Suppression of Interference-Induced Reflectivity Fluctuations in GaN-Based Semiconductor Saturable Absorber Mirror

Abstract

Simulations and experiments were carried out to suppress the interference-induced reflectivity fluctuations in a nonmonolithically grown broad-band GaN-based semiconductor saturable absorber mirror ͑SESAM͒. Laser lift-off of the sapphire substrate, inductively coupled plasma etching of the GaN buffer, and plasma enhanced chemical vapor deposition of the antireflective coating were conducted according to the simulated optimization processes. The interference-induced reflectivity fluctuations were effectively reduced. Characterizations by atomic force microscopy, X-ray diffraction, scanning electron microscopy, and photoluminescence further indicated that the SESAM after the optimization processes exhibited a good surface morphology, crystal quality, and emission property.Passive mode-locking by semiconductor saturable absorber mirrors ͑SESAMs͒ has been successfully developed in solid-state lasers, 1-5 fiber lasers, 6-10 and semiconductor lasers 5,11-13 in a wide wavelength region to produce ultrashort optical pulses. For data storage applications, in order to achieve high-speed operation as well as high-density storage, the high-repetition-rate, ultrashort optical pulses in the blue/ultraviolet ͑UV͒ region have become indispensable. However, the blue/UV ultrashort optical pulses have so far been obtained mainly by frequency conversion methods from infrared solid-state lasers, such as Ti:sapphire lasers 14 or Cr:LiSAF lasers. 15 The direct generation of ultrashort blue/UV optical pulses by passive mode-locking has not been reported yet. One of the major reasons is the difficulty in monolithically fabricating broad-band high-reflective GaN-based distributed Bragg reflectors ͑DBRs͒, due to the lack of suitable semiconductor DBR materials lattice matched to GaN. Though GaN/AlGaN DBRs monolithically grown on GaN have been reported, 16-20 they had very narrow stopbands ͑about 15 nm near the 415 nm wavelength͒ 19,20 due to the small refractive index difference between GaN and AlGaN.To overcome the limitation of the monolithic DBR, we have recently reported the fabrication of a broadband SESAM with a SiO 2 /Si 3 N 4 dielectric DBR. 21 Taking advantage of the dielectric DBR, an over-100 nm stopband with a high reflectivity was achieved. However, this nonmonolithic SESAM suffered from severe interference-induced reflectivity fluctuations within the stopband. For example, in a small wavelength span of about 5 nm, the magnitude of the interference-induced reflectivity fluctuations could be ϳ6% ͑see Fig. 5 in Ref. 21͒, which was comparable to the modulation depth of the saturable absorber ͑ϳ5%͒. 21 If used for mode-locking, this SESAM would encounter severe instability. Therefore, further optimizations were necessary to suppress the reflectivity fluctuations.In this paper, a series of optimization processes was conducted on the GaN-based nonmonolithic SESAM to suppress the interference-induced reflectivity fluctuations. The optimization processes were first designed by simulation. Laser lift-off of the sapphire substrate, indu...

Determination of the band gap of indium-rich InGaN by means of photoacoustic spectroscopy

“…As can be seen in figure 3 decrease in amplitude at energies higher than 1.1 eV, but a strong Fabry-Perot interference pattern arising from the 2.6 μm thick GaN buffer layer makes it unpractical to determine the optical band gap energy. A similar Fabry-Perot interference has been observed in previous studies of samples with GaN buffer layers [25,26].…”

Absorption recovery time reduction in InGaN/GaN quantum well saturable absorbers

“…15 In addition, it is noted that the as-grown HT GaN buffer was only ϳ500 nm thick in samples B and C, while in our optimized GaN-based SESAM structure reported in Ref. 12, to suppress the interference-induced reflectivity fluctuations, the final thickness of the HT GaN buffer after etching was also ϳ500 nm. Thus, it is expected that the interference effect should be much less severe in samples B and C, as compared to sample A.…”

“…Consequently, reflectivity fluctuations within a small wavelength span would be much smaller in samples B and C, and the reflectivity fluctuations can be further reduced by adding an antireflective coating, as demonstrated in Ref. 12. Hence, to fabricate a GaN-based SESAM with thin GaN buffers, no further optimization process to suppress the interference is needed.…”

“…A postgrowth optimization process, which included laser lift-off and plasma etching, was subsequently conducted to reduce the thickness of the HT GaN buffer so as to suppress the interferences. 12 For passive mode locking by SESAMs, to achieve efficient pulse shaping, the saturable absorption of a saturable absorber should recover to its initial state in a short time ͑a few picoseconds to a few tens of picoseconds͒. The absorption recovery times of those epitaxially grown compound semiconductors normally fall in the nanosecond range.…”