John Lawall

Michelson interferometry with 10 pm accuracy

We demonstrate a new heterodyne Michelson interferometer design for displacement measurements capable of fringe interpolation accuracy of one part in 36 000. Key to this level of accuracy are the use of two acousto-optic modulators for heterodyne frequency generation and digital signal processing demodulation electronics. We make a direct comparison of our interferometer to a commercial interferometer based on a Zeeman-stabilized laser, and show that the residual periodic errors in ours are two orders of magnitude lower than those in the commercial unit. We discuss electronically induced optical cross talk and optical feedback as sources of periodic error. Our new interferometer is simple, robust, and readily implemented.

Heterodyne interferometer with subatomic periodic nonlinearity

A new, to our knowledge, heterodyne interferometer for differential displacement measurements is presented. It is, in principle, free of periodic nonlinearity. A pair of spatially separated light beams with different frequencies is produced by two acousto-optic modulators, avoiding the main source of periodic nonlinearity in traditional heterodyne interferometers that are based on a Zeeman split laser. In addition, laser beams of the same frequency are used in the measurement and the reference arms, giving the interferometer theoretically perfect immunity from common-mode displacement. We experimentally demonstrated a residual level of periodic nonlinearity of less than 20 pm in amplitude. The remaining periodic error is attributed to unbalanced ghost reflections that drift slowly with time.

Fabry–Perot metrology for displacements up to 50 mm

A system designed to apply Fabry-Perot interferometry to the measurement of displacements is described. Two adjacent modes of a Fabry-Perot cavity are probed, and both the absolute optical frequencies and their difference are used to determine displacements via changes in cavity length. Light is coupled to the cavity via an optical fiber, making the system ideal for remote sensing applications. Continuous interrogation is not necessary, as the cavity length is encoded in the free spectral range. The absolute uncertainty is determined to be below 10 pm, which for the largest displacement measured corresponds to a relative uncertainty of 4 x 10(-10). To my knowledge this is the smallest relative uncertainty in a displacement measurement ever demonstrated.

Resolved sideband emission of InAs/GaAs quantum dots strained by surface acoustic waves.

The dynamic response of InAs/GaAs self-assembled quantum dots (QDs) to strain is studied experimentally by periodically modulating the QDs with a surface acoustic wave and measuring the QD fluorescence with photoluminescence and resonant spectroscopy. When the acoustic frequency is larger than the QD linewidth, we resolve phonon sidebands in the QD fluorescence spectrum. Using a resonant pump laser, we have demonstrated optical frequency conversion via the dynamically modulated QD, which is the physical mechanism underlying laser sideband cooling a nanomechanical resonator by means of an embedded QD.

Ultrahigh-finesse, low-mode-volume Fabry–Perot microcavity

We construct microcavities comprising ultralow-loss micromirrors fabricated by laser ablation and reflective coatings. With quality factors of 3.3×10 and finesses of 1.5×10 , strong coupling or lasing with a single quantum emitter may be achieved.

Coupling an epitaxial quantum dot to a fiber-based external-mirror microcavity

We report the coupling of individual InAs quantum dots (QDs) to an external-mirror microcavity. The external mirror is bonded to a fiber and positioned above a semiconductor sample consisting of a QD-containing GaAs layer on top of a distributed Bragg reflector (DBR). This open cavity can be rapidly tuned with a piezoelectric actuator without negatively affecting the QD linewidth. A mirror radius of curvature of 42 microns and a cavity length of 10 microns enable good mode-matching and thus high collection efficiency directly into the fiber. With an improved finesse this system may enter the strong coupling regime.

Emission Spectrum of a Dressed Exciton-Biexciton Complex in a Semiconductor Quantum Dot

The photoluminescence spectrum of a single quantum dot was recorded as a secondary resonant laser optically dressed either the vacuum-to-exciton or the exciton-to-biexciton transitions. High-resolution polarization-resolved measurements using a scanning Fabry-Pérot interferometer reveal splittings of the linearly polarized fine-structure states that are nondegenerate in an asymmetric quantum dot. These splittings manifest as either triplets or doublets and depend sensitively on laser intensity and detuning. Our approach realizes complete resonant control of a multiexcitonic system in emission, which can be either pulsed or continuous wave, and offers direct access to the emitted photons.

High-accuracy Fabry–Perot displacement interferometry using fiber lasers

We describe ongoing work in a Fabry‐Perot interferometry system designed to measure displacements over a range of 50 mm with sub-pm uncertainty. The apparatus involves probing two nearby modes of the Fabry‐Perot cavity with narrow-linewidth fiber telecom lasers and measuring both the mode spacing and the absolute mode frequencies relative to a third, frequency-stabilized, fiber laser. We explore the improvement in resolution obtained as the frequency separation between the two modes is increased and employ an internal consistency requirement to infer the magnitude of residual systematic errors. The measurement uncertainty is sufficiently small that we are easily able to see the Gouy phase shift as the cavity length is changed over several millimeters.

Cavity optomechanics with sub-wavelength grating mirrors

We design, fabricate and study a novel platform for cavity optomechanics: a silicon nitride membrane patterned as a sub-wavelength diffraction grating. Using the grating as one mirror of a Fabry-Perot cavity, we realize an optical finesse of F = 2830±60, corresponding to a grating reflectivity of R = 0.998. The finesse we achieve appears to be within a factor of two of the limit set by material absorption. We study the finesse as a function of wavelength and optical spot size in order to elucidate various optical loss mechanisms. We find that the cavity exhibits birefringence, and establish that it, too, is a source of optical loss. We then characterize the mechanical motion. We observe hundreds of normal modes, and find the fluctuating amplitude of one of them to be very well described by a Boltzmann distribution. By injecting a red- detuned cooling laser, we optically cool all of the modes that we observe. The lowest effective temperature we achieve is Teff 1K.

Laser doppler vibrometer employing active frequency feedback

A laser Doppler vibrometer for vibration measurement that employs active feedback to cancel the effect of large vibration excursions at low frequencies, obviating the need to unwrap phase data. The Doppler shift of a reflective vibrating test object is sensed interferometrically and compensated by means of a voltage-controlled oscillator driving an acousto-optic modulator. For frequencies within the servo bandwidth, the feedback signal provides a direct measurement of vibration velocity. For frequencies outside the servo bandwidth, feedback biases the interferometer at a point of maximal sensitivity, thus enabling phase-sensitive measurement of the high-frequency excursions. Using two measurements, one with a low bandwidth and one with a high bandwidth, more than five decades of frequency may be spanned. This approach is of particular interest for the frequently occurring situation where vibration amplitudes at low frequency exceed an optical wavelength, but knowledge of the vibration spectrum at high frequency is also important.