## Abstract

Acoustical and optical non-diffracting beams are potentially useful for manipulating particles and larger objects. An extended optical theorem for a non-diffracting beam was given recently in the context of acoustics. The theorem relates the extinction by an object to the scattering at the forward direction of the beam’s plane wave components. Here we use this theorem to examine the extinction cross section of a sphere centered on the axis of the beam, with a non-diffracting Bessel beam as an example. The results are applied to recover the axial radiation force and torque on the sphere by the Bessel beam.

© 2013 Optical Society of America

## Corrections

Likun Zhang and Philip L. Marston, "Optical theorem for acoustic non-diffracting beams and application to radiation force and torque: erratum," Biomed. Opt. Express**4**, 2988-2988 (2013)

https://www.osapublishing.org/boe/abstract.cfm?uri=boe-4-12-2988

## 1. Introduction

An idealized non-diffracting (optical or acoustic) beam is a beam whose transverse intensity pattern has the feature of propagation-invariance [1–5]. Beams which are locally approximately non-diffracting are potentially useful for particle manipulation.

One application is the possibility of generating pulling forces. Situations giving pulling forces for spheres in non-diffracting Bessel beams have been computed in acoustics [6–10] and in optics [11–13]. It was noticed early [6–8] that acoustical situations predicting to give negative forces corresponded to a significant reduction of the far-field scattering into the backward hemisphere relative to the forward hemisphere. An analysis of momentum projection and conservation associated with optical far-field scattering [11] motivated an analogous analysis in the acoustical case [10, 14] which shows the relationship between the asymmetry in the scattering and the direction of the radiation force. There have been related theoretical discussions of momentum projection in optics [15] followed by a demonstration of negative optical forces in a beam closely resembling intersecting plane waves [16].

There has been significant recent interest in broader applications of acoustical radiation forces and torques for the manipulation of objects of various sizes [17–22]. Acoustic [23, 24] and optical [25] beams with an extra phase dependence exp(*imϕ*), called vortex beams, have a helicoidal wavefront and carry orbital angular momentum. This feature of angular momentum transport allows the beam to exert a torque to rotate an object. Acoustic vortex beams were analyzed beyond the paraxial approximation in [24] to clarify an analogy with optical vortex beams; the radiation torque on a symmetric object centered on the beam’s axis was related to the absorption of power.

It has long been beneficial to consider possible areas of overlap between some related issues arising in acoustical and optical fields of research [17, 23–27]. In this paper we illustrate the application of an extended optical theorem on acoustic radiation forces and torques associated with a non-diffracting beam. The optical theorem for an incident plane wave is known as one of the central theorems in scattering theory [28–30]; the theorem relates the extinction section to the complex scattering amplitude at the forward direction. An extended theorem for a non-diffracting beam was given recently in the context of acoustics [14]. Here we use this extended optical theorem to examine the extinction cross section on a sphere centered on the axis of a non-diffracting beam, in particular, to examine the extinction for a sphere centered on a Bessel beam as an example. The results, together with a prior result of an asymmetry factor of scattering, are then applied to recover the axial radiation force and torque given in [10, 24].

## 2. Optical theorem of a non-diffracting beam: Review

For a non-diffracting beam (with a speed *c*_{0} in the medium, a
frequency *ω*, and a wavenumber *k* =
*ω/c*_{0}) propagating along the *z* axis with an
axial wavenumber *κ* = *k* cos
*β*,

*A*(

_{s}*θ*,

*ϕ*), an extended optical theorem for the extinction cross section, denoted by

*σ*

_{ext}, was derived in the context of acoustics by [14] as an azimuthal angle integration

*β*,

*ϕ*) by using an angular function

*g*(

*ϕ*) of the beam. This angular function

*g*(

*ϕ*) relates to the beam’s profile through a reduced Whittaker integral

*g*(

*ϕ*), and whose wave-vectors

*β*relative to the beam’s axis (refer to Fig. 1). The scattering in the theorem (3),

*A*(

_{s}**n**(

*β*,

*ϕ*)), is the scattering at the forward direction of the beam’s plane wave component at the azimuthal angle

*ϕ*.

The extinction results from both scattering and absorption. One can write *σ*_{ext} = *σ*_{sca} + *σ*_{abs}, with *σ*_{sca} and *σ*_{abs} denoting the cross sections of scattering and absorption, respectively. The scattering cross section, *σ*_{sca}, is given by the integral of the scattering coefficient over the whole solid angle element *d*Ω = sin*θdθdϕ* as,

On taking the limits, *β* = 0 and *g*(*ϕ*) = 1, our extended theorem (3) reduces to the well-known theorem for a plane wave [28–30],

*A*(0, 0) is the forward scattering of the incident plane wave

_{s}*ψ*=

_{i}*ψ*

_{0}exp(

*ikz*−

*iωt*).

## 3. Application to extinction by a sphere

Here we use the extended theorem (3) to examine the extinction cross section of a sphere centered on an idealized Bessel beam of arbitrary order *m*,

*m*is an integer with

*m*= 0 as a special case giving the ordinary Bessel beam. The angular function of this beam is [10, 14]

For a sphere of radius *a* centered on the axis of the beam (8), the far-field scattering was given in [10] in terms of a partial wave expansion as

*s*in the partial wave coefficients (

_{n}*s*− 1)/2 are the same as that for a plane wave scattering [2]. These coefficients are known for different types of objects, as determined by boundary conditions. Notice that |

_{n}*s*| ≤ 1 and only for non-absorptive scattering are all coefficients |

_{n}*s*| equal to unity.

_{n}By using (9) and (10), and letting *θ* = *β*, it immediately follows from the optical theorem (3) that

*s*| ≤ 1, one always has Re[2(1 −

_{n}*s*)] ≥ 0, and hence

_{n}*σ*

_{ext}≥ 0 as required; only in the limit of no scattering,

*s*= 1, does no extinction

_{n}*σ*

_{ext}= 0 occur. The cross section has the dimension of area. For an object being a sphere of radius

*a*, the dimensionless efficiency factor becomes,

*s*− 1)/2 =

_{n}*α*+

_{n}*iβ*, with

_{n}*α*and

_{n}*β*being the real and imaginary parts).

_{n}The separation of the extinction (12) into the scattering and the absorption is straightforward. One may rewrite the coefficients Re[2(1 − *s _{n}*)] as

*s*− 1|

_{n}^{2}, associated with the modula of scattering coefficients, corresponds to the contribution from the scattering. The second part, 1 − |

*s*|

_{n}^{2}, associated with the difference of |

*s*|

_{n}^{2}from unity, corresponds to the contribution from the absorption. Hence it follows from (12) and (13) that

In the special case of non-absorptive scattering (|*s _{n}*| = 1),

*Q*

_{abs}=

*σ*

_{abs}/(

*πa*

^{2}) = 0. In this case using the phase shifts

*δ*with

_{n}Lastly, the corresponding powers are given as

*I*

_{0}, a function of the amplitude

*ψ*

_{0}in (8), characterizes the intensity of the incident beam. In the context of acoustics, if

*ψ*represents the velocity potential field, one has the acoustic intensity

*I*

_{0}= (

*ρ*/2)(

_{o}c_{o}*kψ*

_{0})

^{2}.

## 4. Application to the axial radiation force on a sphere

By the axial projection of momentum, the axial radiation force exerted by the general nondiffracting beam (4) on an object of arbitrary shape and location was given in [14] as

*Y*as

_{p}*β*and 〈cos

*θ*〉

*, correspond to the axial projections of extracted momentum and scattered momentum [10, 11, 14], respectively, and*

_{s}The factor 〈cos *θ*〉* _{s}* is an asymmetry parameter of the scattering: 〈cos

*θ*〉

*is positive or negative when the scattering at the forward hemisphere is stronger or weaker relative to the scattering at the backward hemisphere. An inspection of (20) shows why even in the idealized case of negligible absorption, conditions to achieve negative forces usually require that the conic angle*

_{s}*β*be large: from the form of (20) the asymmetry must lie between −1 and 1. For a Bessel beam with

*m*= 0 the plane wave limit is recovered by taking

*β*= 0. It follows from the form of (20) that the force must be non-negative in that limit with or without absorption. By taking

*Q*

_{ext}=

*Q*

_{sca}+

*Q*

_{abs}, an implicit term in (20) associated with absorption,

*Q*

_{abs}cos

*β*, degrades the negative force [10].

The cross section and efficiency factor associated with the scattering asymmetry factor 〈cos *θ*〉* _{s}* in (21) for a sphere of radius

*a*are

*Q*

_{asym.sca}, can be read from [10] (

*Q*

_{asym.sca}= −

*Y*

_{1}therein) as

## 5. Application to the radiation torque on a sphere

In an axisymmetric field (traveling or standing waves) with an azimuthal phase dependence
exp(*imϕ*), where the integer *m* is the topological charge,
the axial radiation torque on an axisymmetric object centered on the axis of the field was revealed
in [24] to be associated with the absorption
of energy as

*Q*for a sphere of a radius

_{T}*a*as,

*m*, using the absorption (15) from the optical theorem, it has

In [24], Eq. (25) was generalized to the torque on any axisymmetric object centered in any vortex wave field (traveling or standing waves; where the derivation started from the conservation of of angular momentum [34]). Experiments in [27] confirm (25) for a disk shaped object in a vortex wave field having an adjustable topological charge *m*. For most cases series expansions giving the absorption, like our (15), are not currently available. While some related work has appeared [35, 36], the reader is cautioned the left side of Eq. (18) in [35] is incorrect. Other corrections to [35] were given in an erratum published by Mitri et al. late in 2012. Notice also that since the analysis in [24] included the standing wave case, (25) applies to the standing wave case examined in [35].

## 6. Conclusions and discussion

The applications of the extended acoustic extinction theorem (3) illustrated here concern a sphere placed on the axis of an idealized Bessel beam of arbitrary integer order *m*. In that case the resulting predictions for the extinction cross section and associated efficiency factor (12) agree with results derived a different way in [10]. Also the normalized radiation force (24) agrees with the result from analytical integration of the axial radiation stress projection in [10] (equivalent to prior results in [6–9]). The torque and absorption efficiencies in (27) agree with the results from analytical integration of angular momentum and energy flux in [10, 24].

The extinction theorem (3) and the associated axial stress projection relation (19) involving the scattering asymmetry (21) neither require the object to be a sphere nor to be centered on the axis [14]. In that case, however, more complicated approaches are needed for evaluating the scattering amplitude. In the case of a sphere some computational approaches have been described by other researchers for various choices of beam types [20, 22, 37].

There is an additional complication in the case of small spheres in thermal viscous fluids in that highly intense sound waves can establish a steady flow pattern commonly referred to as acoustic streaming [38] not allowed for in the present analysis since all of the flow induced by the acoustic wave is assumed to oscillate at the frequency *ω* according to (1) and (2). The magnitude and flow pattern of acoustic streaming tends to be somewhat dependent on the transducer geometry and apparatus boundary conditions [38]. Methods have been introduced to reduce the effects of streaming yielding agreement between measured and computed forces for objects in traveling waves [39, 40].

An additional complication resulting from thermal-viscous dissipation in the surrounding fluid are the contributions to the effective absorption efficiency (15) and correspondingly the torque efficiency (27) from certain of the partial waves. For example, in the case of a small solid sphere when *ka* ≲ 0.5 the viscous correction to the dipole scattering can become important when the average density of the sphere differs from that of the surrounding fluid. In the usual case in which the thickness of the oscillating viscous boundary layer is small relative to the radius of the sphere, a simple approximation is available for estimating the viscous correction to *s*_{1}[41]. For solid spheres having small intrinsic absorption, this correction tends to decrease in significance the larger the radius of the sphere [41].

A situation where absorption (either by the sphere or in the adjacent boundary layer) can be beneficial concerns the induced rotation of a sphere on the axis of an acoustic vortex Bessel beam. That is because the radiation torque applied to the sphere is proportional to the absorbed power [23,24] as in the case of a sphere made of a lossy dielectric placed in circularly polarized light [26]. If it is desirable to activate the absorption associated with the aforementioned viscous correction to the dipole term *s*_{1}, it is necessary to use a beam having *m* = 1 [41]. When the effect of the absorption of angular momentum by the surrounding fluid is negligible, the rotation rate for the sphere may be estimated by balancing the radiation torque with the viscous drag torque [41] as in the electromagnetic case [26].

## Acknowledgments

Zhang acknowledges the support from NASA and ONR, and Marston acknowledges support from ONR.

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