Janis Alnis

Analysis of gas dispersed in scattering media.

Monitoring of free gas embedded in scattering media, such as wood, fruits, and synthetic materials, is demonstrated by use of diode laser spectroscopy combined with sensitive modulation techniques. Gas detection is made possible by the contrast of the narrow absorptive feature of the free-gas molecules with the slow wavelength dependence of the absorption and scattering cross sections in solids and liquids. An absorption sensitivity of 2.5 x 10(-4), corresponding to a 1.25-mm air column, is demonstrated by measurements of dispersed molecular oxygen. These techniques open up new possibilities for characterization and diagnostics, including internal gas pressure and gas-exchange assessment, in organic and synthetic materials.

Sum-frequency generation with a blue diode laser for mercury spectroscopy at 254 nm

Blue diode lasers emitting 5 mW continuous-wave power around 400 nm have recently become available. We report on the use of a blue diode laser together with a 30 mW red diode laser for sum-frequency generation around 254 nm. The ultraviolet power is estimated to be 0.9 nW, and 35 GHz mode-hop-free tuning range is achieved. This is enough to perform high-resolution ultraviolet spectroscopy of mercury isotopes. The possibility to use frequency modulation in the ultraviolet is demonstrated; however, at present the ultraviolet power is too low to give advantages over direct absorption monitoring. Mercury detection at atmospheric pressure is also considered which is of great interest for environmental monitoring.

Concentration measurement of gas embedded in scattering media by employing absorption and time-resolved laser spectroscopy

Diode-laser-based absorption spectroscopy for the evaluation of embedded gas concentrations in porous materials is demonstrated in measurements of molecular oxygen dispersed throughout scattering polystyrene foam, used here as a generic test material. The mean path length of light scattered in the material is determined with the temporal characteristics of the radiation transmitted through the sample. This combined with sensitive gas-absorption measurements employing wavelength-modulation spectroscopy yields an oxygen concentration in polystyrene foam of 20.4% corresponding to a foam porosity of 98%, which is consistent with manufacturing specifications. This feasibility study opens many possibilities for quantitative measurements by using the method of gas-in-scattering-media absorption spectroscopy.

Frequency-modulation spectroscopy with blue diode lasers

Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6(p)(2 3)P(2) metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 x 10(-6) and 5 x 10(-6) for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.

Atomic spectroscopy with violet laser diodes

Laser spectroscopy with laser diodes can now also be performed in the violet/blue spectral region. A 5 mW commercially available CW laser diode operating at 404 nm was used to perform spectroscopy on potassium atoms with signal detection in absorption as well as fluorescence when operating on a potassium vapor cell and with optogalvanic detection on a potassium hollow cathode lamp. The 4s 2S1/2–5p 2P3/2,1/2 transitions were observed at 404.5 and 404.8 nm, respectively. The laser diode was operated with a standard laser diode driver, and with or without an external cavity. The 4s 2S1/2–4p 2P1/2 transition at 770.1 nm was also observed with a different laser diode. Here, Doppler-free saturated-absorption signals were also observed, enabling the evaluation of the ground-state hyperfine splitting of about 460 MHz. The data recorded allows an experimental verification of the theory for Doppler broadening at two widely separated wavelengths.

Sub-Doppler spectroscopy of Rb atoms in a sub-micron vapour cell in the presence of a magnetic field

We report the first sub-Doppler study of the magnetic field dependence of laser-induced fluorescence excitation spectra of alkali atoms making use of an extremely thin vapour cell (thickness ∼ 400 nm). This thin cell allows fo rs ub-Doppler resolution without the complexity of atomic beam or laser cooling techniques. This technique is used to study the laser-induced fluorescence excitation spectra of Rb in a 50 G magnetic field. At this field strength the electronic angular momentum J and nuclear angular momentum I are only partially decoupled. As a result of the mixing of wavefunctions of different hyperfine states, we observe a nonlinear Zeeman effect for each sublevel, a substantial modification o ft he transition probabilities between different magnetic sublevels, and the appearance of transitions that are strictly forbidden in the absence of the magnetic field. For the case of right- and left-handed circularly polarized laser excitation, the fluorescence spectra differ qualitatively. Well pronounced magnetic field induced circular dichroism is observed. These observations are explained with a standard approach that describes the partial decoupling of I and J states.

Reverse dark resonance in Rb excited by a diode laser

The origin of recently discovered reverse (opposite-sign) dark resonances has been explained theoretically and verified experimentally. It is shown that the reason for these resonances is a specific optical pumping of the ground state magnetic sublevel in a transition when the ground state angular momentum is smaller than the excited state momentum. An experiment was conducted on 85Rb atoms in a cell, when a diode laser using the ground state hyperfine level Fg = 3 simultaneously excites spectrally unresolved hyperfine levels with total angular momentum quantum numbers Fe = 2,3 and 4. It is shown that due to differences in the transition probabilities the dominant role in total absorption and fluorescence signals is played by absorption on a transition Fg = 3→Fe = 4.

Angular momentum spatial distribution symmetry breaking in Rb by an external magnetic field

Abstract Excited state angular momentum alignment – orientation conversion foratoms with hyperfine structure in presence of an external magnetic field isinvestigated. Transversal orientation in these conditions is reported for thefirst time. This phenomenon occurs under Paschen Back conditions at in-termediate magnetic field strength. Weak radiation from a linearly polarizeddiode laser is used to excite Rb atoms in a cell. The laser beam is polarized atan angle of π/4 with respect to the external magnetic field direction. Groundstate hyperfine levels of the 5S 1/2 state are resolved using laser-induced fluo-rescence spectroscopy underconditions for which all excited 5P 3/2 state hyper-fine components are excited simultaneously. Circularly polarized fluorescenceis observed to be emitted in the direction perpendicular to both to the direc-tion of the magnetic field B and direction of the light polarization E. Theobtained circularity is shown to be in quantitative agreement with theoreticalpredictions.I. INTRODUCTION

Development of optical WGM resonators for biosensors

Whispering Gallery Mode (WGM) resonators are very sensitive to nanoparticles attaching to the surface. We simulate this process using COMSOL Wave Optics module. Our spherical WGM resonators are produced by melting a tip of an optical fiber and we measure optical Q factors in the 105 range. Molecular oxygen lines of the air in the 760 nm region are used as reference markers when looking for the shifts of the WGM resonance lines. We demonstrate WGM microresonator surface coating with a layer of ZnO nanorods as well as with polystyrene microspheres. Coatings produce increased contact surface. Additional layer of antigens/antibodies will be coated to make high-specificity biosensors.

Cavity ring down spectrometry for disease diagnostics using exhaled air

In this paper we report the current stage of the development of a cavity ring-down spectrometer (CRDS) system using exhaled human breath analysis for the diagnostics of different diseases like diabetes and later lung cancer. The portable CRDS system is made in ultraviolet spectral region using Nd:Yag laser 266 nm pulsed light. Calibration of the CRDS system was performed using generated samples by KinTek automated permeation tube system and self-prepared mixtures with known concentration of benzene and acetone in air. First experiments showed that the limits of detection for benzene and acetone are several tens of ppb.