High-power Raman lasing and efficient anti-Stokes generation in mm-sized crystalline disk resonators.

We have previously experimentally observed high-power Stokes and second-order Stokes output from a mm-sized CaF2 disk using stimulated Raman scattering. A pump laser at a wavelength of 1.06 µm was coupled via a tapered fiber to the whispering gallery modes (WGM) of the disk. In this Letter, we extend this work and demonstrate the production of the first anti-Stokes sideband at power levels as high as 60 µW in near continuous-wave (CW) operation. The result is a four-component Raman comb at the output, with a wavelength range covering from 1.023 to 1.14 µm. We discuss the threshold dependence of the observed Raman lines on the crystal orientation and provide experimental validation. These advances enable the use of such mm-sized resonators as compact, efficient sources for terahertz-level frequency modulation.

Phase-dependent ionization of hydrogen by intense sub-cycle pulses.

We have performed simulations and analytic calculations that show strong carrier-envelope phase dependence in the ionization of hydrogen atoms using intense sub-cycle sub-femtosecond laser pulses. When the pulse width is comparable to the classical orbit time of the initial bound state, sine-like pulses can ionize more than cosine-like pulses that have the same fluence. This result is the opposite of what is expected from a tunneling-like model, where the ionization probability primarily depends on the peak amplitude of the electric field during the pulse.

Phase-dependent ionization of hydrogen by intense sub-cycle pulses: publisher's note.

This publisher's note corrects an error in Eq. (2) in Opt. Lett.43, 2583 (2018)OPLEDP0146-959210.1364/OL.43.002583.

Continuous-wave modulation of a femtosecond oscillator using coherent molecules.

We describe a new method to broaden the frequency spectrum of a femtosecond oscillator in the continuous-wave (CW) domain. The method relies on modulating the femtosecond laser using four-wave mixing inside a Raman-based optical modulator. We prepare the modulator by placing deuterium molecules inside a high-finesse cavity and driving their fundamental vibrational transition using intense pump and Stokes lasers that are locked to the cavity modes. With the molecules prepared, any laser within the optical region of the spectrum can pass through the system and be modulated in a single pass. This constitutes a CW optical modulator at a frequency of 90 THz with a steady-state single-pass efficiency of ∼10-6 and transient (10 µs-time-scale) single-pass efficiency of ∼10-4. Using our modulator, we broaden the initial Ti:sapphire spectrum centered at 800 nm and produce upshifted and downshifted sidebands centered at wavelengths of 650 nm and 1.04 µm, respectively.

Broadband spectrum generation using continuous-wave Raman scattering.

We experimentally demonstrate a continuous-wave ro-vibrational Raman spectrum that is two octaves wide with spectral components ranging from 0.8 to 3.2 µm in wavelength. The spectrum is produced in low pressure molecular deuterium inside a high finesse cavity.

Tunable source of terahertz radiation using molecular modulation.

We describe a source of terahertz (THz) radiation that is based on Raman down-shifting of an infrared laser beam using highly coherent molecular vibrations. The source can operate in either the pulsed or the continuous wave (CW) regime and is tunable over much of the THz region of the spectrum (1-10 THz). In the pulsed regime, we predict average output powers of order 10 mW and peak powers approaching 1 MW. In the CW regime, average powers exceeding 100 µW with spectral linewidths at the hertz level are achievable.

Continuous-wave, multiple-order rotational Raman generation in molecular deuterium.

We demonstrate the generation of ≈10 rotational sidebands using continuous-wave stimulated Raman scattering in molecular deuterium. The generation occurs inside a high-finesse cavity at molecular gas pressures of ≈0.1 atm.

Continuous-wave high-power rotational Raman generation in molecular deuterium.

We report the generation of more than 300 mW of rotational Stokes output power in a CW Raman laser. The generation is achieved in low-pressure molecular deuterium inside a high-finesse cavity.

Refractive index enhancement with vanishing absorption in an atomic vapor.

We report a proof-of-principle experiment where the refractive index of an atomic vapor is enhanced while maintaining vanishing absorption of the beam. The key idea is to drive alkali atoms in a vapor with appropriate control lasers and induce a gain resonance and an absorption resonance for a probe beam in a two-photon Raman configuration. The strength and the position of these two resonances can be manipulated by changing the parameters of the control lasers. By using the interference between these two resonances, we obtain an enhanced refractive index without an increase in the absorption.

Rabi oscillations between ground and Rydberg states with dipole-dipole atomic interactions.

We demonstrate Rabi oscillations of small numbers of 87Rb atoms between ground and Rydberg states with n< or =43. Coherent population oscillations are observed for single atoms, while the presence of two or more atoms decoheres the oscillations. We show that these observations are consistent with van der Waals interactions of Rydberg atoms.

Generation of high-power laser light with Gigahertz splitting.

We demonstrate the generation of two high-power laser beams whose frequencies are separated by the ground state hyperfine transition frequency in (87)Rb. The system uses a single master diode laser appropriately shifted by high frequency acousto-optic modulators and amplified by semiconductor tapered amplifiers. This produces two 1 W laser beams with a frequency spacing of 6.834 GHz and a relative frequency stability of 1 Hz. We discuss possible applications of this apparatus, including electromagnetically induced transparency-like effects and ultrafast qubit rotations.

Fast ground state manipulation of neutral atoms in microscopic optical traps.

We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by 8 microm at the level of 10(-3). Ramsey spectroscopy is used to measure a dephasing time of 870 micros, which is approximately 5000 longer than the time for a pi/2 pulse.

Refractive index enhancement in a far-off resonant atomic system.

We demonstrate a scheme where a laser beam which is very far detuned from an atomic resonance experiences a large index of refraction with vanishing absorption. The essential idea is to excite two Raman resonances with appropriately chosen strong control lasers.

Generation of a single-cycle optical pulse.

We make use of coherent control of four-wave mixing to the ultraviolet as a diagnostic and describe the generation of a periodic optical waveform where the spectrum is sufficiently broad that the envelope is approximately a single-cycle in length, and where the temporal shape of this envelope may be synthesized by varying the coefficients of a Fourier series. Specifically, using seven sidebands, we report the generation of a train of single-cycle optical pulses with a pulse width of 1.6 fs, a pulse separation of 11 fs, and a peak power of 1 MW.

Quasiperiodic Raman technique for ultrashort pulse generation.

We report the experimental demonstration of a new Raman technique that produces 200 sidebands, ranging in wavelength from 3 microm to 195 nm. By studying multiphoton ionization of nitric oxide (NO) molecules, we show mutual phase coherence among 15 visible sidebands covering 0.63 octaves of bandwidth.

Rotational Raman generation with near-unity conversion efficiency.

We demonstrate collinear generation of equidistant rotational sidebands in low-pressure molecular hydrogen with near-unity conversion efficiency. The spectrum consists of 37 coherent sidebands covering over 20, 000 cm(-1) of spectral bandwidth and ranging from 1.37mum to 352 nm in wavelength.

Raman self-focusing at maximum coherence.

We demonstrate a type of Raman self-focusing and -defocusing that is inherent in operation at maximum coherence. In this regime the two-photon detuning from the Raman resonance controls the refractive index of the medium.

Femtosecond light source for phase-controlled multiphoton ionization.

We describe a femtosecond Raman light source with more than an octave of optical bandwidth. We use this source to demonstrate phase control of multiphoton ionization under conditions where ionization requires eleven photons of the lowest frequency of the spectrum or five photons of the highest frequency. The nonlinearity of the photoionization process allows us to characterize the light source. Experiment-to-theory comparison implies generation of a near single-cycle waveform.

Optical frequency conversion by a rotating molecular wave plate.

We demonstrate efficient four-wave mixing in low-pressure molecular deuterium without the need for phase matching. We use two laser fields with opposite circular polarizations to produce a strong excitation of a rovibrational transition at a frequency of 3167 cm(-1) . The coherent molecular motion, in turn, modulates a third laser field (also circularly polarized) and results in highly efficient single-sideband conversion.

Raman generation by phased and antiphased molecular states.

We use molecular deuterium and two driving lasers to demonstrate collinear generation of mutually coherent equidistant sidebands, covering 50 000 cm(-1) of spectral bandwidth and ranging from 2.94 microm to 195 nm in wavelength. The essential idea is the adiabatic preparation of a single, highly coherent (|rho(ab)| = 0.33) molecular eigenstate.