Pump Laser Frequency Research Articles

Nonlinear atom-photon-interaction-induced population inversion and inverted quantum phase transition of Bose-Einstein condensate in an optical cavity

In this paper we explore the rich structure of macroscopic many-particle quantum states for Bose- Einstein condensate in an optical cavity with the tunable nonlinear atom-photon interaction [Nature (London) 464, 1301 (2010)]. Population inversion, bistable normal phases and the coexistence of normal{superradiant phases are revealed by adjusting of the experimentally realizable interaction strength and pump-laser frequency. For the negative (effective) cavity-frequency we observe remark- ably an inverted quantum phase transition (QPT) from the superradiant to normal phases with the increase of atom-field coupling, which is just opposite to the QPT in the normal Dicke model. The bistable macroscopic states are derived analytically in terms of the spin-coherent-state variational method by taking into account of both normal and inverted pseudospin states.

Raman Line Broadening with Graphene Coated Broad-Band Nano-Antennas

: Raman spectroscopy is a well-established tool providing for molecular fingerprints. Here we examine the Raman signal of graphene coated nano-antennas. The antennas studied were in the shape of bow-tie and diamond. Substantial signal amplification and line broadening have been observed signifying the nonlinear interaction between the antenna gain profile and the Raman line. Summary: Surface enhanced Raman scattering (SERS) [1] is a major spectroscopic tool for an efficient molecular identification down to the level of a few molecules [2,3]. Driven by the notion of localized electric field intensities, research turned to aggregates of metal particles and then to the more structured nano-optical antennas [4-6]. Yet, most antennas used so far were narrow band; covering mostly the laser frequency. Enhancement of the Raman signal has been further hindered by uncertainties in analyte concentration and location relative to the antenna. Here we examined a new antenna type, the diamond-shape antenna (D-ant) [7] with a bandwidth capable of accommodating both the pump laser and scattered Raman frequencies. The role of the antenna at resonance becomes two-fold: enhancing the absorption of the pump laser frequency and imposing unique conditions for plasmonic scattering of the Raman signal at the far-field. By using graphene [8] as a test film we eliminated much of the uncertainty in molecule’s location and concentration, thus enabling better comparative studies. Fig. 1 is a scanning electron microscope (SEM) image of our graphene-coated antennas. The metal layout included markers, which exhibited photoluminescence (PL) when illuminated by the 633 nm HeNe laser. The PL signal of gold was small when illuminated with the 785 nm laser and thus, mostly the antennas glow was accentuated near resonance. The background reflection has substantially elevated near resonance: this occurred because of antenna’s reflectivity and antenna’s directivity.The Raman signal from a graphene coated diamond antenna exhibited a substantial enhancement and line broadening, associated which every Raman line. The broadening becomes smaller as the Raman shift becomes larger. It was as-if the enhancement and line broadening mapped the gain-profile of the diamond shaped antenna.If we zoom at the 1300 cm-1graphene line we may observe an enhancement factor of ca 70 and line broadening by a factor of almost 5.In summary, diamond shape antennas exhibited substantial signal enhancement and line broadening compared to bow-tie antennas with similar dimensions.1. M. Fleischmann, P. J. Hendra, A. J. McQuillan, Chem. Phys. Letters 26 163-166 (1974).2. S. Nie and S. R. Emory, Science 275, 1102-1105 (1997).3. K. Kneipp et al, Phys. Rev. Letts, 78, 1667-1671 (1997).4. P. J. Schuck, D. P. Fromm, A. Sundaramurthy, G. S. Kino, and W. E. Moerner, , Phys. Rev. Lett. 2005, 94 (1), No. 017402-4.5. Katherine A. Willets and Richard P. Van Duyne, Annual Review of Physical Chemistry, 58: 267-297 (2007) DOI: 10.1146/annurev.physchem.58.032806.1046076. Jon A. Schuller, Edward S. Barnard, Wenshan Cai, Young Chul Jun, Justin S. White and Mark l. Brongersma, Nature Materials 9, 193-204 (2010). DOI: 10.1038/nmat2630.7. Nan Ni and H. Grebel, IEEE Antennas and Wireless Propagation Letters 5 (4), pp. 127-129 (2006).8. A. K. Geim, Science, 324 (5934), 1530-1534 (2009): DOI: 10.1126/science.1158877.

Multi-gigahertz picosecond optical parametric oscillator pumped by 80-MHz Yb-fiber laser

We report a multi-gigahertz (GHz) repetition-rate picosecond optical parametric oscillator (OPO) based on MgO:PPLN, synchronously pumped by a Yb-fiber laser operating at 80 MHz, where the multiplication of repetition frequency is achieved using fractional increment in the OPO cavity length. Using this simple technique, we achieve OPO operation up to the 88th harmonic of the pump laser frequency, corresponding to a repetition rate as high as 7 GHz. Deploying a 5% output coupler, we are able to extract up to 960 mW of average signal power at the fundamental with 600 mW at the 88th harmonic (7 GHz), using a pump power of 5.6 W. The measured relative standard deviations of the fundamental and fifth harmonic signal power are recorded to be 1.6% and 3.5%, respectively, while the fundamental signal pulse duration is measured to be 18.4 ps.

High-efficiency frequency conversion in the single-photon regime

In this Letter we demonstrate frequency conversion in the single-photon regime through Bragg-scattering four-wave mixing with near-unit efficiency in a 750 m long commercially available dispersion-engineered highly nonlinear fiber, where all photons and pump laser frequencies are in the low-loss telecommunications band. We achieve 99.1%±4.9% downconversion and 98.0%±5.0% upconversion of photons by 12 nm using a weak coherent state with an average input of 0.27 photons per detection gate window.

Thermal bistability-based method for real-time optimization of ultralow-threshold whispering gallery mode microlasers

A method based on thermal bistability for ultralow-threshold microlaser optimization is demonstrated. When sweeping the pump laser frequency across a pump resonance, the dynamic thermal bistability slows down the power variation. The resulting line shape modification enables a real-time monitoring of the laser characteristic. We demonstrate this method for a functionalized microsphere exhibiting a submicrowatt laser threshold. This approach is confirmed by comparing the results with a step-by-step recording in quasi-static thermal conditions.

Effect of relativistic self-focusing on plasma wave excitation by a hollow Gaussian beam

AbstractA paraxial-like approach has been invoked to understand the nature of propagation of a hollow Gaussian beam (HGB) propagating in plasma under the influence of relativistic non-linearity. In this approach, the parameters are expanded in terms of the radial distance from the maximum of irradiance rather than that from the axis. This paper investigates the excitation of plasma wave in a hot collision less plasma by HGB. On account of the × force, a plasma wave at 2ω0 (here, ω0 is the pump laser frequency) is generated. The solution of the HGB has been obtained within the paraxial ray approximation. Filamentary structures of the laser beam are observed due to relativistic non-linearity.

Cavity-enhanced optical frequency doubler based on transmission-mode Hänsch–Couillaud locking

A variant of the Hansch–Couillaud technique for locking the pump laser frequency to a cavity-enhanced optical frequency doubler is presented. In contrast to the original scheme in which the pump laser light reflected by the enhancement cavity is monitored, the error signal for locking is obtained by polarization analysis of the light transmitted by the cavity. The cavity serves as a spectral and spatial filter for the pump beam, making the transmission-mode locking method particularly useful if the beam quality is poor. Experimental validation of the method is demonstrated by generating more than 190 mW of stable 461 nm light from a cavity-enhanced frequency doubler that is pumped by a diode laser system.

Femtosecond laser generation and detection of high-frequency acoustic phonons in GaAs semiconductors

The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process (opto-acoustic transformation) and the detection process (acousto-optic transformation). We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.

Non-Gaussian states produced by close-to-threshold optical parametric oscillators: Role of classical and quantum fluctuations

Quantum states with non-Gaussian statistics generated by optical parametric oscillators (OPO) with fluctuating parameters are studied by means of the kurtosis excess of the external field quadratures. The field generated is viewed as the response of a nonlinear device to the fluctuations of pump laser amplitude and frequency, crystal temperature, and cavity detuning, in addition to quantum noise sources. The kurtosis excess has been evaluated perturbatively up to the third order in the strength of the crystal nonlinear coupling factor and the second order in the classical fluctuating parameters. Applied to the device described in Opt. Expr. 13, 948 (2005), the model has given values of the kurtosis excess in good agreement with the measured ones.

Effect of ultrarelativistic laser beam filamentation on third harmonic spectrum

This paper investigates the generation of plasma wave and third harmonic generation in a hot collision less plasma by an intense laser beam. On the account of the V→×B→ force, a plasma wave at 2ω0 (here ω0 is the pump laser frequency) is generated. The solution of the pump laser beam has been obtained within the nonparaxial ray approximation. Filamentary structures of the laser beam are observed due to relativistic nonlinearity. By expanding the eikonal and the other relevant quantities up to the fourth power of r it is observed that the focusing of the laser beams become fast in the nonparaxial region. Interaction of the plasma wave with the incident laser beam generates the third harmonics. The mechanism of the plasma wave, third harmonic generation, and the parameters, which govern the third harmonic yield and hence the spectrum of third harmonics, have been studied in detail. Correlation of the third harmonic spectrum with the filamentation has been pointed out. Therefore, the broadening of the third harmonic spectra can be used as a diagnostic tool to study the presence of the filamentation of laser beams in laser plasma experiments.

A Double Resonance Approach to Submillimeter/Terahertz Remote Sensing at Atmospheric Pressure

The remote sensing of gases in complex mixtures at atmospheric pressure is a challenging problem and much attention has been paid to it. The most fundamental difference between this application and highly successful astrophysical and upper atmospheric remote sensing is the line width associated with atmospheric pressure broadening, ~ 5 GHz in all spectral regions. In this paper, we discuss quantitatively a new approach that would use a short pulse infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral absorptions on the time scale of atmospheric relaxation. We show that such a scheme has three important attributes: (1) The time resolved pump makes it possible and efficient to separate signal from atmospheric and system clutter, thereby gaining as much as a factor of 10^6 in sensitivity, (2) The 3-D information matrix (infrared pump laser frequency, SMM/THz probe frequency, and time resolved SMM/THz relaxation) can provide orders of magnitude greater specificity than a sensor that uses only one of these three dimensions, and (3) The congested and relatively weak spectra associated with large molecules can actually be an asset because the usually deleterious effect of their overlapping spectra can be used to increase signal strength.

1.2- to 2.2-$\mu$m Tunable Raman Soliton Source Based on a Cr : Forsterite Laser and a Photonic-Crystal Fiber

A 1.2- to 2.2-mum tunable femtosecond light source based on the soliton-self-frequency-shift effect of high-power Cr: forsterite laser pulses propagating inside a highly nonlinear photonic crystal fiber is reported. The demonstrated soliton self-frequency shift is higher than 42% of the pump laser frequency, corresponding to a record 910-nm wavelength tuning range. Due to the advantages of simplicity, easy tunability, high-temperature stability, and low cost of this new femtosecond light source, it accordingly, could be widely applicable for many applications.

Quantum noise limits to simultaneous quadrature amplitude and phase stabilization of solid-state lasers

A quantum mechanical model is formulated to describe the coupling between pump intensity noise and laser frequency noise in a solid-state laser. The model allows us to investigate the limiting effects of closed-loop stabilization schemes that utilize this coupling. Two schemes are considered: active control of the quadrature phase noise of the laser and active control of the amplitude noise of the laser. We show that the noise of the laser in the actively stabilized quadrature is ultimately limited by the vacuum noise introduced by the feedback beamsplitter in both schemes. In the case of active control of the quadrature phase noise, the noise is also limited by the intensity noise floor of the detection scheme. We also show that some sources of noise in the passively stabilized quadrature can be suppressed and that it is possible to achieve simultaneous quadrature amplitude and phase stabilization of a solid-state laser. However, the quantum mechanically driven noise in the passively stabilized quadrature cannot be suppressed. While this poses the ultimate limit to the noise in the passively stabilized quadrature, we show that it is experimentally feasible to observe squeezing directly generated by a solid-state laser using this technique.

Limitations of steady state solutions to a two-state model of population oscillations and hole burning

We consider a two-state system driven by an on-resonance, continuous wave pump laser and a much weaker pulsed probe laser that is slightly detuned from the pump laser frequency (usually this detuning is about ${\ensuremath{\omega}}_{p}\ensuremath{-}{\ensuremath{\omega}}_{P}=\ensuremath{\Delta}\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$). The upper state population is assumed to be slowly decaying, but the off-diagonal element of the density matrix decays rapidly due to homogeneous broadening. This model has been solved by others in rare-earth-element-doped fibers and crystals in a usual steady state approximation for slow optical wave propagation. We show that in general the usual steady state approximation does not apply unless either $\ensuremath{\Delta}\ensuremath{\tau}⪢1$ or $(2S+1){\ensuremath{\gamma}}_{2}\ensuremath{\tau}⪢1$ where ${\ensuremath{\gamma}}_{2}$ is the decay rate of the excited state population, $\ensuremath{\tau}$ is the pulse length of the probe field, and $2S$ is the saturation parameter. Both conditions, however, are not satisfied in many population-oscillation- and corresponding group-velocity-reduction-related studies. Our theory and corresponding numerical simulations have indicated that for probe pulses that are much shorter than the lifetime of the upper state, there is no analytical theory for the amplitude, pulse shape, and group velocity of the probe field. In addition, there is no reason to assume that the group velocity remains small when ${\ensuremath{\gamma}}_{2}\ensuremath{\tau}⪡1$ and there is no reason to believe that many pulse length decays can be obtained for such short pulses.

Semiclassical model of stimulated Raman scattering in photonic crystals

We study the stimulated Raman scattering (SRS) of light from an atomic system embedded in a photonic crystal and coherently pumped by a laser field. In our study, the electromagnetic field is treated classically and the atomic system is described quantum mechanically. Considering a decomposition of the pump and Stokes fields into the Bloch modes of the photonic crystals and using a multiscale analysis, we derive the Maxwell-Bloch equations for SRS in photonic crystals. These equations contain effective parameters that characterize the SRS gain, the nonlinear atomic response to the electromagnetic field, and the group velocity and that can be calculated in terms of the Bloch modes of the unperturbed photonic crystal. We show that if the pump laser frequency is tuned near a photonic band edge and the atomic system is carefully chosen such that the Stokes mode matches another photonic band edge, low-threshold, enhanced Raman amplification is possible. Possible physical realizations of SRS in photonic crystals are also discussed.

Velocity-selective resonance dips in the probe absorption spectra of Rb D 2 transitions induced by a pump laser

We report experimental observation of velocity-selective resonances in the Doppler-broadened probe absorption spectra of 85Rb and 87Rb D 2 transitions in the presence of a strong co-propagating pump laser locked to a frequency within the Doppler profile of the transition. The set of three dips having the separation of allowed hyperfine transitions move along the Doppler profile while the pump laser frequency is tuned. This indicates a resonance between the pump laser frequency and the velocity shifted probe laser frequency.

Experimental observation of Autler–Townes splitting of the Na 2[formula omitted] mixed levels

The Autler–Townes (A–T) splittings of the A 1 Σ u +∼ b 3 Π u mixed levels of Na 2 have been experimentally studied by pulsed laser perturbation facilitated optical–optical double resonance (PFOODR) spectroscopy. A pump laser was used to excite the A 1 Σ u +(v A ′,J ′)∼ b 3 Π u (v b ′,J ′)← X 1 Σ g +(v″,J″=J ′±1) transition and a probe laser was used to probe the A–T splitting of the A 1 Σ u +∼ b 3 Π u mixed levels by further exciting from the intermediate levels to the 2 3 Π g state and monitoring the 2 3 Π g → a 3 Σ u + violet fluorescence. When the pump laser frequency was held fixed to a frequency between the A 1 Σ u +(v A ′,J ′)← X 1 Σ g +(v″,J″=J ′±1) and b 3 Π u (v b ′,J ′)← X 1 Σ g +(v″,J″=J ′±1) transitions, three OODR excitation lines were observed: one line via the A 1 Σ u +(v A ′,J ′) intermediate level, one line via the b 3 Π u (v b ′,J ′) intermediate level, and between these lines, a “ creeper” line via a new superposition intermediate state. The relative intensities of the three lines vs. the detuning of the pump laser frequency, the intensities of the pump and probe lasers and the buffer gas pressure were studied.

Enhancement of third-harmonic generation by Stark-chirped rapid adiabatic passage

We report enhancement of third-harmonic generation in a nonlinear optical medium, prepared in maximum coherence by Stark-chirped rapid adiabatic passage (SCRAP). In this technique a strong off-resonant infrared radiation pulse induces an adiabatic passage process by sweeping the transition frequency of a two-photon transition in Krypton atoms through resonance with a near-resonant pump radiation pulse at 213 nm, appropriately delayed with respect to the infrared radiation. During the adiabatic passage process, a transient two-photon maximum coherence is induced, which beats with the pump laser frequency. Detrimental effects of Doppler broadening and collisional dephasing in the medium are reduced during the SCRAP process. The efficiency of third-harmonic generation, yielding vacuum-ultraviolet radiation at 71 nm is enhanced by more than one order of magnitude with respect to the case of conventional frequency conversion in an unprepared medium.

Tunable continuous-wave doubly resonant optical parametric oscillator by use of a semimonolithic KTP crystal.

A temperature-tuned continuous-wave doubly resonant optical parametric oscillator (OPO) consisting of a semimonolithic KTP crystal and a concave mirror has been designed and built. Under single-axial-mode-pair operation, we obtained a combined output power of the signal and idler light fields up to 365 mW at a pump power of 680 mW. The output wavelength of the OPO can be temperature tuned by as much as 9 nm. We achieved 2.8-GHz continuous frequency tuning of the OPO by tuning the pump laser frequency.

Asymmetry of microwave spectrum in an optically-pumped Cs atomic beam frequency standard

The asymmetry of microwave spectrum was investigated in an optically pumped Cs atomic beam frequency standard. The asymmetry was measured with the polarization state and the detuning of the pumping beam, and analyzed by the master equation based on the density matrix for the D/sub 2/ transition line. It was found that the pumping transition, F=4/spl rarr/F'=5 can offer a most sensitive detection scheme for the asymmetry. Finally, it was showed that the asymmetry could be compensated by adjusting the state of polarization of the pumping beam as well as the frequency of pumping laser.