Kyohhei Shigematsu

Orbital Angular Momentum Spectral Dynamics of GaN Excitons Excited by Optical Vortices

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.

Single orbital angular momentum mode emission from vertical cavity surface emitting laser by optical feedback

Single angular momentum (OAM) mode emissions from a vertical cavity surface emitting laser (VCSEL) were demonstrated by an external optical feedback using computer generated holograms, which are optimized on the OAM modal gain of the free-running VCSEL. Side-mode suppression ratio of more than 23 dB was achieved for the OAM modes with l = ±1.

Single orbital angular mode emission from externally feed-backed vertical cavity surface emitting laser

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.

Snap-shot optical polarization spectroscopy using radially polarized pulses

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.

Programmable ultrashort optical-vortex pulse generation using optical parametric amplification and 4-f configuration

We demonstrate the optical parametric amplification of optical-vortex pulses with the programmable topological-charge control. Combination of optical-parametric amplification and 4-f configuration overcomes the low-throughput drawback of the vortex converter, simultaneously compensating for the angular dispersion.

Exciton--Exciton Interactions in Tensile-Strained GaN

We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of ΔAB~4.7 meV and ΔBC~14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A–B and B–C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A–B excitons showed an oscillation with a critical π-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B–C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A–B excitons revealed peaks associated with biexciton transitions with binding energies of EAA = 4.6 meV, EAB = 2.1 meV, and EBB = 1.7 meV.