Investigation of oscillator strength enhancement of electric dipole forbidden transitions in optical near field

1亚波长缝隙阵列产生的光学近场中电偶极子禁止跃迁效率的估计用本项目开发的FDTD(有限差分时间域)方法计算了50 nm厚的周期结构的50 nm缝隙阵列产生的光学近场。对于计算出的光场，对电四极(E_2)和磁偶极(M_1)跃迁的效率进行了评估。在入射光的光偏振垂直于狭缝方向的情况下，E2跃迁效率大大提高，而这种偏振中的M1跃迁看不到这种增强。E_2跃迁效率的大幅提高主要来自于场的集中和狭缝边缘的大的空间调制。在入射光平行于狭缝的情况下，增强…E_2跃迁和M_1跃迁的胶结都较小，这与修正因子(K_0d)~(-1)~(-2)~(gt；)是一致的，其中K_0和d分别是光在真空中均匀传播的波数和狭缝宽度。还计算了亚波长狭缝阵列产生的光学近场中Cu2O中的1s-邻位激子对E_2激发效率的提高。用亚波长狭缝阵列观察了Cu2O中1s-邻位激子的发射光谱。用电子束光刻的方法在Cu2O晶体表面制备了厚度为50 nm的周期性结构的金狭缝阵列。我们证实，通过缝隙阵列可以观察到Cu2O中1s-正激子的电四极(E_2)和偶极(E_1)跃迁的发射。到目前为止，还没有探测到缝隙阵列对E_2发射的影响。观测了O_2WE的磁偶极跃迁。3利用偶极激光建立了一套光谱系统来观测O_2磁偶极(763 Nm)磁偶极(M1)跃迁引起的光吸收。为了使系统具有较高的灵敏度，我们将系统与离轴腔合并。证实了在足够的光谱分辨率下，可以测量到吸收光谱。同时也证实了腔内多次反射对提高灵敏度的作用。较少

1 Estimation of electric dipole forbidden transition efficiency in optical near fields generated by a sub-wavelength slit arrayOptical near fields generation by a periodically structured 50 nm width slit array of a 50 nm thick gold film were calculated with the FDTD (Finite Difference Time Domain) method, which was developed in this project. The efficiencies of the electric quadrupole (E2) and magnetic dipole (M1) transitions, which are modified by the gradient of the excitation light field, were evaluated for the calculated optical fields. The E2 transition efficiency is largely enhanced in the case where the light polarization of the incident light is perpendicular to the slit direction while such enhancement is not seen for the M1 transition in this polarization. The large enhancement of the E2 transition efficiency mainly originates from the field concentration and the large spatial modulation at the slit edge. In the case where the incident light is parallel to the slit, the enhan … More cement is small for both the E2 and Ml transition, which is consistent with the modification factor of (k_0ld)^<-2>, where K_0 and d are the wavenumber of the homogenously propagating light in vacuum and the slit width, respectively. The enhancement of the E2 excitation efficiency was also calculated for the 1s-ortho exciton in Cu_2O in the optical near field generated by a sub-wavelength slit array. It was suggested that the enhancement is affected by the surface plasmon resonance.2 Observation of emission spectra of the 1s-ortho exciton in Cu_2O through a sub-wavelength slit arrayWith an electron beam lithographic method, a periodically structured gold slit array with 50 nm thickness was fabricated on a Cu_2O crystal surface. We confirmed that the emissions of the electric quadrupole (E2) and dipole (El) transitions of the 1s-ortho excitons in Cu_2O can be observed through the slit array. So far, effects of the slit array to the E2 emission have not been detected. The polarization and slit-period dependences of the emission intensities were observed.3 Observation of the magnetic dipole transition of O_2We developed a spectroscopic system with a dipole laser to observe the light absorption due to the magnetic dipole (M1) transition of O_2 (A-band, 763 nm). In order to make the system high sensitive, we merged the system with an off-axis cavity. It was confirmed that the absorption spectrum can be measured with sufficient spectral resolution. The effect of multi-reflection in the cavity for the increase in the sensitivity was also confirmed. Less