Detecting Next to Nothing: Spectroscopy in Optical Cavities

Detecting Next to Nothing: Spectroscopy in Optical Cavities Kevin Lehmann Departments of Chemistry & Physics University of Virginia

Collaborators Daniele Romanini Joan Gambogi John Dudek Greg Engel Wilton Virgo Peter Tarsa Iris Scheele Haifeng Huang (UVa) Paul Johnston (UVa) Paul Rabinowitz Wen-Bin Yan, Calvin Kruzen, Bob Augustine, Chris Wu, Yu Chen, Lisa Bergson

Generic Absorption Spectroscopy Instrument L Lock-In DAS Source Filter Sample Chopper Detector

Beer s Law I out (ν) = I in (ν) exp ( - σ(ν) N L) σ(ν): Absorption cross section N: Number density of absorber (Concentration) L: Optical pathlength through sample Minimum Detectable Concentration: N min = ( I/I) min / (σ(ν) L)

Review of Optical Cavities (aka etalons)

Simple Linear Optical Cavity detector Radii of Curvature: R 1, R 2 Length of Cavity: L Mirror Transmission: T Mirror Reflectivity: R Mirror Loss: A = 1 - R - T

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Stable Optical Cavities For R = 1, modes exist which exactly reproduce themselves upon round trip. 0 < L < R 1 or R 2 < L < R 1 +R 2 (R 1 < R 2 ) If R 2 - R 1, << L, then R 1 < L < R 2 only weakly unstable Optic axis defined by line through centers of curvature of mirrors Light rays will oscillate around optic axis. Spot size of mirror

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The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Transverse resonance modes of Cavities If δ g = M/N, then we have rational cavity with periodic transmission spectrum. Arbitrary pulse inside Cavity will exactly reshape after N round trips -- such cavities are used for Herriott Cells.

Some Uses for Optical Cavities in Spectroscopy Control frequency and linewidth of lasers Monitor laser scan for calibration Laser linewidth is reduced by locking on to the transmission peak of a cavity Cavities are used to build up intensity External c.w. second harmonic generators Pump extremely weak transitions Used to enhance optical absorption

Cavity Ring-Down Absorption Spectroscopy R>99.99% Absorption R>99.99% Intensity I(t) = I 0 exp[ t( c ln R + c σ(λ) N)] L Time L eff =L/(1-R) Where: c : speed of light L : length of cavity R : mirror reflectivity σ : absorption cross section N : number density (concentration) L eff : effective pathlength

Ring Down Cavity Technique First Developed by O Keefe and Deacon Rev. Sci. Instr. 59, 2544 (1988) Theory: Romanini and Lehmann J. Chem. Phys. 99, 6287 (1993) Use a passive optical cavity formed from two high reflective mirrors (T~1-100 ppm) Excite cavity with a pulsed laser to fill with photons Detect exponential decay of light intensity inside resonator Decay rate reflects: Loss due to mirrors (slowly changing with wavelengths) Absorption of gas between mirrors

Advantages of CRDS Method Allows much longer pathlengths than traditional multipass cells Only sensitive to absorption and scattering between mirrors Beer s Law holds for all pathlengths; pathlengths determined by time if resolution exceeds width of absorption lines Calibration samples are not needed Cell is very compact; light contained in narrow spot of ~ 1 mm 2 Cell insensitive to vibration since it is a stable optical cavity Amplitude noise of laser not important Can use low power optical sources

Continue growth- publications per year 1980 1 1992 1 2001 60 1993 5 2002 77 1984 2 1994 12 2003 69 1985 1 1995 18 2004 99 1996 20 2005 95 1988 3 1997 30 2006 144 1998 40 2007 130 1990 3 1999 68 1991 2 2000 48 Based upon searches of Web of Science data base for CRDS, Cavity CRLAS, Ring CEAS, Down ICOS, Spectroscopy NICE-OHMS

Early CRDS Work @ Princeton Used to detect high overtone transitions of HCN and other molecules Provided way to determine absolute absorption strength of extremely weak transitions. Could be done with very simple experimental set-up compared to intracavity photoaccoustic spectroscopy.

D. Romanini and KKL, J. Chem. Phys. 99, 6287-301 (1993)

HCN (106) overtone band L(eff) = 24 km This spectrum is now on the cover of a Spectroscopy text by Hollis

Diode Laser Advantages Low cost, compact, all solid state Low power requirements Wide electronic frequency tuning Single mode diodes in the near-ir are becoming available for sensing apps. H 2 O, C 2 H 2, CH 4, CO 2, NO 2, NH 3, etc.

Experimental Setup Mirror Faraday Isolator Acousto-Optical Modulator Mode Matching Optics Diode Laser HR Mirror Cavity Ring-Down Absorption Cell HR Mirror 3 PZTs Mirror Photodiode Trigger Computer Cavity J. B. Ring Dudek Down et al., Spectroscopy Analytical Chemistry 75, 4599-4609 (2003).

Cavity Ring-Down Decay 0.5 Signal (Volts) 0.4 0.3 τ = 295.82+ 0.20 µsec 0.2 data points fit 0.1 0.0 0 200 400 600 800 1000 1200 1400 1600 Time (µsec)

Water Scan with Lorentz fit 140 135 130 125 120 115 7161.5 cm -1 S=1.5 10-20 ~70 ppb 110 105 7160.8 7161.0 7161.2 7161.4 7161.6 7161.8 7162.0 Wavenumber

How stable is the ring down time? Laser power upon entering the cavity: 1.397 µm ~ 3-5 mw Ensemble Standard Deviation: 10 pts: 201.384 ±0.139 µs 0.069% 100 pts: 201.378 ±0.165 µs 0.082% 1000 pts: 201.304 ±0.146 µs 0.073%

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Allan variance and the detection limit in CRDS Applied Physics B 57, 131-139 (1993) In CRDS the accurate measurement of decay time constant can be limited by slow drift of setup. Allan variance can be used to analyze the stability of instrument. For a N time-series data x i the Allan variance is given by: k is the subgroup size and m+1 is the number of subgroups. The integration time T equals to k/f, where f is sample rate. When white noise is dominant in the system (uncorrelated decays), Allan variance is proportional to 1/T and averaging data can improve the signal to noise ratio. When the drift appears Allan variance will become larger. The longest T during which the instrument can be regarded stable is determined by the drift of the system. The minimum of Allan variance gives the smallest detectable change during the longest integration time period.

What does this mean for water detection? Noise equivalent to 68 pptv divided by the square root of the number of ring down events averaged to get signal, i.e. ~2 pptv for averaging 1000 decays.

CRDS in Practice Size (14 x 19 x 26 ) Weight (45 kg) MTO-1000-H 2 O

MTO Response to Moisture Addition 2.5 Data used with permission from Air Products and Chemicals Inc., Allentown, PA Reading (ppb) 2 1.5 1 0.5 0 6/15/02 3:36 6/15/02 6:00 6/15/02 8:24 6/15/02 10:48 6/15/02 13:12 6/15/02 15:36 6/15/02 18:00 Time

Variations on CRDS Method CRDS (a.k.a. CRLAS, RDCS, cavity leak-out spectroscopy) pulsed CRDS cw CRDS phase shift CRDS Fourier Transform CRDS broad band CRDS evanescent wave CRDS fiber optic CRDS, fiber loop CRDS Cavity Ring-down polarimetry Optical feedback CRDS

Cavity Enhanced Absorption Spectroscopy (CEAS)-Engleln, Meijer, et al. a.k.a Integrated cavity output spectroscopy (ICOS) - O Keefe Frequency chirped CEAS Noise Immune Intracavity optical heterodyne method (NICE-OHMS) Intracavity laser absorption spectroscopy (ICLAS) Intracavity photoaccoustic spectroscopy attractive with optical locking! Excellent Review: C. Vallance, New J. of Chem. 29, 869 (2005)

SUPERCONTINUUM BASED BROADBAND CAVITY ENHANCED ABSORPTION SPECTROSCOPY Paul S. Johnston Kevin K. Lehmann Department of Chemistry University of Virginia

Broad Bandwidth Light sources: broad bandwidth dye lasers, Free electron lasers, fs-lasers, LEDs, arc-source Engeln & Meier, Fourier transform CRDS, 1996 Thorpe & Ye, 2007 Mode lock sources with cavities a multiple of the laser repetition rate allows much improved transmission Cavity dispersion a challenge

Source for Broad Bandwidth Coherent Radiation: Supercontinuum Photonic Crystal Fibers Material: Pure Silica Core diameter: 4.8 + 0.2 µm Cladding diameter: 125 + 3 µm Zero dispersion wavelength: 1040 + 10 nm Nonlinear Coefficient at 1060 nm: 11 (W Km) -1 Cavity Ring www.crystal-fibre.com Down Spectroscopy

Supercontinuum parameters Input Average power: 1.0 W Rep rate: 30 KHz Pulse energy: 34 µj, 10 ns Peak power: 3400 W Output Average output power: 0.29 W Wadsworth, W. J. et al. Opt. Express 2004, 12, 299.

Supercontinuum Output

White light sources http://www.crystal-fibre.com

Output Prism Ring-down Resonator Input θ b P- polarization θ b 6 meter radius of curvature G. Engel et al., in Laser Spectroscopy XIV International Conference, Eds. Cavity R. Blatt Ring et al. Down pgs. 314-315 Spectroscopy (World Scientific, 1999).

Advantages of Prism Cavity Wide spectral coverage - Simultaneous detection of multiple species Compact ring geometry (optical isolation) No dielectric coatings (harsh environments) Coupling can be optimized for broadband

Broadband system using white light from photonic crystal fiber PC Fiber Fiber Output Collimating mirror 20x objective P max =10 W Rep Rate: 20-100 KHz Pulse Width: ~10ns Nd:Vanadate laser Gas inlet Mode Matching Mirrors 1 m monochromator ~0.06 cm -1 /pixel λ Cavity Length: 46 cm Sample Cell Time CCD array

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Loss Due to Dispersion changing Brewster s Angle in Fused Silica R ( n 1) 2 ( n, δθ ) = 6 δθ 4 n 4 2 R (1.46,0.1 ) R (1.46,1.0 ) = 1 ppm = 98.7 ppm **Only Brewster s angle loss**

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Modeling Cavity Loss Model: Loss = scattering + Brewster's angle loss

Ring-down Test for Fused Silica Prism Near-IR Prism Cavity Loss Measurements (Tiger Optics) High Equivalent Fused Silica Prisms (built in 43cm Reflectivity cavity) ring down tau/ppm loss vs. wavelength 70 150 Tau Trend measure Tau ppm loss Trend Measure PPM Loss 60 130 Ringdown(tau-microsecond) 50 40 30 20 110 90 70 50 ppm loss 10 30 0 10 1300 1350 1400 1450 1500 1550 1600 1650 1700 Wavelength(nm) Tau measurenment at 1310, 1368, 1377,1392,1522,1531, 1578,1635,1671nm. Every Diode laser Temp scan from (40 or ) 35~0 Celsisus degree.

The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Cavity enhanced spectroscopy Measure time integrated intensity The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Advantages Relatively high sensitivity Simpler set up Sensitivity limitations Residual mode structure Laser noise Berden, G.; Peeters, R.; Meijer, G. Int. Rev. Phys. Chem. 2000, 19, 565.

Atmospheric Oxygen

Current Status Collaborating with Tiger Optics to commercialize a detector of multiple chemical species. Will try near-ir spectroscopy with InGaAs array detector Will use FT-IR for dispersion Have begun building mode-locked (80 MHz) super-continuum source that we expect > 10 W average power. Frequency comb of source can be matched to frequency comb of cavity transmission to greatly improve transmission Potentially can use Vernier principle to improve Cavity Ring upon Down resolution Spectroscopy of spectrograph

Applications of Broad band CRDS Combustion and Plasma diagnostics parallel detection improves S/N ratio if we have unstable sample single shot determination of temperature Breath Analysis Applications both species and isotopic composition studies Could be combined with optical comb technology for high accuracy metrology.

Other Cavity Spectroscopy related projects Methane Isotope Ratio Instrument 13 C/ 12 C and D/H ratios CH 4 in oceans, air, and emitted from permafrost. Possible mission to Mars Detect NO and other molecules in human breath Fabricate and test Prisms from CaF 2 (UV) and BaF 2 (IR) Collaboration with group looking for WIMPS with liquid Ar detector Need to detect H 2 O, O 2, and N 2 impurities < 1 ppb Plan to try to detect N 2 via 3 Σ state produced in discharge.

Contact Information Phone 243-2130 Office: Rm. 124 in Chemistry Building Email: Lehmann@virginia.edu