imran aziz'laser and non-linear optics


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basic theory of laser and non -linear optics


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Introduction to Nonlinear Optics : 


How to make a laser in three easy steps … : 

How to make a laser in three easy steps … • Pick a medium that has the potential for optical gain – i.e., an amplifying medium. • Select a means of putting energy into that medium – i.e., an excitation system. • Construct an optical feedback system for stimulating further emission, i.e., an optical resonator.

Introduction : 

Introduction Question: Is it possible to change the color of a monochromatic light? Answer: Not without a laser light

Stimulated emission, The MASER and The LASER : 

Stimulated emission, The MASER and The LASER (1916) The concept of stimulated emission Albert Einstein (1928) Observation of negative absorption or stimulated emission near to resonant wavelengths, Rudolf Walther Ladenburg (1930) There is no need for a physical system to always be in thermal equilibrium, Artur L. Schawlow

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The Maser : 

The Maser Two groups were working on Maser in 50s Alexander M. Prokhorov and Nikolai G. Bassov (Lebedev institute of Moscow) Charles H. Townes, James P. Gordon and Herbert J. Zeiger (Colombia University)

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Left to right: Prokhorov, Townes and Basov at the Lebede institute (1964 Nobel prize in Physics for developing the “Maser-Laser principle”)

Slide 10: 

Townes (left) and Gordon (right) and the ammonia maser they had built at Colombia University

The LASER : 

The LASER (1951) V. A. Fabrikant “A method for the application of electromagnetic radiation (ultraviolet, visible, infrared, and radio waves)” patented in Soviet Union. (1958) Townes and Arthur L. Schawlow, “Infrared and Optical Masers,” Physical Review (1958) Gordon Gould definition of “Laser” as “Light Amplification by Stimulated Emission of Radiation” (1960) Schawlow and Townes U. S. Patent No. 2,929,922 (1960) Theodore Maiman Invention of the first Ruby Laser (1960) Ali Javan The first He-Ne Laser

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Maiman and the first ruby laser

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Ali Javan and the first He-Ne Laser

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Properties of Laser Beam : 

Properties of Laser Beam A laser beam Is intense Is Coherent Has a very low divergence Can be compressed in time up to few femto second

Applications of Laser : 

Applications of Laser (1960s) “A solution looking for a problem” (Present time) Medicine, Research, Supermarkets, Entertainment, Industry, Military, Communication, Art, Information technology, …

Start of Nonlinear Optics : 

Start of Nonlinear Optics Nonlinear optics started by the discovery of Second Harmonic generation shortly after demonstration of the first laser. (Peter Franken et al 1961)

2. The Essence of Nonlinear Optics : 

2. The Essence of Nonlinear Optics When the intensity of the incident light to a material system increases the response of medium is no longer linear

Response of an optical Medium : 

Response of an optical Medium The response of an optical medium to the incident electro magnetic field is the induced dipole moments inside the medium

Nonlinear Susceptibility : 

Nonlinear Susceptibility The general form of polarization Dipole moment per unit volume or polarization

Nonlinear Polarization : 

Nonlinear Polarization Permanent Polarization First order polarization: Second order Polarization Third Order Polarization

How does optical nonlinearity appear : 

How does optical nonlinearity appear The strength of the electric field of the light wave should be in the range of atomic fields

Nonlinear Optical Interactions : 

Nonlinear Optical Interactions The E-field of a laser beam 2nd order nonlinear polarization

2nd Order Nonlinearities : 

2nd Order Nonlinearities The incident optical field Nonlinear polarization contains the following terms

Sum Frequency Generation : 

Sum Frequency Generation Application: Tunable radiation in the UV Spectral region.

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Difference Frequency Generation

Phase Matching : 

Phase Matching Since the optical (NLO) media are dispersive, The fundamental and the harmonic signals have different propagation speeds inside the media. The harmonic signals generated at different points interfere destructively with each other.

SHG Experiments : 

SHG Experiments We can use a resonator to increase the efficiency of SHG.

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Third Order Nonlinearities : 

Third Order Nonlinearities When the general form of the incident electric field is in the following form, The third order polarization will have 22 components which their frequency dependent are

The Intensity Dependent Refractive Index : 

The Intensity Dependent Refractive Index The incident optical field Third order nonlinear polarization

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The total polarization can be written as One can define an effective susceptibility The refractive index can be defined as usual

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By definition where

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Typical values of nonlinear refractive index

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Third order nonlinear susceptibility of some material

Processes due to intensity dependent refractive index : 

Processes due to intensity dependent refractive index Self focusing and self defocusing Wave mixing Degenerate four wave mixing and optical phase conjugation

Self focusing and self defocusing : 

Self focusing and self defocusing The laser beam has Gaussian intensity profile. It can induce a Gaussian refractive index profile inside the NLO sample.

Wave mixing : 

Wave mixing

Optical Phase Conjugation : 

Optical Phase Conjugation Phase conjugation mirror

Aberration correction by PCM : 

Aberration correction by PCM

What is the phase conjugation : 

What is the phase conjugation The signal wave The phase conjugated wave

Degenerate Four Wave Mixing : 

Degenerate Four Wave Mixing All of the three incoming beams A1, A2 and A3 should be originated from a coherent source. The fourth beam A4, will have the same Phase, Polarization, and Path as A3. It is possible that the intensity of A4 be more than that of A3

Mathematical Basis : 

Mathematical Basis The four interacting waves The nonlinear polarization The same form as the phase conjugate of A3

Origin of Nonlinearities in Optics : 

Origin of Nonlinearities in Optics The fast response of media to an electromagnetic wave in visible and near IR is caused by a displacement of electrons, both free ones in metals and bound ones in dielectrics.

Origin of Nonlinearities in Optics : Origin of Nonlinearities in Optics The fast response of media to an electromagnetic wave in visible and near IR is caused by a displacement of electrons, both free ones in metals and bound ones in dielectrics.

Slide 46: 1. Free electrons The motion of electron in the field of a light wave: (1 is described by an equation: (2) Because , the vector product is proportional to The solution of (2) can be found in a form: (3) where is linear, are nonlinear polarisabilities. The induced electrical dipole moment is equal to (4)

Slide 47: 2. Bound electrons For the case of bound electron the equation has the following form: (5) where the term takes into account real anharmonisity of the oscillator: Considerin as a small term the solution of (5) can be presented as: (3)

Slide 48: 3. Macroscopic characteristics To describe the media response for the electromagnetic field one must calculate a polarization vector , which is a dipole moment of a unit volume. Where N is the concentration of electrons. If a nonlinear dependence of on takes place the vectors and can be presented in the form: (7) (8) where are tensors of 2 rank, are tensors of 3 rank and so on. are nonlinear susceptibilities (6)

Slide 49: 4. Local field factor In a microscopic model of nonlinearity (we presented two such models) it is important to describe correctly microscopic and macroscopic values. For crystals of cubic symmetry: (9) where the term in brackets is so-called Lorentz factor (local field factor). For nonlinear susceptibility in particular for quadratic nonlinearity: (10)

Slide 50: 5. How high is the nonlinearity If the response of the media is caused by electrons in nonresonant case for the following ratio is valid: (11) where is an interatomic field. For hydrogen One can see from this that appreciable nonlinear effects can be observed at relatively high light intensities, which are the features of pulse lasers. The nonlinear optics experiments became real after innovation of Q-switched laser with pulse duration of 10-8 s and intensities of 1010-1011 W/cm2. Now femtosecond lasers became available, which generate pulses with duration of 6-30·10-15 s at the intensity up to 1017-1020 W/cm2. In this case the electric field in the light wave exceeds the value of EA. It opens completely new branch of optics: physics of superstrong fields.

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Besides the above electronic nature of nonlinear response a strong nonlinearity can be caused by an anharmonisity of atomic oscillation in molecules, orientation of polar molecules in an electric field, heating of medium. The slower is a mechanism responsible for nonlinearity the stronger is the nonlinearity. Let us present the values of characteristic time constants and the values of for different mechanism of nonlinear polarization.

III. Optical HarmonicGeneration : III. Optical HarmonicGeneration The high intensity light wave induces the nonlinear polarization in a medium. The wave of polarization is a source for new electromagnetic waves.

Slide 53: 1. Second-harmonic generation First of all we should notice that the tensor , for centrosymmetric media is equal to zero. The same is valid for all even order The operation of symmetry transforms the terms from (12) in the following way: (12) (13) Then , that can not take place under nonzero .

Slide 54: For a simplicity we assume that the medium is isotropic. Then the polarization: (14) The incident waves propagating in z-direction can be presented as: (15) (16)

Slide 55: A spectrum of polarization waves contains new frequencies:

Slide 56: 2. Third-harmonic generation If the medium possesses cubic nonlinearity, under the action of two monochromatic waves and the polarization would contain the components with frequencies:

IV. Wave Nonlinear Optics : IV. Wave Nonlinear Optics As the optical harmonic generation takes place both induced waves of polarization and free running electromagnetic waves of harmonics are propagating in the medium. If the dimensions of the medium are much larger than pumping wavelength the phase matching determines the efficiency of the energy transfer from the pumping wave to harmonics. Let us consider the phase matching conditions for the case of second harmonic generation.

Slide 58: 1. Maxwell equations The propagation of the light in the medium is described by Maxwell equations: (17) For optical range where (18) (19) Combining first and second equations from (17) one may obtain so-called wave equation: (20)

Slide 59: Inserting (18) into (20) we are getting: (21) The nonlinear polarization term in the right hand side of (21) plays a role of a source of electromagnetic waves 2. Phase mismatch For quadratic media and relatively low nonlinearity the plane wave solution of (21) for the intensity of the second harmonic looks like: (22)

Slide 60: Phase mismatch

Slide 61: For the case of the exact phase matching the energy of the pumping wave can be completely transferred into second harmonic

Slide 62: 3. Phase matching How the condition or can be realized? In an isotropic medium with normal dispersion > and never equals to zero But in birefringent uniaxial crystal there are two beams ordinary and extraordinary. For so-called negative crystal no>ne. If pumping wave is ordinary one and second harmonic is extraordinary one the material dispersion ( > ) can be compensate for the difference in refractive indices for o and e beams:

Slide 63: For the process of third-harmonic generation the condition of phase matching looks the same: As it was mentioned already and values for the fast nonresonant electronic polarization do not much differ for many materials and the only way to enhance the efficiency of nonlinear energy transformation is to phase match the interacting waves.

V. Other Nonlinear Effects : V. Other Nonlinear Effects 1. Modulation of a refractive index Cubic nonlinearity causes not only wave generation with new frequency but also appearance of a wave of nonlinear polarization with the frequency of pumping wave: (23) As a result of such selfaction a nonlinear refractive index n2I appears at the frequency : (24) For the fast nonresonant nonlinearity n2 is relatively small: n2~10-13 cm2/kW. For slower mechanisms of the nonlinearity n2 can be much larger in particular for liquid crystals: n2~0.1 cm2/kW.

Slide 65: 2. Selffocusing If the intensity of a laser beam is high enough instead of diffraction an opposite effect of selffocusing takes place. Phase velocity depends on the intensity through nonlinear refractive index: Vph=c/n0+n2I (25) If n2 > 0 the phase velocity at the axis of the beam is lower and nonlinear medium is working as a lens.

VI. Nonlinear OpticalDiagnostics : VI. Nonlinear OpticalDiagnostics Nonlinear susceptibilities and are tensors and they inherit the symmetry properties of the crystalline medium. It means that nonlinear optical effects are structure sensitive. It can be employed to study different structure transformations. A lot of such experiments were done. I will mention just one related with laser induced melting of semiconductors.

Slide 67: Idea of experiment Metal in liquid state Semiconductor in liquid state 1. Nonlinear optical diagnostics of phase transitions

VIII. Conclusions : VIII. Conclusions Nonlinear optics is an attractive and fast developing part of modern optics. Nonlinear effects are structure sensitive in their nature. It can be used for time-resolved monitoring of structural transformation (up to femtosecond time resolution). Artificial photonic media on the base of porous semiconductors open new exciting possibilities for the control of nonlinear optical processes.

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