Scott A. Holmstrom

Fusion and birth of spatial solitons upon collision

We study experimentally the collision of photorefractive screening solitons in a strontium barium niobate crystal. Depending on the relative phase of the solitons and their intersecting angle, such effects as soliton birth, energy exchange, and soliton fusion have been observed.

Trace gas absorption spectroscopy using functionalized microring resonators

We detect trace gases at parts-per-billion levels using evanescent-field absorption spectroscopy in silicon nitride microring resonators coated with a functionalized sorbent polymer. An analysis of the microring resonance line shapes enables a measurement of the differential absorption spectra for a number of vapor-phase analytes. The spectra are obtained at the near-infrared overtone of OH-stretch resonance, which provides information about the toxicity of the analyte vapor.

Mechanical tuning of holographic polymer-dispersed liquid crystal reflection gratings

Holographic polymer-dispersed liquid crystal reflective structures have been formed using a highly elastic photopolymer. The elasticity allows for mechanical tuning of the reflection notch by over 120nm in the visible spectral range using compressive stress. The shift in the central position of the reflection notch is related to the strain in the system and an effective elastic modulus for the structure is obtained.

Trace gas Raman spectroscopy using functionalized waveguides

Weak scattering and short optical interaction lengths have, until this work, prevented the observation of trace gas Raman spectra using photonic integrated circuitry. Raman spectroscopy is a powerful analytical tool, and its implementation using chip-scale waveguide devices represents a critical step toward trace gas detection and identification in small handheld systems. Here, we report the first Raman scattering measurements of trace gases using integrated nanophotonic waveguides. These measurements were made possible using highly evanescent rib waveguides functionalized with a thin cladding layer designed to reversibly sorb organophosphonates and other hazardous chemical species. Raman spectra were collected using 9.6 mm-long waveguides exposed to ambient trace concentrations of ethyl acetate, methyl salicylate, and dimethyl sulfoxide with one-sigma limits of detection in 100 s integration times equal to 600 ppm, 360 ppb, and 7.6 ppb, respectively. Our analysis shows that the functionalized waveguide Raman efficiency can be enhanced by over nine orders of magnitude compared to traditional micro-Raman spectroscopy, paving the way toward a sensitive, low-cost, miniature, spectroscopy-based trace gas sensor inherently suitable for foundry-level photonic integrated circuit manufacturing.

Characterization of Al2O3 single crystals grown by the laser-heated pedestal growth technique for potential use in radiation dosimetry

Abstract Al 2 O 3 single crystals were grown by the laser heated pedestal growth (LHPG) technique for potential use in radiation dosimetry using optically stimulated luminescence. The crystals were grown in the form of short single crystal fibers under a variety of growth conditions. Thermoluminescence (TL), radioluminescence (RL), cathodoluminescence, photoluminescence and optically stimulated luminescence (OSL) measurements were carried out on all fibers to assess their potential in luminescence radiation dosimetry. Measurements of luminescence spectra and the TL and OSL dose response characteristics lead to the suggestion that LHPG fibers have attractive luminescence dosimetry properties and warrant further study for potential applications in radiotherapy using OSL.

Low-temperature photorefractive effects in Bi12GeO20

Abstract The four-wave mixing (FWM) photorefractive response was investigated in pure and Fe-doped Bi12GeO20 (BGO) crystals. A significant difference was found in the temperature dependence of the FWM diffraction efficiency between the undoped BGO and BGO:Fe. Under the write conditions used for these experiments the decay of the photorefractive signal can be characterized by a single dominant dark decay time. Different decay times are found for undoped and Fe-doped BGO. Combining the present low-temperature results with previous observations, a general model is suggested for describing the photorefractive effect in sillenites and its temperature dependence.

Raman heterodyne detected magnetic resonance: I. CW and coherent transient measurements

We report cw and coherent transient studies on electronic and nuclear magnetic transitions of the nitrogen-vacancy centre in diamond using an optical-rf double resonance technique: Raman heterodyne detection. Emphasis is placed on the coherent transient measurements to illustrate the versatility of the detection technique, which is not only very sensitive but is also ideal in this case since it is based on a coherent process. In this paper we show that Raman heterodyne detection can be used to obtain a wide range of signals including nutation, spin echo, rotary echo and spin locking in a single system. Finally an example of using coherent transients to test the validity of the Bloch equation is given.

Probing a doubly driven two-level atom

A two-level system is driven by two strong fields, one on resonance and one close to resonance. There is a resonance-like response of the coupled atom-field system when the detuning of the second field is an integer fraction of the Rabi frequency of the resonant field. These subharmonic resonances are monitored by a transition to a third level and give a characteristic spectrum dominated by a series of doublets.

Nonlinear absorption of laser light in Bi12GeO20 single crystals

Abstract Two-photon absorption (TPA) at 532 nm was investigated in BGO (Bi 12 GeO 20 ) single crystals using a 16 ps pulse width Nd: YAG laser. The TPA coefficient was measured to be 2.2–2.6 cm/GW for undoped BGO. This suggests that nonlinear absorption plays a significant role in the carrier generation processes induced by short laser pulses. The TPA coefficient for heavily Al-doped BGO is only slightly smaller (1.9–2.3 cm/GW) than that of the undoped BGO. This result has specific importance since linear absorption in the visible wavelength region is strongly suppressed by Al doping.

Effect of the Bloch-Siegert shift in a strongly driven transition: Asymmetric Autler-Townes profile

Under the conditions of the rotating wave approximation (RWA), a transition strongly driven by a resonant oscillating field displays the well known symmetric Autler - Townes doublet. However, if the counter-rotating component, neglected in the RWA, is taken into account, the Bloch - Siegert shift gives rise to an Autler - Townes doublet of unequal intensity even in the case of a resonant driving field. This effect is investigated theoretically in a V-shaped three-level double-resonance configuration and the results are presented in this paper. An interesting observation is that the level of asymmetry not only depends on the driving-field intensity but also on the characteristics of the driven system including relaxation rates and equilibrium population distributions.