Efficient two‐photon absorption of bulk GaN in the near‐infrared region

Abstract

Transient transmission changes in the yellow luminescence (YL) band of GaN were investigated using excitation far smaller than the bandgap. At lower excitation power, time-resolved measurements revealed efficient transmission changes due to two-photon absorption (TPA), whose spectrum clearly indicates the contribution of the YL band. Upon increasing the excitation power, the transient signal showed an additional offset signal. The excitation power dependences of the offset suggested a two-step absorption process in conjunction with TPA in the YL band. The excitation energy dependence also supported this conclusion.The study of defect states in GaN films is important for understanding the device physics because even a device grade sample involves a high density of defects. One optical characteristic concerning the defect states is the so-called "yellow luminescence" (YL). So far, several research groups have obtained two-photon and multiple-photon related optical nonlinearities connected with the YL band. . In addition to the study of defect states, these nonlinear absorption studies also provide important insights into the fundamental physics of materials, such as band structures [1], relaxation processes [3-6], and electron-phonon interactions [7]. However, the role of the YL band in these optical nonlinearities remains largely unclear.In this paper, we investigate TPA in the YL band. Transient pump-probe measurements in the nearinfrared (NIR) excitation reveal efficient transmission changes, despite the lack of a bandgap transition. The TPA spectrum clearly shows that the observed large nonlinearity originates from the YL band. In addition, transmission changes at higher excitation power show an additional asymmetrical offset signal. The offset also reflects the characteristic of efficient TPA in the YL band.The sample studied was a freestanding bulk GaN sample with a thickness of $420 mm. The absence of a substrate allows the nonlinear response to be evaluated precisely. The electron concentration in the sample was estimated to be $ 10 19 cm À3 . A mode-locked Ti : sapphire laser (pulse width $100 fs, repetition rate 76 MHz) was employed as an NIR excitation light source. The tunable wavelength of the laser was 1.25 to 1.77 eV, which covers the excitation of carriers from the middle of the YL band to the middle of the energy gap (E G /2) into the conduction band. Linearly polarized pump