Benjamin Petrak

Feedback-controlled laser fabrication of micromirror substrates

Short (40-200 μs) single focused CO(2) laser pulses of energy ≳100 μJ were used to fabricate high quality concave micromirror templates on silica and fluoride glass. The ablated features have diameters of ≈20-100 μm and average root-mean-square (RMS) surface microroughness near their center of less than 0.2 nm. Temporally monitoring the fabrication process revealed that it proceeds on a time scale shorter than the laser pulse duration. We implement a fast feedback control loop (≈20 kHz bandwidth) based on the light emitted by the sample that ensures an RMS size dispersion of less than 5% in arrays on chips or in individually fabricated features on an optical fiber tip, a significant improvement over previous approaches using longer pulses and open loop operation.

Isotopic gas analysis through Purcell cavity enhanced Raman scattering

Purcell enhanced Raman scattering (PERS) by means of a doubly resonant Fabry-Perot microcavity (mode volume ≈ 100 μm3 and finesse ≈ 30 000) has been investigated as a technique for isotopic ratio gas analysis. At the pump frequency, the resonant cavity supports a buildup of circulating power while simultaneously enabling Purcell spontaneous emission rate enhancement at the resonant Stokes frequency. The three most common isotopologues of CO2 gas were quantified, and a signal was obtained from 13C16O2 down to a partial pressure of 2 Torr. Due to its small size and low pump power needed (∼10 mW) PERS lends itself to miniaturization. Furthermore, since the cavity is resonant with the emission frequency, future improvements could allow it to serve as its own spectral analyzer and no separate spectroscopic device would be needed.

Coherent anti-Stokes Raman scattering in a high-finesse microcavity

We report the measurement of degenerate coherent anti-Stokes Raman scattering (CARS) in a high-finesse optical microcavity, with an atmospheric gas as the nonlinear medium. Unlike the well-known bulk process in which index-dispersion compensation is required for phase-matching, efficient microcavity CARS involves a resonant coupling at the Stokes, pump, and anti-Stokes frequencies which can be described using coupled-mode analysis. We show how the interaction is thereby dramatically enhanced in a microscopic sample volume and illustrate the technique for the measurement of CO₂ in air.

Solid optical ring interferometer for high-throughput feedback-free spectral analysis and filtering

We describe a simple and inexpensive optical ring interferometer for use in high-resolution spectral analysis and filtering. It consists of a solid cuboid, reflection-coated on two opposite sides, in which constructive interference occurs for waves in a rhombic trajectory. Due to its monolithic design, the interferometers resonance frequencies are insensitive to environmental disturbances over time. Additional advantages are its simplicity of alignment, high-throughput, and feedback-free operation. If desired, it can be stabilized with a secondary laser without disturbance of the primary signal. We illustrate the use of the interferometer for the measurement of the spectral Mollow triplet from a quantum dot and characterize its long-term stability for filtering applications.

Correlations in pulsed resonance fluorescence

We investigated the first and second-order correlations of the light scattered near-resonantly by a quantum dot under excitation by a frequency comb, i.e., a periodically pulsed laser source. In contrast to its monochromatic counterpart, the pulsed resonance fluorescence spectrum features a superposition of sidebands distributed around a central peak with maximal sideband intensity near the Rabi frequency. Distinguishing between the coherently and incoherently scattered light reveals pulse-area dependent Rabi oscillations evolving with different phase for each component. Our observations, which can be reproduced theoretically, may impact schemes for remote entanglement based on pulsed two-photon interference.

Laser-Induced Fluorescence from N2+ Ions Generated by a Corona Discharge in Ambient Air

In this work, we present the measurement of laser-induced fluorescence from N2+ ions via the B2Σ+ u − X2Σ+ g band system in the near-ultraviolet. The ions were generated continuously by a plasma glow discharge in low pressure N2 and by a corona discharge in ambient air. The fluorescence decay time was found to rapidly decrease with increasing pressure leading to an extrapolated decay rate of ≈10 10 s−1 at atmospheric pressure. In spite of this quenching, we were able to observe laser induced fluorescence in ambient air by means of a time-gated spectral measurement. In the process of comparing the emission signal with that of N2 spontaneous Raman scattering, ion concentrations in ambient air of order 10 8 -10 10 cm−3 were determined. With moderate increases in laser power and collection efficiency, ion concentrations of less than 10 6 cm−3 may be measurable, potentially enabling applications in atmospheric standoff detection of ionizing radiation from hazardous radioactive sources.

Purcell enhancement of Raman scattering from atmospheric gases in a high-finesse microcavity

We report spontaneous emission enhancement of Raman scattering from CO₂ and O₂ gases in a ≈30 μm-long Fabry-Perot microcavity with a mode volume of 200 μm³ and a peak finesse of 50 000.

Feedback-controlled laser fabrication of micromirrors

Micromirror templates of uniform size were created on silica chips and fibers by a feedback-controlled laser ablation process. The templates were coated with a high reflectivity dielectric coating and assembled into high-finesse microcavities.