This thesis reports the creation and trapping of 87RbCs molecules in the absolute ground state with a temperature of 1 uK. We build a tunable narrow-linewidth laser system at 1550 nm and 980 nm, using a single high-finesse optical cavity as a reference for both colours. We use fibre-coupled electro-optic modulators to continuously tune both lasers. These allow a novel measurement of the free spectral range of the cavity to better than 1 part in 10^6. We perform one- and two-photon spectroscopy on 87RbCs Feshbach molecules and identify a suitable intermediate state for transfer to the molecular ground state. We measure the electric dipole moment of the molecular ground state as 1.225(3)(8) D, and demonstrate the highest lab-frame dipole moment of any ultracold molecular system at the time of measurement. We transfer the molecules to the electronic, rovibrational and hyperfine ground state using stimulated Raman adiabatic passage, with 88% efficiency. We measure the transition strengths and excited state linewidth for this transfer route. We develop a model for the transfer which includes the effect of laser linewidth, and find excellent agreement with experimental data. The molecular sample is trapped in an optical dipole potential, and has a lifetime of 0.89(6) s. We reference the STIRAP lasers to a novel design of frequency comb which uses difference frequency generation to cancel the carrier-envelope offset. We use this to measure the binding energy of the molecules as h x 114 268 135.24(4)(3) MHz. To our knowledge, this is the most precise determination of the dissociation energy of a molecule to date. Finally, we report progress toward loading the molecules into a 1D optical lattice at 1064 nm. We develop the tools and methods to characterise a lattice, and demonstrate trapping of Feshbach molecules in both a 1D optical lattice and a harmonic optical potential at 1064 nm.
The thesis reports transferring the ultracold RbCs molecules to the absolute ground state and a trapping light that eliminates the differential AC Stark shift. We build a laser system to perform Stimulated Raman Adiabatic Passage (STIRAP) for the molecules trapped in tweezers, which transfer the molecules from the weakly- bounded state to the absolute ground state. We use the offset cavity stabilisation technique to stabilise the laser frequencies to a passive cavity to narrow the laser linewidths while keeping the laser frequency tunable. We then align the beams to the molecules. We demonstrate a “magic” laser frequency to eliminate the differential AC Stark shift of the rotational states in the ground vibrational state of RbCs molecule and to increase the rotational coherence time of multiple states. We study the AC Stark effect caused by the trapping light. We calculate the polarisabilities of the rotational states near a narrow electronic from X ^1Σ^+ state to b^3Π_0 state and found at certain laser frequency the polarisabilities become the same. We measure the transition frequencies to different vibrational states in the excited electronic state experimentally. We perform microwave spectroscopy to measure the light shift and show that the shift vanishes at the magic frequency. We confirmed the coherence in 1 ms timescale by measuring the Ramsey fringe contrast.
A narrow-linewidth, dual-wavelength laser system is vital for the creation of ultracold ground state molecules via stimulated Raman adiabatic passage (STIRAP) from a weakly bound Feshbach state. Here we describe how a relatively simple apparatus consisting of a single fixed-length optical cavity can be used to narrow the linewidth of the two different wavelength lasers required for STIRAP simultaneously. The frequency of each of these lasers is referenced to the cavity and is continuously tunable away from the cavity modes through the use of non-resonant electro-optic modulators. Self-heterodyne measurements suggest the laser linewidths are reduced to several 100 Hz. In the context of 87 Rb 133 Cs molecules produced via magnetoassociation on a Feshbach resonance, we demonstrate the performance of the laser system through one- and two-photon molecular spectroscopy. Finally, we demonstrate transfer of the molecules to the rovibrational ground state using STIRAP.
A narrow-linewidth, dual-wavelength laser system is vital for the creation of ultracold ground state molecules via stimulated Raman adiabatic passage (STIRAP) from a weakly bound Feshbach state. Here we describe how a relatively simple apparatus consisting of a single fixed-length optical cavity can be used to narrow the linewidth of the two different wavelength lasers required for STIRAP simultaneously. The frequency of each of these lasers is referenced to the cavity and is continuously tunable away from the cavity modes through the use of non-resonant electro-optic modulators. Self-heterodyne measurements suggest the laser linewidths are reduced to several hundred Hz. In the context of 87Rb133Cs molecules produced via magnetoassociation on a Feshbach resonance, we demonstrate the performance of the laser system through one- and two-photon molecular spectroscopy. Finally, we demonstrate transfer of the molecules to the rovibrational ground state using STIRAP. ar X iv
RbCs molecules in the electronic, rovibrational and hyperfine ground state, using stimulated Raman adiabatic passage to transfer the molecules from a weakly bound Feshbach state. We measure one-way transfer efficiencies of 92(1)% and fully characterise the strengths and linewidths of the transitions used. We model the transfer, including a Monte Carlo simulation of the laser noise, and find this matches well with both the transfer efficiency and our previous measurements of the laser linewidth and frequency stability.
This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of
85Rb and
133Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of
85Rb alone in both, the
(f=2,mf=−2) and
(2,+2) states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a
(2,+2)+(3,+3) spin mixture was experimentally confirmed. We associated Cs
2 Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{
2.1(1)×104} molecules with a temperature of
∼60\,nK were produced from a sample of
3.28(2)×105 Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs
2 clouds. Furthermore, we associated up to
∼5000 heteronuclear
87RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm
−1, using two-photon spectroscopy.
We report the binding energy of
87Rb133Cs molecules in their rovibrational ground state measured using an offset-free optical frequency comb based on difference frequency generation technology. We create molecules in the absolute ground state using stimulated Raman adiabatic passage (STIRAP) with a transfer efficiency of 88%. By measuring the absolute frequencies of our STIRAP lasers, we find the energy-level difference from an initial weakly bound Feshbach state to the rovibrational ground state with a resolution of $\ensuremath{\sim}5$ kHz over an energy-level difference of more than
114THz; this lets us discern the hyperfine splitting of the ground state. Combined with theoretical models of the Feshbach-state binding energies and ground-state hyperfine structure, we determine a zero-field binding energy of $h\ifmmode\times\else\texttimes\fi{}114\phantom{\rule{0.16em}{0ex}}268\phantom{\rule{0.16em}{0ex}}135.24(4)(3)\phantom{\rule{0.28em}{0ex}}\mathrm{M}\mathrm{Hz}$. To our knowledge, this is the most accurate determination to date of the dissociation energy of a molecule.