C, Wang S C et al 2007
J. Crystal Growth 298 687

[16] Liu H C, Wachter M, Ban D, Wasilewski Z R, Buchanan
M, Aers G C, Cao J C, Feng S L, Williams B S and Hu Q 2005
Appl. Phys. Lett. 87 141102

[17] Alton J, Barbieri S, Beere H E, Fowler J, Linfield E H
and Ritchie D A 2004 Proc. SPIE 5354 129

[18] Ban D, W?chter M, Liu H C, Wasilewski Z R, Buchanan M and
Aers G C 2006 J. Vac. Sci. Technol. A 24 778

[19] Walther C, Scalari G, Fasit J, Beere H and Ritchie D 2006
Appl. Phys. Lett. 89 231121