An optical centrifuge with zero rotational acceleration

Abstract

An optical centrifuge is a laser pulse which enables controlled rotational excitation of molecules. Centrifuged molecules rotating with well-defined angular frequencies are ideal candidates to probe many-body quantum systems at the nanoscale. Because the interaction with the quantum environment increases the effective moment of inertia of the embedded molecular probes, the required rotational acceleration of the optical centrifuge must be lowered to accommodate adiabatic spinning. We demonstrate a new design of an optical centrifuge, based on the method of spectral focusing, which enables extremely low, down to zero, rotational accelerations. We discuss the potential of using such constant frequency centrifuge to investigate molecular rotation inside helium nanodroplets.

Figures (4)

• Figure 1: (color online) Time-frequency diagrams of laser pulses, corresponding to (a) conventional optical centrifuge and (b) constant-frequency optical centrifuge (cfCFG). Circles with arrows represent the polarization of the chirped pulses (tilted ellipses), whose interference results in the centrifuge field, rotating with frequency $\Omega(t)$. (c) Effects of third order dispersion (TOD, exaggerated for illustration purposes) on the time-frequency diagram of cfCFG. See text for the definition of $\Omega_0$, $\varepsilon$ and $\tilde{\varepsilon}$. (d) An outline of an optical setup producing the field of the cfCFG, illustrated with a corkscrew shape on the left. CPA: chirped pulse amplifier, P: linear polarizer, PBS: polarizing beamsplitter, $\lambda /2$ and $\lambda /4$: half- and quarter-wave plates.
• Figure 2: (color online) Coherent Raman scattering from the gas of oxygen molecules, rotationally excited by the constant-frequency centrifuge. (a) Example of a Raman signal obtained with the cfCFG tuned to the transition between $J=7$ and $J=9$ rotational levels of O$_{2}$. Red vertical dashed line indicates the central wavelength of probe pulses. (b) Raman spectrogram, recorded by scanning the time delay $\Delta t$ between the two arms of the centrifuge. Horizontal dashed lines, labeled with the rotational Raman resonances of oxygen molecule, indicate the calculated frequencies of the corresponding Raman shifts. Diagonal solid line illustrates the predominantly linear dependence of the Raman shift with respect to $\Delta t$. Inset in the lower right corner outlines the relative timing between the two centrifuge arms (solid red) and a probe pulse (dashed blue).
• Figure 3: (color online) (a) Geometry of the experiment with OCS molecules in seeded molecular jet using the technique of velocity map imaging with a multi-channel plate (MCP) detector. (b) Degree of planar alignment of OCS molecules, rotationally excited by the constant-frequency centrifuge, as a function of the time delay $\Delta t$ between the centrifuge arms. Solid red line is the best fit of experimental data (grey dots with error bars) to the sum of six gaussian peaks, plotted with colored curves and labeled with the corresponding excitation transition. (c) Rotational frequency of cfCFG, $\Omega/2\pi$, extracted from the known transition frequencies of OCS (blue diamonds). Solid red line shows the best fit to the linear dependence on $\Delta t$.
• Figure 4: (a) Ensemble-averaged alignment factor $\langle \cos^2\theta_\text{2D} \rangle{}$ of nitrogen molecules in a molecular jet, exposed to the field of a constant-frequency centrifuge. See text and Fig. \ref{['Fig-VMIcalib']} for details on the VMI-based detection technique. (b) Centrifuge-induced optical birefringence of a dense gas of oxygen molecules at room temperature. See text for the details of the polarimetry-based detection method.