Tunable Laser for 3He Neutron Spin Filter Cells in Research
Explore the cutting-edge research on a wide-range wavelength-tunable laser for studies involving 3He neutron spin filter cells, presented by Takashi Ino from KEK & J-PARC PNCMI. The laser's purpose, principle, absorption measurements, and conclusions are outlined, alongside motivation and details on alkali-hybrid spin-exchange optical pumping. Dive into the innovative use of laser technology for scientific studies.
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Presentation Transcript
Wide-range wavelength-tunable laser for studies of 3He neutron spin filter cells Takashi Ino KEK & J-PARC PNCMI 2025 February 24 28, 2025, Dongguan, China
Outline - Purpose of the wavelength-tunable laser - Principle and structure of the wavelength-tunable laser - Laser absorption measurements - Comparison with calculations - Conclusions
Alkali-hybrid spin-exchange optical pumping (SEOP) Spin polarized photons (Circularly polarized light) Optical pumping Spin polarization of Rb atoms Spin polarization of K Spin exchange Spin polarization of 3He nuclei 3.2 amg 200 hrs E. Babcock et al., PRL 91, 123003 (2003)
Alkali-hybrid spin-exchange optical pumping (SEOP) Spin polarized photons (Circularly polarized light) Optical pumping Spin polarization of Rb atoms Spin polarization of K Spin exchange Spin polarization of 3He nuclei ?SE ?He= ?AM ?SE+ ?AM: alkali metal (AM) polarization ?SE: spin-exchange rate between AM and 3He : spin-relaxation rate of polarized 3He 3.2 amg 200 hrs
Alkali-hybrid spin-exchange optical pumping (SEOP) Spin polarized photons (Circularly polarized light) Optical pumping Spin polarization of Rb atoms Spin polarization of K Spin exchange Spin polarization of 3He nuclei ?SE ?He= ?AM ?SE+ ?AM= ?Rb& ?K ?SE Rb & [K] (number densities) 3.2 amg 200 hrs
External cavity diode laser ECDL External cavity Diode laser Diffraction
External cavity diode laser ECDL External cavity Diode laser ? ? = 2?sin? ? : laser wavelength (~ 780 nm) ? : laser incident angle (~ 44.6 ) ? : groove spacing (1/1800 mm) Pivot
External cavity diode laser ECDL External cavity ? ?= 11 ?m/nm Diode laser ? ? = 2?sin? ? : laser wavelength (~780 nm) ? : laser incident angle (~44.6 ) ? : groove spacing (1/1800 mm) Pivot ? ?= 13.8 nm/deg
Temperature control of diode laser with TEC External cavity Diode laser ? Thermoelectric ? ? 0.2 nm/K cooler ?: 25 to R.T. Pivot
Wavelength tunable laser Actuator Thermistor Cooling plate Mirror Grating DL & lens Thermoelectric cooler Kinematic mount 12 cm
Wavelength tunable range K D2 K D1 (770.1 nm) Rb D2 (780.2 nm) Rb D1 (766.7 nm) (795 nm) Intensity (arb. unit) Wavelength [nm] FWHM < 2 GHz (4 pm)
Measurement setup Photodiode 2 Solenoid Mirror Pumping laser 795 nm Oven ?0 3He cell Mirror Beam sampler Photodiode 1 Optical chopper 3.1 amg 208 hrs Beam sampler K/Rb = 17 (mole) Spectrum analyzer Tunable laser Probe laser : linearly polarized in vertical direction Optical chopper : reduction of background light and/or noise
Light absorption spectra Rb D2 Incident polarization 1 0 Transmittance ?0= K D1 K D2 Cell surface temp. 220 C no OP 230 C no OP ?AM= 0 Wavelength [nm]
Light absorption spectra Rb D2 Incident polarization 1 0 Transmittance ?0= K D1 K D2 Cell surface temp. 220 C no OP 230 C no OP 220 C under OP Wavelength [nm]
Light absorption in alkali metal cell Quantization axis of alkali metal spins ? ? 2 Initial intensity ?0= ?0 ??? cos? sin???? ? ? ?0= ?0 ?,? : polarization of light ?AM 1 1 ? Circular polarization ? 2 Rb, K cos? sin? Linear polarization ? Incident angle ? ?0
Alkali metal energy levels -3/2 -1/2 1/2 3/2 ? = ? + ? =3 P3/2 ? = 1 2 ? = ? + ? =1 P1/2 ? = 0 2 D2transition D1transition ? = ? + ? =1 S1/2 ? = 0 2 Spin Spin
Calculation of transition probabilities -3/2 -1/2 1/2 3/2 Angular momentum state | ? ?? P3/2 P1/2 ?? ? = ?? ? ? = ?0exp(?? ? ???) ?0 (electric dipole approx.) 2 Transition probability ? ?? ? ??? S1/2 Spin Spin
Transition probabilities of alkali metal atoms -3/2 -1/2 1/2 3/2 ? 1 P3/2 2cos2? + sin2? cos2? sin2?cos?sin? ? ?+ P1/2 ? 1 6cos2? + sin2? cos2? sin2?cos?sin? ?? ?? ? 1 3cos2? + sin2? cos2? sin2? cos?sin? ? ?+ ? ?+ ?? 2 3sin2? sin2? ?? ?? ?? 1 3sin2? sin2? cos? sin???? ?0= ?0 S1/2 Spin Spin ? : incident angle to quantization axis of AM spins
Absorption of light by D1transition ? = 1 1 1 1 ? 1 ? +0 + ?r?+?d ?l? ?u ?p?0? ? ? 1 2 2 2 ?r: amplitude of right-handed circularly polarized light component of ?(?) ?l: amplitude of left-handed circularly polarized light component of ?(?) ?p: amplitude of linearly polarized light component of ?(?) (parallel to Q-axis) ?u seen along the light axis ? = ?u ?AM = ?u ? = ?AMcos? ? : normalization coefficient : number densities of spin-up and spin-down alkali metal atoms ,?d + ?d ?d cos? sin???? ? ? = ? ? ?2 ?02 ? =? Transmittance of light ? = ??+ ?? = ?0 ?
Absorption of light by D1transition ? = 1 2 2 ? 1 1 1 1 ? +0 + ?r?+?d ?l? ?u ?p?0? ? ? 1 2 Absorption of light by D2transition ? = 1 1 1 1 ? 1 ? +0 + ? ?u + ?+?d ?r?+?u + ?l? ?d ?p?0? ? ? 1 2 2 2 -3/2 -1/2 1/2 3/2 P3/2 ? ?+ P1/2 ? ?+ ? ?+ ?? ?? ?? ?? S1/2 Spin Spin
Light absorption in alkali metal cell Quantization axis of alkali metal spins ? 1 0 Initial light ?0= ?0 ?AM Incident angle ? = 18.4 deg ? Rb, K Alkali metal polarization ?AM= 0 or 1 ? ?0
Light absorption spectra Incident polarization 1 0 Transmittance ?0= ?0 Calculation ?AM= 0 ?AM= 1 Pressure broadening of D1and D2lines K. A. Kluttz et al., PRA 87, 032516 (2013) Wavelength [nm]
Light absorption spectra Incident polarization 1 0 Transmittance ?0= ?0 ?AM= 0 Measurement Calculation Wavelength [nm]
Light absorption spectra Incident polarization 1 0 Transmittance ?0= ?0 Measurement Calculation ?AM= 1 Wavelength [nm]
Summary - A wide-range wavelength-tunable laser has been developed for studies of alkali metal states in 3He neutron spin filter cells. Wavelength coverage: 762 790 nm (K D2, K D1, Rb D2) Linewidth (FWHM) < 2 GHz or < 4 pm - A clear difference was seen in the light absorption spectra for polarized and unpolarized alkali metal atoms. - Calculations show quantitative agreements with the measurements, but qualitatively. - Need more studies to understand the spectra.
