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Optical Effects in Complex Media

#A-940


New Optical Effects at Frequency Transformation and Wave Irreversibility in Complex Media

Tech Area / Field

  • PHY-OPL/Optics and Lasers/Physics
  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
09.10.2002

Leading Institute
Yerevan State University, Armenia, Yerevan

Collaborators

  • University of Central Florida / Center for Research and Education in Optics and Lasers (CREOL) / School of Optics, USA, FL, Orlando\nNational Renewable Energy Laboratory, USA, CO, Golden\nKent State University / Liquid Crystal Institute, USA, OH, Kent

Project summary

The project refers to linear and non-linear optics of periodic media with a twisted (helical) structure. The widely known representatives of such media are the cholesteric liquid crystals (CLCs) and special varieties of liquid crystals having slightly more complicated structure (so-called C*smectics) as compared with CLCs. Owing to the periodic helical structure, the indicated media possess both the property of diffraction reflection on periodic heterogeneities (in this case the reflection takes place selectively with respect to polarization), and gyrotropy, i.e. the property to rotate the plane of light polarization. The combination of periodic heterogeneity and twisting, i.e. right-left asymmetry of the spatial structure, imparts some peculiarities to the media. These peculiarities reveal themselves in:

а) newly established regularities of the phenomenon of the second harmonic generation (the phenomenon ensuring emission of a wavelength in media under the effect of the light with doubled wavelength). These regularities are conditioned by originating of the diffraction reflection;


b) the recently established phenomenon of wave irreversibility consisting in violation of the well-known principle of light wave reversibility, i.e. uniformity of the velocity of light in mutually opposite directions of propagation. (This phenomenon is conditioned by gyrotropy).

The planned works include both investigations of above-stated peculiarities and further development of their theory and engineering of the new optical elements to be based on the created theory.

The objectives and aims of the Project are presented below.

1. Linear optical phenomena.

The optics, despite its broad coverage of phenomena, is traditionally restricted to media where wave reversibility takes place. Reversibility was reckoned as a fundamental principle on the ground that the wave vector surface (it bears the basic information on optical characteristics of the medium) appeared always as centrosymmetrical one. Therefore, any optical characteristic of the medium (the velocity of light, light transmission by a plate, etc.) appeared identical for any mutually opposite directions of light propagation. Until recently, the reversibility was considered as one and only possible option. However, it was shown lately that in media with the right-left asymmetry of spatial structure (homogeneous gyrotropic media, CLCs) in the presence of magnetic fields, the reversibility is disturbed and the wave irreversibility takes place. Here, the surface of wave vectors appears to be non-centrosymmetrical one. The wave irreversibility results in non-reciprocity effects. Namely, the optical characteristics of media become unequal for mutually opposite directions of light propagation. The Project uses just the inequality of transmission factors in mutually opposite directions as a fundamental basis for creation of miniature optical diodes (including the controlled ones) and storage devices, which could increase the light energy density. In pulse mode they can be used for amplifying the light beam intensity (intensity amplifiers). The action of the storage devices is based on the difference between their output and input power rates. When controlling the parameters of diodes, the latter could work as gates. The film diodes can be used also in vehicle to reduce the dazzle effect of headlights. The main objectives of the project are further study of wave irreversibility, theoretical development of the above mentioned optical diodes, energy storage devices, gates, amplifiers, optimization of their operation conditions (parameters) and fabrication of prototypes of the diodes and storage devices. The prototypes of the intended diodes and storage devices should have very high performance and be rather promising for their introduction in practice. Note that the large non-reciprocity effects could be provided by rather miniature spiral-shaped constructions having thickness of tens of microns.

Here, under certain conditions imposed on the structure, these effects can be realized both due to irreversibility created be magnetic fields and without irreversibility i.e. without magnetic fields. In the presence of magnetic fields, additional structural conditions are not necessary.

The investigation of both non-reciprocity (structural and magnetic ones) cases is planned by the Project. Here the irreversibility stands in two ways: a) as a principally new non-reciprocity source of some practical significance, and b) as an effect of fundamental importance.

Note also that the planned control of the above device parameters can be realized by the same magnetic field (that creates non-reciprocity due to irreversibility).

2. Non-linear optical phenomena.

The right-left asymmetry in combination with the periodical heterogeneity is the main physical factor underlying the studies both in linear and non-linear optics.

One of the important branches of modern non-linear optics, which is very important from both fundamental and applied points of view, is non-linear generation of the second harmonic (generation of a wavelength in media by the light with doubled wavelength). Dielectric permeability of media, that stipulates generation of the second harmonic (i.e. generation with the doubled frequency) under the effect of light radiation, depends on wave frequency, thus complicating the second harmonic generation and accordingly, its practical realization. This is the first problem of non-linear optics of homogeneous media. In practice, the intensity of the second harmonic is also important. Under the conditions of phase matching (phase synchronism), the intensity of the second harmonic is proportional to square of thickness of the medium layer where the second harmonic is generated. The increase in intensity by thickening of the layer contradicts the tendencies of modern engineering that require miniaturization. Besides, the thickening is bounded above by certain physical conditions. That is the second problem of non-linear optics of homogeneous media.

Recently it has became clear that non-linear processes such as generation of the second harmonic, can be carried out more efficiently in periodical media than in homogeneous ones. In particular, it turned out that due to the formation of a standing wave in periodical media (a diffraction effect, characteristic for periodically non-homogeneous media), it is possible to create a special field configuration capable to generate the second harmonic. Namely, it is possible to create such configuration, when the realization of the needed phase matching could be reached regardless of the frequency dependence of dielectric permeability. This opportunity could solve the indicated above first problem of the second harmonic generation for homogeneous media conditioned by the frequency dependence of dielectric permeability. The physical mechanisms of this phenomenon, unusual for non-linear optics, are not clear yet. Therefore, investigations in this area are extremely urgent for creation of the theory of the above phenomenon. The practical value of such investigations is conditioned by the possibility of the second harmonic generation with the intensity, proportional not to the second but to the fourth power of the layer thickness. This effect ensuring unusually large intensity of the second harmonic and solving the second mentioned problem, opens new opportunities of creation of high-performance miniature frequency converters for optics.

It should be noted that a series of the monographs and scientific papers written by the Project participants can make the necessary bases for further studies. The project team is specialized in the fields of non-linear and linear optics. Besides, the project participants are sufficiently experienced in the field of gyrotropy and interaction of waves with periodical media (both in optics and relative areas). They have experience not only in the non-linear generation of radiation, but also in other radiation mechanisms. Such a broad coverage of the team’s scientific expertise is very favorable factor for understanding of new phenomena to be studied and for successful implementation of the present project.

The theoretical investigations envisioned by the project should have, on the one hand, practical results, as they are aimed at the development of new optical elements and devices (film diodes, light energy storages, gates, amplifiers, efficient miniature frequency converters capable to operate regardless of the frequency dependence of dielectric permeability) and the creation of prototypes operating according to new physical principles. On the other hand, these investigations are of fundamental interest, as they are connected with the phenomena that are unusual for traditional optics and require an understanding of new physical mechanisms. In fact, up to now the whole linear optics was based upon reversibility, and the whole theory of the second harmonic generation was based upon the frequency dependence of dielectric permeability.

Thus, taking into account aforesaid, the following objectives of the Project can be formulated:

1. Development of the theory of wave irreversibility phenomenon and non-reciprocity effects in complex structures; search of opportunities of high-performance control of the parameters characterizing irreversibility and non-reciprocity.


2. Theoretical study (not connected with the frequency dependence of dielectric permeability) of the second harmonic (and higher harmonics) generation in periodical non-homogeneous media.
3. Determination (on the basis of the developed theory) of optimum parameters of the supposed optical elements (high-performance frequency converters, optical diodes, storage devices, as well as amplifiers and gates) for their practical application in the future; creation of diode and storage prototypes.
4. Development of a fabrication technique of the films with the structure assumed in it. 1-3.


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