Stokes Beam Research Articles

Highly Sensitive Vibrational Imaging by Femtosecond Pulse Stimulated Raman Loss

Nonlinear vibrational imaging of live cells and organisms is demonstrated by detecting femtosecond pulse stimulated Raman loss. Femtosecond pulse excitation produced a 12 times larger stimulated Raman loss signal than picosecond pulse excitation. The large signal allowed real-time imaging of the conversion of deuterated palmitic acid into lipid droplets inside live cells, and three-dimensional sectioning of fat storage in live C. elegans. With the majority of the excitation power contributed by the Stokes beam in the 1.0 to 1.2 μm wavelength range, photodamage of biological samples was not observed.

Generation of bright quadricolor continuous-variable entanglement by four-wave-mixing process

We propose an experimentally feasible scheme to produce bright quadricolor continuous-variable (CV) entanglement by a four-wave mixing process (FWM) with four-level atoms inside the optical ring cavities operating above threshold. The Stokes and anti-Stokes beams are generated via the pump beam (tuned close to one atomic transition) and the coupling beam (tuned to the resonance of another atomic transition), respectively. The quadruply resonant and narrowed linewidth of the cavity fields with different frequencies are achieved and quadricolor CV entanglement among the four cavity fields is demonstrated according to the criterion proposed by van Loock and Furusawa [Phys. Rev. A 67, 052315 (2003)]. This scheme provides a way to generate bright quadricolor CV entanglement and will be significant for applications in quantum information processing and quantum networks.

Focus on spectroscopy

Focus on spectroscopy

Highly efficient Brillouin amplification of strong Stokes seed

Brillouin amplification of strong Stokes seed, which can achieve the output energy of 400 mJ and amplified efficiency of about 85% under the condition that the energy of Stokes beam and pump beam is in the ratio of 39.5/1, is reported. The relationship between output energy of strong signal Brillouin amplification and the energy of Stokes beam as well as pump beam, the interaction length, absorption coefficient is investigated. The design of strong signal Brillouin amplification system with high output energy is also provided.

Raman beam cleanup in silicon in the mid-infrared

We report evidence of beam cleanup during stimulated Raman scattering in silicon. An amplified near-diffraction-limited Stokes beam is obtained from a severely aberrated pump beam.

Dual-pump CARS temperature and major species concentration measurements in counter-flow methane flames using narrowband pump and broadband Stokes lasers

Dual-pump coherent anti-Stokes Raman scattering (CARS) is used to measure temperature and species profiles in representative non-premixed and partially-premixed CH 4/O 2/N 2 flames. A new laser system has been developed to generate a tunable single-frequency beam for the second pump beam in the dual-pump N 2–CO 2 CARS process. The second harmonic output (∼532 nm) from an injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. The second single-longitudinal-mode pump beam centered near 561 nm is generated using an injection-seeded optical parametric oscillator, consisting of two non-linear β-BBO crystals, pumped using the third harmonic output (∼355 nm) of the same Nd:YAG laser. A broadband dye laser (BBDL), pumped using the second harmonic output of an unseeded Nd:YAG laser, is employed to produce the Stokes beam centered near 607 nm with full-width-at-half-maximum of ∼250 cm −1. The three beams are focused between two opposing nozzles of a counter-flow burner facility to measure temperature and major species concentrations in a variety of CH 4/O 2/N 2 non-premixed and partially-premixed flames stabilized at a global strain rate of 20 s −1 at atmospheric-pressure. For the non-premixed flames, excellent agreement is observed between the measured profiles of temperature and CO 2/N 2 concentration ratios with those calculated using an opposed-flow flame code with detailed chemistry and molecular transport submodels. For partially-premixed flames, with the rich side premixing level beyond the stable premixed flame limit, the calculations overestimate the distance between the premixed and the non-premixed flamefronts. Consequently, the calculated temperatures near the rich, premixed flame are higher than those measured. Accurate prediction of the distance between the premixed and the non-premixed flames provides an interesting challenge for future computations.

Modeling of inter-modal Brillouin gain in higher-order-mode fibers

Finite element calculations of inter-modal Brillouin gain between LP(0n) modes in acoustically-inhomogeneous higher order mode (HOM) fibers are presented. When the pump beam is launched in the LP(08) mode, the LP(01) mode of the Stokes beam experiences the highest gain, approximately 6.7 dB higher than the peak LP(08)-LP(08) gain. An LP(01) Stokes beam experiences successively more Brillouin gain when pumped by higher-order LP(0n) modes.

High Resolution Wide Field Stimulated Raman Scattering Microscopy

Fluorescent imaging modalities, such as STED, PALM and STORM, has demonstrated the feasibility of super-resolution imaging. However, no comparable super-resolution imaging has been achieved based on non-fluorescent contrast mechanisms. We present a novel super-resolution approach based on incorporating stimulated Raman scattering (SRS) contrast into a standing-wave (SW) total internal reflection microscope. SW-SRS microscopy has the potential to improve the lateral resolution of current SRS microscopy in total internal reflection geometry. There is a critical difficulty to implement SW-SRS microscopy. Stimulated Raman gain, SRG, is a weak modulation of the intensity of the Stokes beam. The ratio of the SRG to Stokes beam intensity is a function of pump beam instantaneous intensity. The need for wide field imaging further reduces pump beam flux resulting in very unfavorable SRG to Stokes beam intensity ratio. As an example, using a standard Ti-Sapphire laser exciting a 100 × 100 micron square region, SRG to Strokes beam intensity can be as low as 10ˆ-9. This low signal to noise ratio is particularly challenging for wide field imaging that requires area detectors, such as CCD cameras, with limited dynamic range. To overcome these difficulties, we show that SRG to Stokes beam ratio can be improved to 10ˆ-4 by utilizing mJ pulses using a regenerative amplifier, optimization of pulse durations and bandwidths, and destructive interference of the Strokes beam background. We gratefully acknowledge funding from the Singapore-MIT Alliance (SMA-2), the Singapore-MIT Alliance for Research and Technology (SMART), and the Samsung Scholarship.

Three-color multiplex CARS for fast imaging and microspectroscopy in the entire CHn stretching vibrational region

We present a three-color multiplex coherent anti-Stokes Raman scattering (CARS) setup that facilitates a prompt recording of broadband CARS spectra along with a fast CARS imaging. With separate narrowband Stokes and probe beams being introduced in the near IR, we are able to incorporate a stable, wideband Ti:sapphire femtosecond laser as a pump beam that covers the full range of Raman shift for CHn stretching vibrational modes. Experimentally, high-resolution multiplex CARS signals are allowed to investigate molecular vibrations over the range of 2650 cm-1 - 3050 cm-1, which are spectrally integrated to construct lipid-sensitive images. It is demonstrated that the proposed implementation promises a particular benefit on CARS imaging of lipid-rich tissue structures by providing detailed information on CHn Raman-active vibrations at points of interest on the CARS images that can be obtained at high frame rates.

Three-dimensional theory of stimulated Raman scattering

We present a three-dimensional theory of stimulated Raman scattering (SRS) or superradiance. In particular we address how the spatial and temporal properties of the generated SRS beam, or Stokes beam, of radiation depends on the spatial properties of the gain medium. Maxwell equations for the Stokes field operators and of the atomic operators are solved analytically and a correlation function for the Stokes field is derived. In the analysis we identify a superradiating part of the Stokes radiation that exhibit beam characteristics. We show how the intensity in this beam builds up in time and at some point largely dominates the total Stokes radiation of the gain medium. We show how the SRS depends on geometric factors such as the Fresnel number and the optical depth, and that in fact these two factors are the only factors describing the coherent radiation.

Stimulated Raman scattering spectrum of Gaussian and Bessel light beams

The effect of the real part of the Raman suscep-tibility on the Stokes spectrum excited by Gaussian and Bessel beams is studied theoretically and experimentally. This part of the susceptibility is shown to be responsible for the increase in the overlap integral of the Stokes beam and the pump beam and, as a result, for the increase in the Raman amplification in the region of frequencies higher than the frequency of the exact Raman resonance. This, in particular, is observed in the spectral shift of the axial component of Raman generation in the case of Bessel pump and Stokes radiation in the case of the Gaussian pump to the high-frequency region. It is shown that the shift caused by the Bessel pump significantly exceeds the shift caused by the Gaussian pump. The conical component in the case of the Bessel pump is generated at the frequency of the exact Raman resonance.

Investigation of pump-wavelength dependence of terahertz-wave parametric oscillator based on LiNbO3

This paper investigates the performances of terahertz-wave parametric oscillators (TPOs) based on the LiNbO3 crystal at different pump wavelengths. The calculated results show that TPO characteristics, including the frequency tuning range, the THz-wave gain and the stability of THz-wave output direction based on the Si-prism coupler, can be significantly improved by using a short-wavelength pump. It also demonstrates that a long-wavelength-pump allows the employment of a short TPO cavity due to an enlarged phase-matching angle, that is, an increased angular separation between the pump and oscillated Stokes beams under the THz-wave generation at a specific frequency. The study provides an useful guide and a theoretical basis for the further improvement of TPO systems.

High-power picosecond fiber source for coherent Raman microscopy

We report a high-power picosecond fiber pump laser system for coherent Raman microscopy (CRM). The fiber laser system generates 3.5 ps pulses with 6 W average power at 1030 nm. Frequency doubling yields more than 2 W of green light, which can be used to pump an optical parametric oscillator to produce the pump and the Stokes beams for CRM. Detailed performance data on the laser and the various wavelength conversion steps are discussed, together with representative CRM images of fresh animal tissue obtained with the new source.

Tuning Gold Nanoparticle Self-Assembly for Optimum Coherent Anti-Stokes Raman Scattering and Second Harmonic Generation Response

The research and development of new substrates for use in surface-enhanced spectroscopy is primarily motivated by the ability to tune such substrates to provide maximum signal enhancement and therefore lower detection limits. We examined a series of multilayer nanoparticle (NP) arrays with between 1 and 17 NP layers using two nonlinear optical (NLO) techniques: Coherent anti-Stokes Raman scattering (CARS) and second harmonic generation (SHG). The CARS signal of oxazine 720 was monitored at 1600 cm−1 using a 709-nm pump beam and an 800-nm stokes beam. Maximum signal was observed for 11 NP layers and is attributed to the matching of the CARS signal and the substrate surface plasmon excitation at 637 nm. The CARS signal for the 3000-cm−1 C−H stretching vibration showed maximum enhancement at 13 NP layers. The maximum SHG signal enhancement occurred at 13 NP layers, with a 50-fold overall enhancement of the SHG signal. We demonstrate that the NP-containing substrates can be tuned to provide maximum NLO respon...

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν 1 fundamental rovibrational Raman band of methane gas at ∼2916.5 cm−1. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.

An inverse problem in the theory of stochastic electromagnetic beams

The cross-spectral density matrix of an electromagnetic beam has been playing increasingly important role in studies of changes of spectra, of coherence and of polarization as the beam propagates. In this paper we derive solution to an inverse problem, which makes it possible to determine the cross-spectral density matrix of the beam in the source plane z = 0 , from the knowledge of the matrix in any cross-section z = z 0 > 0 in the half-space into which the beam propagates. We apply the result to the theory of so-called Stokes beams, which were introduced not long ago.

Stabilizing slow light delay in stimulated Brillouin scattering using a Faraday rotator mirror

The delay of the Stokes beam in stimulated Brillouin scattering (SBS) is sensitive to the polarization alignment between the pump and the Stokes. Typically, waveguides exhibit a birefringence that causes beams launched from opposite ends to have fluctuating and suboptimal alignment. We demonstrate in an optical fiber that using a Faraday rotator mirror not only overcomes this problem, but also enhances SBS slow light delay.

Single-laser-shot detection of nitric oxide in reacting flows using electronic resonance enhanced coherent anti-Stokes Raman scattering

Single-laser-shot electronic resonance enhanced coherent anti-Stokes Raman scattering (ERE-CARS) spectra of nitric oxide (NO) were generated using the 532 nm output of an injection-seeded Nd:YAG (yttrium aluminum garnet) laser as the pump beam, a broadband dye laser at approximately 591 nm as the Stokes beam, and a 236 nm narrowband ultraviolet probe beam. Single-laser-shot ERE-CARS spectra of NO were acquired in an atmospheric-pressure hydrogen/air counterflow diffusion flame. The single-shot detection limit in this flame was found to be approximately 30 ppm, and the standard deviation of the measured NO concentration was found to be approximately 20% of the mean.

Beam quality of the Stokes output of continuous-wave Raman fiber amplifiers using multimode fiber

The beam quality of the Stokes output of a Raman fiber amplifier based on a long multimode graded-index fiber is examined. The beam quality of the Stokes output is modeled by considering the relative gain of the Stokes modes of the fiber; this model predicts that the beam quality of the coupled Stokes beam and the beam quality of the coupled pump beam determine the beam quality of the Stokes output. Experimental investigations validate this model by showing that the beam quality of the Stokes output is similar to the beam quality of the Stokes input.

Multi-dimensional differential imaging with FE-CARS microscopy

Focus-engineered coherent anti-Stokes Raman scattering (FE-CARS) microscopy is used to visualize microscopic samples with differential imaging contrast in both longitudinal and lateral directions. Multi-dimensional contrast is achieved by applying a one-dimensional π-phase step in the transverse beam profile of the Stokes beam. The observed longitudinal differential contrast results from fortuitous phase-matching at axial interfaces and is highly sensitive to the resonant part of the third-order susceptibility. Based on phase-sensitive recordings of the vibrational spectrum of dimethyl sulfoxide in the CH 3-stretching range, it is shown that FE-CARS uniquely probes the spectral phase of the Raman bands of the sample.