Formation of Narrow Optical Resonance by Micrometer Thin Rb- Vapor Layer

Formation of Narrow Optical Resonance by Micrometer Thin Rb- Vapor Layer A. Sargsyan Institute for Physical Research, NAS of Armenia, Ashtarak-00, Armenia, sarmeno@mail.ru ABSTRACT Recently developed thin cells containing atomic vapor of micrometric column thickness L allow one to study peculiarities of Electromagnetically Induced Transparency () phenomenon, along with the accompanying velocity selective optical pumping/saturation (VSOP) resonances for the case when < 00 µm. The micrometric thin cells (MTC) are filled with pure Rb and neither buffer gas nor paraffin-coated walls were used. The Λ-systems on D line of 85 Rb have been studied experimentally with the use of bi-chromatic radiation of two separate diode lasers (λ 780 nm, γ L 5MHz). It is demonstrated that when L~60 µm it is still possible to form the resonance with the sub-natural linewidth ~ 5 MHz. The resonance linewidth increases up to 0 MHz, when column thickness L is reduced down to ~ µm. Six VSOP resonances are detected in the fluorescence and absorption spectra when the thickness L ~ 0 µm. Dependence of the resonance linewidth as a function of the atomic vapor column thickness L is presented. Key words: electromagnetically induced transparency, micrometric thin cell, coherent resonances, atomic vapor. INTRODUCTION There has been considerable interest in recent years for the fascinating properties of Coherent Population Trapping (CPT) and the related Electromagnetically-Induced Transparency () -7. These phenomena attract much attention because of its significance for various applications in metrology, magnetometry and fundamental investigations. The and CPT resonances can occur in a Λ-system with two long-lived states and one excited state coupled by two laser fields. In order to achieve narrow resonance two lasers have to be coherently coupled and for this purpose several modulation techniques could be implemented,. However, there are some cases when a large frequency region of the probe laser frequency tuning is needed, and for this case the use of two different lasers for coupling and probe radiation formation could be more convenient. For example, in order to study the peculiarities of sub-doppler satellites (so called velocity Selective Optical Pumping/Saturation (VSOP) resonances), accompanying resonance, the tuning range of probe laser more than GHz might be required. The linewidth of the resonance in the case of small laser intensity is γ Γ + Ω /γ N, where Γ is the relaxation rate between ground hyperfine levels, Ω is the Rabi frequency. As the size of the vapor cell is reduced, the lifetime of the ground-state coherence becomes shorter because of collisions of the atoms with the cell s windows: Γ = (π t) -, where t= L/u, with L being the distance between windows and u - the thermal velocity. Also, the resonance contrast (defined as the ratio of the depth to the height of shoulders of the window) strongly depends on Γ, thus in the case of L ~ µm (Γ >00 MHz), one could expect that the effect will be vanished. In order to prevent atom window collisions for the case of sub-millimeter thin cells, buffer gas has been successfully used in 5. Nevertheless, in the work it has been predicted for the cell with of L ~ 0 µm, that even in the case of pure atomic vapor narrow ICONO 007: Nonlinear Laser Spectroscopy and High-Precision Measurements; and Fundamentals of Laser Chemistry and Biophotonics, edited by Sergey Tikhomirov, Thomas Udem, Valery Yudin, Maxim Pshenichnikov, Oleg Sarkisov, Proc. of SPIE Vol. 677, 6779, (007) 077-786X/07/$8 doi: 0.7/.75 Proc. of SPIE Vol. 677 6779- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

resonance can be observed. The explanation is that the contribution of atoms flying nearly parallel to the MTC s windows is enhanced thanks to their longer interaction time with laser field. Thus, atoms flying nearly parallel to the windows yield a stronger contribution to the resonance. Here we present the experimental results of further study of the peculiarities of and VSOP resonances which are formed in micrometric thin cells with the thickness of the atomic vapor column L< 00 µm. Also, two applications of nano-cells 8- with the thickness of atomic vapor column L=λ and L=λ/ (λ =780 nm) are presented.. Experiment with Micrometric Thin cells (L < 00 µm) The experimental arrangement is sketched in Fig. Both beams ( mm) of two single-frequency diode lasers (coupling and probe) with λ 780 nm (the linewidth is ~ 5 MHz) are well superposed and directed onto the MTC at near-normal incidence with the help of the first Glan prism (the coupling and probe beams have linear and perpendicular polarizations). In some cases (which will be mentioned below) the second Glan prism has been used, ()-are Faraday isolators. The MTC was placed inside the three pairs of mutually perpendicular Helmholts coils () providing possibility to cancel laboratory magnetic field as well as to apply homogeneous magnetic field. The optical radiations were recorded by the photodiodes () and the signal of the photodiodes was intensified and recorded by a two-channel digital storage oscilloscope Tektronix TDS 0B, F-are filters. The power of the coupling and probe lasers was in the range of 0.5 - mw and 0. - 0.5 mw, respectively. An improved Dichroic-Atomic-Vapor-Laser-Locking (DAVLL) nano-cell L=λ/ + - Error signal Ring Magnet λ/ F nano-cell L=λ Oscilloscope ω P F GLAN GLAN DL ω C DL MTC Fig.. Sketch of the experimental Set-Up. method, realized in a separate nano-cell with the thickness L = λ/, is used for the coupling laser frequency stabilization; this technique will be published elsewhere 6. The frequency reference spectra formation has been realized Proc. of SPIE Vol. 677 6779- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

with the help of an auxiliary nano-cell. When the nano-cell thickness is L λ, sub-doppler peaks of reduced absorption appear in the transmission spectra, centered on the hyperfine atomic transitions, 0,, 5 as it is in saturated absorption technique. The physical origin of formation of these peaks is velocity selective optical pumping/ saturation processes. Note that since a single beam is used, the crossover resonances, which are dominant in saturated absorption spectra, are absent. This is important when there are many overlapped spectral lines as for the case of F g = F e =,, transitions of 85 Rb D line. This technique has been used in this work for the frequency reference spectra formation. For the reference formation the laser intensity ~ mw/cm is sufficient. The near-normal incidence of the laser beam is important 8, 5. The operation temperature of the reference nano-cell is T W ~ 0 o C at the window and T SA ~ 00 o C at the side-arm. As shown in Fig., our Λ-type system consists of two ground hyperfine levels of 85 Rb spaced by 06 MHz, and an excited 5P / state, which serves as a common upper level. In Fig. the transmission spectrum of the probe laser is presented (upper curve), when the couple laser (~ 0.5mW) is resonant with the 85 Rb F g = F e = transition, while the probe laser (~0. mw) is scanned across Fg= Fe =,, transitions (see the diagram in Fig.). The thickness of the vapour column for this case is L 60 µm. The operation temperature are T W ~ 90 o C and T SA ~70 o C. Transmission (arb.un.) L=60 µm 5 9MHz ω C 5 ω P L=λ, -' -9 -' -' 0 Frequency difference (MHz) Fig. and five VSOP resonances, L=60 µm. The lower curve is the transmission spectrum of the nano-cell, L= λ. Fig..Partial diagram of 85 Rb, D levels. In the upper curve the resonance is seen together with five sub-doppler VSOP resonances 7 marked by the pointing numbers. The narrowest resonance, which was formed in the 60 µm-thick cell, has a sub-natural linewidth ~ 5 MHz (see the inset in Fig.). The VSOP s resonance linewidth is ~ times larger. VSOP resonances,, are formed by the atoms flying parallel to the MTC s windows, while, 5 are formed by the atoms flying with v 50 m/s in the direction opposite to laser radiation propagation k (see Fig.). In Fig. the experimental transmission spectrum of the probe laser is presented for 85 Rb, D line for the thickness L=0 µm. The coupling laser is resonant with the 85 Rb F g = F e = transition, while the probe laser is scanned in a wide region (more than GHz) across Fg= Fe =,, and F g = Fe =,, transitions. The inset shows the Proc. of SPIE Vol. 677 6779- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

resonance along with sub-doppler VSOP resonances. The lower curve is the transmission spectrum of the nano-cell, L= λ. As it is seen when the starting ground level F g = is the same for the coupling and the probe, then the VSOP resonance demonstrates reduction of the absorption (right side of the upper curve), while when the starting ground level is different for the coupling (Fg= ) and the probe (Fg= ), then the VSOP resonance demonstrates an increase of the absorption (left side of the upper curve). It is interesting to note that despite the frequent atom-wall collisions, VSOP Transmission (arb.un.) L = 0 µm L=λ : -' -' -' 500 MHz Frequency -' -' -' Fig.. 85 Rb, D line. L=0 µm, the coupling laser is resonant with the 85 Rb F g = F e = transition, the probe laser is scanned in a wide region across Fg =, Fe =,,,. The inset shows the resonance along with sub-doppler VSOP resonances. The lower curve is the transmission spectrum of the nano-cell, L= λ. resonances are still well observable. In Fig.5 two diagrams are presented which show that the VSOP resonances of increased absorption (left diagram) and reduced absorption (right diagram)are formed by the three following atomic groups: i)atoms flying parallel to the MTC s windows, ii) the atoms flying with v 50 m/s and v 7 m/s in the ω C 5 6 7 ω P ω C 5 6 7 ω P Fig. 5. Diagrams of 85 Rb, D levels, which are involved in the and VSOP resonance formation. Proc. of SPIE Vol. 677 6779- Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

same direction with laser radiation propagation k. In Fig.6 it is shown the fluorescence spectrum (upper curve), transmission spectrum (middle curve) of the probe laser when the coupling laser (~ 0.6 mw) is resonant with Fg= Fe= transition, while the probe laser (~0. mw) frequency is scanned across Fg= Fe=,, transitions. The lower curve is the reference transmission spectrum of the nano-cell for L = λ. The MTC thickness L ~ 0 µm. In the upper and middle curves the resonance is seen together with six sub-doppler VSOP resonances marked by the pointing numbers. The resonance, which was formed in the 0 µm-thick cell, has a linewidth ~ 7 MHz. The VSOP s resonance linewidth is times larger. VSOP resonances,, 5 are formed by atoms flying parallel to the MTC s windows. VSOP resonances, are formed by the atoms flying with v 50 m/s in the same direction with laser radiation propagation k ; VSOP resonances 6, 7 (they are present only in the coupling laser spectrum 7 ) are formed by the atoms flying with v 95 m/s in the direction opposite to laser radiation propagation k (see Fig.7). Transmission Fluorescence L=0µm 7 6 5 ω C 5 6 7 ω P L=λ, -' -' 6 MHz 0 -' Frequency Difference Fig.6 The and six VSOP resonances are seen in Fluorescence (upper curve) and transmission (middle curve)spectra. The couple is resonant with Fg= Fe= transition, the probe is scanned across Fg= Fe=,,. The MTC thickness L ~ 0 µm. The lower curve is the reference spectrum. Fig.7 Diagram of relevant levels of 85 Rb. As it is mentioned above, the polarizations of the coupling laser and the probe laser are perpendicular to each other, thus with the help of Glan prism (see Fig.) it is possible to separate either probe spectrum or coupling laser spectrum. This allows one to detect separately the coupling and probe spectra. In Fig.8 the experimental transmission spectra are presented for 85 Rb, D line for the thickness L= 5λ (~ µm). The coupling laser (~mw)) is resonant with the 85 Rb F g = F e = transition, while the probe laser (0.5 mw) is scanned acrossed Fg= Fe =,, transitions (see the left diagram of Fig.5). The upper curve shows transmission spectra when the both coupling and probe spectra are detected ( Proc. of SPIE Vol. 677 6779-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

the curve is mentioned as Couple +Probe ). The curve Only Probe means that the coupling laser has been blocked after after being passedthrough the MTC, thus the process has been realized.. The curve Only Couple means that the probe laser has been blocked after passage through the MTC. The curve Couple is blocked before the cell means that only probe laser spectrum has been detected for the thickness L= 5λ. The lower curve is the reference transmission spectrum of the nano-cell for L = λ. First of all one sees a difference between two lower curves for the case L = 5λ and L = λ : the ratio of the amplitudes of the VSOP peaks for atomic transition Fg= Fe = and Fg= Fe = is greater than two in the case of L = λ and is ~ 0.5 in the case of L = 5λ. This is caused by the fact that atomic transition Fg= Fe = is cycling, while transition Fg= Fe = is non-cycling. Note that the VSOP peak of the increased absorption (the left one on the spectrum) on the curve Only Couple is caused by the probe laser when it is resonant with Fg= Fe = and provides transfer (optical pumping) of the atoms from the ground Fg= to Fg= through the upper level Fe = (see the left diagram of Fig.5). The VSOP peak of the increased absorption is absent when the probe frequency is resonant L= 5λ Couple + Probe Transmission (arb.un.) Only Probe Only Couple Couple blocked before the cell L=λ, -' -' -' -6 0 Frequency difference (MHz) Fig.8 Transmission spectra of 85 Rb, D line, for the thickness L= 5λ (~ µm). The coupling laser is resonant with the 85 Rb F g = F e = transition, the probe is scanned acrossed Fg= Fe =,, transitions (see the text). with F g = Fe =, since transition from Fe = to Fg= is forbidden. Note that the resonance is well seen both in the curves Only Probe and Only Couple. Also, there is some difference in the spectrum of Couple + Probe and only Probe. In the first case the VSOP peaks of increased absorption are larger when the probe frequency is resonant with the atomic transitions Fg= Fe = and Fe =. In Fig.9 the experimental transmission spectra are presented for 85 Rb, D line for the thickness L= λ (~. µm). The coupling laser (~mw) is resonant with the 85 Rb, F g = F e = transition, while the probe laser (0. mw ) is scanned acrossed F g = Fe =,, transitions (see the left diagram of Proc. of SPIE Vol. 677 6779-6 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

Fig.5). The curve Only Probe means that the coupling laser has been blocked after being passed through the MTC, thus L~λ Transmission (arb.un.) Only Probe Couple is Blocked before the cell L=λ, -' -' -' Frequency difference (MHz) -6 0 Fig. 9 Transmission spectra of 85 Rb, D line for the MTC thickness L= λ (~. µm). the process has been already realized in the cell. The resonance with the linewidth ~0 MHz is well seen (compare with the VSOP peak when the couple is blocked). The curve Couple is blocked before the cell means that only probe laser spectrum has been detected for the thickness L= λ. The lower curve is the reference transmission spectrum of the nano-cell for L = λ. The VSOP peak of the increased absorption is well pronounced when the probe linewidth (MHz) 0 8 6 0 0 00 000 Thickness of Column Vapor (µm) Fig. 0. Dependence of the resonance linewidth as a function of the atomic vapor column thickness. Proc. of SPIE Vol. 677 6779-7 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms

frequency is resonant with the atomic transitions Fg= Fe =. Taking into account the results which are presented above as well as the results obtained earlier and presented in 7, 8, 9, we have plotted dependence of the resonance linewidth as a function of the Rb atomic vapor thickness which is shown in Fig.0. Theoretical model which describes well the peculiarities of the and VSOP resonances formation in micrometric thin cell is presented in [9]. ACKNOWLEDGEMENTS The author is grateful to A. Sarkisyan for his valuable participation in fabrication of the MTC as well as to D.Sarkisyan and A.Papoyan for very useful discussions. This work is supported, in part, by the ANSEF Grant PS-nano- 657 and SCOPES Grant IB70-068/. REFERENCES. E. Arimondo, Coherent population trapping in laser spectroscopy, in Progress in Optics edited by E. Wolf (Elsevier, Amsterdam) 5, 57 5 (996).. S. Harris, Electromagnetically induced transparency, Physics Today 50(7) 6 (997).. R. Wynands, A. Nagel, Precision spectroscopy with coherent dark states, Appl. Phys. B: Lasers Opt. 68, 5 (999) and references therein.. D. Petrosyan, Yu. Malakyan, Electromagnetically induced transparency in a thin vapor film, Phys. Rev. A 6, 0580 0588 (000). 5. S. Knappe, L. Hollberg, J. Kitching, Dark-line atomic resonances in submillimeter structures, Opt. Lett. 9, 88 90 (00). 6. K. Fukuda, A. Toriyama, A. Izmailov, M.Tachikawa, Dark resonance of Cs atoms velocity-selected in a thin cell, Appl. Phys..B, 80, 50-509 (005). 7. A.Sargsyan, D.Sarkisyan, A.Papoyan, Dark-line atomic resonances in sub-micron thin Rb vapor layer, Phys. Rev. A 7, 080 080 (006). 8. D. Sarkisyan, D. Bloch, A. Papoyan, M. Ducloy, Sub-Doppler spectroscopy by sub-micron thin Cs- vapor layer Opt. Commun. 00, 0 (00). 9. G. Dutier, A.Yarovitski, S. Saltiel, A. Papoyan, D. Sarkisyan, D. Bloch, M. Ducloy,, Collapse and revival of a Dicke-type coherent narrowing in a sub-micron thick vapor cell transmission spectroscopy Europhys. Lett. 6(),5 (00). 0. D. Sarkisyan, T. Varzhapetyan, A. Sarkisyan, Yu. Malakyan, A. Papoyan, A. Lezama, D. Bloch, M. Ducloy Spectroscopy in an extremely thin vapour cell: comparing the cell length dependence in fluorescence and in absorption techniques, Phys. Rev. A 69, 06580 (00).. D. Sarkisyan, T. Varzhapetyan, A. Papoyan, D. Bloch, M. Ducloy, Absorption and fluorescence in atomic submicron cell: high laser intensity case, Proc. SPIE 657, 6570 65706 (006).. G. Nikogosyan, D. Sarkisyan, Yu. Malakyan, Resonant absorption and fluorescence of atomic layer of the thickness of the order of light wavelength, J. of Opt. Tech. 7, 60 (00).. D. Sarkisyan, A. Papoyan, T. Varzhapetyan, K. Blush, M. Auzinsh, Fluorescence of Rb in a sub-micron vapor cell: spectral resolution of atomic transitions between Zeeman sublevels in moderate magnetic field, JOSA B,, 88-95 (005).. K. L. Corwin, Z. T. Lu, C. F. Hand, R. J. Epstein, C. E. Wieman, Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor, Appl. Opt. 7, 95 98 (998). 5. S. Briaudeau, D. Bloch, M. Ducloy, Sub-Doppler spectroscopy in a thin film of resonant vapor, Phys. Rev. A 59, (999) 7 and references therein. 6. A. Sargsyan, A. Papoyan, D. Sarkisyan, to be published. 7. A. Sargsyan, D. Sarkisyan, D. Staedter, and A. Akulshin, Optics and Spectroscopy 0, 76-768 ( 006) 8. D.Sarkisyan, A.Sargsyan, A.Papoyan, Y.Pashayan-Leroy, Formation of narrow optical resonances using submillimeter and sibmicron-thin atomic vapor layer Proc. SPIE, V. 660, 66005 (007). 9. Y. Pashayan-Leroy, C. Leroy, Sargsyan, A. Papoyan, D. Sarkisyan Electromagnetically induced transparency: the thickness of the vapour column is of order of light wavelength to be published in JOSA B, 007. Proc. of SPIE Vol. 677 6779-8 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 0/0/05 Terms of Use: http://spiedl.org/terms