James W. Chan

Structural changes in fused silica after exposure to focused femtosecond laser pulses

Using in situ Raman scattering in a confocal microscopy setup, we have observed changes in the network structure of fused silica after modifying regions inside the glass with tightly focused 800-nm 130-fs laser pulses at fluences of 5-200 J cm(-2). The Raman spectra show a large increase in the peaks at 490 and 605cm(-1), owing to 4- and 3-membered ring structures in the silica network, indicating that densification occurs after exposure to the femtosecond laser pulses. The results are consistent with the formation of a localized plasma by the laser pulse and a subsequent microexplosion inside the glass.

Spectroscopic characterization of different femtosecond laser modification regimes in fused silica

Structural changes associated with femtosecond laser fabrication of waveguides and Bragg gratings in fused silica were analyzed using optical microscopy and laser spectroscopy. Using 800 nm femtosecond lasers with a kilohertz repetition rate and various pulse energies, both smooth and rough modifications were induced. The different modification regimes were characterized by measuring the spectra of the light emitted during writing with the femtosecond laser and collecting fluorescence spectra after femtosecond writing using a low power 488 nm laser as an excitation source. The spectral features observed during and after writing can be used to distinguish the smooth and rough modification regimes, and they assist in understanding the underlying modification mechanisms..

Optical trapping and coherent anti-Stokes Raman scattering (CARS) spectroscopy of submicron-size particles

Optical trapping combined with coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time as a new technique for the chemical analysis of individual particles over an extended period of time with high temporal resolution. Single submicron-size particles suspended in aqueous media are optically trapped and immobilized using two tightly focused collinear laser beams from two pulsed Ti:Sapphire laser sources. The particles can remain stably trapped at the focus for many tens of minutes. The same lasers generate a CARS vibrational signal from the molecular bonds in the trapped particle when the laser frequencies are tuned to a vibrational mode of interest, providing chemical information about the sample. The technique is characterized using single polystyrene beads and unilamellar phospholipid vesicles as test samples and can be extended to the study of living biological samples. This novel method could potentially be used to monitor rapid dynamics of biological processes in single particles on short time scales that cannot be achieved by using other vibrational spectroscopy techniques.

Mid-Infrared Trace Gas Analysis with Single-Pass Fourier Transform Infrared Hollow Waveguide Gas Sensors:

A hollow core optical fiber gas sensor has been developed in combination with a Fourier transform infrared (FT-IR) spectrometer operating in the spectral range of 4000–500 cm−1, enabling continuous detection of small volume gas-phase analytes such as CH4, CO2, C2H5Cl, or their mixtures at trace levels. Ag/Ag-halide hollow core optical fibers simultaneously serve as an optical waveguide for broad-band mid-infrared radiation and as a miniaturized absorption gas cell. Specifically, carbon dioxide, methane, and ethyl chloride as well as binary mixtures in a carrier gas were analyzed during exponential dilution experiments. In the studies reported here, the integration of an optical gas sensor with FT-IR spectroscopy provides excellent detection limits for small gas volumes (∼1.5 mL) of individual analytes at a few tens of parts per billion (ppb, vol/vol) for carbon dioxide and a few hundreds of ppb (vol/vol) for methane. Furthermore, the broad-band nature of the radiation source and of the hollow core optical waveguide provides the capability of multi-constituent analysis in mixtures.

Multipass Capillary Cell for Enhanced Raman Measurements of Gases

A simple Raman multipass capillary cell (MCC) is described that gives 12-to 30-fold signal enhancements for non-absorbing gases. The cell is made by coating the inside of 2-mm inner diameter silica capillary tubes with silver. The device is very small and suitable for remote and in situ Raman measurements with optical fibers. Application of the MCC is similar to previously described liquid core waveguides but, unlike the latter devices, the MCC is generally more applicable to a wide range of non-absorbing gases.

Confocal fluorescence and Raman microscopy of femtosecond laser-modified fused silica

Modified lines were written inside Corning 7940 fused silica with 130 fs laser pulses from an amplified Ti–sapphire laser operating at 800 nm at a repetition rate of 1 kHz. The sample was scanned at 20 µm s−1 with laser pulse energies ranging from 1 to 35 µJ, resulting in modified lines with diameters ranging from 8 to 40 µm. Confocal fluorescence and Raman microscopy was used to probe for spatial variations in defect concentration and glass structure across the modified lines. The fluorescence intensity decreased with increasing distance from the line centre, whereas the Raman intensity increased. No significant variations in the concentration of three- and four-membered ring structures were observed.

Quantitative measurements of CO2 and CH4 using a multipass Raman capillary cell.

Raman measurements of two common gases are made using a simple multipass capillary Raman cell (MCC) coupled to an unfiltered 18 around 1 fiber-optic Raman probe. The MCC, which is fabricated by chemical deposition of silver on the inner walls of a 2 mm inner diameter glass capillary tube, gives up to 20-fold signal enhancements for nonabsorbing gases. The device is relatively small and suitable for remote and in situ Raman measurements with optical fibers. The optical behavior of the MCC is similar to previously described liquid-core waveguides and hollow metal-coated waveguides used for laser transmission, but unlike the former devices, the MCC is generally applicable to a very wide range of nonabsorbing gases.

Fs Laser Fabrication of Photonic Structures in Glass: the Role of Glass Composition

The use of fs lasers to directly write phonic structures inside a glass has great potential as a fabrication method for three-dimensional all-optical integrated components. The ability to use this technique with different glass compositions --specifically tailored for a specific photonics application -- is critical to its successful exploitation. Consequently, it is important to understand how glass composition effects waveguide fabrication with fs laser pulses and how different glasses are structurally modified after exposure to fs laser pulses. We have used confocal laser spectroscopy to monitor the changes in glass structure that are associated with waveguide fabrication. Using a low power continuous wave (cw) Ar laser as excitation source we have measured both Raman and fluorescence spectra of the modified regions. Raman spectroscopy provides us with information on the network structure, whereas fluorescence measurements reveal the presence of optically active point defects in the glass. In this paper we review our work on fs-laser fabrication and characterization of photonic structures in glass and discuss the effect of glass composition on processing parameters and structural modification.

Hollow Waveguide Gas Sensor for Mid-Infrared Trace Gas Analysis

A hollow waveguide mid-infrared gas sensor operating from 1000 cm-1 to 4000 cm-1 has been developed, optimized, and its performance characterized by combining a FT-IR spectrometer with Ag/Ag-halide hollow core optical fibers. The hollow core waveguide simultaneously serves as a light guide and miniature gas cell. CH-4 was used as test analyte during exponential dilution experiments for accurate determination of the achievable limit of detection (LOD). It is shown that the optimized integration of an optical gas sensor module with FT-IR spectroscopy provides trace sensitivity at the few hundreds of parts-per-billion concentration range (ppb, v/v) for CH-4.

Raman Analysis of Common Gases Using a Multi-Pass Capillary Cell (MCC)

The Raman analysis of common, non-absorbing gases was performed using an 18@1 fiber-optic probe coupled to a multi-pass capillary cell (MCC) for signal enhancement. The MCC is fabricated by metal-coating, using silver or other highly reflective metals, the inside of a 1-2 mm diameter glass capillary using commercially available silvering solutions and provides enhancements up to 30-fold over measurements using the fiber-optic probe alone. The design of the MCC is simple and the device is easy to incorporate into an experimental setup making it suitable for remote and in-situ analysis. Although the MCC is functionally similar to liquid-core waveguides that have been previously described in the literature, the MCC is not based on total internal reflection and so the refractive index of the analyte is not important to the operation of the device. The principle of operation of the MCC is similar to mirror-based multiple pass Raman cells, however, the MCC is not expensive, alignment is trivial and an optical path length up to several meters in length is possible. With our first-generation silver-coated MCCs, limits of detection were determined to be 0.02% and 0.2% for CH4 and CO2 respectively. In this talk we will discuss optimization of the MCC and issues involved in its use.