Hypergeometric-Gaussian Modes

Hypergeometric-Gaussian Modes

Ebrahim Karimi, Gianluigi Zito, Bruno Piccirillo, Lorenzo Marrucci, and Enrico Santamato Dipartimento di Scienze Fisiche Università degli Studi di Napoli “Federico II”, Complesso di Monte S. Angelo,
80126 via Cintia Napoli, Italy
Consiglio Nazionale delle Ricerche-INFM Coherentia, Napoli, Italy
Corresponding author: enrico.santamato@na.infn.it
Abstract

We studied a novel family of paraxial laser beams forming an overcomplete yet nonorthogonal set of modes. These modes have a singular phase profile and are eigenfunctions of the photon orbital angular momentum. The intensity profile is characterized by a single brilliant ring with the singularity at its center, where the field amplitude vanishes. The complex amplitude is proportional to the degenerate (confluent) hypergeometric function, and therefore we term such beams hypergeometric gaussian (HyGG) modes. Unlike the recently introduced hypergeometric modes (Opt. Lett. 32, 742 (2007)), the HyGG modes carry a finite power and have been generated in this work with a liquid-crystal spatial light modulator. We briefly consider some sub-families of the HyGG modes as the modified Bessel Gaussian modes, the modified exponential Gaussian modes and the modified Laguerre-Gaussian modes.
ocis: 050.1960, 230.6120

In the recent years there has been an increasing interest in laser beams especially tailored to experiments. In particular, experimentalists are looking for laser beams which are either nondiffracting or have a definite value of the photon orbital angular momentum (OAM) along the propagation direction. Such special laser beams have found useful applications in optical trapping, image processing, optical tweezers, metrology, microlithography, medical imaging and surgery, wireless and optical communications he95 (); durnin87 (); salo99 (); durnin87a (); davis00 (). Moreover, beams carrying definite photon OAM present novel internal degrees of freedom that are potentially useful for quantum information applications molinaterriza02 (); molinaterriza07 (). Motivated by these issues, there has been an increasing interest in generating and studying laser beams corresponding to particular solutions of the scalar Helmholtz paraxial wave equation other than the well known Hermite-Gaussian (HG) and the Laguerre-Gaussian (LG) modes. Miller has solved the 3D Helmholtz paraxial wave equation in 17 coordinate systems, 11 of which were based on orthogonal coordinates miller (). Other examples of recently investigated paraxial beams are Parabolic, Mathieu, Ince-Gaussian, Helmholtz-Gaussian, Laplace-Gaussian, and pure light vortices (see kotlyar07 () and references therein).

In this Letter we introduce a novel family of paraxial beams which are solutions of the scalar Helmoltz paraxial wave equation and are also eigenstates of the photon OAM. The field profile of these beams is proportional to the confluent hypergeometric function so we call them Hypergeometric-Gaussian modes (HyGG). Unlike the hypergeometric modes studied in kotlyar07 (), our HyGG modes carry a finite power so that they can be realized in practical experiments.
The field of the HyGG modes is given by

(1)

where is integer, is real valued, is the gamma function and is a confluent hypergeometric function abramowitz (). In Eq. (1) we used dimensionless cylindrical coordinates , , , where is the beam waist and is the beam Rayleigh range. We notice the characteristic factor in the field, so that the integer values of are identified with the eigenvalues of the photon OAM in units of . The normalization condition inner_product () fixes the constant to up to a constant phase factor. The HyGG modes are an overcomplete not orthogonal set of modes. The inner product of two normalized HyGG modes is given by . The asymptotic behavior of the intensity of the HyGG modes as at fixed is in general, and changes into the Gaussian behavior when is a non negative even integer (see below). At the beam center () the field of the HyGG modes vanishes as as expected for the eigenmodes of the photon OAM (for ). Because all zeroes of the hypergeometric function occur for real values of , a characteristic feature of the HyGG modes given by Eq. (1) is that their intensity never vanishes in the transverse plane, except at the beam axis and at infinite. This confers to the intensity of the HyGG modes the typical doughnut shape (a single brilliant ring) for any values of and , except when the HyGG mode reduces to the TEM Gaussian mode.
The limit of the field at the pupil plane is given by

(2)

This feature is very useful because this pupil function can be generated by applying the singular phase factor to a Gaussian-parabolic transmittance profile of the order . In particular, the HyGG modes with are simply generated by applying the phase factor to a TEM laser field at its waist plane.
The HyGG modes can be expanded in the complete basis of the Laguerre-Gauss (LG) modes. In general, the mode HyGG is a superposition of the infinite LG modes with same and any integer . In fact, when both the HyGG and the LG modes are normalized, we have with coefficients given by

(3)

The HyGG modes can exhibit different features when the mode parameters and are changed. It is then convenient to separate the HyGG modes in a few subfamilies having similar properties.

Figure 1: Experimentally observed intensity distributions of the mode for in the transverse plane . The Rayleigh range was  cm.

  1. This mode is the well known Gaussian mode TEM.

  2. , odd.
    The pupil field of these modes at is . At planes the modes are linear combinations of the modified Bessel functions and , where . We may call this subfamily of modes the modified Bessel Gauss (MBG) modes. Unlike the well known Bessel modes, these modes carry a finite power and are not diffraction free. When at fixed , the intensity of these modes vanishes according to .

  3. , even.
    The pupil field of these modes at is . At planes the modes are linear combinations of exponential -dependent terms. We may call this subfamily of modes the Modified Exponential Gauss (MEG) modes. When at fixed , the intensity of these modes vanishes according to .

  4. , even.
    The pupil field of these modes is given by Eq. (2). When is a non negative even integer, the confluent hypergeometric function reduces to a Laguerre polynomial. We will refer to these modes as to the modified Laguerre-Gauss modes (MLG). The asymptotic behaviour of the intensity of the MLG modes as at fixed is the same as for the usual LG modes (i. e. ). Unlike the LG modes, however, the MLG modes have a single-ring intensity profile for any admitted value of . The MLG modes can be expressed as the linear superposition of a finite number of LG modes, namely, the LG modes having the same and integer . In fact, when is a non negative even integer, Eq. (3) reduces to

    (4)

    where . The quantities form the entries of a non singular matrix. It is then obvious that this sub-family of HyGG modes forms a complete, yet not orthogonal, set of functions in the transverse plane and that the full set of HyGG modes is therefore overcomplete.

In our experiment, a TEM linearly polarized laser beam from a frequency doubled Nd:YVO ( nm, Model Verdi V5, Coherent) was used to illuminate a grey scale computer generated hologram (CGH) sent onto the LCD microdisplay of a spatial light modulator (SLM) (HoloEye Photonics LC-R 3000), with pixels in a rectangle 18.2411.40 mm wide. The SLM was located in the waist of the incident beam. We performed two series of measurements, according to the beam waist values at the SLM position. We measured by best fit between the error function and the integral curve of the intensity profile of incident beam as seen by CCD camera.The measured values were  mm and  mm, corresponding to Rayleigh ranges cm and  cm, respectively. The theory was compared with the experimental data just using as parameters the measured values reported above, without further best fitting. We focused our attention on the HyGG modes with .

Figure 2: The ring diameter of the mode for measured at plane . The Rayleigh range was  cm. The reported values of the diameters were scaled with respect to the value for . – theory, – experiment.

In accordance with the theoretical predictions, we observed an intensity profile in the transverse plane essentially made of a single bright annulus, whatever the value of we used or the observation -plane. Some instances of the observed intensity profiles are shown in Fig. 1. The ring diameter of the beam as a function of is reported in Fig 2. The diameter of the ring was defined as the maximum distance between any two opposite maxima of the intensity profile. The scaling law of versus turned to be in good agreement with the theoretical predictions. We measured also the ratio between the diameter of the luminous ring and the gaussian beam size at different planes. The measurements were made by switching on and off the CGH to compare the intensity profile of the HyGG mode with the gaussian profile TEM beam profile at the same plane. Figure 3 shows that the ratio was the same in all planes, as predicted by theory when . The constant value of the ratio obtained from the experiment was , which is close to the theoretical prediction .

Figure 3: The ratio between the diameter of the mode and the intensity radius of the generating TEM gaussian beam as a function of . The Rayleigh range was  cm.

In summary, we studied a novel family of paraxial beams having hypergeometric field profile. This set of modes is overcomplete and nonorthogonal and all modes carry a finite power. In spite of their complicated field profile, these mode have a simple profile at the pupil plane. Finally we have experimentally produced these modes for different values of indexes. The agreement between experimental results and theoretical predictions turned to be satisfactory.

References

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  • (8) W. Miller Jr., Symmetry and Separation of Variables (Addison-Wesley, Boston, MA, 1977).
  • (9) V. V. Kotlyar, R. V. Skidanov, S. N. Khonina, and V. A. Soifer, Opt. Lett. 32, 742 (2007).
  • (10) M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions (Dover Publications Inc., New York, 1970).
  • (11) The Hilbert inner product is defined according to .
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