Nath Diffraction Research Articles

Nonlinear Raman-Nath diffraction of femtosecond laser pulses.

We study the nonlinear Raman-Nath diffraction (NRND) of femtosecond laser pulses in a 1D periodic nonlinear photonic structure. The calculated second-harmonic spectra represent frequency combs for different orders of transverse phase matching. These frequency combs are in close analogy with the well-known spectral Maker fringes observed in single crystals. The spectral intensity of the second harmonic experiences a redshift with a propagation angle, which is opposite the case of Čerenkov nonlinear diffraction. We analyze how NRND is affected by the group-velocity mismatch between fundamental and second-harmonic pulses and by the parameters of the structure. Our experimental results prove the theoretical predictions.

Diffraction and shaping analysis of excimer laser through an ultrasonic grating

Abstract A novel numerical simulation method for the shaping of an excimer laser beam by an acoustic grating is proposed. Partially coherent theory and extended Huygens–Fresnel principle are used to analyze the light intensity diffraction pattern. Fast Fourier transform function FFT in the Matlab is used to calculate the numerical integral, which makes the integral operation simple and efficient. It is shown by a numerical simulation that the output intensity distribution is closely related to the coherence width, Raman–Nath comprehensive parameter, diffraction distance and ultrasonic wave frequency. The simulation results show that a flat top beam can be obtained by controlling these parameters. On the other hand, the results are used as an effective analytic tool for the determination of the design parameters of acousto-optic modulator.

Experimental research on the feasibility of a portable Q-switch for ultraviolet lasers in Raman–Nath diffraction

The optimal acousto-optic interaction length in Raman–Nath diffraction is investigated by using a deliberately designed apparatus working with pure water. Then the optimum interaction length, the suitable dimensions for the transducer, and the required index difference dominated by the ultrasonic frequency and power to achieve the ultimate efficiency are analyzed. The portable device analogy to a solid appliance is designed based on the optimization. Taking advantage of the device a highest diffraction efficiency of more than 98% is obtained. The feasibility of using the portable device to act as a Q-switch for ultraviolet and visible lasers is discussed.

Spatial modulation of second-harmonic generation via nonlinear Raman–Nath diffraction in an aperiodically poled lithium tantalite

We propose and experimentally demonstrate a colorful nonlinear Raman-Nath second-harmonic generation by engineering the quadratic nonlinearity χ((2)) in an aperiodically poled lithium tantalite. The engineered nonlinear structure allows multicolored Raman-Nath second-harmonic signal outputs along a uniform direction, which cannot be achieved in a uniform nonlinear grating. The diffraction angles are independent of the beam waist and the position of incidence. This verifies that nonlinear Raman-Nath diffraction does not depend on the local superlattice structure where the fundamental frequency beam locates, but on the whole nonlinear χ((2)) crystal.

Parametric wave interaction in one-dimensional nonlinear photonic crystal with randomized distribution of second-order nonlinearity

We theoretically study the parametric wave interaction in nonlinear optical media with randomized distribution of the quadratic nonlinearity \(\chi ^{(2)}\). In particular, we discuss the properties of second and cascaded third harmonic generation. We derive analytical formulas describing emission properties of such harmonics in the presence of \(\chi ^{(2)}\) disorder and show that the latter process is governed by the characteristics of the constituent processes, i.e. second harmonic generation and sum frequency mixing. We demonstrate the role of randomness on various second and third harmonic generation regimes such as Raman–Nath and Cerenkov nonlinear diffraction. We show that the randomness-induced incoherence in the wave interaction leads to deterioration of conversion efficiency and angular spreading of harmonic generated in the processes relying on transverse phase matching such as Raman–Nath interaction. On the other hand, the Cerenkov frequency generation is basically insensitive to the domain randomness.

Design and process test of a novel MOEMS accelerometer based on Raman—Nath diffraction

A novel micro-opto-electro-mechanical system (MOEMS) accelerometer based on Raman—Nath diffraction is presented. It mainly consists of an FPW delay line oscillator and optical strip waveguides. The fundamental theories and principles of the device are introduced briefly. A flexural plate-wave delay-line oscillator is designed to work as an acousto-optic (AO) shifter, which has a Klein—Cook parameter of 0.38. Single-mode optical strip waveguides of 2 μm in width and thicknesses of 0.6 μm are designed by using the effective index method for light transmission. The Ey00 mode waveguide polarizers are designed to ensure the consistency of the light polarization in the waveguides. The fabrication process, based on (100) oriented, 450-μm-thick silicon wafers is proposed in detail, and some difficulties in the process are discussed carefully. At last, a series of process tests are undertaken to solve the proposed problems. The results indicate that the proposed design and fabrication process of the device is dependable and realizable.

Laser frequency stabilisation by the Pound — Drever — Hall method using an acousto-optic phase modulator operating in the pure Raman — Nath diffraction regime

Frequency stabilisation of diode laser radiation has been implemented by the Pound — Drever — Hall method using a new acousto-optic phase modulator, operating in the pure Raman — Nath diffraction regime. It is experimentally shown that, as in the case of saturated-absorption spectroscopy in atomic vapour, the spatial divergence of the frequency-modulated output spectrum of this modulator does not interfere with obtaining error signals by means of heterodyne frequency-modulationspectroscopy with a frequency discriminator based on a high-Q Fabry — Perot cavity with finesse of several tens of thousands.

Theoretical analysis and concept demonstration of a novel MOEMS accelerometer based on Raman—Nath diffraction

The design and simulation of a novel microoptoelectromechanical system (MOEMS) accelerometer based on Raman—Nath diffraction are presented. The device is planned to be fabricated by microelectromechanical system technology and has a different sensing principle than the other reported MOEMS accelerometers. The fundamental theories and principles of the device are discussed in detail, a 3D finite element simulation of the flexural plate wave delay line oscillator is provided, and the operation frequency around 40 MHz is calculated. Finally, a lecture experiment is performed to demonstrate the feasibility of the device. This novel accelerometer is proposed to have the advantages of high sensitivity and anti-radiation, and has great potential for various applications.

Theoretical investigations of nonlinear Raman–Nath diffraction in the frequency doubling process

We theoretically study the second-harmonic generation via nonlinear Raman–Nath diffraction in an optical medium with the spatial modulation of quadratic nonlinearity. We derive analytical equations that govern the emission properties of this nonlinear wave phenomenon. We also discuss how a substantial range of parameters such as the thickness of a nonlinear medium and the condition of a pump laser affect the strength of the emitted harmonic signals.

Third-harmonic generation via nonlinear Raman–Nath diffraction in nonlinear photonic crystal

We report on the observation of multiple third-harmonic conical waves generated in an annular periodically poled nonlinear photonic crystal. We show that the conical beams are formed as a result of the cascading effect involving two parametric processes that satisfy either the transverse and/or longitudinal phase-patching conditions. This is the first experimental observation of third-harmonic generation based on nonlinear Raman-Nath diffraction.

Electric-field-controlled two-dimensional Raman–Nath diffraction from paraelectric potassium lithium tantalate niobate

Electric-field-controlled two-dimensional Raman–Nath diffraction has been realized using a photorefractive diffraction grating. The grating was produced by two-wave coupling (at a wavelength of 632.8 nm) at small incidence angles using a potassium lithium tantalate niobate single crystal. Results for the Raman–Nath diffraction from the g 44 grating are presented, in which the externally applied field is perpendicular to both the grating vector and the wave vector of the incident beam. Two pairs of coherent beams were used to record the grating for two-dimensional Raman–Nath diffraction. The wave vector and the polarization of one pair lay in the ( x, z) plane, and those of the other pair lay in the ( y, z) plane. The influence of the applied electric field was studied, and the results show that the intensity of the Raman–Nath diffraction could be controlled by the direction and intensity of the applied field.

Chronological Investigations of Raman–Nath Diffraction Grating Inscribed by Direct Laser Writing in Photoreactive Monomer Base Mixtures

Phase-type diffraction gratings, inscribed using an ultraviolet laser beam, were grown in a thin film of a photoreactive monomer base mixture over a period of several hours. The photopolymerization of monomers and subsequent molecular migrations generated both surface relief gratings and inner refractive index modulation. The changes in the diffraction intensity over time revealed the pronounced behavior of grating formation, which can be explained using a time-dependent model based on the Raman–Nath diffraction theory. Homogeneous irradiation with ultraviolet light during the growth enabled us to fix the diffraction characteristics of the gratings at an arbitrary moment in time, allowing us to tailor the resultant diffraction gratings according to a desired specification. This sequential process can have practical applications.

Compact acousto-optic modulator operatingin the purely Raman — Nath diffraction regime as a phase modulator in FM spectroscopy

We report fabrication of a new acousto-optic modulator (AOM-RN) operating purely in the Raman — Nath diffraction regime. This device can be used as an external phase modulator in frequency-modulation (FM) optical heterodyne spectroscopy for fast and broadband frequency control of diode lasers. The AOM-RN design is significantly simplified, and its dimensions are minimised due to a decrease (by almost an order of magnitude in comparison with the existing AOMs) in the acousto-optic interaction length and the absence of impedance matching circuit. The FM spectroscopy based on AOM-RN makes it possible to analyse both absorption and dispersion properties of optical resonances under study; this possibility is shown by the example of saturated-absorption resonances in cesium vapour. The possibility of detecting coherent population trapping resonances using FM spectroscopy with AOM-RN as an external phase modulator is experimentally demonstrated.

Acoustic and Acousto-Optic Characteristics of Silicon Nanofoam

Silicon nanofoam is a porous material with a nanometer structure produced through a sol–gel process, and is used as a heat insulator. It is expected that the nanofoam may work as a good acoustic matching layer of an airborne ultrasonic transducer for highly sensitive and wideband ultrasound transmission/detection since the nanofoam has an extremely low acoustic impedance. The nanofoam may also have a possibility as an acousto-optic device because of its very low sound speed and optical transparency. In this study, we have estimated the fundamental acoustic characteristics of the nanofoam through acousto-optic measurements. Sound speed and acoustic attenuation were measured in the frequency range from 130 to 444 kHz using rectangular samples attached to a piezoelectric transducer. The sound speed and acoustic attenuation constant were approximately in the 140–150 m/s range and 4.3 ×10-11f1.9 dB/(mm·Hz1.9), respectively. It was observed that the change rate in the optical refractive index of the nanofoam owing to sound pressure was approximately in the range of (1.2–1.6) ×10-8 1/Pa. Raman–Nath diffraction occurred at a relatively low frequency since the sound speed is low. We also observed modulation in the polarization of the transmitted light owing to ultrasonic waves.

Beam coupling in photorefractive liquid crystal light valves

Photorefractive liquid crystal light valves (LCLVs) are hybrid devices that combine a nematic liquid crystal layer with a thin monocrystalline Bi12SiO20 (BSO) photorefractive crystal in the form of a cell wall. The device behaves as an optically addressed spatial light modulator, where the photoconductive layer is made of the BSO crystal. Differently from conventional types of spatial light modulators, usually working in retroreflective configuration, the photorefractive light valves work in transmission, thus allowing new applications related to the coupling of the optical beams when they pass through the liquid crystal layer. Here, we review some recent experiments of beam coupling in photorefractive LCLVs. After a characterization of the device in terms of its spatial resolution, which is related to the features of pattern formation in an optical feedback configuration, we present two-beam coupling and optical amplification in single pass experiments. Then, we develop a theoretical model by taking into account the Raman–Nath diffraction of the incoming beams over the thin liquid crystal layer. By using two or more light valves in cascade, we show, both experimentally and theoretically, that the two-wave mixing gain can be enhanced by the Talbot effect related to the multi-passage of the beams through the successive layers of the nematic liquid crystal. Finally, we show that self-pumped phase conjugation can be realized by placing the light valve in a tilted feedback configuration. In this case, the four-wave mixing is spontaneously created through the scattering of the signal beam onto the feedback induced grating.

Phase-sensitive detection of optical resonances by using an acousto-optic modulator in the Raman – Nath diffraction mode

A new method for frequency control of an external cavity diode laser without direct modulation of the injection current is proposed. The Pound – Drever optical heterodyne technique or the method of frequency control by frequency-modulated sidebands, in which an acousto-optic modulator operating in the Raman – Nath diffraction mode is used as an external phase modulator, can be employed to obtain error signals upon automatic frequency locking of the diode laser to the saturated absorption resonances within the D2 line of cesium atoms or to the optical cavity resonances.

The spatial properties of atomic Raman–Nath diffraction

This paper obtains the exact analytical solution of atomic Raman-Nath diffraction in the coordinate representation and discusses the influence of different initial conditions and detunings on the atomic spatial population distribution. The phase difference between the dipole matrix element and initial atomic population may influence the atomic spatial population distribution after diffraction, which has never been discussed before as far as we know. It offers a method to measure the phase by the spatial population distribution, which is interesting in the study of quantum optics.

Light diffraction by two parallel superposed ultrasonic waves of the frequency ratio 1:2

The paper constitutes a concise study of light diffraction by two superposed ultrasonic waves of the frequency ratio 1:2, beyond the Raman–Nath diffraction regime. Relevant numerical simulations of light intensity distribution within separate diffraction orders are shown based on the Nth order approximation method (NOA). A comparison is given between generalized theoretical predictions ( N = 7) and simplified formulas of Mertens ( N = 2) [R. Mertens, Diffraction of light by two parallel superposed supersonic waves, one being the nth harmonic of the other: A critical study of the methods leading to approximate solutions in finite form, Proc. Indian Acad. Sci. 55 (1962) 63–98.]. Moreover, the results of the performed experiment are presented which indicate limited applicability range of Mertens’ pioneer description of 1962.

Mixed phase and absorption thin gratings diffraction

AbstractThe coupled wave theory of Raman and Nath diffraction is extended to the case of thin anisotropic holographic media with grating vector parallel to the medium boundaries. Solutions for the wave amplitudes, diffraction efficiencies, and angular mismatch sensitivities are given in transmission geometries for the case of mixed dielectric and absorption modulations. For an apparent distinction, the results are given only for dielectric modulation. The main difference of the new results, with respect to the expressions valid for isotropic media, arises due to the walk-off between the wave-front and energy propagation directions. The difference is particularly important in materials with large birefringence, such as organic crystals, ordered polymers, and liquid crystalline cells.

The Theory of Light Diffraction in Thin Anisotropic Medium

The coupled wave theory of Raman and Nath diffraction is extended to the case of thin anisotropic holographic media with grating vector parallel to the medium boundaries. Solutions for the wave amplitudes, diffraction efficiencies, and angular mismatch sensitivities are given in transmission geometries for the case of dielectric modulation. The main difference of the new results with respect to the expressions valid for isotropic media arises due to the walk-off between the wave-front and energy propagation directions. The difference is particularly important in materials with large birefringence, such as organic crystals, ordered polymers, and liquid crystalline cells.