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WO2012031208A2 - Détection ultrasensible de vapeurs chimiques par spectroscopie photothermique à microcavités - Google Patents

Détection ultrasensible de vapeurs chimiques par spectroscopie photothermique à microcavités Download PDF

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WO2012031208A2
WO2012031208A2 PCT/US2011/050338 US2011050338W WO2012031208A2 WO 2012031208 A2 WO2012031208 A2 WO 2012031208A2 US 2011050338 W US2011050338 W US 2011050338W WO 2012031208 A2 WO2012031208 A2 WO 2012031208A2
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micro
cavity
chemical vapor
probe
directing
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WO2012031208A3 (fr
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Juejun Hu
Hongtao Lin
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University of Delaware
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/171—Systems in which incident light is modified in accordance with the properties of the material investigated with calorimetric detection, e.g. with thermal lens detection
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
    • G01N21/7703—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides
    • G01N21/7746—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides the waveguide coupled to a cavity resonator

Definitions

  • IR infrared
  • PhotoThermal Spectroscopy has been recognized as a highly sensitive and precise method for measuring infrared molecular absorption. Since optical scattering/reflection does not generate a photothermal signal, PTS is particularly suitable for field applications where scattering is often a major concern. For example, PTS has been successfully applied to aerosol absorption measurement where optical scattering almost completely overshadows optical absorption. In addition to its immunity to scattering interference, the measured optical signal can be amplified by photothermal effects in PTS. Such amplification is quantified using an enhancement factor, defined as the ratio of optical signal magnitudes caused by photothermal effects and by direct absorption.
  • an enhancement factor defined as the ratio of optical signal magnitudes caused by photothermal effects and by direct absorption.
  • Enhancement factors up to ⁇ 2000 with respect to conventional transmission-based IR spectroscopy have been experimentally demonstrated, making PTS a highly sensitive technique for trace chemical analysis.
  • the sensitivity of the PTS technique can be further improved by introduction of optical resonant cavity enhancement. Improved PTS sensitivity has been demonstrated by placing the sample to be analyzed inside a Fabry-Perot etalon optical cavity. When the optically absorbing sample is irradiated by a pump laser beam, the etalon resonance modification due to thermo-optic effects is detected as the photothermal signal.
  • Such cavity-enhanced photothermal spectroscopy is described in H. A. Schuessler, S. H. Chen, Z. Rong, Z. C. Tang, and E. C. Benck, "Cavity-enhanced photothermal spectroscopy: dynamics, sensitivity, and spatial resolution," Appl. Opt.
  • One aspect of the invention comprises an apparatus for detecting a chemical vapor species having an absorption wavelength in the mid- to far-infrared range.
  • the apparatus comprises a micro-cavity, an ambient region surrounding the micro-cavity for receiving the chemical vapor species, means for directing a pump beam having a wavelength in the near- to far-infrared range into the micro-cavity, means for directing a near-infrared probe beam through the micro-cavity, a detector for detecting the probe beam after transmission through the micro-cavity, and a processor connected to an output of the detector.
  • the micro-cavity is transparent to infrared light having a wavelength greater than about 2 ⁇ , is thermally connected to a heat sink, and has at least a first resonant frequency and a second resonant frequency.
  • the first resonant frequency corresponds to the absorption wavelength of the chemical vapor species.
  • the pump beam has a resonant mode coincident with the first resonant frequency and is configured to induce optical absorption by the chemical vapor species that causes photothermal effects in the micro-cavity, including a shift in at least the second resonant frequency and corresponding second resonant wavelength.
  • the probe beam is tuned to the second resonant frequency.
  • the processor is configured to identify the shift in the second resonant wavelength and correlate that shift to a characteristic photothermal signal for the chemical vapor species.
  • the micro-cavity may comprise a traveling wave cavity, such as a micro-ring, a micro-disk or a micro-sphere, or it may comprise a standing wave cavity, such as a multi-layer Bragg cavity or photonic crystal.
  • the micro-cavity may comprise a micro-disk cavity defined by a pedestal, an overhanging portion of a suspended micro-disk supported by the pedestal, and a substrate to which the pedestal is attached, wherein the substrate comprises the heat sink, and the apparatus further comprises a waveguide on the substrate to evanescently couple the pump and probe beams and transmit the beams into the cavity.
  • the apparatus may be configured to detect a plurality of chemical vapor species, in which case the apparatus comprises a plurality of micro-cavities, each micro-cavity having a first resonant frequency and a second resonant frequency different from the first and second resonant frequencies of another of the plurality of micro-cavities.
  • Such an apparatus produces a plurality of pump beams and directs each pump beam into a corresponding one of the plurality of micro-cavities, each pump beam having a resonant mode coincident with the first resonant frequency of the corresponding one of the micro- cavities.
  • the apparatus further produces a plurality of probe beams and directs each probe beam into a corresponding one of the micro-cavities, each probe beam tuned to the second resonant frequency of the corresponding one of the micro-cavities.
  • a plurality of detectors are configured to detect the plurality of probe beams after transmission through the plurality of micro-cavities, and the processor is configured to identify shifts in the corresponding second resonant wavelengths and to correlate the shifts to characteristic photothermal signals for each of the chemical vapor species present in the sample.
  • Another aspect of the invention comprises a method of detecting a chemical vapor species having an absorption wavelength in the mid- to far-infrared range.
  • the method comprising the steps of providing the apparatus described herein, introducing the chemical vapor species into the ambient region surrounding the micro-cavity;
  • the resonant mode of the pump beam evanescently interacts with the chemical vapor species and converts the optical energy of the pump beam into heat via optical absorption by the chemical vapor species, causing the temperature of the micro-cavity to increase from an initial temperature to an increased temperature that causes a shift in at least the micro-cavity second resonant wavelength.
  • a reference micro-cavity may be provided to cancel out temperature fluctuation noise.
  • the method may further comprise pre- concentrating the chemical vapor species prior to introducing the chemical vapor species into the micro-cavity.
  • Intensity interrogation techniques or frequency interrogation techniques may preferably be used with the probe beam to detect the wavelength shift.
  • Yet another aspect of the invention comprises a method of detecting a plurality of chemical vapor species in a sample, each chemical vapor species having an absorption wavelength in the mid- to far-infrared range, the method comprising the steps of providing an apparatus as described herein comprising the plurality of micro-cavities, pump beams, and probe beams, introducing the sample into the ambient region surrounding the micro-cavity; directing the plurality of pump beams into the plurality of micro-cavities; directing the plurality of probe beams into the plurality of micro-cavities, detecting the shift in second resonant wavelengths in one or more of the plurality of micro-cavities; and correlating the shifts to characteristic photothermal signals for the plurality of chemical vapor species present in the sample.
  • Fig. 1. illustrates the generic configuration of an exemplary micro-cavity device MC-PTS embodiment.
  • Fig. 2 depicts the flow of an exemplary MC-PTS detection process.
  • Fig. 3A depicts a schematic tilted view of an exemplary on-chip pedestal microdisk cavity made of chalcogenide glass.
  • FIG. 3B depicts a cross-section of the exemplary pedestal micro-disk cavity (not to scale).
  • FIG. 4 depicts a schematic of a multiplexed set of micro-cavities designed to detect a plurality of chemical vapor species.
  • Fig. 5A depicts a top view of an exemplary suspended waveguide beam photonic crystal (PhC) dual resonance photothermal cavity.
  • PhC suspended waveguide beam photonic crystal
  • Fig. 5D depicts an exemplary temperature profile as a result of photothermal heating due to molecular IR absorption, showing low thermal leakage, which leads to large photothermal enhancement.
  • One aspect of the invention comprises a Micro-Cavity PhotoThermal Spectroscopy (MC-PTS) technique, where the bulk mirror assembly of a conventional etalon cavity is replaced by a micro-cavity such as a microsphere, micro-ring, micro-disk, or photonic crystal cavity.
  • MC-PTS Micro-Cavity PhotoThermal Spectroscopy
  • the pump laser spot (and hence the photothermal interaction volume) is typically much smaller than the cavity mode volume; therefore, the photothermal effect becomes spatially localized and a large fraction of the cavity volume remains 'cold' and is not utilized.
  • both the pump and probe light are tuned to resonate inside the micro- cavity to maximize their optical mode spatial overlap.
  • micro-cavities used in MC-PTS can be engineered through material and geometry design to achieve record large photothermal enhancement factors, as illustrated in the example below. Additional benefits of using a micro-cavity include short response time due to its small thermal mass (which also helps to reduce 1/f noise by using high frequency chopped
  • a systematic theoretical analysis on the MC-PTS technique is provided herein, first using a generic micro-cavity model to derive the general properties of the MC-PTS technique, then analyzing the photothermal enhancement factor and noise contributions from different mechanisms to predict the limit of detection (LOD) of the technique, and then applying the theoretical insight gained by this analysis to develop an optimization strategy of material and device design for MC-PTS.
  • An exemplary embodiment using chalcogenide glass micro-disk cavities shows that by proper selection of material and cavity designs, dramatic LOD improvement over conventional cavity-enhanced infrared absorption spectroscopy can be achieved .
  • a preferred embodiment comprises a micro- /nano-cavity which is transparent to infrared and has high photothermal enhancement, such as a micro-ring/disk micro-cavity that is thermally isolated from the environment by using a suspended structure, as further described in Example 1, or a photonic crystal nano-cavity, as further described in Example 2.
  • micro-cavity refers to any structures that may properly be considered a micro-cavity by one of skill in the art as well as any structure of lesser size, such as those that may more accurately be referred to as “nano-cavity" structures by those of skill in the art.
  • a core component of the device is an optical micro-cavity 30, which in the most general case can either be a traveling wave cavity such as micro-ring, microdisk or micro-sphere, or a standing wave cavity such as a multi-layer Bragg cavity (as is known in the art) or a point defect in a photonic crystal slab (shown and discussed later herein).
  • the cavity is thermally connected to a heat sink 32 (e.g . the substrate on which the micro-cavity device is fabricated) through a thermal conductance G.
  • the heat sink may be regarded to be held at a constant temperature T 0 , which will be used as a reference point for the temperature rise in the micro-cavity.
  • a high-power pump beam 12 such as created using a laser source (not shown) is locked to one of the resonant frequencies of the cavity, and is used to induce optical absorption and photothermal effects in the cavity.
  • the optical resonant mode of the pump beam interacts with these molecular species evanescently: such interaction converts optical energy of the pump beam into heat via optical absorption.
  • thermo-optic effect the temperature of the cavity will increase from T 0 to T 0 + dT, when thermal equilibrium is established between the photothermal heat generation and heat dissipation through the thermal conductance.
  • the resulting temperature change dT translates to a cavity resonant wavelength change dAp due to the thermo-optic effect:
  • ⁇ ⁇ K - dT (D where ⁇ is the thermo-optic coefficient of the micro-cavity (the subscript p following ⁇ denotes that the wavelength ⁇ ⁇ is associated with the probe beam).
  • a resonant wavelength shift 18 is then detected by a low-power, near-I probe beam, using a wavelength or intensity interrogation technique, such as those described in J. Homola, S. Yee, and G. Gauglitz, "Surface plasmon resonance sensors: review,” Sens. Actuators, B 54, 3- 15 (1999), incorporated herein by reference.
  • a near-IR detector 40 shown in reference to Fig.
  • a processor 50 detects the wavelength shift, determine the wavelength shift and correlate the shift to a characteristic photothermal signal for the chemical vapor species.
  • the source of the low-power probe beam may be any near-infrared source known in the art.
  • the basic operating principle of MC-PTS is summarized in the flowchart of Fig. 2.
  • the micro-cavity has a loaded quality factor Q and is critically coupled to the pump beam, i.e. the fraction of pump light entering the cavity is unity, the total optical energy stored in the cavity at steady state is then given by:
  • e c gives the spatial dielectric constant distribution of the cavity
  • E 0 is the electric field complex amplitude of the cavity mode
  • P is the pump light power coupled into the cavity
  • CO is the angular frequency of pump light
  • the integrals are carried out across the entire cavity, as is denoted by the subscript c.
  • the decay time constant of optical energy in the cavity due to molecular absorption may be written as: and the optical energy loss rate (which is equal to the photothermal heat generation rate) in the cavity is given by:
  • c 0 is the velocity of light in vacuum
  • n is the index of refraction (real part) of the sensing medium surrounding the cavity
  • ⁇ represents the pump wavelength
  • the subscript e in the integrals specifies that the integration domain covers only the evanescent wave in the surrounding medium.
  • the photothermal enhancement factor E for the pump- probe configuration can be defined as the ratio of the fractional transmitted power change of the probe beam dl p to the fractional transmitted power change of the pump ⁇ beam dl, since the former is induced by the photothermal effect whereas the latter is the direct consequence of optical absorption. Assuming that the cavity quality factor Q is the same at both pump and probe wavelengths, maximum photothermal enhancement factor is achieved when the following conditions are met: 1) the cavity also operates near the critical coupling regime at the probe wavelength; and
  • optical absorbance experienced by the pump beam in the limit of ⁇ ⁇ ⁇ ⁇ /Q is given by: dl Q (7)
  • n c denotes refractive index of the cavity material.
  • the enhancement factor E is given by the ratio of dl p to dl:
  • the surrounding medium where the target molecular species is present.
  • enhancement effect to the photothermal signal dl p is two-fold : the optical absorption of the pump and hence photothermal heat generation is enhanced roughly by a factor of Q; and the resonant peak width is inversely proportional to Q such that even a small resonant wavelength shift d ⁇ p translates to a large probe intensity change dl p , which is increased by a factor of Q (the first equality in Eq. 6), leading to another factor of Q boost of di p .
  • Eq. (8) suggests that the enhancement factor is proportional to the cavity Q-factor. This is also consistent with previous analysis by other authors. Therefore, we can expect very large photothermal enhancement with respect to conventional cavity- enhanced absorption spectroscopy by using a high-Q micro-cavity for MC-PTS.
  • Eq. (6) and Eq. (8) provide the photothermal signal at steady state.
  • chopped measurement is employed to suppress noise at unwanted frequencies (e.g. 1/f noise at low frequencies) and hence to improve Signal-to-Noise Ratio (SNR). Therefore, it is important to understand the frequency dependence of the photothermal signal in MC-PTS.
  • Photothermal signal frequency dependence may be derived by solving the heat flow equation with a sinusoidal input of heat flux.
  • the photothermal enhancement factor of a micro-cavity with a heat capacity C measured using a sinusoidal input at a frequency f is given by:
  • ⁇ C/G is the thermal time constant of the micro-cavity.
  • TF temperature fluctuation
  • AF absorption fluctuation
  • HS heat sink
  • the magnitude of the temperature fluctuation noise can be derived from the fluctuation-dissipation theorem as:
  • ks represents the Boltzmann constant and B denotes the measurement bandwidth. Since the sensor needs to operate in a sensing medium (typically air) rather than vacuum, the dominant heat transfer mechanism is most likely to be thermal conduction rather than radiation. Therefore radiative contribution to the factor G, and hence background fluctuation noise, can be ignored.
  • ⁇ A F (f ) - ⁇ (nra bg + n c a c ) - (SP(f)) (13) in wh ' rch ⁇ 3 ⁇ 4 ⁇ gives the background absorption from the sensing medium, a c denotes the micro-cavity material absorption coefficient, and ( ⁇ 5P(/)) is the power spectral density function of pump laser intensity noise (only considering the laser power coupled into the cavity).
  • scattering e.g. scattering from roughness or material density non- uniformity
  • the heat sink e.g. the substrate on which the device is fabricated
  • T 0 the heat sink temperature
  • the micro-cavity and the heat sink are thermally connected through the thermal conductance G, heat sink temperature fluctuation also translates to micro-cavity temperature change.
  • the micro-cavity resonant wavelength is determined by the micro-cavity temperature rather than the temperature difference between the micro-cavity and the heat sink, it is expected that the heat sink temperature fluctuation will add to sensor noise. Power spectral density of this noise contribution can be derived by solving the heat flow equation in the frequency domain and the results give:
  • Intensity interrogation looks at the intensity modulation caused by photothermal resonant wavelength shift.
  • the enhancement factor analysis discussed above is based on intensity interrogation.
  • the read-out noise superimposed on dkp is linked to the coupled pump beam power fluctuation by dividing the fractional power change of the probe beam ( ⁇ ⁇ ⁇ /) ⁇ ⁇ with the maximum slope of the cavity transmission curve:
  • the subscript ' ⁇ ' denotes quantities associated with the probe beam, as read-out noise originates from probe beam power fluctuations and is thus independent of the pump beam properties. Equivalently, the read-out noise may be expressed in terms of cavity temperature fluctuati
  • the constituent materials of the micro-cavity should be transparent at these wavelengths to enable full spectrum fitting and target species identification. Therefore, the selection favors materials consisting of heavy elements to reduce phonon absorption in the infrared. Further, to minimize excess free carrier absorption, dielectric materials, such as but not limited to glasses such as silica, silicon nitride, and chalcogenides, and wide- bandgap semiconductors, such as but not limited to ZnO, ZnS, ZnSe, SiC, and diamond, are preferred over narrow-gap semiconductors. Examples of IR-transparent materials include chalcogenides, halides, heavy metal oxides, and certain compound
  • semiconductors e.g . ZnSe, ZnO, ZnS, ZnSe, SiC, and diamond.
  • Low thermal conductivity preferably, but not limited to a range lower than 5 W/(mK).
  • High thermo-optic coefficient preferably, but not limited to a range higher than 10 '4 RIU/K
  • Eq. (8) suggests that the photothermal enhancement factor is maximal in a thermally isolated (small thermal conductance ⁇ 10 ⁇ 5 W/K or lower, high-Q (Q > 10,000) cavity with high thermo-optic coefficient.
  • the thermal conductance may be preferably, but not limited to, a G of about ⁇ 10 "5 W/K or lower, and Q may be preferably, but not limited to, a range of about 10,000 and higher,
  • micro-cavity should be thermally isolated from the heat sink.
  • micro-fabricated suspended structures employed in infrared bolometers have been proven to be highly effective in minimizing thermal leakage to the substrate, and thermal conductance in the order of 10 "8 W/K has been demonstrated.
  • the same design concept may be implemented for MC-PTS as well.
  • Some possible design variations include pedestal structures and thin film devices suspended on an insulating membrane.
  • the device figure-of-merit for improving MC-PTS sensor detection limit is the cavity Q-factor rather than finesse.
  • the proposed MC-PTS sensing mechanism can operate in both gaseous and aqueous environment, it is ideally suited for vapor detection given the much lower thermal conductivity of air (0.024 W/mK) as compared to that of liquid water (0.6 W/mK).
  • the strong infrared optical absorption of water also prohibits most infrared spectroscopic applications.
  • An exemplary MC-PTS device embodiment used a pedestal micro-disk resonator made of IG3 chalcogenide glass (Ge3oAs 13 Se32Te25).
  • IG3 chalcogenide glass Ga3oAs 13 Se32Te25.
  • the rationales for selecting this material include the wide infrared transparency, low thermal conductivity, and high thermo-optic coefficient of this glass composition.
  • the relevant thermal and optical properties of the IG3 glass are listed in Table 1.
  • Chalcogenide glasses can be fabricated into planar high-Q micro-disk resonators by combining CMOS backend compatible lift-off patterning and post-fabrication thermal reflow treatment, thereby opening up the prospect for high volume manufacturing, as described, for example, in J . Hu, N. Carlie, N . Feng, L. Petit, A. Agarwal, K. Richardson, and L. C. Kimerling, "Planar waveguide-coupled, high-index-contrast, high-Q resonators in chalcogenide glass for sensing," Opt. Lett. 33, 2500-2502 (2008) and J. Hu, N. Feng, N .
  • FIGs 3a and 3b show schematic diagrams of the proposed chalcogenide glass micro-cavity device for MC-PTS applications.
  • the device consists of a micro-disk cavity (resonator) 30 made of IG3 glass, a pedestal 31 which mechanically supports the suspended micro-disk cavity, and a planar bus waveguide 34 on the substrate 32 to evanescently couple the pump 12 and probe 14 beams into the cavity.
  • resonator micro-disk cavity
  • pedestal 31 which mechanically supports the suspended micro-disk cavity
  • a planar bus waveguide 34 on the substrate 32 to evanescently couple the pump 12 and probe 14 beams into the cavity.
  • both pump and probe wavelengths are assumed to be around 4 pm (although in practice the pump and probe beams are preferably different resonant wavelengths), the overhang length to be about 10 pm (Fig. 3b), and the micro-disk thickness and diameter to be approximately 1 pm and approximately 100 pm, respectively.
  • the thermal conductivity of IG3 glass is almost an order of magnitude larger than that of air. If it is assumed that the substrate and the pedestal have large thermal conductivity so that they are considered to be held at a constant temperature T 0 all the time, the overall thermal conductance can be approximated as the direct sum of two parts: the thermal conductance through the glass micro-disk and through the surrounding air:
  • the two thermal conductance components may be separately calculated by mapping the thermal diffusion problem into a cross-section (Fig. 3b) in a cylindrical coordinate and numerically solving the 2-d Laplace equation. Using an air thermal conductivity value of 0.024 W/mK and the IG3 glass thermal conductivity listed in Table 1, the numerical solution gives a total effective thermal conductance of
  • intensity noise figures of mid-IR quantum cascade lasers calculated using a semi- classical model are taken as the noise characteristics of the pump and probe sources, which give a Relative Intensity Noise (RIN) value of -170 dB/Hz for the
  • the Wiener-Khintchine theorem was used to correlate the power spectral density function ( ⁇ 5T 0 (/)) to the RMS amplitude of heat sink temperature fluctuation ( ⁇ 0 ) :
  • f is taken to be the measurement chopping frequency
  • a heat sink RMS temperature fluctuation amplitude of 1 mK corresponds to:
  • Heat sink thermal noise is more than one order of magnitude larger than the other noise sources, given the assumed conditions. It is noted that this noise does not present a fundamental limit to the MC-PTS technique, however, as in practice it may be partially circumvented by selecting a chopping frequency where (ST 0 (f)) is minimal, or by using a reference micro-cavity to cancel out the temperature fluctuation effect, as discussed in A. Densmore, M. Vachon, D.-X. Xu, S. Janz, . Ma, Y.-H. Li, G. Lopinski, A. Delage, J.
  • MC-PTS sensor limit of detection in terms of analyte concentration is a figure commonly used in the sensing community. Assuming a measurement bandwidth window of 1 Hz, Eq. (9) and Eq. (25) yield an estimated absorption detection limit of
  • the concentration detection limit of the proposed MC-PTS device is ⁇ 40 ppt (parts-per-trillion). No pre- concentration is assumed in deriving this LOD number; thus orders of magnitude further improvement is expected when the MC-PTS technique is coupled with gas pre- concentration schemes, which will bring the LOD well into the sub-ppt range.
  • a suspended waveguide beam photonic crystal (PhC) cavity may be provided as the micro-cavity rather than a pedestal micro-disk.
  • This geometry offers simultaneously sub-diffraction optical confinement, high-finesse resonance enhancement at both pump and probe wavelengths, and superior thermal isolation.
  • the photonic crystal may be fabricated using standard DUV lithography or ion milling, as is well known in the art.
  • the 5.27 pm pump wavelength matches the absorption line of nitric oxide (NO) and is chosen only as an example for illustration here.
  • NO nitric oxide
  • Both of the nested cavities adopt a size-modulated design to achieve high-Q operation.
  • a lower pump wavelength Q-factor was adopted by reducing the number of mirror pairs, because the absorption saturation effect poses an upper limit of ⁇ 20,000 for the pump Q, beyond which further increasing pump Q has a diminishing return on sensitivity improvement.
  • the cavities are preferably strongly coupled to the input/output waveguides by satisfying the condition that the coupling coefficient (mirror loss) is much larger than radiative loss inside the cavity, which guarantees efficient pump power delivery into the cavity.
  • Coupling loss of both pump and probe may be minimized by using a Bloch mode engineering approach to design the taper structure, such as disclosed in Q. Quan, P. Deotare, and M . Loncar, "Photonic crystal nanobeam cavity strongly coupled to the feeding waveguide," Appl. Phys. Lett. 96, 203102 (2010), incorporated herein by reference.
  • micro-cavity photothermal spectroscopy may be used for ultra-sensitive detection of chemical species.
  • the doubly-resonant pump-probe configuration leads to efficient, resonantly enhanced infrared absorption, as well as superior spectroscopic resolution, and gives rise to record large photothermal enhancement factors exceeding 10 4 .
  • Quantitative numerical analysis performed based on a chalcogenide glass micro- cavity sensor design yielded an absorption detection limit down to 2 x 10-9 cm '1 for a cavity with a moderate quality factor of 2 x 10 5 and at a pump laser power of 0.1 W, which corresponds to ⁇ 40 ppt chemical vapor molecular detection limit without pre- concentration.
  • the pump Q may be designed to be lower than the probe Q so that the pump beam remains in resonance despite the probe resonance shift.
  • the pump wavelength may be feedback loop locked to the cavity's 1st resonance.
  • sensor performance may be further improved by selecting a measurement chopping frequency where the noise spectral density is minimal, or by using a reference cavity to cancel out the temperature fluctuation effect.
  • a suitable reference cavity will have an identical configuration as the main cavity and placed in close proximity to the main cavity, except shielded from the molecule flux occurring in the main cavity so that the resonance shift is due entirely to thermal noise.
  • each wavelength component in the mid-IR pump beam 12 split using any beam splitting optics 38 known in the art, with each wavelength pump beam component 12 t .. n then directed into the respective micro-cavity in which the wavelength resonates.
  • Each probe beam 14i... n preferably comprises a single wavelength, near-IR beam (e.g. such as from a thermally tuned distributed feedback laser or any other suitable near-IR source known in the art) that can be split, directed through the micro-cavities, and detected at the output end using inexpensive near-IR sensors 40 ! ... n .
  • the plurality of micro-cavities are preferably designed with different first resonant frequencies corresponding to the absorption wavelengths of the chemical species intended to be detected, with all of the micro-cavities sharing the same second resonant frequency so that a single probe beam can be used for detection.
  • a single probe beam and detector may be used without splitting the beam into components directed through each micro-cavity simultaneously, wherein the single beam can be configured to sequentially scan each of the micro-cavities and be detected by the single detector.
  • a single beam with multiple detectors could be used, or more than one beam and/or detector but fewer than the number of micro-cavities.
  • the multiple micro- cavities may also include one or more micro-cavities used solely for noise compensation, as discussed elsewhere herein .
  • the pump-probe configuration using a mid-IR pump beam with a near-IR probe beam eliminates the need for costly, low-performance mid-IR detectors in spectroscopic sensing.
  • Processor 50 stores and processes data captured by the detectors 40, determines the shifts in the micro-cavity resonant wavelengths, and correlates the shifts to characteristic photothermal signals for the chemical vapor species present and detected in the sample.

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Abstract

Cette invention concerne un appareil pour détecter des espèces chimiques en phase vapeur comprenant une microcavité ayant une première fréquence de résonance correspondant à la longueur d'onde d'absorption de l'espèce et une seconde fréquence de résonance ; une zone ambiante autour de la microcavité pour recevoir l'espèce ; une source pour diriger un faisceau de pompe mi-infrarouge/infrarouge lointain ayant un mode de résonance coïncidant avec la première fréquence dans la microcavité ; une source pour diriger un faisceau de sonde proche-infrarouge réglé sur la seconde fréquence de résonance dans la microcavité ; et des détecteurs pour détecter le faisceau de sonde après émission à travers la microcavité. Le faisceau de pompe induit une absorption optique par l'espèce de la vapeur, créant des effets photothermiques dans la microcavité qui décalent la seconde fréquence de résonance. Un processeur identifie et corrèle le décalage de longueur d'onde à un signal photothermique caractéristique de l'espèce chimique en phase vapeur. Un appareil pour détecter de multiples espèces et des procédés pour détecter des espèces uniques ou multiples sont également décrits.
PCT/US2011/050338 2010-09-02 2011-09-02 Détection ultrasensible de vapeurs chimiques par spectroscopie photothermique à microcavités Ceased WO2012031208A2 (fr)

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WO2014120316A1 (fr) * 2013-02-01 2014-08-07 Battelle Memorial Institute Spectromètre à absorption capillaire et processus l'analyse isotopique d'échantillons de petite taille
CN105675529A (zh) * 2016-01-21 2016-06-15 电子科技大学 微小型中红外光波导气体传感器
US10132743B2 (en) 2016-01-25 2018-11-20 General Electric Company Fixed optics photo-thermal spectroscopy reader and method of use
US10732097B2 (en) 2016-07-13 2020-08-04 Technische Universität Wien Photothermal interferometry apparatus and method
AT525495A3 (de) * 2021-09-17 2023-09-15 Univ Wien Tech Ausgleichsdetektion mit ICAPS innerhalb einer optischen Kavität
CN119354889A (zh) * 2024-12-24 2025-01-24 中国计量大学 一种基于双增强光热光谱的高灵敏微小气量气体检测装置及方法

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US6507684B2 (en) * 2000-06-28 2003-01-14 The Charles Stark Draper Laboratory, Inc. Optical microcavity resonator system
US7384797B1 (en) * 2000-10-12 2008-06-10 University Of Utah Research Foundation Resonant optical cavities for high-sensitivity high-throughput biological sensors and methods
US7781217B2 (en) * 2002-10-02 2010-08-24 California Institute Of Technology Biological and chemical microcavity resonant sensors and methods of detecting molecules
WO2008034118A2 (fr) * 2006-09-15 2008-03-20 President And Fellows Of Harvard College Procédés et dispositifs destinés à des mesures utilisant une spectroscopie pompe-sonde dans des microcavités de haute qualité
US7667200B1 (en) * 2007-12-05 2010-02-23 Sandia Corporation Thermal microphotonic sensor and sensor array

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2014120316A1 (fr) * 2013-02-01 2014-08-07 Battelle Memorial Institute Spectromètre à absorption capillaire et processus l'analyse isotopique d'échantillons de petite taille
US9297756B2 (en) 2013-02-01 2016-03-29 Battelle Memorial Institute Capillary absorption spectrometer and process for isotopic analysis of small samples
CN105675529A (zh) * 2016-01-21 2016-06-15 电子科技大学 微小型中红外光波导气体传感器
US10132743B2 (en) 2016-01-25 2018-11-20 General Electric Company Fixed optics photo-thermal spectroscopy reader and method of use
US10732097B2 (en) 2016-07-13 2020-08-04 Technische Universität Wien Photothermal interferometry apparatus and method
AT525495A3 (de) * 2021-09-17 2023-09-15 Univ Wien Tech Ausgleichsdetektion mit ICAPS innerhalb einer optischen Kavität
AT525495B1 (de) * 2021-09-17 2023-12-15 Univ Wien Tech Ausgleichsdetektion mit ICAPS innerhalb einer optischen Kavität
CN119354889A (zh) * 2024-12-24 2025-01-24 中国计量大学 一种基于双增强光热光谱的高灵敏微小气量气体检测装置及方法
CN119354889B (zh) * 2024-12-24 2025-06-20 中国计量大学 一种基于双增强光热光谱的高灵敏微小气量气体检测装置及方法

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