Stimulated Raman Radiation Research Articles

Multifrequency stimulated Raman scattering in ethanol under picosecond laser excitation

For the first time, stimulated Raman scattering (SRS) was detected in rectified ethanol upon excitation by ultrashort (55 ps) pulses of a YAG:Nd3+ laser operated at a wavelength of 355 nm. SRS of light is observed in the form of several Stokes components on the symmetric mode of the CH3 group. The large Raman shift of the Stokes components, as well as the relatively high gain of stimulated Raman radiation, makes it possible to effectively use ethanol for the conversion of laser radiation into different spectral ranges.

Controlling cross pumping between C-N and C-H vibration in nitromethane by selective fluorescence-enhanced stimulated Raman scattering.

To investigate the vibrational features of nitromethane (NM), which is a kind of energy material and a well known low-sensitivity and high explosive, experiments are performed to obtain the stimulated Raman scattering (SRS) of NM by employing a 532 nm pulsed pump laser. The Raman signal involves two stimulated emissions at 918 and 2,963 cm-1, attributed to the C-N and C-H stretching vibrations, respectively. To overcome the complexity of cross pump in the pure NM, one stimulated Raman radiation is chosen as a pump source to excite the other Raman mode. Two fluorescence dyes were added to selectively enhance each Raman cross section. By internally seeding the Raman gain medium with fluorescent photons, a significant modification in the stimulated Raman scattering spectrum has been observed. The enhanced Stokes emission at 918 cm-1 was able to induce the 2,963 cm-1 vibration mode when the all-trans-β-carotene was internal seeding in the NM, while the Raman radiation at 2,963 cm-1 was enhanced to excite the C-N mode with the addition of m-Cresol purple. The output energy of both 918 and 2,963 cm-1 under different input energy was also measured to illustrate this result.

Investigation of collisionally induced stimulated scattering in sodium vapor with temporal and spectral resolution

In this work we present fully time-resolved spectra of Amplified Spontaneous Emission (ASE) and Stimulated Raman Scattering (SRS) in sodium vapor. We observe that the ASE signal precedes the SRS emission, and the peak of the amplified Stokes pulse is delayed with respect to the peak of the laser pulse. The time evolution of the line widths of the ASE and Raman signals is also shown. We find that the line widths of the stimulated fluorescence component (ASE) and the stimulated Raman radiation change with time. Both spectra start as small narrow fluctuating lines, then the line widths reach maximum values and thereafter decrease with time.

Third-order sum-frequency generation in droplets: experimental results

New experimental third-order sum-frequency generation (TSFG) spectra are presented for CCl4 droplets. The droplet radii are discretely tuned over a size range in increments of 1 part in 6000. For each droplet radius only one or a few of the many possible TSFG peaks are dominant. Both low- and high-resolution TSFG spectra of D2O droplets are measured. The possible generating waves are the stimulated Raman radiation [known to occur at the morphology-dependent resonance (MDR) frequencies] and the internal fields at the incident laser frequency, which is usually not on a MDR. The experimental findings suggest that the resultant TSFG frequency must be near an output MDR. A physical description of the third-harmonic generation (THG) process in droplets is presented. MDR’s are treated as traveling waves that rotate in the azimuthal direction. Their azimuthal phase velocities are derived, and the concept of phase matching among the MDR’s is presented. The relation between phase matching and spatial overlap of MDR’s is discussed. Reasons are presented for the low probability of having the THG or TSFG frequency on or near a MDR.

PHOTODISSOCIATION OF PbCl2

The population on 6p2 3P2 metastable state and 6p2 3P0 ground state of Pb atoms are de-monstrated, which are originated from dissociating PbCl2 by the fourth harmonic of YAG lasers. The population ratio of 3P2 to 3P0 is measured in terms of laser-induced fluorescence. The results indicate that the dissociation of PbCl2 is a two photon process and the population inversion between 3P2 and 3P0 exists. The stimulated Raman radiation based on the population of 3P2 metastable state is observed. The possible way of dissociation is studied.

Efficient generation of picosecond coherent tunable radiation between 190 and 212 nm by sum-frequency mixing from Raman and optical parametric radiations

An efficient sum-frequency mixing of optical parametric radiation and stimulated Raman radiation has been used to generate picosecond continuously tunable coherent radiation ranging from 190 to 212 nm with energies of 20-40 \mu J. An ordinary KDP crystal was phase-matched by using a combination of long and short wavelength incident waves.

Tunnel generation and locking stimulated Raman radiation components

Stimulated Raman radiation is considered in a ring optical resonator. It is supposed that the resonator is partly filled with a condensed medium at the full length of the medium close to several tunnel lengths. A Raman-active medium and an optical waveguide are assumed to occupy a part of the condensed medium. The feasibility is shown of tunnel generation and synchronization of a great number of Stokes components of stimulated Raman radiation. Here, femtosecond light pulses are shown to be formed in a spacing adjacent to the Raman-active medium, the pulses being single over the period <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T = 2\pi/\omega_{R}</tex> (ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> is the Raman frequency shift). Predicted and considered also are tunnel generation and locking of new light modes in an optical parametric oscillator. The effects are due to a correction for the dispersion of the full effective refractive index of the medium inside the resonator. Numerical calculations of the correction for a glass thin-film optical waveguide, quartz, and the Raman-active media CaCO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> , C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> , CS <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> inside the resonator are represented. Here, five or six Stokes components are shown to be generated and locked for CaCO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> and C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> , eight Stokes components for CS <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . A similar numerical calculation is represented for the active medium LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> in the corresponding optical parameteric oscillator. Here, new modes covering a frequency bandwidth of 5000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are shown to be feasible. Noted and discussed is also a higher-order correction for the effective refractive index dispersion making it feasible to broaden the frequency bandwidth for tunnel generation and locking.

Small-stokes-shift frequency conversion in single-mode birefringent fibers

We report experimental observation of nondegenerate four-photon mixing (FPM) via a new phase-matched process in birefringent fibers. This phase-matching process is characterized by relatively long coherence lengths, and by small pump-to-Stokes frequency shifts (of the order of 100 cm -1 in high birefringence fibers) that are directly proportional to the fiber birefringence. Pump-to-Stokes energy conversion efficiencies of over 40 percent have been obtained with moderate pump powers (≈1 kW) from a visible cw mode-locked dye laser coupled into single-mode polarization-preserving high-birefringence fibers of a few meters length. In addition, we report the first observation of a totally non-degenerate phase-matched four-photon process in a fiber; this invokes mixing of the original FPM Stokes component with the pump and stimulated Raman radiation.

'Tunnel' Locking Stimulated Raman Radiation Components in an Optical Resonator

Stimulated Raman radiation is considered in an optical resonator excited by an external light beam. It is assumed that the resonator is filled with a dispersive (condensed) medium for a length of the medium (along the resonator axis) close to several ‘tunnel’ lengths. A Raman-active medium is assumed to fill part of the resonator length along the resonator axis. Numerical calculations are presented of tunnel locking stimulated Raman radiation components. The shape of light pulses is calculated, and the band of locking Stokes components is derived. The feasibility of generating a light pulse of duration 3 × 10-14 −4 × 10-15 s in a frequency region close to the point ∂2 n eff / ∂ω2 = 0, located usually in the infrared spectral region (n eff is the effective refractive index of the substance inside the resonator) is discussed. Also discussed is the feasibility of correcting the refractive index dispersion and generating femtosecond light pulses in the visible spectral region.

Nonlinear optical phenomena in a multiple interferometer

A multiple interferometer is proposed and stimulated Raman radiation, stimulated Mandel’stam–Brillouin radiation, and Cerenkov-type optical parametric oscillation are considered in this interferometer, which is excited by an external light beam. An extremely low threshold intensity of the exciting beam is noted. Optical bistability and hysteresis, self-pulsing of the radiation intensities, amplification of amplitude-phase deviations in the incident pumping beam, limitation of the intensity of the beam transmitted by the interferometer, and nonlinear reflection of the beam from the multiple interferometer are predicted and discussed. It is also noted that the threshold for generation of the second Stokes component of stimulated Raman radiation and stimulated Mandel’stam–Brillouin radiation under the conditions discussed is usually not achieved, even for high excess of the exciting beam intensity over the threshold for generation of the first Stokes component.

On the Theory of Stimulated Raman Radiation in an Optical Resonator

Stimulated Raman radiation involving an arbitrary number of Stokes components in an optical resonator is considered for the case when a pumping wave is directly incident on a Raman-active condensed substance at an angle θ to the resonator axis. The expressions are derived for the steady-state radiation intensities of all the Stokes components generated, and the number of the components versus the pumping wave intensity is discussed. The conditions of the effective conversion of the pumping wave into radiation of Stokes components are elucidated. It is shown that in a frequency region close to the point ∂2 n/∂ ω2 = 0 (n is the effective refractive index of the substance inside the resonator) locking Stokes components is feasible due to a ‘tunnel’ traversal of femtosecond pulses through a dispersive medium, provided the latter occupies a characteristic tunnel length along the resonator axis. For the case of the three components generated, the wave band over which locking occurs is derived and a frequency mo...

Spectral properties of stimulated Raman radiation from an intense relativistic electron beam

Spectra have been obtained in the range of wavelengths from 1–3 mm for the stimulated Raman scattering from an intense relativistic electron beam which propagates in a rippled magnetostatic field (’’zero frequency pump’’). The Raman scattering from the plasma space charge wave and the cyclotron wave idler modes has been observed and the frequency dependence upon beam energy, rippled period, and guiding magnetic field is found to be in accord with theory. Results suggest that the spectral linewidth for the scattered radiation is influenced by beam energy spread and the amplitude of the rippled field pump.

Generation of Raman radiation in a long stimulated-Raman-scattering medium

An experimental study was made of the generation of stimulated Raman radiation excited in a long gaseous medium. The use of such medium made it possible to increase considerably the efficiency of excitation of high-order anti-Stokes components, to obtain strong Raman radiation at low gas pressures, and to construct a multifrequency Raman laser with a uniform distribution of the intensity between the spectral components.

Stimulated Raman Radiation and Stimulated Mandel'stam-Brillouin Radiation in a Laser Resonator

In this paper we consider the process of the generation of stimulated Mandel'stam-Brillouin radiation and of stimulated Raman radiation in the laser itself at an arbitrary number of components generated for the case where only one axial mode in each component is excited. The expressions for the steady-state radiation intensities of all the components generated versus the speed of active particle excitation in laser material, as well as the expressions for threshold values of this speed necessary to generate a given number of components, have been obtained. The conditions of possible effective conversion of laser light into radiation of stimulated scattering components have been cleared out. The feasibility of producing super-short light pulses due to synchronization of stimulated Mandel'stam-Brillouin radiation components by introducing a non-linear absorber into a resonator has been established.

Gain, Frequency Shift, and Angular Distribution of Stimulated Raman Radiations under Multimode Excitation

A theoretical explanation of anomalous gain, large frequency shift, and variable angular distribution in the stimulated Raman radiation from liquids under multimode excitation is presented. The laser output under multimode oscillation is treated as a series of very sharp pulses which come out at the recurrence frequency of axial mode separation. The anomalous Stokes gain and its apparent independence of the mode structure are explained when the relaxation time of the molecule is between 10-9 and 10-8s. The intensity-dependent index of refraction of the medium produces phase shifts in the Raman radiation. This is shown to produce considerable frequency shifts and changes in the angular distribution of the Raman radiations which are emitted from a filamentary region under multimode excitation.

Angular distribution and frequency shift of stimulated Raman radiation

Angular distribution and frequency shift of stimulated Raman radiation

Calculation of the polar diagram and power output of stimulated Raman radiation

A calculation is outlined which gives simple formulae for the polar diagram and power output of stimulated Stokes radiation from a cylindrical focal region.

Angular Distribution of Stimulated Raman Radiation

The angular distribution of Stokes and anti-Stokes radiations generated by a laser beam in a filamentary region is calculated. Three cases are discussed: one is the surface radiation which is found to appear if the filament is very thin and long. Secondly, if the first-order Stokes is broadly generated off-axis and the filament is thick, the theory gives volume phase-matching which has been observed as class I radiation. Finally, if the first-order Stokes is strongly directed along the axis and the filament is thin, Raman radiations at angles between those of surface and volume phase-matching conditions are found. They represent class II radiations which have been normally observed in many liquids. Theoretical cone-angles of higher-order Stokes and anti-Stokes show very good agreement with experimental observations by E. Garmire. Some properties of class I and class II radiations are also explained by this theory.

Laser Generation of Excitons and Fluorescence in Anthracene Crystals

Experimental and theoretical studies are reported of the short-lived and delayed fluorescence of anthracene single crystals, excited by single- and double-photon absorption. A giant-pulse ruby laser provides the primary source of radiation of 14 400 cm−1 (up to 1027 photons/cm2·sec) and is also used to generate second-harmonic radiation from ADP, as well as stimulated Raman radiation of 12 800 and 17 500 cm−1 from liquid oxygen. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temperature and excitation wavelength. The very intense fast fluorescence with a half-life of 30 nsec at 300°K, characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are separated using sectored disks. It is concluded that the triplet state at 14 750 cm−1 can be populated (i) by direct absorption of laser photons involving an activation energy of 350 cm−1; (ii) via two-photon absorption, presumably leading to a vibrationally excited state of the 1B2u exciton, followed by intersystem crossing; (iii) via one-photon (second-harmonic) excitation from levels≥700 cm−1 into the singlet absorption band, followed by conversion of the singlet exciton into a triplet pair. The latter process is suggested by the observed activation energy of 700 cm−1. In agreement with these interpretations, the delayed fluorescence intensity is found to vary with the second to fourth power of the laser intensity depending on the experimental conditions. Also, light of 17 500 cm−1 leads exclusively to Process (i), light of 12 800 cm−1 exclusively to (ii). Triplet lifetimes from 2–17 msec are obtained, depending on crystal purity, which indicates that unimolecular triplet decay is an extrinsic, radiationless process. A singlet—triplet intersystem crossing rate constant of about 3×10−5 sec−1 is estimated. The triplet—triplet annihilation rate constant is found to be about 5×10−11 cm3 sec−1. This value considered together with the triplet-pair creation process suggests a triplet exchange rate ≳ 1013 sec−1 and a triplet diffusion constant ≳o5×10−4cm2/sec.

Quantum theoretical treatment of stimulated raman radiation

Quantum theoretical treatment of stimulated raman radiation