Optical control of coherent magneto-optical resonances in potassium

The influence of Light Induced Atomic Desorption (LIAD) on the potassium D2 line magneto-optical resonances in uncoated buffer-gas optical cell is investigated. LIAD effect reduces the drawbacks of conventional heating for achieving high atomic density that is essential for many spectroscopy-based applications. Another feature of LIAD is the impact on the dwelling time of the atoms when colliding with the surface of the cell. In this work we investigate LIAD from point of view to distinguish the influence of LIAD on the atomic density from the dwelling time as well as to control and improve the parameters of magneto-optical resonances in potassium vapor. The results are interesting for development of new precise optical sensors and devices for various applications.


Introduction
The rapidly accelerating development of sensors in every field of modern life from space exploration to nanotechnologies demands constantly improving the existing technological advancements and searching for new ones pushing the existing knowledge of physics to its limits and even beyond.The study of atomic vapours in optical cells is a constantly expanding field of physics with diverse applications.Quantum memories [1], atomic optical clocks [2], vapor magnetometers [3], optical isolators [4], Faraday rotation filters [5] and atomic gyroscopes [6] are just a few to mention.Optical glass-or quartzmade cells usually confine alkali-metal vapours in vacuum.Potassium (K), Rubidium (Rb) and Cesium (Cs) are widely applied in spectroscopy mostly because of their well-resolved spectra either in absorption or emission, fairly high density at room temperature and the possibility to be excited to higher energy state by diode lasers with wavelengths equal to their resonant lines.Some of these applications like miniaturization of atomic clocks and magnetometers require relatively high atomic densities usually at much higher than room temperature and long spin lifetime of alkali-metal atoms.However, as the spin-polarized lifetime of the atoms is reduced by collisions between the atoms and the walls of the alkali-metal vapor cell, reducing the size of the cell leads to higher depolarization of the atomic spin.In order to minimize the depolarization and keep the atom at a certain energy state for longer time cells with buffer gas and/or anti-relaxation (AR) coating are used.
Light Induced Atomic Desorption (LIAD) [7] is an all-optical non-resonant phenomenon usually applied to increase and keep the atomic density higher and steady in optical cells.This is usually achieved by illuminating the cell's coated walls with relatively high intensity light.Some experiments show that the efficiency of LIAD may increase by homogeneous illumination of the cell and might be effective even in uncoated bare glass cells in some cases.Moreover, it can serve for indication of the purity of the cell's walls [8,9] and even decrease the depolarization rate of the atoms [10].
Magneto-optical spectroscopy of alkali atoms is extensively applied in many fields including atomic frequency standards and atomic magnetometers [11].The Magneto-Optical Resonances (MOR) on 766.7 nm D2 line of potassium are very promising candidate for magnetometry, as the hyperfine groundlevel frequency difference is smaller than the Doppler width of the optical transitions and the overlapping of the Doppler profiles, starting from both ground-state hf levels can provide re-population of the resonantly excited by the light ground hf levels and enhance the efficiency of magneto-optical transitions [12,13].
In previous works [13,14] we have investigated the coherent MOR on the potassium D2 line in Hanle configuration with potassium excited by circularly polarized laser beam in cylindrical cell with length and diameter 8 mm and 30 Torr Ne buffer gas.The capability of the laser system to work with magnetically unshielded cell was demonstrated.
The forementioned properties of LIAD suggest that investigations of LIAD phenomenon might be beneficial for MOR experiments even in a buffer-gas cell [13,14] although as the cell's pureness is presumably high and LIAD effect is not expected to influence the density significantly, but the impact on the depolarization rate of the atoms and their interaction with the cell's surface could be beneficial.The purpose of this work is to investigate the possibilities of optical control of the MOR by LIAD [14 and references therein] in an unshielded potassium buffer-gas cell.The results are interesting for development of new precise optical sensors and devices for different applications.

Experimental setup
The experimental setup for investigation of the MOR (figure 1) is similar to the one described in [14].A circularly polarized laser beam with 2 mm 2 spot of a single mode 766.7 nm DFB diode laser is aligned parallel to the horizontal component of the laboratory magnetic field and passes through an optical cell (8 mm in length and diameter) filled with K and 30 Torr Ne buffer gas to increase the time of interaction of the alkali atoms with the laser beam [15].The laser beam power is controlled by an attenuator.The transmitted light is registered by a power meter.In section 3, when the LIAD influence on MOR is investigated, the magnetic field along the laser beam direction is induced by Helmholtz coil, scanned linearly around zero value by second signal generator and an amplifier (not shown).Another Helmholtz coil compensates the vertical component of the laboratory magnetic field (not shown).To investigate LIAD influence on the MOR, the upper part of the temperature control box from [14] is removed, and a LED array is mounted on top of the box, so that the blue desorbing light is directed downwards to the cell.The LED array is cooled by a heatsink and a fan above it to avoid damage of the LEDs by the heat from the heating element of the cell and the heat of the LED array itself.

LIAD dynamics and LIAD yield in an uncoated potassium buffer gas cell
In figure 2 (a) the pressure and Doppler broadened profiles which encompass the D2 line hyperfine transitions as a result of the laser frequency scan at 36 o C without LIAD (black) and with LIAD (blue) are shown.As observed from figure 2 (a) the absorption profile is not visibly enhanced by LIAD.The results show that the difference in absorption is of the order of 0.5%.As expected for an uncoated cell, the influence of LIAD on the K vapor density in negligible.
Figure 2 (b) illustrates the LIAD dynamics (the change of absorption of the laser light with frequency at the apex point of the Doppler broadened profile when the illumination of the cell is switched on and off).The phenomenological model of the LIAD dynamics in uncoated cell [16] is mainly characterized by the short time constants τ1 and τ3, which illustrates the evolution of the absorption (respectively the density of atoms) when the desorbing light is switched on and off by fitting following equation to the experimental data: where   and   are the start and stop times of the desorbing illumination,  0 is the density in equilibrium before LIAD; τ 1 is the time constant of the exponential growth of the density after switching the illumination and τ 3 characterizes the exponential decay of the density when the LIAD is off.τ 2 is the long-time constant of the decrease of the vapor density after the initial increase, but it cannot be determined by figure 2b because of the short time interval between   and   .The time constants τ 1 and τ 3 obtained after fitting the curve shown in figure 2 (b) with an exponential function are 3.7ms and 4.9ms respectively.By increasing the scanning time interval of the LIAD up to 30 Hz similar results are obtained for τ 1 and τ 3 .These desorbing and adsorbing rates and the small LIAD yield are typical for uncoated cells [8] and confirm our presumption of the purity of the cell.

LIAD influence on MOR in the buffer gas potassium cell
In our previous investigations on MOR in Hanle configuration [14], where the laser frequency is fixed at the top of the Doppler broadened profile of the potassium D2 and the magnetic field is scanned linearly in the same direction as the laser beam (the same as in figure 1), it has been shown that the sign of MOR observed in fluorescence perpendicular to the laser beam direction depends on the atomic density.When In figure 3 the EIA resonances with and without LIAD are compared.The black curve is the MOR before the illumination of the cell, and the blue one is registered after the light is switched on.The two experimental curves have similar shapes, and both are fitted with Lorentzian functions (yellow dashed curve and green dashed curve respectively).With LIAD illumination the resonance contrast increases up to 140% for the blue curve (if we presume the black one is 100%).This increase is significant and cannot be related to increase of density, which changes just 0.5% of the initial one due to the LIAD effect.The observed results could be attributed to shorter dwelling time of atoms or the so called "sticking time", which is usually related to binding energy in coated cells [7,17].We can assume that LIAD has similar impact on atoms in uncoated cells too, leading to increased contrast of the Hanle resonance unalike to [18].In [18], this result could be related not only to different dwelling time, but also to the higher density, as the cell there is coated.
The FWHM of the EIA resonances with and without LIAD is almost equal (about 3% difference).The centre of the resonance is shifted from zero as the cell is unshielded and the offset corresponds to the laboratory magnetic field in direction of the laser beam.

Conclusions
In our investigation we have shown that the LIAD effect might be very useful even in uncoated buffer gas optical cells.The LIAD yield and the time constants are indicative for the pureness of the cell and confirm our initial assumption that our cell is uncoated and clear.Moreover, the LIAD effect could reduce the dwelling time of atoms on the surface due to influence of LIAD on the binding energy of atoms even on a bare glass surface.As a result, we observe MOR with much higher contrast at the same temperature in the buffer-gas uncoated cell.This could improve the long-term stability of the sensors, as the coating of the cells deteriorates over time and the use of buffer gas cells could be advantageous.The investigations could be helpful also to distinguish the influence of LIAD on desorption and dwelling time of atoms and as a result may be beneficial in precision optical control of magneto-optical sensors.

Figure 2 .
Figure 2. (a) Doppler broadened profile in absorption without (red curve) and with LIAD (blue curve).(b) LIAD dynamics (switching on and off the illumination with 200ms period) at 36 o C.

Figure 3 .
Figure 3. EIA resonances registered in absorption when the laser frequency is fixed at the top of the Doppler broadened profile of the potassium D2 line and the magnetic field is scanned linearly across the laser beam direction.