Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet
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Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet Ningning Song, Renxian Li, Han Sun, Jiaming Zhang, Bojian Wei, Shu Zhang, F.G. Mitri PII: DOI: Reference:
S0022-4073(19)30801-5 https://doi.org/10.1016/j.jqsrt.2020.106853 JQSRT 106853
To appear in:
Journal of Quantitative Spectroscopy & Radiative Transfer
Received date: Revised date: Accepted date:
27 October 2019 22 January 2020 23 January 2020
Please cite this article as: Ningning Song, Renxian Li, Han Sun, Jiaming Zhang, Bojian Wei, Shu Zhang, F.G. Mitri, Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet, Journal of Quantitative Spectroscopy & Radiative Transfer (2020), doi: https://doi.org/10.1016/j.jqsrt.2020.106853
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Highlights
Optical force on a sphere of arbitrary size illuminated by an Airy lightsheet is investigated using GLMT.
The BSCs of the Airy light-sheet are calculated using ASDM and VSWFs.
The negative longitudinal optical force is particularly emphasized
The two kinds of polarizations (TE and TM) of Airy light-sheet are discussed.
The influences of the transverse scale parameter ω 0 and attenuation parameter γ of the Airy light-sheet are discussed.
Optical radiation force on a dielectric sphere of arbitrary size illuminated by a linearly polarized Airy light-sheet Ningning Songa , Renxian Lia,b,∗, Han Suna , Jiaming Zhanga , Bojian Weia , Shu Zhanga , F. G. Mitric a School b Collaborative
of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China Innovation Center of Information Sensing and Understanding, Xidian University, Xi’an 710071, China c Santa Fe, NM 87508, USA
Abstract Based on the generalized Lorenz-Mie theory (GLMT) and the angular spectrum decomposition method (ASDM), we calculate the optical radiation force exerted on a lossless dielectric sphere of arbitrary size illuminated by an Airy light-sheet. The beam shape coefficients (BSCs) of the Airy light-sheet are calculated using the vector angular spectrum decomposition and vector spherical wave functions methods. The optical radiation force acting on the spherical particle is obtained by the integral of Maxwell’s stress tensor. The transverse (Fy ) and longitudinal (Fz ) forces are numerically computed. Two kinds of polarization (TE and TM) are considered for the Airy light-sheet, and the negative longitudinal optical (pulling) force is particularly emphasized. The influence of the transverse scale parameter w0 and attenuation parameter γ of the Airy light-sheet on the force is discussed. The results of the present theory are verified using the dipole approximation method in which the gradient force has been also computed for a Rayleigh sphere. The numerical results show that when the transverse scale parameter w0 and attenuation parameter γ increase, the transverse and longitudinal forces decrease. Furthermore, the force caustic (i.e., maximum) shifts to the direction of y < 0 as the transverse scale parameter w0 increases. As the dimensionless size parameter of the sphere ka increases (where k is the wavenumber and a is the radius), the resonance peaks of the optical forces become larger. The results of this paper are of practical significance for the development of Airy light-sheet based optical manipulation technologies. Keywords: Optical force, Airy light-sheet, GLMT, optical tweezers, TE and TM polarization
1. Introduction The development of optical manipulation technologies include optical trapping and immobilization, optical binding and attraction/repulsion, rotation and particle transport. Among such different methods, light trapping is used most widely, namely by the means of optical tweezers. In 1966, the angular stabilization of dust particles in a He-Ne laser cavity was observed, suggesting that the term ”photodynamic stability” is appropriate [1]. Subsequently, it was verified that photophoresis is the source of driving forces and stabilizing torques [2]. In 1970, dielectric particles were manipulated in water with two focusing laser beams propagating in opposite directions, proving that there is indeed a force exerted from the laser on the particles [3]. In 1986, the stable capture of dielectric particles in water by using a single beam of strongly focused laser was achieved, by a gradient force trap, marking the birth of optical tweezers technology [4]. Optical tweezers technology is a invention of great significance, which enriches and promotes the development of the optical field further, making optics one of the most cutting-edge and most potential scientific fields in the 21st century. The advantage of optical tweezers is the contact free mode, high precision and the ability to measure the magnitude of forces in a scale of 10−10 N. It is widely used in manipulation of particles ranging ∗ Corresponding author at: School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China. E-mail address:
[email protected] (R. Li).
Preprint submitted to Journal of Quantitative Spectroscopy and Radiative Transfer
January 30, 2020
from atomic size to hundreds of micrometers order. It has been applied in cell biology, single molecular biology, soft material colloid science, physics and other fields [5–16]. The theoretical basis of optical manipulation is to study the interaction between different specific light fields and particles by various calculation methods. Traditional optical tweezers used the focused Gaussian beam as a specific structured light field [17–20]. On the other hand, laser-sheets (i.e. light-sheets ) [21–26] have been developing in recent years, which have important applications including particle manipulation [27, 28], evaluation of a particle sizing [29], laser microsurgery [30], light-sheet microscopy [31], fluorescence microscopy [32], optical tomography sectioning [33, 34], flow visualization [35], etc, where Hermite-Gaussian light-sheet [27], Bessel light-sheet [36], Bessel-Gauss light-sheet [37], bottle light-sheets [38] and Airy lightsheet [28, 31] are suggested. The optical radiation force (particularly negative optical radiation force) and torques are mainly computed in the previous analysis. Research shows that a plasmonic layer of optimal thickness coating a small sphere can boost the longitudinal and transverse force components as well as the axial torque induced by Bessel pincers light-sheets [36]. The generation of negative optical forces and torques depend on the beam parameters, derivative order of the beam, the polarization of the electric/magnetic vector potentials, the orientation of subwavelength spheroid in space and its aspect radio [37, 39]. Negative or positive axial torques can arise depending on the choice of the size and the location of the particle in the field [27]. The Airy light-sheet is a kind of non-diffracting beam, which has the characteristics of self-healing [40], non-diffraction [41], and self-acceleration [42, 43], providing a possibility for the special and complex manipulation of particles. These intrinsic features have made the Airy light-sheet the focus of several works in the interaction of light-sheet with particles. The subwavelength sphere [28, 39] and non-spherical particle [44] in the fields of Airy light-sheet have been studied. The purpose of this investigation is to extend the scope of the previous studies for the case of a lossless dielectric sphere of arbitrary size illuminated by an Airy light-sheet, using the GLMT to investigate the optical radiation force (particularly the generation of a negative longitudinal optical force), including the transverse (Fy ) and longitudinal (Fz ) forces, with emphasis on two kinds of polarization (TE and TM). The influences of transverse scale parameter w0 and attenuation parameter γ of the Airy light-sheet are also examined. The rest of this paper is organized as follows. The beam shape coefficients (BCSs) for the Airy light-sheet with TE and TM polarizations are derived using the angular spectrum decomposition method (ASDM) in section 2. The general expressions for the components of the optical radiation force exerted on a sphere of arbitrary size/radius involving the BSCs of an Airy light-sheet are provided. Section 3 discusses numerical results of the optical force exerted on a sphere of arbitrary size, with particular emphasis on polarization, the transverse scale parameter w0 and attenuation factor γ of the polarized Airy light-sheet. The negative longitudinal optical forces are emphasized. Finally, a conclusion of the present work is given in Section 4. 2. Theory Consider a polarized Airy light-sheet propagating toward the half free space z > z0 , with the z−axis taken to be the axis of wave propagation, and focused on point (y0 , z0 ) (Fig.1). When the polarization is TE, the incident electric field is polarized along the x−direction, propagating in the yz plane. Thus, we have Eiy = Eiz = Hix = 0. On the other hand, when the polarization is TM, its incident magnetic field is polarized along the x−direction. Thus, we have Hiy = Hiz = Eix = 0. The incident electric field of an Airy light-sheet can be expressed using the ASDM [41, 45–48]. 2.1. Airy light-sheet Assuming a TE-polarized Airy light-sheet, the only non-zero component of the incident electric field propagating along the z-axis can be expanded into an angular spectrum of plane waves as [45] Z +∞ Z +∞ i[ky y+kz (z−z0 )] ik[qy+p(z−z0 )] Eix (y, z) = E0 AE dky = E0 k AE dq (1) x (ky ; z0 )e x (q)e −∞
−∞
1 2
where q = sin α sin β, p = cos α, k = (µ) ω/c, µ and ε are the permeability and permittivity of the surrounding medium, respectively. The parameters (ky , kz ) are the transverse and axial components of the 2
Figure 1: Definition of k, r, and the corresponding angles
Figure 2: The graphical representation of the interaction of an Airy light-sheet with a sphere of arbitrary size located arbitrarily in space
wave vector (Fig.2), ky = kq = k sin α sin β, and kz = kp = k cos α. AE x (q) is the angular spectrum, given by the Fourier transform of the initial electric field in the z = z0 plane. Assuming TE-polarization, the incident electric field vector of the Airy light-sheet is described as [41, 46, 47] y − y0 γ(y − y0 ) Ei (y, z0 ) = ex Eix (y, z0 ) = ex Ai exp (2) w0 w0 where w0 is the transverse scale, Ai(·) is the Airy function, (y0 , z0 ) are the coordinates of a point in the transverse plane (yz) of the incident field, and γ is the attenuation parameter. Using the Fourier transformation, the angular spectrum at the source can be obtained as Z +∞ y − y0 1 γ(y − y0 ) −ikqy AE (q) = e Ai exp e dy x x 2π −∞ w0 w0 (γ − ikw0 q)3 −iky y0 w0 exp e (3) = ex 2π 3 Considering Eqs. (1) and (3) and ignoring the contribution of evanescent waves as they decay away from the source and do not contribute to the radiated field, the incident electric field vector is expressed as Z +∞ ik[qy+p(z−z0 )] Ei (y, z) = E0 k e x AE dq x (q)e −∞
=
E0 k
Z
0
π 2
ik·r −ikpz0 ex AE e cos αdα x (α)e
(4)
The term ex eik·r can be expanded using vector spherical wave functions as[49] ex eik·r =
∞ X n X
n=1 m=−n
where Dmn = p0mn = −in+1 e−ipβ
h i 0 (1) Dmn p0mn N(1) mn + qmn Mmn
(5)
(2n + 1)(n − m)! n(n + 1)(n + m)!
(6)
τmn (cos α)eθ (α) − iπmn (cos α)eφ (α) 3
· ex
(7)
0 qmn = −in+1 e−ipβ
πmn (cos α)eθ (α) − iτmn (cos α)eφ (α)
· ex
M(1) mn = [iπmn (cos θ)eθ − τmn (cos θ)eφ ] jn (kr) exp(imφ)
(8) (9)
1 d [rjn (kr)] exp(imφ) kr dr jn (kr) +er n(n + 1)Pnm (cos θ) exp(imφ) kr
N(1) mn = [τmn (cos θ)eθ + iπmn (cos θ)eφ ]
(10)
with
Pnm (cos α) (11) sin α dPnm (cos α) τmn (cos α) = (12) dα where jn (kr) is the spherical Bessel function of the first kind, Pnm (cos α) represents the associated Legendre functions of degree n and order m, and e(r,θ,φ) are the radial, polar, and azimuthal unit vectors. Substituting Eq.(5) into Eq.(4), the incident electric field of the Airy light-sheet is expanded in terms of vector spherical wave functions (VSWFs) as πmn (cos α) = m
Ei (r, θ, φ) = −
n ∞ X X
n=1 m=−n
h i (1) pol (1) iEmn ppol m,n Nmn + qm,n Mmn
(13)
where the superscript pol corresponds to the type of polarization, i.e., T E or T M , respectively, and Emn is p (14) Emn = E0 in Dmn pol The following coefficients ppol m,n and qm,n are obtained as[50]
ppol mn =
i1−n p kr Dmn 4πE0 jn (kr)
pol qmn =−
Z
kr i−n Z p Dmn 4πE0 jn (kr)
π
θ=0
Z
Z
π
θ=0
2π
φ=0
Z
[er · Ei (r, θ, φ)] Pnm (cos θ)e−imφ sin θdθdφ
(15)
[er · Hi (r, θ, φ)] Pnm (cos θ)e−imφ sin θdθdφ
(16)
2π
φ=0
p where Z = µ/ε is the impedance of the medium of wave propagation surrounding the sphere. For a pol TE-polarized field, the coefficients ppol m,n and qm,n become E pTm,n
TE qm,n
√
Z π2 Dmn −ikpz0 = k n+1 p0mn AE cos αdα x (α)e −i 0 √ Z π2 Dmn 0 −ikpz0 = k n+1 qmn AE cos αdα x (α)e −i 0
(17)
(18)
Since the electric field of the Airy light-sheet is polarized along the x−axis, ex = −eφ (α), and the E TE expressions for pTm,n and qm,n become p0mn
−in+1 e−ipβ
τmn (cos α)eθ (α) − iπmn (cos α)eφ (α)
πmn (cos α)
· ex
= −in+1 e−ipβ
πmn (cos α)eθ (α) − iτmn (cos α)eφ (α)
· ex
= =
0 qmn
n −ipβ
i e
n −ipβ
= i e
τmn (cos α)
4
(19)
(20)
The substitution of Eqs. (19) and (20) into Eqs. (17) and (18) leads to E pTm,n = ik
TE qm,n
Z p Dmn
= ik
π 2
0
p
Dmn
Z
−ikpz0 e−ipβ πmn (cos α)AE cos αdα x (α)e
(21)
−ikpz0 cos αdα e−ipβ τmn (cos α)AE x (α)e
(22)
π 2
0
E TE The final expressions of pTm,n and qm,n are given as E pTm,n = ik
Z p π Dmn e−ip 2
TE qm,n = ik
π 2
0
p
π
Dmn e−ip 2
Z
π 2
0
−ikpz0 cos αdα πmn (cos α)AE x (α)e
(23)
−ikpz0 cos αdα τmn (cos α)AE x (α)e
(24)
For a TM-polarized Airy light-sheet, the direction of propagation of the incident electric field is the z−axis, and its incident magnetic field is polarized along the x−direction. Based on Maxwells equations M TM and the duality principle, we acquire the expressions of pTm,n and qm,n as M pTm,n = −k TM qm,n = −k
Z p π Dmn e−ip 2
π 2
0
p
π
Dmn e−ip 2
Z
0
π 2
−ikpz0 τmn (cos α)AH cos αdα x (α)e
(25)
−ikpz0 πmn (cos α)AH cos αdα x (α)e
(26)
where AH x (α) is the angular spectrum of the magnetic field polarized along the x-axis of the TM-polarixed Airy light-sheet. 2.2. Optical Force In GLMT, the incident, internal and scattered electromagnetic fields can be expended, respectively, using VSWFs as n ∞ X h i X (1) pol (1) iEmn ppol (27) Ei (r, θ, φ) = − m,n Nmn (kr) + qm,n Mmn (kr) n=1 m=−n
Hi (r, θ, φ) =
∞ n h i ik X X pol pol (1) iEmn qm,n N(1) mn (kr) + pm,n Mmn (kr) ωµ n=1 m=−n
El (r, θ, φ) = − Hl (r, θ, φ) =
∞ X n X
n=1 m=−n
h i (1) pol (1) iEmn dpol m,n Nmn (mkr) + cm,n Mmn (mkr)
∞ n h i imk X X (1) pol (1) iEmn cpol m,n Nmn (mkr) + dm,n Mmn (mkr) ωµ n=1 m=−n
Es (r, θ, φ) =
∞ X n X
n=1 m=−n
Hs (r, θ, φ) = −
h i (3) pol (3) iEmn apol m,n Nmn (kr) + bm,n Mmn (kr)
∞ n h i ik X X (3) pol (3) iEmn bpol m,n Nmn (kr) + am,n Mmn (kr) ωµ n=1 m=−n
(28)
(29)
(30)
(31)
(32) (3)
where m in Eqs.(29) and (30) is the index of refraction of the sphere. The VSWFs of the third kind (Nmn (3) (1) (1) and Mmn ) are described by replacing jn (kr) by hn (kr) in Eqs.(9) and (10), where hn (kr) is the spherical 5
pol Hankel function of the first kind. The expansion coefficients of the internal and scattered field apol m,n , bm,n , pol pol cm,n and dm,n are obtained via the Mie coefficients an , bn , cn and dn [51, 52], namely pol apol m,n = an pm,n ,
pol bpol m,n = bn qm,n
pol cpol m,n = cn qm,n ,
pol dpol m,n = dn pm,n
(33)
According to GLMT, the optical radiation force acting on the sphere can be obtained by integrating the time-average Maxwell’s stress tensor as [53] I D↔E b · T dS F= n (34) S
bEis the unit normal vector on a surface enclosing the sphere, dS is the differential surface element, where D ↔n and T is the time-averaged Maxwell stress tensor, defined as [53] D↔E 1 ↔ 1 T = Re εE ⊗ E∗ + µH ⊗ H∗ − (εE · E∗ + µH · H∗ ) I 2 2
(35) ↔
where the symbol ⊗ denotes a tensor product, the superscript ∗ denotes a complex conjugate and I is the unit tensor. E and H are the sum of incident and scattered fields, respectively. Considering the lossless background medium, the optical force vector components (Fx ,Fy ,Fz ) can be expressed by substituting the total (incident + scattered) electromagnetic field into Eq.(35) as shown in [54]. Therefore, the expressions for the force components where Re and Im denote, respectively, the real and imaginary parts of a complex number, become, Fx = Re [F1 ] , Fy = Im [F1 ] , Fz = Re [F2 ] (36) where ! ( 1 ∞ X n X ˜∗pol ˜pol ˜∗pol 2πε [(n − m)(n + m + 1)] 2 a ˜pol 2 m,n bm+1,n + bm,n a m+1,n F1 = 2 |E0 | × ∗pol pol ∗pol k n(n + 1) p˜m+1,n ˜m+1,n − q˜m,n −˜ ppol m,n q n=1 m=−n " # 12 ! ˜pol ˜∗pol n(n + 2)(n + m + 1)(n + m + 2) a ˜pol ˜∗pol m,n a m+1,n+1 + bm,n bm+1,n+1 − × 2 pol ∗pol −˜ ppol ˜∗pol ˜m,n q˜m+1,n+1 (n + 1) (2n + 1) (2n + 3) m,n p m+1,n+1 − q # 12 " ! ∗pol ˜bpol ˜b∗pol n(n + 2)(n − m)(n − m + 1) a ˜pol a ˜ + m,n+1 m+1,n m,n+1 m+1,n × + pol ∗pol pol ∗pol 2 −˜ pm,n+1 p˜m+1,n − q˜m,n+1 q˜m+1,n (n + 1) (2n + 1) (2n + 3) ∞ X n X m 4πε 2 pol ˜∗pol pol ∗pol |E | × a ˜ b − p ˜ q ˜ 0 m,n m,n m,n m,n k2 n(n + 1) n=1 m=−n " ! # 12 ∗pol ˜bpol ˜b∗pol n(n + 2)(n − m + 1)(n + m + 1) a ˜pol a ˜ + m,n m,n+1 m,n m,n+1 + × 2 pol ∗pol pol ∗pol −˜ pm,n p˜m,n+1 − q˜m,n q˜m,n+1 (n + 1) (2n + 1) (2n + 3)
(37)
F2 = −
and
1 pol pol a ˜pol m,n = am,n − pm,n , 2 ˜bpol = bpol − 1 q pol , m,n m,n 2 m,n 6
1 pol p 2 m,n 1 pol = qm,n 2
p˜pol m,n = pol q˜m,n
(38)
(39)
3. Numerical results and discussions Initially, in the first subsection, the theory and numerical code are verified. We first verify the BSCs by comparing the original field to the reconstructed field. Also, the differential scattering cross section (DSCS) calculated using this theory is compared to a previous result available in the literature [55]. In addition, the optical forces calculated using the dipole approximation method and the gradient force for a Rayleigh sphere are compared to verify our theory and code. In the second subsection, the optical radiation force components are evaluated based on the GLMT for a spherical particle with different radii. 3.1. Verification and validation 2 U L J L Q D O
2 U L J L Q D O
5 H F R Q V W U X F W H G
0
_ (i ( _
0
_ (i ( _
2
2
5 H F R Q V W U X F W H G
í
í
í
í
í
\ μ P
í
í
í
\ μ P
(a) TE-polarized
(b) TM-polarized
Figure 3: Comparision of the original incident electric field intensity and its reconstructed counterpart 2
Initially, a comparison of the original relative incident electric field intensity |Ei /E0 | and its reconstructed form using the BSCs is performed, based on Eqs.(13), and (23)-(26). The original field is expressed by Eqs.(3) and (4). The wavelength of the incident Airy light-sheet is λ = 632.8 nm, and its amplitude is E0 = 1 × 106 V/m, y varies in the range −20 6 y 6 20 µm, and z = 10 µm. The TE and TM-polarized incident fields are considered. The transverse scale parameter of the Airy light-sheet is w0 =1.1 µm, and the attenuation parameter is γ=0.1. Fig. 3 displays the results, where a complete agreement is observed between the original and reconstructed data. Then, the differential scattering cross section is calculated. According to Refs. [55] and [56], the wavelength of the Airy light-sheet is λ = 633 nm, the radius of the sphere is 1 µm, and the scattering angle varies in the range 0 6 θ 6 180 ◦ . Results in Fig. 4 agree with the plots shown in panel (b) of Fig. 4 previously presented in [55] . Finally, the optical forces calculated using the exact GLMT are compared to the results obtained by two theoretical models for a relatively small particle. In the first theoretical model, the optical radiation force using the dipole approximation method is expressed as [57] 3 X 1 Fζ = εRe αEij ∇ζ Eij ∗ (40) 2 j=1 where ε is the permittivity of the medium of wave motion. ∇ζ ≡ ∂r∂ζ , ζ and j denote the three different directions, and Fζ is the ζ-th component of the optical radiation force vector, Eij is the j-th component of the incident electric vector field, where Ei means the incident electric field, and α is the polarizability of the object given by [57] α0 α= (41) 3 1 − 32 k4πα0 with
α0 = 4πa3 7
m2 − 1 m2 + 2
(42)
ka=5 ka=10
' 6 &