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arXiv:1412.2349 · Journal of Optics

Field theory of monochromatic optical beams: II. Classical and quantum paraxial fields

Andrea Aiello

Published Nov 27, 2019• 3 citations
arXivHTMLPDF

Abstract

Abstract This work is the second part of an investigation aiming at the study of optical wave equations from a field-theoretic point of view. Here, we study classical and quantum aspects of scalar fields satisfying the paraxial wave equation. First, we determine conservation laws for energy, linear and angular momentum of paraxial fields in a classical context. Then, we proceed with the quantization of the field. Finally, we compare our result with the traditional ones.

DOI
10.1088/2040-8986/ab5c6d

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•Jan 1, 2006•3 citations
Abstract This work is the second part of an investigation aiming at the study of optical wave equations from a field-theoretic point of view. Here, we study classical and quantum aspects of scalar fields satisfying the paraxial wave equation. First, we determine conservation laws for energy, linear and angular momentum of paraxial fields in a classical context. Then, we proceed with the quantization of the field. Finally, we compare our result with the traditional ones.
The scalar wave equation is analyzed in the relativistic front form, appropriate for paraxial-wave optics. The group-theoretical basis for this treatment is uncovered. The formal similarity of the propagation of paraxial beams through optical systems to the quantum mechanics of particles in two dimensions subject to harmonic impulses, and the role of the metaplectic group of Bacry and Cadilhac, are both traced back to the structure of the Poincar\'e group. Light rays are defined in this context as in statistical-wave optics, and the laws for their free propagation as well as transmission through lenses are derived.
A companion paper Beetle and Jongewaard de Boer (The paraxial approximation in quantum optics I: henochromatic modes of a scalar field. J Mod Opt. 2025) has argued that the best way to associate single-particle quantum states of a scalar field to the modes of a narrowly collimated beam of classical radiation modelled in the paraxial approximation uses the ‘henochromatic’ states previously introduced by Sudarshan et al. (Paraxial-wave optics and relativistic front description. II. The vector theory. Phys Rev A. 1983;28:2921–2932. doi: 10.1103/PhysRevA.28.2921, Paraxial-wave optics and relativistic front description. I. The scalar theory. Phys Rev A. 1983;28:2933–2942. doi: 10.1103/PhysRevA.28.2921). This paper extends that result to Maxwell fields, again emphasizing the central role of unitarity in defining the association. The principal new technical element in the present discussion has to do with the intertwining of polarization and spatial degrees of freedom in the resulting single-photon states.
This paper examines how best to associate quantum states of a single particle to different modes of a narrowly collimated beam of classical radiation modeled in the paraxial approximation. Our analysis stresses the importance of the relationship between two inner products naturally arising in the problem. These are the inner product used to expand a general beam as a superposition of orthogonal modes in the paraxial approximation, on the one hand, and the canonical inner product on which the statistical interpretation of quantum (field) theory is founded, on the other. While several candidates for the sort of association between beam modes and single-particle quantum states have been proposed in the literature, here we argue that one of them is uniquely well suited to the task. Specifically, the mapping of beam modes to the ‘henochromatic’ quantum states previously introduced by Sudarshan, Simon and Mukunda [Paraxial-wave optics and relativistic front description. I. The scalar theory. Phys Rev A. 1983;28:2921–2932; Paraxial-wave optics and relativistic front description. II. The vector theory. Phys Rev A. 1983;28:2921–2932.] is unique within a large class of similar mappings in that it is unitary in a mathematically precise sense.
This paper draws a comparison between atom optics and light optics, based on the observation that one can derive a Schrödinger equation for the slowly varying amplitude of the electromagnetic field in the paraxial approximation, the `optical Schrödinger equation'. A correction scheme between (monochromatic) light waves and matter waves is established and applied to cases of interest in atom or fibre optics. First the propagation in a periodic -potential is investigated, where well known effects, such as Bragg scattering or channelling are re-examined from a `matter wave versus light wave' point of view. In a second application the general theory is extended to axisymmetrical materials such as step-index fibres or graded-index fibres. In particular, a matter wave analogue of the so-called non-diffractive beams of electrodynamics is discussed.
The classical concept of a paraxial wave is extended to quantum optics as a condition on the state vector, and the family of vectors obeying this condition defines a paraxial Hilbert space. Field operators acting on this space are approximated by the product of a slowly-varying envelope operator and a classical carrier wave. This permits the discussion of Gaussian pulses and paraxial wave packets. This treatment is extended to obtain an expansion of state vectors and field operators in powers of the opening angle of the cone of paraxial rays. The leading term in the expansion yields the component of the angular momentum along the propagation axis as a sum of well defined orbital- and spin-parts. With the same technique, it is shown that approximate localization of photons is possible for localization volumes much larger that the mean wavelength in the paraxial state.
We introduce a generalized angular spectrum representation for quantized light beams. By using our formalism, we are able to derive simple expressions for the electromagnetic vector potential operator in the case of: a) time-independent paraxial fields, b) time-dependent paraxial fields, and c) non-paraxial fields. For the first case, the well known paraxial results are fully recovered. c ○ 2008 Optical Society of America OCIS codes: 000.1600, 270.0270. Propagation of nonclassical states of the electromagnetic field is an issue of growing interest in quantum optics for both fundamental and technological purposes. 1 Consider, for instance, the relevance of propagation of entangled photons to quantum cryptographic systems. 2 The purpose of the present Letter is to provide a novel perfectly general formalism for the representation of quantized light beams which can be used in any regime of propagation. Such objective is achieved by using a dispersion relation that some of us have recently introduced in Ref., 3 and a generalized angular spectrum representation for the field operators. 4 The usefulness of our approach5 becomes manifest whenever one deals with quantum systems for which both paraxial and nonparaxial regimes of propagation may be relevant as, e.g., down-converted photon pairs. 6 Consider the plane-wave expansion of the positivefrequency part of the electromagnetic vector potential operator Â(r, t) = Â(+) (r, t) + H.c. in the Coulomb gauge7 Â (+) ∫ (r, t) = d 3 (