In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Summary

Presented here is a method to measure the birefringence of vacuum windows by maximizing the fluorescence counts emitted by Doppler cooled ²⁵Mg⁺ ions in an ion trap. The birefringence of vacuum windows will change the polarization states of the laser, which can be compensated by changing the azimuthal angles of external wave plates.

Abstract

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of the vacuum windows will affect the polarization states of laser light inside the vacuum system, and it is very difficult to measure and optimize the polarization states of the laser light in situ. The purpose of this protocol is to demonstrate how to optimize the polarization states of the laser light based on the fluorescence of ions in the vacuum system, and how to calculate the birefringence of vacuum windows based on azimuthal angles of external wave plates with Mueller matrix. The fluorescence of ²⁵Mg⁺ ions induced by laser light that is resonant with the transition of |3²P_3/2,F = 4, m_F = 4 → |3²S_1/2,F = 3, m_F = 3 is sensitive to the polarization state of the laser light, and maximum fluorescence will be observed with pure circularly polarized light. A combination of half-wave plate (HWP) and quarter-wave plate (QWP) can achieve arbitrary phase retardation and is used for compensating the birefringence of the vacuum window. In this experiment, the polarization state of the laser light is optimized based on the fluorescence of ²⁵Mg⁺ ion with a pair of HWP and QWP outside the vacuum chamber. By adjusting the azimuthal angles of the HWP and QWP to obtain maximum ion fluorescence, one can obtain a pure circularly polarized light inside the vacuum chamber. With the information on the azimuthal angles of the external HWP and QWP, the birefringence of the vacuum window can be determined.

Introduction

In many research fields such as cold atom experiments¹, measurement of the electric dipole moment², test of parity-nonconservation³, measurement of vacuum birefringence⁴, optical clocks⁵, quantum optics experiments⁶, and liquid crystal study⁷, it is important to precisely measure and accurately control the polarization states of laser light.

In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of vacuum windows will affect the pola....

Protocol

1. Set up the reference directions for polarizers A and B

1. Put polarizer A and polarizer B into the laser beam (280 nm fourth harmonic laser) path.
2. Ensure that the laser beam is perpendicular to the surfaces of the polarizers by carefully adjusting the polarizer holders to keep the back-reflection light coincident with the incident light.
   NOTE: All the following alignment procedures for the optics components must follow the same rule. The placement of polarizer A and B in the laser path is not important. The spacing between them should be large enough for the future convenient adjustment.
3. Put a power meter behind polarizer A and r....

Results

Figure 3 shows the beam path of the experiment. Polarizer B in Figure 3a is removed after angle initialization (Figure 3b). The laser passed through a polarizer, an HWP, a QWP, and the vacuum window, sequentially. The Stokes vector of laser is , where

Discussion

This manuscript describes a method to perform in situ measurement of the birefringence of the vacuum window and the polarization states of the laser light inside the vacuum chamber. By adjusting the azimuthal angles of the HWP and the QWP (α and β), the effect of the birefringence of the vacuum window (δ and θ) can be compensated so that the laser inside the vacuum chamber is a pure circularly polarized light. At this point, there exists a definite relationship between the birefringence of the vacuum .......

Materials

Name	Company	Catalog Number	Comments
280 nm Doppler cooling laser	Toptica	SYST DL-FHG Pro 280	Doppler cooling laser
285 nm ionization laser	Toptica	SYST DL-FHG Pro 285	ionization laser
Ablation laser	Changchun New Industries Optoelectronics Technology	EL-532-1.5W	Q-switched Nd:YAG laser
AOM	Gooch & Housego	AOMO 3200-1220	wavelengh down to 257 nm
EMCCD camera	Andor	iXon3 897	imaging of 25Mg+ in ion trap
Glan-Taylor polarizer	Union Optic	Custom	distinction ratio 1e-6
Half waveplate	Union Optic	Custom	made of quartz
Photon multiplier tube	Hamamatsu	H8259-09	fluorescent counting
Power meter	Thorlabs	PM100D	laser power monitor
Quarter waveplate	Union Optic	Custom	made of quartz
Mirror	Union Optic	Custom	dielectric coated for 280 nm
Stepper motor roation stage	Thorlabs	K10CR1/M	rotating wave plates
Vacuum chamber	Kimball Physics	MCF800-SphSq-G2E4C4	made of Titanium
Vacuum window	Union Optic	Custom	made of fused silica