Radiation-balanced lasing in Yb3+:YAG and Yb3+:KYW.

Radiation-balanced lasing and thermal profiling is reported in two Yb-doped laser crystals at room temperature. In 3% Yb3+:YAG a record efficiency of 30.5% was achieved by frequency-locking the laser cavity to the input light. Both the average excursion and axial temperature gradient of the gain medium were maintained within 0.1 K of room temperature at the radiation balance point. By including saturation of background impurity absorption in the analysis, quantitative agreement was obtained between theory and the experimentally measured laser threshold, radiation balance condition, output wavelength, and laser efficiency with only one free parameter. Radiation-balanced lasing was also achieved in 2% Yb3+:KYW with an efficiency of 2.2% despite high background impurity absorption, losses from Brewster end faces that were not parallel, and non-optimal output coupling. Our results confirm that relatively impure gain media can be operated as radiation-balanced lasers, contrary to earlier predictions which ignored background impurity properties.

Laser cooling of Yb3+:KYW.

We report the first observation of laser cooling in Yb3+:KYW and validate the results by comparison with experiments in the well-studied material Yb3+:YAG. Radiation from a single-mode Ti:Al2O3 laser was used to achieve cooling of 1.5 K/W in 1% Yb:KYW at 1025 nm, comparing well with the reference material 3% Yb:YAG which cooled by 3.5 K/W at 1030 nm under open lab conditions. Experimental results for KYW crystals mounted on aerogels and doped with 1-20% Yb were in excellent agreement with the theoretical dependence of cooling power on the Yb absorption spectrum. Elimination of thermal conduction through the sample support structure was found to permit the attainment of lower temperatures and to simplify modeling of radiation balance conditions in self-cooled lasers with longitudinal thermal gradients. Contrary to the notion that more coolant ions yield higher cooling power, concentrations of Yb over 1% caused re-absorption of luminescence in KYW crystals, leading to a progressive red shift in the optimal cooling wavelength and the prevention of laser cooling altogether in a 20% sample at room temperature. The prospect of attaining radiation-balanced lasing in commercially-available tungstate crystals is evaluated.

Optical torque induces magnetism at the molecular level.

We report experimental observations of a mechanism that potentially supports and intensifies induced magnetization at optical frequencies without the intervention of spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered light, recorded at moderate intensities (108 W/cm2) and short timescales (<150 fs) in a series of non-magnetic molecular liquids, reveal the signature of torque dynamics driven jointly by the electric and magnetic field components of light at the molecular level. While past experiments have recorded radiant magnetization from magneto-electric interactions of this type, no evidence has been provided to date of the inelastic librational features expected in cross-polarized light scattering spectra due to the Lorentz force acting in combination with optical magnetic torque. Here, torque is shown to account for unpolarized rotational components in the magnetic scattering spectrum under conditions that produce only polarized vibrational features in electric dipole scattering, in excellent agreement with quantum theoretical predictions.

Optical magnetization, part III: theory of molecular magneto-electric rectification.

A fully quantized analysis is presented of induced magneto-electric rectification in individual diatomic molecules in the steady-state regime. Good agreement is obtained between this quantum theory and a classical model that includes the same key kinematic elements but predicts temporal dynamics as well. At the molecular level, an enhanced magneto-electric optical interaction driven by dual optical fields E and H* is shown to give rise to a static electric dipole (ED) moment oriented along the propagation direction of linearly-polarized light in dielectric materials. This longitudinal Hall effect or "charge separation" interaction is quadratic with respect to the incident field strength and exhibits an induced moment that is limited by the ED transition moment of the molecular resonance. Overall, the two-photon dynamics can be described as first establishing an electric polarization and imparting orbital angular momentum on which the magnetic field exerts torque in the excited state of the molecule. Magnetic torque mediates an exchange of orbital and rotational angular momenta that de-excites the molecule and simultaneously enhances magneto-electric rectification. Material properties that affect magneto-electric response at the molecular level are identified.

Evanescent coupling between refillable ring resonators and laser-inscribed optical waveguides.

We investigated theoretically and experimentally the evanescent coupling between photonic waveguides of arbitrary shapes and refillable optical ring resonators on the same chip. The resonator hosts were designed to facilitate whispering gallery modes and etched by using a single-mask standard lithography process, whereas the waveguides were imprinted in the proximity of the ring resonator by using 3D ultrafast laser-writing technology. Finite element analysis in conjunction with coupled-mode theory revealed a coupling Q-factor (QC) of approximately 106. The polymer core ring resonator exhibited a loaded Q-factor (QT) as high as 5.4×104 and a free spectral range (FSR) of 406 pm at a center wavelength of 775 nm. Long-term stability of the ring resonator was repeatedly tested by examining the spectral location of optical resonances and the constancy of Q-factors and FSRs under ambient laboratory conditions for 1 month. We recorded consistent Q-factors and repeatable FSRs for all measurements. Renewability of the polymer core was demonstrated by removing and redepositing the polymer in the cavity, followed by measurements of Q-factors and FSRs. This work promises to enable reconfigurable and renewable photonic devices for on-chip lasers, 3D integrated optical signal processing, chip-scale molecular sensing, and the investigation of new optical phenomena.

Optical magnetization, Part I: Experiments on radiant optical magnetization in solids.

Linearly-polarized magnetic dipole (MD) scattering as intense as Rayleigh scattering is reported in transparent garnet crystals and fused quartz through a magneto-electric interaction at the molecular level. Radiation patterns in quartz show the strongest optical magnetization relative to electric polarization ever reported. As shown in an accompanying paper, quantitative agreement is achieved with a strong-field, fully-quantized theory of magneto-electric (M-E) interactions in molecular media. The conclusion is reached that magnetic torque enables 2-photon resonance in an EH* process that excites molecular librations and accounts for the observed upper limit on magnetization. Second-order M-E dynamics can also account for unpolarized scattering from high-frequency librations previously ascribed to first-order collision-induced or third-order, all-electric processes.

Optical magnetization, Part II: Theory of induced optical magnetism.

A fully quantized analysis is presented on the origin of induced magnetic dipole (MD) scattering in two-level diatomic molecules. The interaction is driven by dual optical fields, E and H*, and is universally allowed in dielectric optical materials, including centrosymmetric media. Leading terms of the interaction are shown to be quadratic and cubic with respect to the intensity, predicting an upper limit for the induced magnetic dipole scattering intensity (IMD∝m2) that is equal to the electric dipole scattering (IED∝p2). The optical dynamics proceed by first establishing an electric polarization in the system. Then the magnetic field exerts torque on the orbital angular momentum of the excited state, mediating an exchange of orbital and rotational angular momenta that enhances the magnetic moment. The magneto-electric interaction also accounts for second-order, unpolarized scattering from high-frequency librations previously ascribed to third-order, all-electric processes.

Dynamic gain aperture modelocking in picosecond regime based on cascaded second-order nonlinearity.

The operation of a cascaded second-order mode-locked Nd:YVO4 laser has been investigated considering it as a soft-aperture Kerr lens type and using complex beam parameters. A self consistent complex beam propagation method is used to incorporate the effect of cascaded Kerr nonlinearity on radially varying gain aperturing. The analysis deduces a stable pulsewidth of ~9.5 ps which agrees well with the experimental value of 10.3 ps.

Holographic imaging through a scattering medium by diffuser-aided statistical averaging.

We introduce a practical digital holographic method capable of imaging through a diffusive or scattering medium. The method relies on statistical averaging from a rotating ground glass diffuser to negate the adverse effects caused by speckle introduced by a static diffuser or scattering medium. In particular, a setup based on Fourier transform holography is used to show that an image can be recovered after scattering by introducing an additional diffuser in the optical setup. This method is capable of recovering object information from behind a scattering layer in biomedical or military imaging applications.

Backscatter analysis based algorithms for increasing transmission through highly scattering random media using phase-only-modulated wavefronts.

Recent theoretical and experimental advances have shed light on the existence of so-called "perfectly transmitting" wavefronts with transmission coefficients close to 1 in strongly backscattering random media. These perfectly transmitting eigen-wavefronts can be synthesized by spatial amplitude and phase modulation. Here, we consider the problem of transmission enhancement using phase-only-modulated wavefronts. Motivated by biomedical applications, in which it is not possible to measure the transmitted fields, we develop physically realizable iterative and non-iterative algorithms for increasing the transmission through such random media using backscatter analysis. We theoretically show that, despite the phase-only modulation constraint, the non-iterative algorithms will achieve at least about 25π%≈78.5% transmission with very high probability, assuming that there is at least one perfectly transmitting eigen-wavefront and that the singular vectors of the transmission matrix obey the maximum entropy principle such that they are isotropically random. We numerically analyze the limits of phase-only-modulated transmission in 2D with fully spectrally accurate simulators and provide rigorous numerical evidence confirming our theoretical prediction in random media, with periodic boundary conditions, that is composed of hundreds of thousands of non-absorbing scatterers. We show via numerical simulations that the iterative algorithms we have developed converge rapidly, yielding highly transmitting wavefronts while using relatively few measurements of the backscatter field. Specifically, the best performing iterative algorithm yields ≈70% transmission using just 15-20 measurements in the regime, where the non-iterative algorithms yield ≈78.5% transmission, but require measuring the entire modal reflection matrix. Our theoretical analysis and rigorous numerical results validate our prediction that phase-only modulation with a given number of spatial modes will yield higher transmission than amplitude and phase modulation with half as many modes.

Iterative, backscatter-analysis algorithms for increasing transmission and focusing light through highly scattering random media.

Scattering hinders the passage of light through random media and consequently limits the usefulness of optical techniques for sensing and imaging. Thus, methods for increasing the transmission of light through such random media are of interest. Against this backdrop, recent theoretical and experimental advances have suggested the existence of a few highly transmitting eigen-wavefronts with transmission coefficients close to 1 in strongly backscattering random media. Here, we numerically analyze this phenomenon in 2D with fully spectrally accurate simulators and provide rigorous numerical evidence confirming the existence of these highly transmitting eigen-wavefronts in random media with periodic boundary conditions that are composed of hundreds of thousands of nonabsorbing scatterers. Motivated by bio-imaging applications in which it is not possible to measure the transmitted fields, we develop physically realizable algorithms for increasing the transmission through such random media using backscatter analysis. We show via numerical simulations that the algorithms converge rapidly, yielding a near-optimum wavefront in just a few iterations. We also develop an algorithm that combines the knowledge of these highly transmitting eigen-wavefronts obtained from backscatter analysis with intensity measurements at a point to produce a near-optimal focus with significantly fewer measurements than a method that does not utilize this information.

Observation of magneto-electric rectification at non-relativistic intensities.

The subject of electromagnetism has often been called electrodynamics to emphasize the dominance of the electric field in dynamic light-matter interactions that take place under non-relativistic conditions. Here we show experimentally that the often neglected optical magnetic field can nevertheless play an important role in a class of optical nonlinearities driven by both the electric and magnetic components of light at modest (non-relativistic) intensities. We specifically report the observation of magneto-electric rectification, a previously unexplored nonlinearity at the molecular level which has important potential for energy conversion, ultrafast switching, nano-photonics, and nonlinear optics. Our experiments were carried out in nanocrystalline pentacene thin films possessing spatial inversion symmetry that prohibited second-order, all-electric nonlinearities but allowed magneto-electric rectification.

Self assembly and optical properties of dendrimer nanocomposite multilayers.

Ultrathin multilayers are important for electrical and optical devices, as well as for immunoassays, artificial organs, and for controlling surface properties. The construction of ultrathin multilayer films by electrostatic layer-by-layer deposition proved to be a popular and successful method to create films with a range of electrical, optical, and biological properties. Dendrimer nanocomposites (DNCs) form highly uniform hybrid (inorganic-organic) nanoparticles with controlled composition and architecture. In this work, the fabrication, characterization, and optical properties of ultrathin dendrimer/poly(styrene sulfonate) (PSS) and silver-DNC/PSS nanocomposite multilayers using layer-by-layer (LbL) electrostatic assembly techniques are described. UV-vis spectra of the multilayers were found to be a combination of electronic transitions of the surface plasmon peaks, and the regular frequency modulations attributable to the multilayered film structure. The modulations appeared as the consequence of the highly regular and non-intermixed multilayer growth as a function of the resulting structure. A simple model to explain the experimental data is presented. Use of DNCs in multilayers results in abrupt, flat, and uniform interfaces.

Fusion of Renewable Ring Resonator Lasers and Ultrafast Laser Inscribed Photonic Waveguides.

We demonstrated the monolithic integration of reusable and wavelength reconfigurable ring resonator lasers and waveguides of arbitrary shapes to out-couple and guide laser emission on the same fused-silica chip. The ring resonator hosts were patterned by a single-mask standard lithography, whereas the waveguides were inscribed in the proximity of the ring resonator by using 3-dimensional femtosecond laser inscription technology. Reusability of the integrated ring resonator - waveguide system was examined by depositing, removing, and re-depositing dye-doped SU-8 solid polymer, SU-8 liquid polymer, and liquid solvent (toluene). The wavelength reconfigurability was validated by employing Rhodamine 6G (R6G) and 3,3'-Diethyloxacarbocyanine iodide (CY3) as exemplary gain media. In all above cases, the waveguide was able to couple out and guide the laser emission. This work opens a door to reconfigurable active and passive photonic devices for on-chip coherent light sources, optical signal processing, and the investigation of new optical phenomena.

Intense nonlinear magnetic dipole radiation at optical frequencies: molecular scattering in a dielectric liquid.

We report white-light generation and intense linear scattering from magnetic dipoles established by the time-varying magnetic flux of an incident light field in a dielectric medium. Large magnetic response is very unexpected at optical frequencies, and should lead to the discovery of new magneto-optical phenomena and the realization of low-loss homogeneous optical media with negative refractive indices.