Raman-Nath Regime Research Articles

Composite Raman-Nath heterodyne interferometry with relevance for precise spectroscopy

Atomic state manipulation by laser radiation requires a stringent control of the light phase noise for precise spectroscopy and quantum computation. In this work we describe a novel interferometer recently employed to estimate the phase noise induced by an optical frequency shifter based on serrodyne modulation for optical frequency standards (Barbiero et al., 2023). The interferometer is generated by an acousto-optic actuator working in the Raman-Nath regime. Using the heterodyne detection between the unshifted optical mode with the first higher order optical components, we are able to detect the differential phase between the two optical path arms with a reduction of the phase noise induced by the actuator. We provide a theoretical treatment for the actuator phase suppression and we estimate the detectable phase noise limit of this interferometer. Finally, we discuss the possible applications in the field of precision measurements.

STAR-FDTD: space-time modulated acousto-optic guidestar in disordered media

Abstract We developed a 2D Finite-difference time-domain (FDTD) method for modeling a space-time modulated guidestar targeting wavefront shaping applications in disordered media. Space-time modulation in general (a particular example being the acousto-optic effect) is used here as a guidestar for the transverse confinement of light around the tagged region surrounded by disorder. Together with the guidestar, the iterative optical phase conjugation (IOPC) method is used to overcome the diffusion of light due to multiple scattering. A phase sensitive lock-in detection technique is utilized to estimate the steady-state amplitude and phase of the modulated wavefronts emerging from the guidestar region continuously operating in the Raman-Nath regime. As the IOPC scheme naturally converges to the maximally transmitting eigenchannel profile, one could use the position of the guidestar within the disorder to channelize the maximal transmission through the tagged region. The associated code developed in MATLAB® is provided as an open source (The MIT License) package. The code package is referred by the acronym STAR-FDTD where STAR stands for Space-Time modulated Acousto-optic guidestaR.

Full-color eye-box expansion via holographic volume gratings recorded in photo-thermo-refractive glass.

Conventional head-up displays (HUDs) suffer from a limited exit pupil and a lack of compactness mainly due to the use of bulky optics. HUDs need a high-quality image with a large field of view (FOV) in small packaging to gain commercial acceptability. Holographic HUDs are phase-only devices that allow vision correction and focus adjustment while having a wide FOV. However, the limited bandwidth of a spatial light modulator (SLM) imposes a trade-off between the FOV and eye-box size. Combining a holographic system with an image-replicating element eliminates such a tradeoff. For image replication, we designed and fabricated a compact 2D diffractive beam splitter formed from two perpendicular volume gratings operating in the Raman-Nath regime. The gratings were recorded holographically in photo-thermo-refractive (PTR) glass, with optimized index modulation, thickness, and period to provide uniform intensity distribution across all desired orders for the fundamental red, green and blue (RGB) colors. We demonstrated a full-color holographic projection with an eye-box expanded by the designed 2D diffractive beam splitters.

Second‐Harmonic Generation via Nonlinear Raman–Nath Diffraction in an Optical Fibonacci Superlattice

AbstractNonlinear Raman–Nath diffraction is an effective method for frequency conversion in the presence of phase mismatch. In a structure featuring periodically modulated quadratic nonlinearity, the available frequency‐doubled patterns generated from nonlinear Raman–Nath diffraction constitute uniformly spaced points due to the one parametric decided reciprocal lattice vectors. Herein, nonlinear Raman–Nath second‐harmonic generation from the optical Fibonacci superlattice is theoretically investigated, and a propagation equation that describes the properties of frequency‐doubled waves is presented. Abundant diffraction peaks that are substantially more varied than the traditional Raman–Nath regime are observed. The dependence of the generated second‐harmonic signals on the structural parameters and fundamental laser performance is also comprehensively discussed.

Large-angle two-dimensional grating with hybrid mechanisms

We demonstrate a large-diffraction-angle two-dimensional (2D) grating based on cholesteric liquid crystal (CLC). One dimension is a polarization volume grating (PVG) working in the Bragg regime, which is produced by a patterned photoalignment layer. The other dimension is a CLC grating working in the Raman-Nath regime, which is introduced by CLC self-assembly under a weak anchoring energy condition. The condition for the coexistence of the CLC Raman-Nath grating (RNG) and PVG is analyzed, and the efficiency and grating period of the CLC RNG are also characterized. Potential application of this 2D grating for enlarging the eyebox of augmented reality displays is discussed.

Birefringent optofluidic gratings.

A set of parallel microfluidic channels behaving as a diffraction grating operating in the Raman-Nath regime has been fabricated and studied. The diffraction efficiency of such structure can be tuned by selecting a liquid with a particular refractive index and/or optical anisotropy. Alternatively the optical properties of the liquid can be characterised by measuring the diffraction efficiency and the state of polarization of the diffracted beam. In this work, the microfluidic channels under study have been filled with penicillin molecules dissolved in water. Due to the chirality of the penicillin, the liquid has been found to have circular birefringence of 2.14 × 10-7. The addition of the anisotropic liquid modifies the polarization properties of the microfluidic diffraction grating. The diffraction efficiency of the grating has been characterised for different probe beam wavelengths and states of polarization. Currently the diffraction efficiency of the device is low - 1.7%, but different approaches for its improvement have been discussed.

Observation of sub-wavelength phase structure of matter wave with two-dimensional optical lattice by Kapitza-Dirac diffraction

We report an experimental demonstration of generation and measurement of sub-wavelength phase structure of a Bose-Einstein condensate (BEC) with two-dimensional optical lattice. This is implemented by applying a short lattice pulse on BEC in the Kapitza-Dirac (or Raman-Nath) regime, which, in the classical picture, corresponds to phase modulation imprinted on matter wave. When the phase modulation is larger than 2π in a lattice cell, the periodicity of phase naturally forms the sub-wavelength phase structure. By converting the phase information into amplitude, we are able to measure the sub-wavelength structure through the momentum distribution of BEC via the time-of-flight absorption image. Beyond the classical treatment, we further demonstrate the importance of quantum fluctuations in the formation of sub-wavelength phase structure by considering different lattice configurations. Our scheme provides a powerful tool for exploring the fine structure of a lattice cell as well as topological defects in matter wave.

Perfectly asymmetric Raman-Nath diffraction in disordered atomic gratings.

We investigate the effects of geometrical and structural disorders on perfectly asymmetric diffraction (PAD) in Raman-Nath regime. The two types of disorders are realized by introducing random fluctuations in the position and width of one-dimensional (1D) driven atomic lattices. Raman-Nath diffraction is modified differently with respect to the geometrical and structural disorders. It is shown that the PAD is observed with a certain strength range of geometrical disorder, exceeding which it can be destroyed, while the PAD is rather robust against structural disorder. The different behaviors originate from the disorder-induced random variations of the spatial phase shifts of the standing-wave (SW) coupling field and atomic lattices with Gaussian profile. Furthermore, we find that, in the presence of geometrical disorder, the PAD is more susceptible to correlated disorder than to uncorrelated disorder. Our scheme may be useful for understanding the effects of disorder on the diffraction of light and matter waves in disordered potentials..

Characteristics of ultrasonic modulation of coherent light

We find that the zero-order diffracted light without optical frequency shifts in the Raman-Nath regime is phase-modulated by the ultrasound wave. The finding gives us a chance to reveal the characteristics of ultrasonic modulation of coherent light. We reveal that the temporally periodic variations of ultrasound-modulated light intensity depend on and vary with the ultrasonic power. Additionally, the light intensity varies periodically along the ultrasound beam. Finally, the light intensity varies periodically at twice the ultrasonic frequency for the ultrasound wave of low power.

Diffraction of an atom laser in the Raman-Nath regime

Atom interferometry is the most successful technique for precision metrology. However, current interferometers using ultracold atoms allows one to probe the interference pattern only momentarily and has finite duty cycle, resulting in an aliasing effect and a low-bandwidth measurement -- also known as Dick effect. Interferometry with a continuous-wave atom laser shows promise in overcoming these limitations due a continuous monitoring of the interference pattern. In this work, we demonstrate a key step towards such an interferometry by demonstrating a diffraction of an `atom laser' in the Raman-Nath regime. We outcouple a continuous beam of coherent atoms from a reservoir of $^{87}$Rb Bose-Einstein condensate (BEC) upto 400 ms. The `atom laser' interacts with a grating formed by a standing wave of a far detuned laser light. The atom laser diffracts into several orders going up to 9$^{th}$ order or up to momenta of $\pm 18\ \hbar k$. We have characterized the diffraction of atom laser for different conditions and the results match with numerical simulations. Such atom laser will allow for construction of an atom-interferometer to probe physics phenomenon continuously up to a time of the order of few hundred millisecond.

Characterization of pulsed ultrasound using optical detection in Raman-Nath regime

In this paper, we describe an optical detection method for the characterization of pulsed ultrasound based on acousto-optic interaction. We deduce the relationship between the ultrasound and the diffracted light from the principle of acousto-optic diffraction in the Raman-Nath regime, which is verified experimentally. Five ultrasonic transducers with different central frequencies and different focusing types are measured to show the method's performance regarding linearity, sound pressure measurement, phase measurement, frequency response, and spatial resolution. The experimental results show a good agreement with simulation data by CIVA (ultrasonic simulation software, M2M NDT, Inc.) and the pulse-echo method.

Transitions Between Quantized Standing Waves at Interactions with an Atom in the Raman–Nath Regime

The interaction of an atom with an electromagnetic field induces transitions between its quantized modes, in which cos- and sin-standing waves are chosen as the basis for quantization. The number of photons in the modes were calculated in the Raman–Nath regime in the approximation of adiabatic following. It is shown that in the course of transitions an initially populated mode is generally not completely emptied, and the oscillating part of photons increases with the total number in the system.

Small dosage of holographic exposure via a He-Ne laser to fabricate tunable liquid crystal phase gratings operated with low electric voltages

ABSTRACTThe fabrication processes of tunable liquid crystal (LC) phase gratings via small dosage of holographic exposure of a He-Ne laser beam were investigated. The initial LC cells were filled with various ratios of ingredients mainly including LCs and photocurable monomers. The fabricated LC phase gratings shown in the Raman–Nath regime possessed a maximum value of first-order diffraction efficiency close to 33.3% at 5.8 Vrms. Furthermore, optimised grating was demonstrated and used as an interference recorder in holography.

Signatures of spatial inversion asymmetry of an optical lattice observed in matter-wave diffraction

The structure of a two-dimensional honeycomb optical lattice potential with small inversion asymmetry is characterized using coherent diffraction of $^{87}$Rb atoms. We demonstrate that even a small potential asymmetry, with peak-to-peak amplitude of $\leq 2.3\%$ of the overall lattice potential, can lead to pronounced inversion asymmetry in the momentum-space diffraction pattern. The observed asymmetry is explained quantitatively by considering both Kapitza-Dirac scattering in the Raman-Nath regime, and also either perturbative or full-numerical treatment of the band structure of a periodic potential with a weak inversion-symmetry-breaking term. Our results have relevance for both the experimental development of coherent atom optics and the proper interpretation of time-of-flight assays of atomic materials in optical lattices.

Liquid crystal light valve operating at near infrared spectral range

A near infrared sensitive liquid crystal light valve, based on Ru-doped Bi12TiO20 (BTO:Ru) inorganic crystal and liquid crystal is proposed. The two-beam mixing experiments at Raman–Nath regime show promising gain value at 1064 nm. An image evolution through the proposed device is demonstrated supporting all optical switching property.

The Klein-Cook parameter in analyzing acousto-optic interaction in acoustically anisotropic media

A theoretical study of acousto-optic interaction in acoustically anisotropic media is performed. A new criterion for estimating the Raman-Nath and Bragg regimes of light diffraction by means of ultrasound in acoustic crystals is proposed. The modified Klein-Cook parameter is found to consider the walkoff of ultrasonic wave energy as a result of the elastic anisotropy of a medium.

QUANTUM CATALOGUING VIA ATOM INTERFERENCE

A quantum cataloguing scheme to identify any unknown quantized cavity field state using the atom interference is proposed. The scheme consists of sending a two-level state atom through two cavities, with Bragg and Raman-Nath regimes, respectively. We inveterate the atom interference from the second cavity to achieve different interference patterns which depend on the photons statistics.

Light diffraction by acoustophotorefractive dynamic gratings in Raman―Nath regime

Raman–Nath light diffraction on acoustophotorefractive holographic gratings that was recorded in cubic photorefractive crystals according to a synchronous detection mechanism was investigated. It was shown that the diffraction efficiency in sillenite-type gyrotropic crystals depended weakly on the incident light polarization and the external electric field strength. The highest diffraction efficiency in non-gyrotropic crystals was reached for p-polarized light (writing and reading) and a strong external electric field.

Free space diffraction on active gratings with balanced phase and gain/loss modulations

A theoretical analysis of asymmetrical diffraction in Raman-Nath, intermediate and Bragg diffraction regimes is presented. The asymmetry is achieved by combining matched periodic modulations of the phase and of the loss/gain of the material, which enables the breakdown of optical symmetry and redirects all resulting optical energy in only positive or only negative diffraction orders, depending on the quarter period shift directions between the phase and the loss/gain modulations. Analytic expressions for the amplitudes of the diffraction orders are derived based on rigorous multimode coupled mode equations in slowly varying amplitude approximation.

Deterministic secure communication via atomic momentum state

Abstract We present a secure direct communication scheme via atomic external degrees of freedom. The scheme consists of transmitting the secret message by superposition of atomic center-of-mass momentum states, quantum bit, via atomic scattering in the Bragg regime and Raman-Nath regime, respectively. The information can be coded/decoded during the transmission, without basis reconciliation, privacy amplification, and end of the communication. The security of information against any eavesdropper is ensured by the interference pattern.