Raman-active Medium Research Articles

Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be Mx2=2.54,My2=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.

High-efficiency anti-stokes Raman blue laser in CO2 enables high-luminance RGB laser-driven white light

Stimulated Raman scattering (SRS) is an efficient nonlinear frequency conversion method, enabling simultaneous generation of red, green, and blue (RGB) lasers. In order to synthesize a white light source for laser display by SRS, a 532 nm laser was used as pump source, and high purity gaseous carbon dioxide (CO2) was used as the Raman active medium. First, by optimizing experimental parameters, with an f = 1.5 m focal lens, in 0.8 atm CO2 pumped at 304 mJ, a first-order anti-Stokes (AS1) 495 nm blue laser was achieved, with an energy of 31.9 mJ, peak power of 10.9 MW and conversion efficiency (CE) of 10.5 %. Then, Then, the second-order Stokes light (S2) at 624 nm, residual pump laser (S0) at 532 nm, and AS1 laser were utilized as RGB primary colors. By the variation of pressure below 1 atmosphere (atm), the laser-driven white light (LDWL) with adjustable correlated color temperature (CCT) below 4700 K were simulated. Finally, LDWL up to 2.6 × 1018cd/m2 of a CCT of 3300 K could be synthesized at an RGB power ratio of PR: PG: PB = 0.447:0.094:0.459, resulting in a white light power CE of 44.4 % and luminous efficacy of 113.7 lm/W. In addition, use of the 574 nm yellow Raman light is expected to realize a four-primary (RGBY) laser display scheme with higher Luminance and broader color gamut. Moreover, the feasibility of using a 515 nm Yb: YAG laser as pump source to widen the range of CCT and improve the brightness of LDWL was discussed.

Asymmetric manipulations on the coherent Raman sideband generation under molecular modulations

We study the light evolution through a Raman-active medium, where a pair of fields pump molecules near-resonantly and excite the molecular coherence. A synthetic lattice including multiple frequency sidebands via the coherent Raman process is constructed. By varying the density of molecules in the light propagation direction, the system can present a lattice model undergoing dynamic modulations. Further considering the frequency-dependent couplings between synthetic lattice sites, we explore intriguing phenomena, including spectral Bloch oscillation and super Bloch oscillation with asymmetric patterns, which leads to interesting frequency-conversion phenomena with the Raman sideband generations. Our work could be useful for future experiments in coherent Raman sideband processes towards ultrashort pulse generations and higher-frequency conversion.

High efficiency ethane Raman laser pumped by 532 nm laser

In this work, 532 nm laser was used as the pump source and pressurized ethane was used as Raman active medium; 631.3 nm first Stokes (S1) and 776.0 nm second Stokes (S2) Raman lasers were generated. By the optimization of ethane pressure and focus lens, S2 and backwards first Stokes Raman conversion could be reduced, while the conversion of S1 could be improved. Under 2.0 MPa ethane and focal length of 1.5 m, the maximum S1 conversion efficiency of 84.5% was achieved. Up to 68.7 mJ S1 Raman laser was obtained, and the corresponding peak power is 17.2 MW. By the comparison of the threshold of methane Raman laser, Raman gain coefficients of ethane were estimated to be 0.50 × 10−12 m/W at 0.5 MPa and 0.76 × 10–12 m/W at 1.4 MPa ethane respectively.

Stimulated vibrational-rotational Raman scattering of hydrogen pumped at a 1064-nm laser.

A ∼2.1-µm laser is within an atmospheric transmission window and can be used in remote sensing. In this work, a 1064-nm laser was used as the pump source, pressurized hydrogen was used as the Raman active medium, and a dual-wavelength ∼2.1-µm Raman laser was generated. The 2147-nm laser was generated by a combination processes of stimulated vibrational Raman scattering and stimulated rotational Raman scattering, while a 2132-nm laser was generated by stimulated S-branch vibrational Raman scattering. Optimizing experimental conditions yielded a maximum pulse energy of 76.1mJ, a peak power of ∼9.2M W, and a photon conversion efficiency of 29.8%.

Generation of waveform-tunable unipolar pulses in a nonlinear resonant medium

We theoretically demonstrate the possibility to produce unipolar pulses of varying temporal profile in an extended layer of a resonant medium with the nonlinear coupling to the driving electric field. The proposed approach relies on the coherent control of the low-frequency medium oscillations by a sequence of ultrashort excitation pulses as well as the collective emission of multiple resonant centers along an optically thick medium layer. In particular, we show that creating an inhomogeneous spatial profile of the density of resonant centers allows obtaining unipolar pulses of adjustable waveform, such as rectangular or triangular ones. Such optical response could be realized in Raman-active media, e.g., in molecular crystals with well-resolved vibrational terahertz resonances.

Near IR Raman laser beams generated by 1064 nm pumped Stimulated Raman Scattering (SRS) of pressurized nitrogen

Nitrogen is a nonflammable, nonexplosive and cheap Raman active gas. N2 has a vibrational Raman shift of 2330 cm −1, and there are no other Raman active media with a similar Raman shift. Therefore, the SRS of N2 plays an essential role in expanding the spectral range of Raman lasers. In this work, a pulsed 1064 nm laser was used as a pump source, and by the first and second Stokes SRS of pressurized N2, 1415 nm and 2110 nm laser beams were generated. By optimizing N2 pressure and focus configuration, the maximum pulse energies of 1415 nm and 2110 nm Raman lasers reached 109.9 mJ and 22.5 mJ, respectively. Their corresponding peak powers obtained were 20.7 MW and 4.4 MW, respectively. Comparing the thresholds of H2 (hydrogen) and N2, the first Stokes (S1) Raman gain coefficient of 1.5 MPa N2 was estimated to be about 1/9 of H2.

Enhanced Raman gain coefficients of semiconductor magneto-plasmas

Considering the origin of stimulated Raman scattering in Raman susceptibility, we obtain expressions for Raman gain coefficients (under steady-state and transient regimes) of semiconductors magneto-plasmas under various geometrical configurations. The threshold value of excitation intensity and most favourable value of pulse duration (above which transient Raman gain vanishes) are estimated. For numerical calculations, we consider n-InSb crystal at 77 K temperature as a Raman active medium exposed to a frequency doubled pulsed CO2 laser. The variation of Raman gain coefficients on doping concentration, magnetic field and its inclination, scattering angle and pump pulse duration have been explored in detail with aim to determine suitable values of these controllable parameters to enhance Raman gain coefficients at lower threshold intensities and to establish the suitability of semiconductor magneto-plasmas as hosts for compression of scattered pulses and fabrication of efficient Raman amplifiers and widely tunable Raman oscillators.

Deep- to near-ultraviolet Raman frequency conversion pumped by femtosecond pulses in a hollow-core waveguide.

Broadband vibrational/rotational Raman generation ranging from deep ultraviolet (DUV) to blue wavelengths is demonstrated by using molecular hydrogen in a hollow-core waveguide as a Raman-active medium pumped by a femtosecond DUV laser. We find the high-order transient stimulated Raman scattering is drastically enhanced for input beams including a circularly polarized component; a circularly polarized input beam achieves the highest conversion efficiency. Coherent vibrational anti-Stokes Raman emission is observed only for a circularly polarized pump beam, indicating that the waveguide effect also contributes to the upconversion of a DUV pulse via transient stimulated Raman scattering.

Suppression of self-induced thermal lensing in stimulated Raman scattering of liquids

Stimulated Raman scattering (SRS), due to the interaction of intense laser beams with Raman active media, provides an effective operational mode for frequency conversion, downgraded (primarily in liquids) by self-induced thermal lensing (TL). Here, a unique setup, including a pump laser beam [green (532 nm), 10 Hz, 5 ns] passing through a rotating cell, filled with a saturated aqueous solution of sodium nitrate ( N a N O 3 ) was tested, for conversion to a yellow (564 nm) beam. This distinct geometry allowed each pump pulse to pass through a “fresh” solution, practically eliminating TL. A sharp enhancement in SRS generation ( ∼ 20 % ) and observation of unexpanded beams’ cross sections at the cell exit verified TL removal. The results and the obtained physical insight from the N a N O 3 model system point to the promise of this approach for suppression of impeding TL in frequency conversion by SRS in liquids.

Investigation of multispectral SF6 stimulated Raman scattering laser

In this work, SF6 as a Raman-active medium is investigated to generate a multispectral Raman laser by the combination of cascade stimulated Raman scattering (SRS) and four wave mixing. The Raman frequency comb from the 10th-order anti-Stokes to the 9th-order Stokes was generated, and its spectral range covered 377–846 nm. The photon conversion efficiency of 16.4% for the first Stokes was achieved, and the Raman gain coefficient at 1.5 MPa of SF6 under the 532 nm pump laser was calculated to be 0.83 cm/GW by the SRS threshold comparison with H2. Using helium as the carrier gas, the thermal effect of the SF6 Raman laser was improved dramatically under a repetition rate of 10 Hz.

Preparation and Decay of a Single Quantum of Vibration at Ambient Conditions

A single quantum of excitation of a mechanical oscillator is a textbook example of the principles of quantum physics. Mechanical oscillators, despite their pervasive presence in nature and modern technology, do not generically exist in an excited Fock state. In the past few years, careful isolation of GHz-frequency nano-scale oscillators has allowed experimenters to prepare such states at milli-Kelvin temperatures. These developments illustrate the tension between the basic predictions of quantum mechanics that should apply to all mechanical oscillators existing even at ambient conditions, and the complex experiments in extreme conditions required to observe those predictions. We resolve the tension by creating a single Fock state of a vibration mode of a crystal at room temperature using a technique that can be applied to any Raman-active system. After exciting a bulk diamond with a femtosecond laser pulse and detecting a Stokes-shifted photon, the 40~THz Raman-active internal vibrational mode is prepared in the Fock state $|1>$ with $98.5\%$ probability. The vibrational state is read out by a subsequent pulse, which when subjected to a Hanbury-Brown-Twiss intensity correlation measurement reveals the sub-Poisson number statistics of the vibrational mode. By controlling the delay between the two pulses we are able to witness the decay of the vibrational Fock state over its $3.9$ ps lifetime at room temperature. Our technique is agnostic to specific selection rules, and should thus be applicable to any Raman-active medium, opening a new generic approach to the experimental study of quantum effects related to vibrational degrees of freedom in molecules and solid-state systems.

Stimulated Raman Scattering at Induced Acoustic Vibrations of Elastic Particles in Suspensions Placed in an Optical Cavity

The stimulated low-frequency Raman scattering at acoustic vibrations of elastic inclusions in a liquid heterogeneous medium placed in an external open optical cavity has been investigated. The amplitudes of stationary field oscillations in the cavity at the frequencies of the excitation and the first Stokes component are found within a two-dimensional model. The specific features of the oscillation in dependence of the parameters of the cavity and Raman-active medium have been studied based on the simulation results.

Transmission spectra of one-dimensional porous alumina photonic crystals

The optical properties of a one-dimensional photonic crystal based on porous alumina with a crystal lattice constant of 460 nm were studied. The experimentally measured transmission spectrum of a one-dimensional photonic crystal film was compared with the theoretical spectrum in the spectral regions corresponding to the first, second, and third stop bands. The potential for using one-dimensional porous alumina photonic crystals as selective narrow-band filters and sensors was analyzed. Embedding a Raman-active medium into the voids of porous photonic crystal should provide an essential increase of spontaneous Raman scattering signal and low-threshold stimulated Raman scattering. The amount of substance needed for recording Raman spectra may be substantially decreased.

Vibrational spectroscopy via stimulated Raman induced Kerr lensing

We present a new method for the measurement of the stimulated Raman spectrum based on time-dependent spatial modulation of a laser beam as it passes through a Raman active medium. This effect is similar to the instantaneous Kerr lensing and Kerr deflection yet involves resonant vibrations which result in a time-dependent refractive index change. We use sub-nanojoule pulses together with a sensitive pump-probe measurement apparatus to excite and detect the fine (10−5–10−4) temporal and spatial variations in intensity resulting from the Raman-induced Kerr effect. We demonstrate the effect by changing the spatial overlap between the pump and probe at the sample and measuring the time-dependent deformation of the probe beam’s cross section. This method is particularly useful for detection of low-frequency Raman lines, as we demonstrate by measuring the Raman spectrum of neat liquids in a cuvette.

On the analytical description of self-frequency shift of a pulse propagating in the region of zero group-velocity dispersion

An analytical expression describing the frequency shift of nonsoliton pulses in a nonlinear Raman-active medium is obtained by the method of moments. The regime of propagation in which the central frequency of the signal falls into the region of zero group-velocity dispersion is analyzed.

Nonlinear mixing of optical vortices with fractional topological charge in Raman sideband generation

We studied the nonlinear parametric interaction of femtosecond fractionally-charged optical vortices in a Raman-active medium. Propagation of such beams was described using the Kirchhoff–Fresnel integrals by embedding a non-integer 2π phase step in a Gaussian beam profile. When using fractionally-charged pump or Stokes beams, we observed the production of new topological charge and phase discontinuities in the Raman field. These newly generated fractionally-charged Raman vortex beams were found to follow the same orbital angular momentum algebra derived by Strohaber et al (2012 Opt. Lett. 37 3411) for integer vortex beams.

Generation of unipolar pulses in a circular Raman-active medium excited by few-cycle optical pulses

We study theoretically a new possibility of unipolar pulses generation in Raman-active medium excited by a series of few-cycle optical pulses. We consider the case when the Raman-active particles are uniformly distributed along the circle, and demonstrate a possibility to obtain a unipolar rectangular video pulses with an arbitrarily long duration, ranging from a minimum value equal to the natural period of the low frequency vibrations in the Raman-active medium.

Particular features of the emission of radiation by a superluminally excited Raman-active medium

Particular features of the emission of radiation by a Raman-active medium excited by a sequence of ultrashort superluminal pulses have been studied theoretically. It is shown that such an excitation gives rise to the possibility of obtaining unipolar rectangular videopulses the duration and amplitude of which depend on the velocity of propagation of the excitation over the medium.

Generation of unipolar optical pulses in a Raman-active medium

Response of a Raman-active media (RAM) to the excitation by a series of ultrashort (few-cycle) optical pulses propagating at a superluminal velocity is studied theoretically. It is shown that under certain conditions rectangular unipolar pulses (video-pulses) can be generated as the RAM response. The duration, shape and amplitude of these video-pulses can be widely tuned by modifying the pump pulse parameters.