Feedback-controlled nonresonant laser cooling

We present a new approach to nonresonant laser deceleration and cooling of atoms based on their interaction with a bistable optical cavity. The cooling mechanism presents a photonic version of Sisyphus cooling, in which the conservative motion of atoms is interrupted by sudden transitions between two stable states of the cavity mode. The mechanical energy is extracted due to the hysteretic nature of those transitions. The bistable character of the cavity may be achieved by an external feedback loop, or by means of nonlinear intracavity optical elements. In contrast to the conventional cavity cooling, in which atoms experience a viscoustype force, bistable cavity cooling imitates “dry friction” and stops atoms much faster. Based on this novel approach, we explore the prospects of using optical bistability for efficient radiation pressure cooling of micromechanical devices that are modeled as a Fabry-Perot resonator with one fixed and one oscillating mirror. In all cases, analytical results are presented, supported by realistic numerical examples.     

Stimulated radiative laser cooling

Building a refrigerator based on the conversion of heat into optical energy is an ongoing engineering challenge. Under well-defined conditions, spontaneous anti-Stokes fluorescence of a dopant material in a host matrix is capable of lowering the host temperature. The fluorescence is conveying away a part of the thermal energy stored in the vibrational oscillations of the host lattice. In particular, applying this principle to the cooling of (solid-state) lasers opens up many potential device applications, especially in the domain of high-power lasers. In this paper, an alternative optical cooling scheme is outlined, leading to the radiative cooling of solid-state lasers. It is based on converting the thermal energy stored in the host into optical energy by means of a stimulated nonlinear process, rather than a spontaneous process. This should lead to better cooling efficiencies and a higher potential of applying the principle for device applications.     

Transient behaviour of laser

The transient behaviour of laser is discussed using the quantum Scully-Lamb model. The generating functions approach is proposed to solve the diagonal part of the Scully-Lamb equation of motion for the density matrix of the radiating field. The phenomenological model due to Arecchi and Degiorgio is justified. The results of the theory are compared with the experimental data. A good agreement is reached, as far as the time evolution of mean photon number and variance is concerned.Supported by Polish Ministry of Higher Education, Science and Technology, Project M.R.I.7     

Solid-state laser system for laser cooling of sodium

We demonstrate a frequency-stabilized, all-solid laser source at 589 nm with up to 800 mW output power. The laser relies on sum-frequency generation from two laser sources at 1064 nm and 1319 nm through a PPKTP crystal in a doubly resonant cavity. We obtain conversion efficiencies as high as 2 W/W² after careful optimization of the cavity parameters. The output wavelength is tunable over 60 GHz, which is sufficient to lock on the sodium D₂ line. The robustness, beam quality, spectral narrowness and tunability of our source make it an alternative to dye lasers for atomic physics experiments with sodium atoms.     

Atom lithography without laser cooling

Using direct-write atom lithography, Fe nanolines are deposited with a pitch of 186 nm, a full width at half maximum (FWHM) of 50 nm, and a height of up to 6 nm. These values are achieved by relying on geometrical collimation of the atomic beam, thus without using laser collimation techniques. This opens the way for applying direct-write atom lithography to a wide variety of elements.     

Blackbody-radiation-assisted laser cooling of molecular ions

The translational motion of molecular ions can be effectively cooled sympathetically to temperatures below 100 mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The distribution of internal rovibrational states, however, gets in thermal equilibrium with the typically much higher temperature of the environment within tens of seconds. We consider a concept for rotational cooling of such internally hot, but translationally cold, heteronuclear diatomic molecular ions. The scheme relies on a combination of optical pumping from a few specific rotational levels into a "dark state" with redistribution of rotational populations mediated by blackbody radiation.     

Transient effects in pulsed laser irradiation

Theoretical analysis of the influence of the temporal profile (rectangular, triangular, Gaussian) of the laser pulse on heating/cooling and phase transition velocities and quantity of ablated material was performed on the basis of a multifront Stephan problem. Modeling showed that material removal under stationary conditions (that correspond to long pulses) is entirely controlled by specific heat and material density, while in the case of transient regimes (short pulses) thermal conductivity and heat capacity play a predominant role. Interaction of the melting and evaporation fronts characterized by an evaporation front velocity far exceeding the melting front one is one of the examples of the transient nature of the phenomena influenced by the laser pulse parameters.     

基于相变冷却的大功率二极管激光器技术

 设计了一种基于相变冷却方式工作的大功率二极管激光器,该激光器的散热器是基于节流式喷射微槽道相变冷却的原理,使冷却液在微槽中的气化率达到了70%,大幅度提高了冷却效果,减小了冷却液流量,在同样制冷功率条件下,冷却液流量仅为水冷方式的1/10。利用相变冷却器进行了背冷式半导体激光器叠阵封装工艺的研究,采用复合热沉与AuSn硬焊料结合的新型封装工艺,完成了准连续3 kW叠阵的封装。实验测试表明,单元叠阵的输出功率达到3.01 kW,占空比10%,封装间距为1.3 mm,光谱宽度小于3.5 nm。最大功率输出时所需R134a冷却液的流量仅为110 mL/min。     

高功率二极管激光器喷雾冷却实验研究

通过以液氨为制冷工质的开式喷雾冷却系统,在相同实验条件下研究了二极管激光器（DL）热沉喷雾表面分别为光滑表面、均匀密排微孔、深孔和多孔表面结构时的冷却效果,实验结果表明：在热流密度达到300 W/cm2时,冷却表面温度均保持在28 ℃以内,适用于高热流密度下的DL热管理问题;喷雾表面均布微结构能显著强化喷雾冷却性能,当采用均匀密排多孔表面时,散热功率达511.5 W/cm2,对流传热系数为346 701.1 W/(m2℃),传热系数较光滑表面时提高了83.9%。     

薄片激光器喷流冷却技术

为了分析喷流冷却技术对薄片激光器的冷却效果,从湍流换热理论出发,定义了评定喷流冷却换热能力和冷却均匀性的两个参数：面均对流换热系数和平均最大温差。利用ANSYS CFX流体力学仿真软件,建立喷流冷却的物理模型,对多种喷板结构进行求解计算。设计加工了整套喷流冷却实验装置,并进行了模拟热源喷流冷却实验。分析结果表明,喷流冷却换热能力和冷却均匀性主要受喷板结构参数的影响,仿真计算结果可以定性地指导喷板结构参数的优化设计。     

高功率固体激光器喷雾冷却技术

喷雾冷却作为一种解决高功率激光器散热需求的技术得到越来越多的关注。结合近几年的研究工作,综述了喷雾冷却技术的研究现状。针对高功率激光器的散热需求,主要从传热机理、影响因素、温度均匀性方面进行阐述,分析了存在的问题。提出将制冷系统和喷雾系统结合、R600a等制冷剂为冷却剂的冷却方案,设计了气助式制冷喷雾冷却系统,分析了适用于制冷喷雾冷却系统的工质,提出了喷雾冷却技术在高功率固体激光器散热中的发展方向。     

High-power broad-area diode lasers for laser cooling

Received: 1 April 1996/Revised version: 24 September 1996     

Blue laser cooling transitions in Tm I

We have studied possible candidates for laser cooling transitions in ¹⁶⁹Tm in the spectral region of 410–420 nm. By means of saturation absorption spectroscopy, we have measured the hyperfine structure and rates of two nearly closed cycling transitions from the ground state 4f¹³6s²(²F₀)(J_g=7/2) to upper states 4f¹²(³H₅)5d_3/26s²(J_e=9/2) at 410.6 nm and 4f¹²(³F₄)5d_5/26s²(J_e=9/2) at 420.4 nm and evaluated the life times of the excited levels as 15.9(8) ns and 48(6) ns, respectively. Decay rates from these levels to neighboring opposite-parity levels are evaluated by means of Hartree–Fock calculations. We conclude that the strong transition at 410.6 nm has an optical leak rate of less then 2×10^-5 and can be used for efficient laser cooling of ¹⁶⁹Tm from a thermal atomic beam. The hyperfine structure of two other even-parity levels, which can be excited from the ground state at 409.5 nm and 418.9 nm, is also measured by the same technique. In addition, we give a calculated value of 7(2) s^-1 for the rate of magnetic-dipole transition at 1.14 μm between the fine structure levels (J_g=7/2)↔(J’_g=5/2) of the ground state which can be considered as a candidate for applications in atomic clocks. PACS 32.70.Cs; 32.10.Fn; 32.80.Pj