可调谐二极管激光器和频率梳,用于原子激光器的冷却和陷波

原子的激光冷却和磁光俘获

  • 多普勒冷却
  • Sisyphus / 极化梯度冷却及光频饴
  • 拉曼冷却及其余特殊激光冷却方案
  • 磁光阱(MOT)
  • 原子喷泉
  • 原子光刻和光学偶极子阱

上世纪关于物质波以及光与原子相互作用的许多实验和理论研究为现代原子光学及其在研究和工业中的应用铺平了道路。 1985年左右人类首次实现了中性原子的激光冷却。1997年诺贝尔奖授予“开发用激光冷却和捕获原子方法”的Steven Chu,Claude Cohen-Tannoudji和William D. Phillips。随后,多种激光冷却和捕获机制得到实现,激光冷却使用范围甚至拓展到更多的原子种类。激光冷却最终温度在微开量级,这比外太空中的任何地方都要冷许多个数量级,并且仅比绝对零度高几百万分之一度。这样看来,激光冷却的原子云真的很酷!它们通常用于研究原子与光的相互作用,并作为冷原子源用于其他实验(如玻色 - 爱因斯坦凝聚和简并费米气体;原子干涉测量;碰撞研究和计量,如时间和频率的精确测量,加速度和旋转,同位素比率,基本物理常数测量等)。

激光冷却是通过向原子集合施加特殊光场或光脉冲来实现的,原子集合可通过背景蒸汽或热束产生。 光子和原子可通过不同的机制交换能量和动量。一种机制是原子受阻尼直接冷却,另一种则是通过光泵浦使原子跃迁至特殊量子态(即谐振势阱中的振荡态)以获取低动量。原子光学中最常用的激光冷却方案是多普勒冷却和极化梯度(或西西弗斯)冷却(详情参见应用说明)。 拉曼激光冷却有时与被捕获的原子一起使用,更常用于光阱中的离子。 另外还有比较少见的VSCPT和去极化/退磁冷却方案,后者可能是最古老的光冷却方案(2006年)。

激光冷却中性原子 按原子数大小排列:(欢迎提供相关应用其他原子种类,您将获得TOPTICA定制口杯)
He*, Li, Ne*, Na, Mg, Al, Ar*, K, Ca, Cr, Fe, Ga, Kr, Rb, Sr, Ag, Cd, In, Xe, Cs, Ba, Eu*, Dy, Ho, Er, Tm, Yb, Hg, Fr, Ra (未完待续)

磁光阱 将激光光场的阻尼与同原子在磁场中位置相关的束缚力相结合。 为了产生与位置相关的合力,我们把磁四极场与三对正交反向传播的激光束对相结合。激光束对在磁场为零的磁光阱(MOT)中心相交。 梯度磁场与激光偏振相结合通过塞曼效应产生束缚力,使得原子总是被推向MOT中心。 大多数可通过激光冷却的元素均可被MOT束缚,典型的原子数为几千至几十亿,温度范围在微开至毫开范围内,并且密度通常为108到1011原子每立方厘米。

原子喷泉 可通过磁光阱(MOTs)产生。在MOT内积累并热激发足够多原子时,通过调节磁场并且调控上下两方向传播激光束之间的频率差,可将原子在移动框架中进行冷却并且以精确调控的速度向上移动。在上升过程中,重力使原子减速,到达最高点后下落。原子喷泉的主要优势在于它可以极大地延长释放原子的观察以及反应时间(典型值1秒),这对于精密测量来说非常重要。原子最终将被重新捕获并重新发射,实现真正的原子操控。

原子光刻及偶极子光阱 是建立在偶极力基础上的。激光对原子引入谐振电偶极子,通过与激光束本身的电场相作用而产生保守光势场。光势场强度随激光强度改变,其梯度导致了偶极力的产生。当激光频率低于原子共振频率时,原子偶极子及电场同相振荡,光势场为负。这时,原子受到偶极力的作用向光场亮度最高点移动。强力聚焦的激光束可将原子束缚在其焦点处,从而形成光镊或偶极子光阱。更复杂的光阱可通过在一维,二维甚至三维空间中利用相对传播的激光束对构成的光晶格而实现。这些光晶格——由光产生的理想晶体(光强焦点分布于晶格结构上每个半波长距离处,一般间隔小于1微米)—— 常用于凝聚态物理中与束缚原子相关的研究和模拟。光势场亦可用于原子束整形,其作用可类比于物质与光作用中的传统整形光学器件(例如,玻璃透镜)。在原子光刻应用中,原子束通过特殊光场构造,经过“光透镜”聚焦入生长基板,并在基板上形成纳米级别原子构架。

TOPTICA在原子光学方向提供的附加值 原子光学领域对激光应用来说是充满挑战的。为了应对科研课题的不断更新进展,研究人员一般需要特殊或者用户定制的激光。激光系统需要在所需波长上提供足够能量. 激光线宽必须保持在原子跃迁频率(通常为兆赫,然而最新实验亦可达千赫兹量级)或目标激光频率与原子跃迁间差值以下。 激光系统还需保证无跳模精准调谐,这种精确的调谐方法允许用户将激光频率设置并且稳定在原子跃迁频率附近的特定位置。而即便是复杂的激光系统仍需操作简单,因为现在的实验操作往往要求越来越多的激光可靠地同步运行以保证实验成功。

TOPTICA是这类复杂激光系统的主要供应商,我们的客户不仅包括多位诺贝尔获奖者,更包括全世界大部分相关领域研究团队。自十多年以来,我们就因高质量的产品和弹性的服务而闻名于科研前线。我们不断地在开发新的激光系统并且能够为客户提供个性化特殊解决方案。最关键的是,作为一个质检客服五脏俱全,管理专业,以工业以及OEM产业闻名的公司,我们依旧保持着科学家的本心。与全世界许多科研团队保持活跃的伙伴关系对我们来说不仅是必须的,更是一种乐趣。因为我们依旧对科学充满激情。在TOPTICA您可以找到具有深厚原子光学背景的专家,他们理解您的应用和要求。请联系我们并与我们讨论您实验的相关细节。

Magneto-optical trapping

Magneto-optical trapping combines the friction force of laser cooling with a “restoring” force depending on the atomic position. In order to realize the position dependent force, a magnetic quadrupole field combined with three orthogonal pairs of counter-propagating laser beams intersect at the center of the magneto-optical trap (MOT), the location where the magnetic field is zero. The therefore position-dependent Zeeman effect interplays with the circular polarization and the frequency of the laser beams such that atoms are always pushed towards the MOT center. Most of the laser cooled elements have also been trapped in MOTs. Typical atom numbers range from a few thousand to a few billion of atoms at temperatures in the micro to millikelvin range and densities of typically 108 to 1011 atoms/cm3.

Atom lithography and optical dipole traps

Atom lithography and optical dipole traps are based on the so-called dipole force. The laser induces an oscillating electric dipole in the atom which interacts with the electric field of the laser beam itself to form a conservative optical potential. The optical potential varies with the laser intensity and its gradient gives rise to the dipole force. If the laser frequency is below the atomic resonance frequency, the atomic dipole and the electric field oscillate in phase and the optical potential is negative. Atoms experience a dipole force towards the intensity maximum of the light field. A tightly focused laser beam can then trap atoms at the center of its focus and form a so-called optical tweezer or optical dipole trap. More complex optical traps are created by counter-propagating laser beams in one, two or even three dimensions forming 1-d, 2-d or even 3-d optical lattices. These optical lattices – perfect crystals made out of light with intensity maxima at every half of the optical wavelength (typ. < 1 µm) – are used to study and simulate solid state physics with trapped atoms. Optical potentials can also be used to generate lenses for rays of atoms reversing the traditional roles of matter and light where an arrangement of matter (e.g. a glass lens) is used to focus a ray of light. In atom lithography, a beam of atoms is sent through a special light configuration and focused by this light lens onto a substrate where it forms atomic structures on the nm scale.

TOPTICA’s added value in laser cooling-based quantum technologies

Quantum technology applications require special or even customized lasers with always constantly evolving demands according to the newest developments and changing scientific research topics: The laser systems have to provide enough power at the desired  wavelength. The linewidth has to be below the linewidth of the atomic transition (typ. MHz but in recent experiments also in the kHz or even Hz range) or the difference between atomic transition and desired laser frequency. Mode-hop-free fine tuning, that is very precise adjustment of the laser frequency, allows one to set or even stabilize the laser frequency at a well-defined position close to the atomic transition frequency. Even complex laser systems have to be easy to operate since nowadays more and more lasers have to work simultaneously and reliably in order to have experimental success. Remote, digital control of the laser is another feature becoming increasingly essential.
TOPTICA is key supplier of such laser systems to most research groups and to quantum technology industry all around the world. Since more than two decades, we are well known for the quality of our products always at the front line of research and for our flexibility. We are constantly developing new laser systems and are open for special solutions according to customers’ demands. You will find experts at TOPTICA with profound quantum technology background that understand your application and your requirements. Please contact us to discuss details of your plans.