Two-photon absorption study of GaN Chi-Kuang Sun, a) Jian-Chin Liang, and Jiun-Cheng Wang Department of Electrical Engineering and Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China Fu-Jen Kao Department of Physics, National Sun Yat-Sen University, Kaoshiung 80424, Taiwan, Republic of China Stacia Keller, Michael P. Mack, Umesh Mishra, and Steven P. DenBaars Department of Electrical and Computer Engineering and Materials Department, University of California, Santa Barbara, California 93106 Received 21 July 1999; accepted for publication 24 November 1999 Two-photon absorption coefficients of GaN for below band gap ultraviolet wavelength and midgap infrared wavelength were measured by using femtosecond pulsewidth autocorrelation and Z-scan techniques. Large two-photon absorption coefficients were obtained. Taking advantage of the large two-photon absorption, we have demonstrated two-photon confocal imaging of a GaN thin film. Direct correlation was found between the yellow luminescence and suppression of bandedge luminescence. © 2000 American Institute of Physics. S0003-69510004404-1 GaN and its related materials have attracted great inter- est for their applications in light emitters in the green to ultraviolet UV wavelength. 1 Recently there has been a growing attention on the studies of the two-photon absorp- tion TPA process in GaN, which will lead to optical power limitation and optical damages in nitride based optoelectron- ics. Taking advantage of the TPA process in GaN thin films and photodetectors, autocorrelation and cross-correlation of visible and infrared IR femtosecond pulses have been dem- onstrated by several research groups. 2–5 TPA induced photo- luminescence PL of GaN was also previously studied 6 us- ing tunable picosecond pulses. A large TPA coefficient  of 17.5 cm/G W at 600 nm wavelength was reported. 2 A value of 1.5 cm/G W was also obtained for  at photon energies above E gap/2 in the IR wavelength region. 7 In this letter, we report our study of the TPA of GaN in below band gap UV wavelength region using TPA-type au- tocorrelation techniques. The study of TPA process for be- low band gap UV-blue wavelength is important due to the fact that most nitride based optoelectronics are operated in this wavelength regime instead of in red or IR wavelength. On the other hand, optical autocorrelation of ultrashort pulses using TPA provides a convenient, sensitive, and inex- pensive alternative to standard techniques using nonlinear crystals. 8–10 UV TPA autocorrelation is especially important due to the difficulty of finding a suitable second-harmonic generation crystal in this wavelength regime. In order to avoid possible systematic errors by the two-photon autocor- relation technique, we have also performed single-beam Z-scan measurements for below band gap UV wavelengths. The GaN sample was grown by atmospheric-pressure metalorganic chemical-vapor deposition on a doubleside pol- ished c-plane sapphire substrate. After annealing the sub- strate and deposition of a nucleation layer, unintentionally doped GaN layer of 5 m thickness was grown. The crystal structure is wurzite. A room-temperature transmission spec- trum indicated that the band gap of the sample was located 365 nm. The TPA study was performed by demonstrating UV TPA autocorrelation using a standard transmission-type pump-probe technique. The laser output from a femtosecond mode-locked Ti:sapphire laser was frequency doubled in a 500-m-thick betabarium-borate crystal to reach UV wave- length. The frequency-doubled UV pulses were tunable be- tween 350 and 400 nm with a repetition rate of 82 MHz. The UV pulses were split as pump and probe pulses by a beam- splitter. These two beams were then focused onto the same point on the GaN sample by using an objective. The pump beam was chopped and the detected probe signal was mea- sured as a function of the temporal delay between the pump and the probe by a lock-in amplifier. Complimentary tran- sient reflection measurements were also performed in order to remove the transient contributions due to surface reflec- tion. Figure 1a shows the measured transient response using UV femtosecond pulses centered at a wavelength of 390 nm open circles. Around zero time delay, a large transmission decrease was observed. This was induced by the TPA pro- cess. The differential transmission change of probe beam de- creased linearly with the pump excitation power, indicating a TPA process consisted of one pump photon and one probe photon. The shape of the TPA-induced differential transmis- sion decrease will mimic the pulse autocorrelation function. However, after the TPA decrease, the measured transient re- sponse did not return to zero but was followed by a small step-like positive transmission increase within our measured time delay 1 ps. This transmission increase is attributed to the absorption saturation of the bandtail states. This band- tail state absorption might be the origin of the nonideal in- terferometric autocorrelation pulse shape a 6:1 ratio instead of an 8:1 ratio obtained from a GaN photodetector using 410 nm pulses. 4 In order to remove the contribution from the bandtail state saturation, we have performed a convolu- tion fit. The fitting considers two components: a Gaussian pulse shape negative, representing the TPA induced pulse- a Electronic mail: sun@cc.ee.ntu.edu.tw APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 4 24 JANUARY 2000 439 0003-6951/2000/76(4)/439/3/$17.00 © 2000 American Institute of Physics