Voigt effect is a nonlinear magneto-optical phenomenon originating from the rotation and ellipticity of linearly-polarized light as it travels in optically active media. In this study, the detailed features and capabilities of a newly developed ultra-sensitive Voigt rotation device consisting of a GaN-based diode emitting light of wavelength range 400 – 480 nm as the light source. A transverse 1 T magnetic field is used to trigger the optical media response. A sensitive and fast semiconducting detector is employed to detect the finest rotation in light polarization. The detector is also coupled with an electronic circuit board which is configured to record changes in the polarization direction of the transmitted light relative to the reference polarizer, in addition to measuring the absolute magnetic field strength. The device sensitivity and modes of operation will also be presented. Moreover, a theoretical model to simulate polarized light transmission through optically active media will be presented and compared with experimental results.
In this paper, an external cavity diode laser (ECDL) in Littrow configuration with narrowband emission is presented. The laser system is based on a commercially available GaN Fabry-Perot laser diode. Longitudinal mode selection is performed using a reflective holographic grating. Tuning range over 3.4 nm is achieved with a short linewidth of 0.02 nm at the rated current. The ECDL system is integrated into an optical sensor for remote detection of Nitrogen Dioxide (NO2) gas.
Chiral molecules are typically optically active with inherently non-superimposable mirror images. Henceforth, these molecules can exhibit differences in absorption between left-and-right circularly polarized light. This difference in light absorption is terminologically called circular dichroism. In this paper, a comparative theoretical and spectroscopic study on the one-photon absorption circular dichroism (OPCD) and two-photon absorption circular dichroism (TPCD) of an optically active chiral ketone over the far and near-ultraviolet region is presented. As molecular transitions associated with OPCD and TPCD obey different selection rules with feasibly different spectral features, the two processes are thus complementary. The strengths and weaknesses of employing one and two-photon circular dichroism of R-(+)-3-Methylcyclopentanone to investigate optical activity are discussed. Moreover, the basic theory of OPCD and TPCD is also shown and compared against corresponding spectroscopic results.
In this paper, the ultraviolet output of frequency-doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) and mass spectra of supersonically-expanded and jet-cooled molecular pulses of iodomethane. The molecular sample consisted of a 3% neat iodomethane gas dissolved in helium. The REMPI and mass spectra of iodomethane were obtained using a two meter time-of-flight (TOF) mass spectrometer coupled with a microchannel plate detector that was kept at 4K volts. The laser power density effect on the (2+1) REMPI process was also investigated by focusing and defocusing the laser pulses using 30 cm and 75 cm biconvex lenses, respectively. By monitoring I+ and CH3+ fragments, it was evident that 3 photons were involved in the REMPI process.
GAs in Scattering Media Absorption Spectroscopy (GASMAS) is used to correlate the average pore size within mesoporous alumina samples to the broadening of the absorption lines of oxygen gas and water vapor entrapped within the pores. Collisions of gas molecules cause extra broadening to the absorption linewidths if the average time between collisions is smaller than the inverse of the linewidth of the absorption line. A gas molecule can collide either with another molecule or with the walls of its container. Hence, for a gas entrapped within a porous medium that has an average pore size comparable to the mean free path of intermolecular collisions, collisions of the gas molecules with the walls of the pores can cause extra broadening. This extra broadening is used to estimate the average size of the pores. At atmospheric pressure, the mean free path of intermolecular collision is about 100 nm and thus broadening due to collision with the walls of the pores should be noticeable for pore sizes of order of 100 nm or less. In this work, high resolution tunable singlemode diode lasers at 761 nm and 936 nm are employed to study the absorption from oxygen gas and water vapor, respectively. The samples used are made from porous pure 𝛼-alumina with average pore sizes ranging from 50 to 150 nm.
The broadening of the P9P9 absorption line of oxygen molecules entrapped in the pores of nanoporous alumina is studied using Gas in Scattering Media Absorption Spectroscopy (GASMAS). A narrow band tunable vertical-cavity surface-emitting laser (VCSEL), with emission peaking around 763 nm, is used to scan over the absorption line of the oxygen A-band. The oxygen line broadening in alpha alumina discs of pore sizes 150 nm, 80 nm and 50 nm, are measured to be 3.8 GHz, 4.2 GHz, and 4.8 GHz, respectively, and compared with the measured open-air oxygen line broadening of 3.3 GHz. The oxygen line broadenings are correlated with studied samples pore sizes and are found to agree well with the line broadenings evaluated using a model based on collisions of confined oxygen molecules with the bulk sample pore walls.
Highly-resolved spectra of (2+1) resonance-enhanced multiphoton ionization (REMPI) for nozzle-jet expanded molecular beams of H2O and D2O molecules seeded in Ar and He gases of 1% concentration are presented. The third Harmonic output (λ=355 nm) of a seeded Q-switch Nd:YAG laser was employed to pump an OPO laser, consequently, the VUV output of frequencydoubled OPO radiation was directly used to photo-ionize the jet-cooled sample molecules. By monitoring the H2O+ and D2O+ parent ions signals, highly resolved mass and photoionization spectra were recorded by probing the C-band via a (2+1) REMPI process and over the spectral range 80400 – 81000 cm-1. In this study, highly resolved mass and REMPI spectra of the C-band of H2O and D2O will be presented. Moreover, the effect of seeding gas effect on sample molecules as well as the nature of the photoionization spectra will be discussed.
The commercial availability of laser diodes emitting in the blue spectral region offer excellent opportunity for using them in the detection of nitrogen dioxide (NO2) gas. Nitrogen dioxide, which is one of the main air pollutants, has strong light absorption cross-section in the blue spectral region. In this paper, a tunable blue diode laser in Littrow external-cavity configuration is investigated. The output power, spectral line-width and tunable range are measured. The results will be used to select a suitable external cavity diode laser (ECDL) system based on a multimode blue laser diode to be employed for the detection of NO2 gas.
Limonene (C10H16) is the most common six-membered ring monoterpene that is daily consumed through the digestive system of humans and most living substances. The chiro-optical properties of chiral molecules are optimally investigated by employing the nonlinear optical method known as vibrational circular dichroism (VCD). In this paper, the spectra of differential infrared (IR) absorption between left-and right circularly polarized light of limonene’s R and S mirror-image configurations are presented for the vibrational energy levels. Furthermore, high resolution and well-resolved temperature-dependent infrared absorption spectra of limonene’s individual enantiomers are also reported and thoroughly discussed. Lastly, a comparison will be drawn between the density functional theory calculations of the IR and VCD (correlation function type B3LYP with basis set aug-cc-pVDZ) for limonene’s most possible individual conformers (Eq (T), Eq (C) and Ax (T)) against corresponding experimental results.
High-intensity nanosecond Nd:YAG laser pulses were used to induce crystallization in saturated solutions of the nitrate salts; sodium nitrate (NaNO3), potassium nitrate (KNO3) and calcium nitrate [Ca(NO3)2] to produce small micro-meter in size crystals. The crystallization of nitrate salts has been specifically chosen to study as these salts have tremendous applications over a wide spectrum of industries such as food, agriculture, dyes, and solar cells production. The induced crystallization in the nitrate salts solutions was mainly triggered by shock waves produced in the solution by directly focusing the laser pulses of 80 mJ pulse energy and 532 nm wavelength into nitrate salts solutions for a period of time ranging from 1 to 15 minutes. The yielded small crystals were characterized using different techniques, namely; x-ray diffraction (XRD), polarized light microscopy (PLM) as well as scanning electron microscope (SEM). A comparison has been drawn between crystals formed conventionally without photochemical intervention versus crystals formed by laser-induced shock wave crystallization mechanism. Finally, the grown crystals size and size distribution were related to laser irradiation time and energy in the three solutions.
The linear and nonlinear chiro-optical properties of chiral molecules are typically investigated by employing a few sensitive to optical activity spectroscopic methods, such as circular dichroism (CD), optical rotation, infrared absorption and vibrational circular dichroism (VCD). In this study, the spectra of differential absorption between left-and right circularly polarized light of carvone’s R and S mirror-image configurations of enantiomers in vapor and solution phases are presented for both electronic (CD) and vibrational (VCD) energy levels. Furthermore, well-resolved ultraviolet, visible and infrared absorption spectra of carvone’s individual enantiomers in vapor and solution phases are also reported and thoroughly discussed. Lastly, a comparison will be drawn between the R and S enantiomers linear versus nonlinear optical activity.
Faraday rotation is a magneto-optical phenomenon defined as the rotation of the polarization plane of light passing through a transparent isotropic sample in the presence of an external longitudinal magnetic field that causes an induced difference between the refractive indices for right and left circularly polarized light inside the sample. In this paper, we present the details of a newly developed ultra-sensitive Faraday rotation device consisting of a GaN-based light emitting diode as the light source of wavelength range 400 – 480 nm. Two linear polarizers are used; the first polarizer is placed before the quartz sample cell to set a reference polarization angle while the other polarizer is connected to a stepper motor which is configured to change the polarization angle of the light beam exiting the cell. A ring permanent magnet is coaxially fitted around the quartz cell and is employed to generate a strong external magnetic field of ~ 1 Tesla. The detection system consists of a sensitive and fast light detector coupled with an electronic circuit board which is configured to record the finest Faraday rotation angle in the polarization direction of the transmitted light relative to the reference polarizer. In addition to the experimental details, the modes of operation and sensitivity of the device will be also presented.
Collisions of gas molecules with the walls of small pores in nanoporous materials can cause the width of gas absorption lines to become wider. Also, the effective absorption path lengths through the gas become longer due to multi-scattering within the nanoporous materials. GAs in Scattering Media Absorption Spectroscopy (GASMAS) is an effective technique that can differentiate between the absorption from the gas within pores and the absorption from the bulk of scattering materials. A Vertical-Cavity Surface-Emitting Laser (VCSEL) emitting around 760 nm is used to investigate the absorption from molecular oxygen gas trapped within mesoporous alumina Al2O3. The pore size of the alumina samples are also characterized based on Barrett, Joyner and Halenda (BJH) method. In this work, we will present the correlation between the pore sizes of the mesoporous alumina samples the width of the absorption line of molecular oxygen gas under different experimental conditions.
In molecular spectroscopy, imaging of supersonically expanded and jet-cooled molecular pulses to evaluate their time of flight and velocity is useful in obtaining photoionization spectra of higher resolution, well-resolved, and enhanced signal to noise ratio. In this paper, the ultraviolet output of frequency doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra for cooled molecular pulses of methyl iodide (CH3I) sample seeded in helium gas. The recorded photoionization spectra were manipulated to study the shape, duration and structure of the jet cooled molecular beam pulses. A two meter time- of- flight (TOF) mass spectrometer was employed to identify and record the ions produced while varying the time delay between molecular pulses and laser shots. Imaging of CH3I molecular pulses yielded lorentzian distributions of full width at base and flight time peak location corresponding to pulses duration of ~ 0.3 milliseconds and 931 m/s cooled molecules translational velocity, respectively.
The external differential quantum efficiency, defined as the ratio of number of photons emitted per unit time to number of carriers passing the laser diode junction, is known to be sensitive to laser diode’s operating temperature. In this paper, high-resolution spectral emissions of a commercially available GaN-based blue laser diode are measured and utilized to study the effect of temperature on the external differential quantum efficiency and over the operating temperature range of 270 – 330 °K. Upon studying the L-I curves and over the full range of laser diode’s operating current and temperature, three distinct temperature regimes of the quantum efficiency were identified with the regime of temperature range 285 -301 °K yielding the highest temperature stability. In addition to experimentally determining the characteristic temperature of the laser diode, the effect of non-radiative recombination and free carrier absorption processes on external differential quantum efficiency will be discussed.
Nitrogen dioxide, one of the main air pollutants, has strong light absorption cross section in the blue region of the optical spectrum. Recent availability of blue laser diodes provides possibility of detecting NO2 in open-air paths with very low detection limits. However, in the blue region, the sharp features of the NO2 spectrum is relatively broad with typical width of tenths of nanometer. This poses a serious challenge for implementing traditional direct or wave-modulated tunable diode laser absorption spectroscopy. In this study, we report the usage of a blue laser diode with multi longitudinal modes tuned over about one nanometer in detecting trace amount of NO2 gas. Details of the setup and its optimization will be presented along with a comparison of its performance with other NO2 detection optical methods.
Typical emission spectra of GaN-based blue laser diodes are known to have irregular shapes. Hence, well-resolved study of their spectra may help in understanding the origin of their spectral shapes irregularity. In this paper, the spectra of a commercial GaN-based blue laser diode are studied as a function of injection current and temperature using a spectrometer with highresolution of 0.003-nm over the spectral region 440 – 450 nm. The obtained laser spectra are used to track the longitudinal modes evolution as a function of operating currents and temperatures as well as to precisely map single mode operation. In addition, yielded laser spectra will be utilized to evaluate few parameters related to the laser diode, such as mode spacing, optical gain, slope efficiency and threshold current at certain temperature.
Temperature effects on the spectral lines of two Fabry-Perot GaN-based blue laser diodes obtained from Toptica and Roithner Laser Tech are experimentally investigated over the temperature range 5 °C to 60 °C in steps of 0.5 °C. A high resolution monochromator SPEX 1403 with a nominal resolution of 0.003 nm is used in this study. A detailed comparison on the number of modes, mode spacing, emission range and change of emission wavelength per degree Celsius will be presented in this paper. The results of this comparison are used to investigate the suitability of the employment of these laser diodes in open-path detection of NO2 gas pollution.
Availability of high intensity light emitting diodes in the blue region offer excellent opportunity for using them in active Differential Optical Absorption Spectroscopy (DOAS) to detect air pollution. Their smooth and relatively broad spectral emissions as well as their long life make them almost ideal light sources for active DOAS. In this study, we report the usage of a blue light emitting diode in an active DOAS setup to measure traces of NO2 gas and achieving few parts per billion detection limit for a path length of 300 m. Details of the setup will be presented along with the effects on measurement accuracy due to shifts in the measured spectra calibration and due to using theoretical instrument Gaussian function instead of the measured instrument function.
High resolution spectral lines study is performed on the emissions of a blue laser diode as a function of applied current. The range of applied current used is between the threshold current of 20 mA and 100 mA with a 0.2 mA increment. With this range of current, the observed emission spectra are between 446 and 448 nm. Typically, 21 longitudinal modes are observed with a mode spacing of 0.05 nm. This mode spacing is found to be in good agreement with the predicted values calculated using the GaN index of refraction and the length of laser cavity. The peak location of each longitudinal mode is measured to shift uniformly with a rate of 0.0045 nm/mA. The intensity and wavelength of each longitudinal mode are observed to be stable over extended period of time. Selected longitudinal modes will be employed to detect traces of pollution gases.
Generating highly validated and well-resolved vertical profiles of water vapor is crucially important to understand short
and long term global climate changes. Latest results of a newly developed water vapor Raman lidar instrument at the
Environment Canada’s Centre for Atmospheric Research Experiments (CARE) (44°14'02" North, 79°45'40" West) will
be presented. The CARE Raman lidar setup utilizes third harmonic (355 nm) output of employed YAG laser to probe
aerosols, water vapor, and nitrogen profiles. By manipulating inelastic backscattering lidar signals of the Raman nitrogen
channel (386.7 nm) and Raman water vapor channel (407.5 nm), vertical profiles of water vapor mixing ratio (WVMR)
from the near ground up to 9 km geometrical altitude are routinely deduced, calibrated, validated, and compared against
WVMR profiles obtained from simultaneously performed and collocated radiosonde launches. Seasonal effects and
variations of WVMR will be also discussed and related to Raman lidar setup efficiency.
Obtaining high resolution vertical profiles of water vapor is crucially important to understand short and long term global
climate changes. Raman lidar technique is widely recognized as the most effective tool to study water vapor and aerosols
profiles in the lower atmosphere. The Great lakes area is one of the ideal areas to study the environmental impact of
water vapor and aerosols profiles on air quality due to its dynamic ecological system, and proximity to most North
American industrial centers. Latest results of a newly developed water vapor Raman lidar instrument at the Environment
Canada's Centre for Atmospheric Research Experiments (CARE) (44°14'02" North, 79°45'40" West) will be presented.
In this study, the instrument is described and its capabilities are illustrated along with preliminary measurements. The
CARE Raman lidar setup utilizes third harmonic (355 nm) output of employed YAG laser to probe aerosols, water
vapor, and nitrogen profiles. By manipulating inelastic backscattering lidar signals of the Raman nitrogen channel (386.7
nm) and Raman water vapor channel (407.5 nm), a vertical profile of water vapor mixing ratio from the near ground up
to 12 km geometrical altitude is deduced. Vertical profile of the backscattering ratio obtained at 1064 nm using another
elastic lidar will be shown and related to the Raman lidar results.
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