Physica Scripta ACCEPTED MANUSCRIPT Azimuthal modulation of probe absorption and transfer of optical vortices. To cite this article before publication: Aamen Shujaat et al 2020 Phys. Scr. in press https://doi.org/10.1088/1402-4896/aba2b3 Manuscript version: Accepted Manuscript Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors” This Accepted Manuscript is © 2020 IOP Publishing Ltd. During the embargo period (the 12 month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. 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All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements. This content was downloaded from IP address 144.82.238.225 on 06/07/2020 at 09:33 Page 1 of 5 Azimuthal modulation of probe absorption and transfer of optical vortices. Aamen Shujaat1,2 , Urgunoon Saleem1,2 , Muqaddar Abbas1 ,∗ and Rahmatullah1† 1 Quantum Optics Lab. Department of Physics, COMSATS University, Islamabad, Pakistan and 2 Both authors contributed equally to this work. (Dated: July 2, 2020) I. INTRODUCTION an In recent era, the coherent control of optical properties of a medium has received substantial interest. Coherent interaction of light with multilevel atoms can dramatically modify the systems response to an optical field due to quantum interference between the excitation amplitudes of optical transitions. This leads to an interesting phenomenon known as electromagnetically induced transparency (EIT) in which the absorption of a weak probe field is suppressed via a strong coupling field [1, 2]. EIT has important applications in quantum information processing [3], non-linear optics [4, 5], slow light [6], optical switching and storage of the photons [7–10]. around the propagation axis with an undefined phase at the center known as an optical vortex or phase singularity [14–18]. Recently, the tight focusing properties of a hollow Gaussian beam (HGB) [19], a linearly polarized circular Airy Gaussian vortex beam (CAiGVB) [20] and a radially polarized symmetric Airy beam (RPSAB) [21] have been investigated. Optical beams with OAM also called structured light have widely been explored in optical communications [22, 23], optical trapping [24, 25], optical data storage and quantum technologies [26, 27], quantum information [28] and optical tweezers [29]. us PACS numbers: cri pt We analyze azimuthal modulation of probe field absorption profile in a closed four-level inverted Y-type atomic system. We employ a Laguerre-Gaussian beam and a microwave field to realize spatially varying atomic susceptibility, which is responsible for the generation of structured light. An extra nonvortex control beam is used to ensure electromagnetically induced transparency at the core of Laguerre-Gaussian beam. Our proposed model exhibits both absorption and gain at different azimuthal angles, and orbital angular momentum of light can be identified via absorption profile of the probe field. We further discuss the transfer of optical vortices from Laguerre-Gaussian beam to the transmitted probe field. ce pte dM Light is an electromagnetic wave which can carry linear and angular momentum. Linear momentum of light is always in the direction of propagation and on the other hand, angular momentum of light consists of both spin angular momentum (SAM) and orbital angular momentum (OAM). The SAM is responsible for the circular polarization of light while OAM gives light a helical shape corresponding to the phase lΦ, where Φ is the azimuthal angle with respect to the beam axis and l represents the winding number or the azimuthal quantum number. The most common example of light carrying OAM is Laguerre-Gaussian (LG) beams [11]. When LG-beams interfere, different types of shapes can be appeared in the interfered plane of a vortex beam which depends on winding numbers of both interfering beams, their relative angle, coinciding or not coincident their centers. These patterns can be classified into classes fork-shape, starlike shape, spiral zone plate and also like electric field lines of two separated charges. These patterns are similar to moiré patterns of superimposing two gratings consisting of azimuthal dependent terms in their transmission phase functions [12, 13]. The amplitude is distributed ∗ Electronic address: muqaddarabbas@comsats.edu.pk address: rahmatktk@comsats.edu.pk Ac 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 AUTHOR SUBMITTED MANUSCRIPT - PHYSSCR-110782.R1 † Electronic It is possible to identify the OAM of light in an azimuthally modulating atomic susceptibility where the information about OAM is obtained by mapping the spatially modulating probe absorption profile [30, 31]. Recently, Radwell et al. experimentally demonstrated spatially dependent EIT in cold rubidium atoms driven by the vortex probe light beam [32]. This scheme is further theoretically analyzed using density matrix formalism [33]. As the intensity of a vortex beam is zero at the center of the vortex, therefore, EIT vanishes at core center and an extra control field having no vortex is needed to maintain EIT [34]. Here we are going to mention that several systems have been reported where the vortices present in the control field have been transferred to probe light beam. Such transferred of optical vortices have been observed in atomic systems [35–40], semiconductor quantum dots [41] and semiconductor quantum wells [42, 43]. Motivated by the above studies, we present a scheme for the generation and detection of structured light in a closed four-level inverted Y-type atomic system. We employ LG beam to realize azimuthal phase-dependent variation of the probe field absorption profile. We also consider a nonvortex beam which maintains the EIT at core of the vortex beam, and this feature is missing in Refs [30, 32, 33]. We observe both absorption and gain at different azimuthal angles and also analyze spatially structured transparency. Moreover, our proposal can be implemented to distinguish different modes of LG beam. Due to closed-loop structure, optical vortices can be transferred from control to probe beam. AUTHOR SUBMITTED MANUSCRIPT - PHYSSCR-110782.R1 Page 2 of 5 2 (a) (b) 0.15 ρ̇cc = (γa − γc ) ρcc + iΩ∗c1 ρac + iΩB eiϕ ρbc −iΩ∗B e−iϕ ρcb + iΩc1 ρca , (5) ρ̇dd = iΩc2 ρad − iΩ∗c2 ρda − γd ρdd , (6) 0.10 y/w 0.05 ρ̇ab = (i∆p − γab ) ρab + iΩp (ρbb − ρaa ) + iΩc1 ρcb cri pt 0 +iΩ∗c2 ρdb − iΩB eiϕ ρac , x/w FIG. 1: (a) Schematic diagram of a closed four-level inverted Y-type atomic system. Here, Ωp , Ωc1 , Ωc2 and ΩB are the Rabi frequencies of probe, LG, control and microwave fields respectively. (b) Intensity profile of LG beam for l = 1. +iΩ∗c2 ρdc − Ω∗B e−iϕ ρab , ρ̇da = (i∆c2 − γda ) ρad − iΩ∗p ρdb − iΩ∗c1 ρdc +iΩc2 (ρaa − ρdd ), THEORY AND DISCUSSION A. Model and Equations (7) ρ̇ac = (i∆c1 − γac ) ρac + iΩc1 (ρcc − ρaa ) + iΩp ρbc (8) (9) ρ̇cb = (i(∆p − ∆c1 ) − γcb ) ρbc − iΩp ρca + iΩ∗c1 ρab +iΩB eiϕ (ρbb − ρcc ) , (10) ρ̇dc = (i(∆c1 + ∆c2 ) − γdc ) ρcd − iΩc1 ρda − iΩ∗B e−iϕ ρdb +iΩc2 ρac , (11) ρ̇db = (i(∆p + ∆c2 ) − γdb ) ρdb − iΩp ρda − iΩB eiϕ ρdc +iΩc2 ρab (12) dM an We consider a four-level inverted-Y type atomic system consisting of two excited states |ai and |di and two closely lower lying level |bi and |ci as shown in Fig. 1 (a). A weak probe field Ep drives the atomic transition |bi to |ai and two control fields Ec1 and Ec2 are coupled to atomic transitions |ci ↔ |ai and |ai ↔ |di respectively. A microwave beam EB acts along |bi ↔ |ci transition. We consider Ep , Ec2 and EB are non-vortex beams and Ec1 is a LG beam. Fig. 1 (b) shows the intensity profile of LG beam. The corresponding Hamiltonian for the proposed system under the rotating-wave and dipole approximation is written as; us II. H = −h̄[∆p |ai ha| + (∆p − ∆c1 ) |ci hc| +(∆p + ∆c2 ) |di hd|] − h̄[Ωp |ai hb| +Ωc1 |ai hc| + Ωc2 |di ha| +ΩB eiϕ |ci hb| + H.c.], (1) pte where ϕ = ϕB + ϕc1 − ϕp is the relative phase and ∆p = ωab − υp , ∆c1 = ωac − υc1 and ∆c2 = ωda − υc2 are the detunings of probe (Ep ) and control (Ec1 , Ec2 ) fields. The corresponding Rabi frequencies are defined ℘ E as Ωp = abh̄ p , Ωc1 = ℘ach̄Ec1 , Ωc2 = ℘adh̄Ec2 and ℘bc EB . Here, ℘ab , ℘ac , ℘ad and ℘bc are transiΩB = h̄ tion dipole elements. We utilize the Liouville equation to describe the dynamics of the proposed system ce i 1 ρ̇ = − [H, ρ] − {Γ, ρ} . (2) h̄ 2 The second term represents various incoherent decay processes. Therefore, the equations of motion can be obtained as ρ̇aa = (γd − γa ) ρaa + iΩp ρba − iΩ∗p ρab + iΩc1 ρca −iΩ∗c1 ρac + iΩ∗c2 ρda − iΩc2 ρad , (3) B. Perturbative Analysis of Equations of Motion In this subsection, we solve Eqs. (3-12) under weak probe field approximation where the intensities of the probe and microwave fields are low as compared to the control fields, i.e., ΩB < Ωp < Ωc1 , Ωc2 . The density matrix equations can be solved by expressing them in terms of the perturbation expressions (0) (1) (2) ρij = ρij + λρij + λ2 ρij + ......(i, j = a, b, c, d), (13) (0) (1) where λ represents the interaction order. Here ρij , ρij (2) and ρij are the zeroth, first and second order solutions in Ωp respectively. Under the weak field approximation (1) only the first-order terms ρij are important. In order to (1) obtain ρij , we substitute Eq. (13) into Eqs. (3-12). Our aim is here to study the absorption profile of the probe field corresponding to the atomic transition (1) |ai ←→ |bi and hence we need ρab . We assume that initially all the populations are in ground state |bi, i.e., (0) (0) ρbb = 1 and ρij = 0. After applying the perturbation, Eqs. (3-12) are reduced to the following equations (1) (1) (1) (1) ρ̇ab = (i∆p − γab )ρab + iΩp + iΩc1 ρcb + iΩ∗c2 ρdb , (14) ρ̇bb = (γa + γc ) ρbb + iΩ∗p ρab − iΩp ρba +iΩB eiϕ ρcb − iΩ∗B e−iϕ ρbc , Ac 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (4) (1) (1) (1) ρ̇cb = (i(∆p − ∆c1 ) − γcb )ρcb + iΩ∗c1 ρab + iΩB eiϕ ,(15) Page 3 of 5 AUTHOR SUBMITTED MANUSCRIPT - PHYSSCR-110782.R1 3 (a) (b) (a) -2 -2 (b) x10 x10 1.0 1.0 1.5 1.5 0.5 0.5 1.0 1.0 0.0 0.0 0.5 -1.0 y/w -0.5 0 1 2 3 4 5 6 -1.0 0 F(radian) 1 2 3 4 5 0 6 F(radian) 0.5 y/w 0 -0.5 -0.5 -1.0 -1.0 cri pt -0.5 -1.5 FIG. 2: Absorption profile of probe field as a function of the azimuthal angle Φ for (a) l = 1 and (b) l = 2. The other parameters are γab = γ = 1MHz, γcb = 10−3 γ, γdb = 10−2 γ, Ωc1 = Ωc2 = γ, Ωp = 0.1γ, ΩB = 0.01γ, ϕ = 0 and ∆p = ∆c1 = ∆c2 = 0. x/w (1) (1) (16) We solve the Eqs. (14-16) under steady state condition (1) ρ̇ij = 0 and get required expression for ρab as iΓcb Γdb Ωp (1) ρab = 2 2 Γab Γcb Γdb + Γdb |Ωc1 | + Γcb |Ωc2 | Γdb Ωc1 ΩB eiϕ − 2 2 , (17) Γab Γcb Γdb + Γdb |Ωc1 | + Γcb |Ωc2 | (d) -2 x10 1.5 1.5 1.0 1.0 0.5 y/w 0 0.5 0 -0.5 -0.5 -1.0 -1.0 -1.5 x/w -1.5 x/w FIG. 3: Absorption profile of the probe field as a function of x/w and y/w for (a) l = 1, (b) l = 2, (c) l = 3 and (d) l = 4. We choose ǫc1 = γ. The remaining parameters are the same as those given in Fig. 2. one gain and one absorption peak for l = 1 as shown in Fig. 2 (a). The maxima of the gain and absorption peak lies at Φ = π/2 and Φ = 3π/2 respectively. Fig. 2 (a) also shows zero absorption (EIT) at different Φ, e.g., 0, π and 2π. The absorption profile exhibits two gain and two absorption peaks by increasing the OAM from l = 1 to l = 2, see Fig 2 (b). To study spatially modulating probe field absorption, we consider control field of Rabi frequency Ωc1 as a LG doughnut type beam C. dM an where Γab = γab − i∆p , Γcb = γcb − i(∆p − ∆c1 ) and Γdb = γdb − i(∆p + ∆c2 ). The first term on the right (1) hand of ρab corresponds to standard EIT in a four level inverted Y -type atomic system. The second term arises due to microwave field and related to the gain process. -2 x10 us (1) ρ̇db = (i(∆p + ∆c2 ) − γdb )ρdb + iΩc2 ρab . -1.5 x/w (c) y/w Azimuthal modulation of probe absorption pte The density matrix element ρab is a complex quantity consists of both real and imaginary parts. The real part is related to the dispersion, and imaginary part corresponds to the absorption of the probe field. In this section, we study the absorption profile of the probe field corresponding to the atomic transition |ai ←→ |bi and (1) only analyze the imaginary part of ρab . First, we investigate the effect of the azimuthal angle on absorption profile of probe field. In that case we consider control field Ωc1 carries OAM l and other fields have no vortices. The Rabi frequency Ωc1 is defined as Ωc1 = |Ωc1 | eilΦ , (18) ce where Φ is the azimuthal angle. In Fig. 2, we plot the probe field absorption profile versus azimuthal angle Φ for two different values of OAM, i.e., (a) l = 1 and (b) l = 2. The other parameters are γab = γ = 1MHz, γcb = 10−3 γ, γdb = 10−2 γ, Ωc1 = Ωc2 = γ, Ωp = 0.1γ, ΩB = 0.01γ, ϕ = 0 and ∆p = ∆c1 = (1) ∆c2 = 0. We observe optical transparency (Im[ρab ] = (1) (1) 0), gain (Im[ρab ] < 0) and absorption (Im[ρab ] > 0) at different values of azimuthal angle Φ. There are only Ac 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 r2 r Ωc1 = ǫc1 ( )|l| e− w2 eilΦ , (19) w p where Φ = tan−1 (y/x), r = x2 + y 2 , w is the beam waist parameter and ǫc1 is the strength of the beam. We consider the other fields are independent of azimuthal angle Φ. As the intensity of the LG beam is zero as r → 0 (see Fig. 1(b)), therefore, in a three-level atomic system, EIT vanishes at the center of the LG beam. In order to maintain EIT, we consider an extra non-vortex beam Ωc2 . In the absence of the first control field (Ωc1 = 0), Eq. (17) reduces to (1) ρab = iΓdb Ωp Γab Γdb + |Ωc2 | 2, (20) which represents standard EIT in three-level ladder type atomic system. Therefore, in our proposed model, EIT still maintains even at the core of LG beam. We further proceed to investigate the absorption profile of the probe field for different OAM numbers. Fig. 3 shows the spatially dependent probe absorption profile Page 4 of 5 AUTHOR SUBMITTED MANUSCRIPT - PHYSSCR-110782.R1 4 Transfer of optical vortices ∂Ωp (z) αp γab (1) =i ρ , ∂z 2L ab (22) dM iκΩc1 ΩB eiϕ 1 − e−βz , β here Ωp (0) is the probe field at the entrance of the cell and αp γab Γcb Γdb , 2L Γab Γcb Γdb + Γdb |Ωc1 |2 + Γcb |Ωc2 |2 (23) κ= Γdb αp γab . 2L Γab Γcb Γdb + Γdb |Ωc1 |2 + Γcb |Ωc2 |2 (24) pte β= y/w -2 x10 -2 1.0 1.0 0.5 0.5 y/w 0 -0.5 -0.5 -1.0 -1.0 ce 0 y/w -1 -1 -2 -2 x/w -3 x/w (c) (d) 3 3 2 2 1 1 0 y/w 0 -1 -1 -2 -2 -3 -3 x/w FIG. 5: Phase profile of the probe field as a function of x/w and y/w for (a) l = 1, (b) l = 2, (c) l = 3 and l = 4. Here we consider αp = 100. The remaining parameters are the same as those given in Fig. 3. We consider control field Ωc1 carries the OAM and corresponding Rabi frequency Ωc1 is defined by Eq. (19). The Eq. (22) shows that OAM l of the control field Ωc1 is transferred to the transmitted probe field. Initially at the entrance of the atomic medium, the incoming probe field does not carry any vortex. After traveling some distances, vortex of the control beam shifts to the probe beam due to the closed-loop structure of the atomic system. The presence of the nonvortex beam Ωc2 avoid the losses of the probe field. In Fig. 5, we plot phase profile of the transmitted probe field at z = L for l = 1, 2, 3, 4 and αp = 100. The singularity at the center shows transfer of the optical vortex, and corresponding phases shift from 0 to 2πl. III. CONCLUSION x10 1.5 -1.5 -1.5 x/w (b) 1.5 0 1 an (21) where αp is the optical depth of probe field and L is the length of the atomic vapor cell. With Eq. (17), the transmitted probe field can be expressed as (a) 1 x/w In this section, we demonstrate the transfer of optical vortices from control to probe field. The time independent propagation equation of the probe field under slowly varying envelope approximation can be written as Ωp (z) = Ωp (0)e−βz − 2 -3 y/w 3 2 0 y/w (b) 3 us D. (a) cri pt for (a) l = 1, (b) l = 2, (c) l = 3 and (d) l = 4. We obtain pleasing petal-like structures proportional to 2l. The bright petals indicate to absorption and the dark petals reveal the gain and orange area corresponds to the optical transparency. The above results imply that the behavior of the absorption profile of the probe field depends on the OAM l. The absorption profile displays 2l-symmetry and unknown vorticity of a LG beam can be identified by counting the number of petal structures. Further, we investigate the effect of the relative phase on the probe field absorption spectrum. The petal-like structures rotate clockwise as we change the relative phase, see Fig. 4. The patterns rotate through an angle of 45◦ for ϕ = π/2 as shown in Fig. 4(a). By changing the relative phase from ϕ = π/2 to ϕ = π, absorption in Fig. 3 (b) converts into gain and vice versa as shown in Fig. 4(b). x/w FIG. 4: Absorption profile of the probe field as a function of x/w and y/w for (a) ϕ = π/2 and (b) ϕ = π. Here we consider l = 2. The remaining parameters are same as those given in Fig. 3. Ac 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 In conclusion, we have studied spatially structured transparency in a four-level inverted Y type atomic configuration illuminated by a pair of control fields, a probe and a microwave field. A LG beam is used to periodically modulates the absorption profile in the azimuthal plane. Such a periodic oscillation is the key factor behind the generation of structured light. 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