连续极紫外激光的产生

Generation of continuous extreme ultraviolet laser

产生短波长的激光是激光技术中的一大瓶颈问题.目前可用的桌面极紫外(extreme ultraviolet,EUV)激光都是通过多光子过程产生的,效率较低,并且无法产生连续短波长极紫外激光.通过高压直流放电激发产生的亚稳态氦原子与连续近红外(near-infrared,NIR)激光结合泵浦的方式可以产生波长为58.4 nm的连续极紫外激光.该过程为单光子相关的反斯托克斯拉曼散射(anti-Stokes Raman scattering,ASRS).连续极紫外激光的光通量可以接近目前同步辐射在该波段的数值,并且其拥有与泵浦的近红外激光近乎相同的发散度,显示了其受激特性.该光源有望抑制光电子能谱测量中的空间电荷效应.目前连续极紫外激光的强度在气压合适的情况下,随着放电激发功率、泵浦激光注入功率的升高而升高.本文结果为单光子过程上转换产生桌面连续极紫外激光提供了一种可能的方法.

Generating laser with short wavelength is a bottleneck problem in laser technology.The current applicable table-top extreme ultraviolet(EUV)lasers are all generated by multi-photon processes with low efficiency,which cannot produce continuous short wavelength EUV lasers.High density metastable helium atoms are generated by high voltage direct current(HVDC)discharge excitation,and then a continuous extreme ultraviolet laser with a wavelength of 58.4 nm can be produced by combining a continuous near-infrared(NIR)pumped laser to pump the metastable helium atoms to the excited state and then back to the ground state.This process is called single-photon related anti-Stokes Raman scattering(ASRS).The flux of the EUV laser can approach the current value of synchrotron radiation in this band,which is 2×1011 ph/s.The spot distribution of the generated extreme ultraviolet light is measured,and it is found that some of it has almost the same divergence as the pumped near infrared laser,which is produced by stimulated radiation.The other part is isotropic spontaneous radiation with the same divergence as the extreme ultraviolet light produced by the plasma.Through the change of DC discharge power,it is found that the intensity of the generated extreme ultraviolet laser increases with the increase of discharge power,and the power absorption of the continuous near-infrared pumped laser increases with the increase of discharge power.By changing the wavelength of the NIR laser to observe the above experimental phenomenon,it can be seen that the absorption of the infrared laser power is positively correlated with the generation of the extreme ultraviolet laser intensity,indicating that the enhancement of the extreme ultraviolet light is not caused by the thermal effect of the infrared laser,but the resonance pumping between the metastable and excited states of the continuous NIR laser makes the generation of the extreme ultraviolet laser.Through the change of the injection power of the continuous nearinfrared pumping laser,it is observed that with the increase of the injection power of the continuous near-infrared pumping laser,the intensity of the generated extreme ultraviolet laser also increases,but the corresponding production efficiency slowly decreases,which is theorized to be due to the fact that the metastable atomic density increase rate cannot match after the laser pumping.Through the change of helium pressure,it is observed that the extreme ultraviolet laser has the best helium pressure value at a certain discharge power and continuous near-infrared pumped laser injection power.This is because the increase rate of metastable states density has a certain relationship with the electric power and helium pressure,and the extreme ultraviolet laser intensity is affected by the pump laser,so the optimal helium pressure point exists.Compared with the EUV light generated by traditional high power pulsed seed laser,the CW EUV laser source has the characteristics of high total light intensity and low instantaneous light intensity.This light source is expected to suppress the space charge effect caused by high peak power light sources in photoelectron spectroscopy measurements.Our results provide a possible method for the desktop CW EUV laser by single photon process upconversion.