Spontaneous Raman Emission Research Articles

Monitoring Raman emission through state population in cold atoms confined inside a hollow-core fiber.

We study the spontaneous Raman emission in an ensemble of laser-cooled three-level Λ-type atoms confined inside a hollow-core photonic-bandgap fiber using a novel approach to observe the process. Instead of detecting the emitted light, we measure the number of atoms in the ground state as a function of Raman pump time, which eliminates the need to suppress the pump photons with a high-resolution filter. We describe how this measurement can be used to detect superradiant emission from the atomic ensembles and estimate the number of atoms required to observe Raman superradiance in atomic clouds inside a hollow-core fiber.

Gas-phase Raman spectroscopy of non-reacting flows: comparison between free-space and cavity-based spontaneous Raman emission.

We report on a comparison of free-space and cavity-enhanced Raman spectroscopy for gas-phase measurements of nitrogen and oxygen in ambient air. Real-time analysis capabilities and continuous Raman signals with low power diodes make the technique non-invasive, affordable, compact, and applicable for usage in non-reacting flows. We derive a comprehensive model for estimation of photon emission for both free-space and cavity-based signals and discuss trade-offs in how to organize the cavity geometry for maximum gain relative to free space. Measurements in both free and cavity configurations are compared to the expected signals, demonstrating the usefulness of the model in predicting amplification. The present results can serve as a quick guide on how to use low-power continuous wave lasers in a cavity setup to obtain enhanced laser-induced spontaneous Raman scattering.

Generation of ideal thermal light in warm atomic vapor

We present the experimental generation of light with directly observable close-to-ideal thermal statistical properties. The thermal light state is prepared using a spontaneous Raman emission in a warm atomic vapor. The photon number statistics are evaluated by both the measurement of second-order correlation function and by the detailed analysis of the corresponding photon number distribution, which certifies the quality of the Bose–Einstein statistics generated by a natural physical mechanism. We further demonstrate the extension of the spectral bandwidth of the generated light to hundreds of MHz domain while keeping the ideal thermal statistics, which suggests a direct applicability of the presented source in a broad range of applications including optical metrology, tests of robustness of quantum communication protocols, or quantum thermodynamics.

Partially coherent noise-like pulse generation in amplified spontaneous Raman emission.

Amplified spontaneous Raman emission under picosecond pulse pumping is studied in detail both experimentally and numerically. It is found that the Raman output pulses are noise-like with partial coherence, which has important implications for various applications. Numerical simulation reproduces the finding well, along with the temporal and spectral dependence of the Raman pulse on the pump pulse energy. The numerical simulations of the temporal, spectral, and energy evolution of the Raman and pump pulse along the fiber gives insight on how to design such sources.

Pulsed amplified spontaneous Raman emission at 2.2 μm in silica-based fiber

All-fiber source at 2.2 μm is investigated with amplified spontaneous Raman scattering process in highly Ge-doped silica fiber. By optimizing the gain fiber length, the second-order Raman Stokes light at 2.43 μm is suppressed and 3 W first-order Raman Stokes light at 2.2 μm is obtained with a homemade 2-μm Q-switched Tm3+-doped fiber laser as pump source. The conversion efficiency is 35.9 % from 2.0 to 2.2 μm, and the peak power of the 2.2-μm laser is about 400 W.

Photon-pair source with controllable delay based on shaped inhomogeneous broadening of rare-earth-metal-doped solids

Spontaneous Raman emission in atomic gases provides an attractive source of photon pairs with a controllable delay. We show how this technique can be implemented in solid state systems by appropriately shaping the inhomogeneous broadening. Our proposal is eminently feasible with current technology and provides a realistic solution to entangle remote rare-earth doped solids in a heralded way.

Fabrication of Low-Loss Silicon Photonic Wires by Self-Profile Transformation and Applications in 3-D Photonic Integration and Nonlinear Optics

A new technology using self-profile transformation is introduced to make subwavelength silicon photonic wires with very smooth surface. Due to the smooth surface, the propagation loss was measured to be as low as 1.26 dB/cm. Moreover, 2-D beam spot-size converters and 2-D multimode interference couplers were demonstrated to monolithically integrate the photonic wires, showing the possibility of 3-D photonic integration. Owing to the low-insertion loss of the wires, various nonlinear optical processes such as four-wave mixing and spontaneous Raman emission were observed with low-pump power of just tens of milliwatts.

Creating single collective atomic excitations via spontaneous Raman emission in inhomogeneously broadened systems: Beyond the adiabatic approximation

The creation of single collective excitations in atomic ensembles via spontaneous Raman emission plays a key role in several quantum communication protocols, starting with the seminal DLCZ protocol [L.-M.Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature {\bf 414}, 413 (2001).] This process is usually analyzed theoretically under the assumptions that the write laser pulse inducing the Raman transition is far off-resonance, and that the atomic ensemble is only homogeneously broadened. Here we study the impact of near-resonance excitation for inhomogeneously broadened ensembles on the collective character of the created atomic excitation. Our results are particularly relevant for experiments with hot atomic gases and for potential future solid-state implementations.

Prospects for a waveguide Raman amplifier in porous silicon at 1.5 μm

AbstractRecently, the possibility of light generation and/or amplification in silicon, based on Raman emission, has achieved significant results. However, limitations inherent to the physics of silicon have been pointed out, too. In order to overcome these limitations, a possible option is to consider low dimensional silicon. In this paper, an approach based on Raman scattering in porous silicon is investigated. First, we point out two significant advantages with respect to silicon: the broadening of the spontaneous Raman emission and the tuning of the Stokes shift. Then, we discuss about the prospect of Raman amplifier in porous silicon. Finally the design of a Raman amplifier in porous silicon waveguide is proposed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

RAMAN APPROACH IN POROUS SILICON AT 1.5 μM

In the last years, the possibility of light generation and/or amplification in silicon, based on Raman emission, has achieved great results. However, some significant limitations, inherent to the physics of silicon, have been pointed out, too. In order to overcome these limitations, a possible option is to consider low dimensional silicon. Along this line of argument, an approach based on Raman scattering in porous silicon is presented. We prove two significant advantages with respect to silicon: the broadening of spontaneous Raman emission and the tuning of the Stokes shift. Finally, we discuss the prospect of Raman amplifier in porous silicon.

Spontaneous Raman emission in porous silicon at 1.5 µm and prospects for a Raman amplifier

In the last three years, the possibility of light generation and/or amplification in silicon, based on Raman emission, has achieved significant results. However, limitations inherent to the physics of silicon have also been pointed out. One possible option to overcome these limitations is to consider low dimensional silicon. In this paper, an approach based on Raman scattering in porous silicon is theoretically and experimentally investigated. We prove two significant advantages with respect to silicon: the broadening of the spontaneous Raman emission and the tuning of the Stokes shift. Finally, we discuss the prospect of a Raman amplifier in porous silicon.

Broadening and tuning of spontaneous Raman emission in porous silicon at 1.5μm

In the last three years, the possibility of light generation and/or amplification in silicon, based on Raman emission, has achieved significant results. However, limitations inherent to the physics of silicon have been pointed out, too. In this letter, an approach based on Raman scattering in porous silicon is investigated. Two significant advantages with respect to silicon are proved: the broadening of spontaneous Raman emission and the tuning of the Stokes shift. Finally, we discuss about the prospect of Raman amplifier in porous silicon.

Raman amplification and lasing in SiGe waveguides

We describe the first observation of spontaneous Raman emission, stimulated amplification, and lasing in a SiGe waveguide. A pulsed optical gain of 16dB and a lasing threshold of 25 W peak pulse power (20 mW average) is observed for a Si1-xGex waveguide with x=7.5%. At the same time, a 40 GHz frequency downshift is observed in the Raman spectrum compared to that of a silicon waveguide. The spectral shift can be attributed to the combination of composition- and strain-induced shift in the optical phonon frequency. The prospect of Germanium-Silicon-on-Oxide as a flexible Raman medium is discussed.

Laser cooling of trapped Fermi gases far below the Fermi temperature

We study the collective Raman cooling of a polarized trapped Fermi gas in the Festina Lente regime, when the heating effects associated with photon reabsorptions are suppressed. We predict that by adjusting the spontaneous Raman emission rates and using appropriately designed anharmonic traps, temperatures of the order of 2.7% of the Fermi temperature can be achieved in 3D.

Optically induced pulse delay in a solid-state Raman amplifier

The pump-induced group velocity reduction of subnanosecond pulses is calculated and measured in a Ba(NO3)2 solid-state Raman amplifier. 1.197 μm probe pulses with 90 ps duration were generated using a Raman-shifted mode-locked and Q-switched Nd:YAG laser, and propagated through a Ba(NO3)2 crystal synchronously pumped by 7-ns-long 1.06 μm pulses. The time delay of the pulse peak was measured with varying pump intensity and was compared with theoretical calculation up to the point where amplified spontaneous Raman emission becomes dominant. The maximum time delay was found to be 105 ps.

Theory of Raman scattering with pulses: Application to continuum Raman spectroscopy

A theory of real-time dependence of Raman scattering for a pulse-mode laser is developed within second-order perturbation theory and using the wavepacket terminology. The rate of spontaneous Raman emission with a pulse correctly reduces to the dynamical equivalent of the Kramers-Heisenberg-Dirac expression in the monochromatic limit. We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency. The rate of Raman emission as a function of time and pulse carrier frequency, from an initial ground vibrational state to various final vibrational states, is shown to be structureless for all pulses, and for pulses that are longer than the dissociation time the rate also rises and decays with the pulses. This is contrary to recent reports of recurring resonance fluorescence-type structures at long times after the pulse has vanished. We explain why such structures are unphysical for continuum Raman scattering. Results are also presented for excitation from an initial first excited vibrational state.

Detection of Molecular Hydrogen by Stimulated Raman Emission

Molecular hydrogen is a very difficult molecule to detect in a noninvasive manner. The molecule has no dipole moment. Excitation of its first electronic excited state requires light in the vacuum ultraviolet region, ~110 nm. Hydrogen has been shown to produce spontaneous Raman emission at pressures as low as 550 Torr, but the process occurs with a low quantum efficiency, and efficient collection of the emissions is difficult. Lower partial pressures of hydrogen can be detected with coherent anti-Stokes Raman spectroscopy (CARS), but the experimental apparatus can be prohibitively expensive.

Random occurrence of stimulated Raman scattering emission from liquid water microdroplets

Stimulated Raman scattering (SRS) spectra from micrometer-sized water droplets have been obtained in the range 2100 < Δν < 5100 cm-(1). A number of Raman bands have been individually identified (to our knowledge, for the first time), corresponding to fundamental OH- and OD-stretching vibrations and to vibrations of hydrogen-bonded molecular complexes. All bands exhibit the intense morphologydependent resonance features that are characteristic of SRS emission from microdroplets. SRS emission is apparently random from all bands; however, the frequency of occurrence varies widely, from bands where emission is seen on practically every laser shot to bands where emission is seen only once in > 10(4) laser shots. Possible causes of these noteworthy emission features are discussed, including the difficulty of coupling weak spontaneous Raman emission to both the intense pump beam and the morphologydependent resonances within the droplet.

Measurement of aerosol/gas reaction rates by microparticle Raman spectroscopy

Chemical reactions between a reactive gas and single solid and liquid aerosol particles have been studied using an electrodynamic balance coupled to a Raman spectrometer. Electrodynamic levitation was used to suspend a single charged microparticle in a polarized laser beam, which was used as the light source for spontaneous Raman emission. The highly exothermic reaction between bromine vapor and an olefin microdroplet, 1-octadecene, and the reaction between a sorbent particle, calcium oxide, and a humid air stream containing sulfur dioxide have been explored by following the formation of a Raman-active chemical bond.

Observations of Descartes ring stimulated Raman scattering in micrometer-sized water droplets

Stimulated Raman scattering (SRS) from micrometer-sized droplets, which results from coupling of spontaneous Raman emission to droplet morphology-dependent resonances (MDR's), exhibits unique characteristics. Spatial patterns consist of bright SRS arcs on the droplet rim. A second source of SRS emission has recently been observed from a ringlike region encircling the droplet axis near the geometric focus (the Descartes ring). Investigation of the time and spectral characteristics of Descartes ring SRS and its suppression by the addition of absorptive dye to the droplet reveals it to be an additional manifestation of droplet MDR's. We conjecture that the Descartes ring results when the MDR light is scattered by refractive-index inhomogeneities produced by the intense pump field within the droplet.