The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge ( ) dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different values under the same excitation conditions. The OAM decay times of the = 1 component are usually longer than those of the = 0 component. The -dependent OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing.
We report on the coherent dynamics of the photoexcited orbital angular momentum (OAM) states of GaN excitons. The measurements were performed by a pump-probe four-wave mixing (FWM) using optical vortex (OV) pulses, where the pump OV pulse transfers its OAM to the center of mass momentum of excitons and the conversion of OAM via FWM with the probe pulse is detected. Especially, we performed analysis of the signal using an OAM-resolved spectroscopy, which allows us to evaluate the dephasing dynamics of excitons in time and space. The results indicate that near perfect conversion is achieved using the OV pulses with nonzero OAM and its spatial coherence remains high during the dephasing time, clearly indicating that the excitons keep the OAM transferred by the pump OV. In contrast, the component associated with the imperfect conversion becomes significant when one of the incident pulses intentionally includes an OAM-free component. The effect is clearly confirmed by the FWM excited by the pump with multiple OAM.
By using radially polarized pulses for excitation in degenerate four-wave mixing (DFWM) spectroscopy, we realize highly sensitive snap-shot measurement of optical anisotropies in semiconductor films. Because the radially polarized pulses exhibit spatially varying linear polarizations, the optical anisotropies in samples can be evaluated from the spatial distributions of DFWM signals without rotating the polarization of the excitation pulses. We measure the excitons in GaN layers and evaluate the energies of uniaxial strain and the spin-exchange interaction constant. Our results prove both the accuracy and sensitivity of the snap-shot measurement.
The single orbital angular momentum mode emission from a commercially available broad-area vertical cavity surface emitting laser (VCSEL) is realized with an optical feedback technique using a spatial light modulator, where a computer generated hologram (CGH) is optimized by taking into account the mode properties of the external cavity VCSEL. The flexibility with which the chirality of the orbital angular momentum mode can be selected is also demonstrated simply by changing the chirality of the CGH. The technique is very simple and easily applicable to other VCSELs without processing.
The orbital angular momentum (OAM) resolved spectroscopy is being applied to various research fields associated with optical vortices or LaguerreGaussian (LG) beams, in which the key technique is to convert LG beams to a fundamental Gaussian (HG≡HG 00 ) beam. In this work, we propose a method to realize efficient mode conversion by using spiral phase modulations. Numerical simulations and experiments are carried out to compare our proposal with the standard technique using rotational phase modulations. The results reveal that our proposed method improves the uniformity of the conversion efficiency across various OAM, allowing us to obtain precise OAM spectra.
Keywords
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.