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Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects, ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in the life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nano-particle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement single crystal nucleation spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a nonsupervised spectral decomposition technique, uncovered the Raman spectrum of prenucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of prenucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of linear hydrogen-bonded networks. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.

We theoretically investigate the damping and trapping forces in a three-dimensional magneto-optical trap (MOT), by numerically solving the optical Bloch equations. We focus on the case where there are dark states because the atom is driven on a 'type-II' system where the angular momentum of the excited state, F ′ , is less than or equal to that of the ground state, F. For these systems we find that the force in a three-dimensional light field has very different behaviour to its one dimensional counterpart. This differs from the more commonly used 'type-I' systems ( F ′ = F + 1 ) where the 1D and 3D behaviours are similar. Unlike type-I systems where, for red-detuned light, both Doppler and sub-Doppler forces damp the atomic motion towards zero velocity, in type-II systems in 3D, the Doppler force and polarization gradient force have opposite signs. As a result, the atom is driven towards a non-zero equilibrium velocity, v0, where the two forces cancel. We find that v 0 2 scales linearly with the intensity of the light and is fairly insensitive to the detuning from resonance. We also discover a new magneto-optical force that alters the normal MOT force at low magnetic fields and whose influence is greatest in the type-II systems. We discuss the implications of these findings for the laser cooling and magneto-optical trapping of molecules where type-II transitions are unavoidable in realising closed optical cycling transitions.

Laser cooling is used to produce ultracold atoms and molecules for quantum science and precision measurement applications. Molecules are more challenging to cool than atoms due to their vibrational and rotational internal degrees of freedom. Molecular rotations lead to the use of type-II transitions ( F ⩾ F ′ ) for magneto-optical trapping (MOT). When typical red detuned light frequencies are applied to these transitions, sub-Doppler heating is induced, resulting in higher temperatures and larger molecular cloud sizes than realized with the type-I MOTs most often used with atoms. To improve type-II MOTs, Jarvis et al (2018 Phys. Rev. Lett. 120 083201) proposed a blue-detuned MOT to be applied after initial cooling and capture with a red-detuned MOT. This was successfully implemented (Burau et al 2023 Phys. Rev. Lett. 130 193401; Jorapur et al 2024 Phys. Rev. Lett. 132 163403; Li et al 2024 Phys. Rev. Lett. 132 233402), realizing colder and denser molecular samples. Very recently, Hallas et al (2024 arXiv:2404.03636) demonstrated a blue-detuned MOT with a ‘1+2’ configuration that resulted in even stronger compression of the molecular cloud. Here, we describe and characterize theoretically the conveyor-belt mechanism that underlies this observed enhanced compression. We perform numerical simulations of the conveyor-belt mechanism using both stochastic Schrödinger equation and optical Bloch equation approaches. We investigate the conveyor-belt MOT characteristics in relation to laser parameters, g -factors and the structure of the molecule, and find that conveyor-belt trapping should be applicable to a wide range of laser-coolable molecules.

The working mechanism of single-beam optical tweezers is revisited using a recently established method. The optical force is split into conservative and nonconservative components, and these components are explicitly calculated for particles in the Rayleigh, Mie and geometrical optics regimes. The results indicate that optical trapping is attributable to the formation of an ‘optical trapping core’. Stable trapping is achieved when the conservative forces are larger than the nonconservative forces in the core region centered at the beam centers for all particle sizes. According to the conventional understanding, stability is a result of the conservative force overcoming the nonconservative force. In comparison, the concept of the optical trapping core more accurately illustrates the physical mechanism of optical trapping, for not only single-beam optical tweezers but also optical trapping settings.

For the first time, we experimentally determine the infrared magic wavelength for the 40Ca+4s2S1/2→3d2D5/2 electric quadrupole transition by observation of the light shift canceling in 40Ca+ optical clock. A ‘magic’ magnetic field direction is chosen to make the magic wavelength insensitive to both the linear polarization purity and the polarization direction of the laser. The determined magic wavelength for this transition is 1056.37(9) nm, which is not only in good agreement with theoretical predictions (‘Dirac–Fock plus core polarization’ method) but also more precise by a factor of about 300. Using this measured magic wavelength, we also derive the differential static polarizability to be −44.32(32) a.u., which will be an important input for the evaluation of the blackbody radiation shift at room temperatures. Our work paves a way for all-optical-trapping of 40Ca+ optical clock.

Plasmonic and dielectric tweezers represent a common paradigm for an innovative and efficient optical trapping at the micro/nanoscale. Plasmonic configurations provide subwavelength mode confinement, resulting in very high optical forces, at the expense of a higher thermal effect, that could undermine the biological sample under test. On the contrary, dielectric configurations show limited optical forces values but overcome the thermal challenge. Achieving efficient optical trapping without affecting the sample temperature is still demanding. Here, we propose the design of a silicon (Si)-based dielectric nanobowtie dimer, made by two tip-to-tip triangle semiconductor elements. The combination of the conservation of the normal component of the electric displacement and the tangential component of the electric field, with a consequent large energy field confinement in the trapping site, ensures optical forces of about 27 fN with a power of 6 mW/µm2. The trapping of a virus with a diameter of 100 nm is demonstrated with numerical simulations, calculating a stability S = 1, and a stiffness k = 0.33 fN/nm, within a footprint of 0.96 µm2, preserving the temperature of the sample (temperature variation of 0.3 K).

We demonstrate rapid loading of a magneto‐optical trap (MOT) of cadmium atoms from a pulsed cryogenic helium buffer gas beam, overcoming strong photoionization losses. Using the 1S0→1P1 $ ^1S_0 \\rightarrow {} ^1P_1$transition at 229 nm, we capture up to 1.1(2)×107112Cd $ 1.1(2) \\times 10^{7\\;\\;112}{\\rm Cd}$atoms in 10 ms, achieving a peak density of 2.5×1011 $2.5 \\times 10^{11}$cm−3 $^{-3}$and a phase‐space density of 2×10−9 $ 2 \\times 10^{-9}$ . The large scattering force in the deep ultraviolet enables Zeeman slowing within 5 cm of the trap, yielding a capture velocity exceeding 200 m/s. We measure the MOT trap frequency and damping constant, and determine the absolute photoionization cross‐section of the 1P1 $^1P_1$state. Photoionization losses are mitigated via dynamic detuning of the trapping light's frequency, allowing efficient accumulation of multiple atomic pulses. Our results demonstrate the benefits of deep ultraviolet (DUV) transitions and cryogenic beams for loading high‐density MOTs, especially for species with significant loss channels in their main cooling cycle. The cadmium MOT provides a robust testbed that benchmarks our DUV laser cooling system and establishes the foundation for trapping and cooling polar AlF molecules, which share many optical and structural properties with Cd. We demonstrate laser cooling and trapping of cadmium atoms using deep‐ultraviolet light at 229 nm, capturing more than 1×107 $1\\times10^{7}$atoms within 10 ms from a compact 5 cm Zeeman slower and cryogenic beam source. We achieve peak densities of 2.5×1011cm−3 $2.5\\times10^{11} \\,\\text{cm}^{-3}$and phase‐space densities of 2×10−9 $2\\times10^{-9}$ . By dynamically tuning the cooling light's frequency we suppress photionization losses enbling efficient accumulation from multiple atomic pulses. These results establish cadmium as a powerful new platform for precision measurement and quantum science, with direct applications to optical lattice clocks, isotope‐shift spectroscopy, and the laser cooling of molecules.

One challenge of optical trapping of nanoparticles is the weak trapping force compared to the destabilizing pushing force. Here we enhance the optical gradient force (GF), which is responsible for trapping, to achieve stable nanoparticle trapping through aberration compensation. The optical forces are calculated using multipole expansion theory and the focused fields are determined using Debye focusing theory accounting for interface aberrations between oil, glass, and water. With typical oil immersion objectives, the glass-water interface aberration reduces the GF relative to the scattering force (SF), leading to unstable trapping. By optimizing the refractive index of the immersion oil, the interface aberrations can be compensated. This significantly enhances the GF while moderately improves the SF, enabling stable nanoparticle trapping. The enhancements are particularly notable for large probe depths. Further improvement can be achieved with a thicker oil layer. With optimized conditions, the GF exceeds the SF by over two-fold. And the minimum axial force and axial stiffness increased approximately three-fold. Our study provides theoretical guidance to improve nanoparticle trapping efficiency through aberration compensation and force optimization.

The manipulation of single molecules has attracted extensive attention because of their promising applications in chemical, biological, medical, and materials sciences. Optical trapping of single molecules at room temperature, a critical approach to manipulating the single molecule, still faces great challenges due to the Brownian motions of molecules, weak optical gradient forces of laser, and limited characterization approaches. Here, we put forward localized surface plasmon (LSP)-assisted trapping of single molecules by utilizing scanning tunneling microscope break junction (STM-BJ) techniques, which could provide adjustable plasmonic nanogap and characterize the formation of molecular junction due to plasmonic trapping. We find that the plasmon-assisted trapping of single molecules in the nanogap, revealed by the conductance measurement, strongly depends on the molecular length and the experimental environments, i.e., plasmon could obviously promote the trapping of longer alkane-based molecules but is almost incapable of acting on shorter molecules in solutions. In contrast, the plasmon-assisted trapping of molecules can be ignored when the molecules are self-assembled (SAM) on a substrate independent of the molecular length.