Patent Application: US-201514884716-A

Abstract:
configurations for in - situ gas detection are provided , and include miniaturized photonic devices , low - optical - loss , guided - wave structures and state - selective adsorption coatings . high quality factor semiconductor resonators have been demonstrated in different configurations , such as micro - disks , micro - rings , micro - toroids , and photonic crystals with the properties of very narrow nir transmission bands and sensitivity up to 10 − 9 . the devices are therefore highly sensitive to changes in optical properties to the device parameters and can be tunable to the absorption of the chemical species of interest . appropriate coatings applied to the device enhance state - specific molecular detection .

Description:
one embodiment of a present rot is shown in fig1 , the basic structure of which is a semiconductor - based edge - emitting laser , eel 200 . the basic eel , which is well known in the art , is augmented with a state - selective thin - film functionalized adsorption layer , thereby resulting in the trace - gas sensor . the eel is comprised of a substrate , 210 , upper ( gain ) guiding ridge layer 220 , and top and bottom contacts , 230 and 240 , respectively , for biasing . in some cases , the top contact can be partitioned , so that different regions within the laser can possess differing amounts of optical gain or loss . the basic guided - wave optical mode , 250 , is confined vertically by the basic waveguide structure and laterally , via gain guiding . the eel optical cavity consists of a pair of fresnel reflective air / semiconductor interfaces , 255 , formed at the front and rear substrate facets , respectively , normal to the optical axis . the semiconductor materials and structure of the basic laser are well known in the art . as an example , the eel laser can be comprised of group iii - v materials , e . g ., inas , inp and gaas , and its ternary and quaternary alloys ( e . g ., ingaas and ingaasp , respectively ); and the gain medium can be configured in a variety of architectures , an example of which is a double quantum - well structure ( dqw ) using similar materials . in this embodiment , a set of functionalized surface regions , 260 and 270 , respectively , are each coated with a suitable state - selective adsorptive thin - film layer . these coatings are deposited on the upper surface of the eel , and positioned on either side of the ridge guiding layer , 220 . by changing the coating , different gases can be detected with a well - defined selectivity and specificity as a function of the coating material . during operation , as the desired species is adsorbed by the thin film , the optical properties of the material will be modified , such as its refractive index and / or absorption . these trace - quantity induced thin - film optical perturbations will , in turn , affect the output of the eel , in terms of its laser output power , output spectrum and modal characteristics . the sensitivity to the adsorbed material is enhanced by the highly nonlinear dependence of the laser properties to the optical cavity characteristics . in essence , the internal laser photons traverse the optical cavity multiple times ( 100s to 1 , 000s of traversals , depending on the specific laser design ), thereby probing the modified cavity parameters in a like manner , and , thusly , effectively amplifying the sensitivity of the sensor . various regions on the coatings can be masked via optional coverings , 265 , as may be deemed necessary for a given structure . an example of a coating , with application to the detection of trace quantities of h 2 gas , is a thin film of either pd or pt . the optical and electrical properties of these materials change in the presence of h 2 which can be detected even in a background of atmospheres of n 2 . in a separate set of experiments , ellipsometric measurements were performed on pd and pt thin films to quantify the complex index of refraction at various optical wavelengths of interest , with and without the presence of h 2 . these measurements enable more accurate quantitative information to be obtained using the thin - film coated trace - gas sensors , discussed herein . in addition , they provide more accurate data as input to the sensor simulations so that their performance can be optimized . fig2 shows data of the measured fractional change in reflection ( fcr ), δr / r 0 , and transmission ( fct ), δt / t 0 for an optically thin pt 11 . 83 nm film , in the presence of h 2 . the fractional change technique is beneficial because it normalizes out collection efficiency proportionality factors . the steady state fcr and fct values for 1 % h 2 were 2 . 06 % and − 2 . 32 %, respectively . a pd 26 . 94 nm film had a similar response with fcr = 1 . 33 % and fct =− 1 . 42 %. the anomalous reflectance increase in optically thin films can be attributed to reflections from multiple interfaces . for the thick films , statistically significant changes were not observed . the pt and pd thin - film rise times were quite different ; 10 minutes and 10 seconds , respectively . after the h 2 is shut off , the reflection and transmission for both films begin moving away from the baseline . this behavior is under investigation . with the measured 1550 nm values for n and κ for each film , the spectrally averaged fcr and fct were simulated for δn = 0 . 001 and δκ = 0 . 001 using the transmission matrix technique . assuming linear dependencies , the combination of δn and δκ that yield the measured fcr and fct can be solved algebraically from a 2 × 2 matrix . estimates of δn and δκ made this way were very close to the solutions obtained from iterative searching . for 1 % h 2 in n 2 , we find δn =− 0 . 089 and δκ = 0 . 181 for the pt 11 . 8 3 nm film , compared to δn =− 0 . 033 and δκ = 0 . 067 for the pd 26 . 94 nm film . the increase in extinction coefficient is unexpected given previous discussions and trends for pd films . additional experiments are needed to confirm this trend and the parameter extraction technique . nonetheless , these measurements provide useful input to the modeling and simulation of the h 2 trace - gas sensors . the performance of the embodiment shown in fig1 was modeled using a numerical simulation code . as a specific example of an eel sensor , a laser with a graded - index separate confinement heterostructure ( grinsch ) configuration was considered , using a standard double quantum well ( dqw ) structure . lasers without lateral surface coatings have been previously fabricated and thoroughly characterized and modeled . for this choice of material and geometry , the laser wavelength varies from 950 - 970 nm , depending on bias and temperature . the eel geometry that was modeled is a multiple section 2 μm × 250 μm ( eel ) with a 50 nm thick pd surface coating that is located 100 nm above the active region and extends 3 μm laterally on either side . thus , the total sensor width is 8 μm ( see fig1 for labeling of the dimensions ). the optical mode has a small overlap , γ pd ≈ 1 × 10 − 4 , with this thin coating . h 2 gas reacts with pd to form pdh thereby reducing the internal loss seen by the laser mode , α i , from 58 cm − 1 at a rate of 0 . 03 cm − 1 per 100 ppm of h 2 for our example sensor . thus , the laser &# 39 ; s output power increases rapidly in response to the amount of adsorbed h 2 . fig3 shows results of a simulation to model the laser sensor . a 1 - d rate equation model , previously verified against experimental l - i data , was used to calculate the response for l = 250 μm , w = 2 μm , v = 100 nm , and t = 50 nm for three types of pd - coated sensors : a passive waveguide ( wg ), a single section eel , and a multiple section eel , the latter of which includes a 37 . 5 μm long unbiased saturable absorber with ≈ 15 % optical absorption . ( fig1 shows the dimensional labeling .) for a fair comparison , we adjust the biases so that the sensor &# 39 ; s output optical power is fixed at 10 μw for zero h 2 . it is also assumed that laser output power measurement has an instrument uncertainty of ≈ 10 − 3 . for this set of parameters , the predicted limits of detection ( lods ) for the three structures were 138 ppm , 4 ppm , and 1 ppm , respectively . the passive wg requires 43 □ w of input optical power , whereas the eels need no input optical power but use 8 mw and 13 mw , respectively , of electrical power , given an operation voltage of 1 . 5 v . shorter cavity lengths used less power but had higher minimum detection limits ( mdls ). the resonant cavity of the eel provides a strong nonlinearity in the sensor response , i . e ., the lasing knee , and thereby reduced the lod compared to the passive wg . the saturable absorber further amplifies this lasing knee nonlinearity according to the gain - lever effect . the simulation also provided spatial information as to the guided - wave mode supported by the eel structure . fig4 shows a typical “ pear shaped ” spatial mode profile for the case w = 2 □ m and t pd = 50 nm and the inset shows t pd = 0 nm . both have elliptical central field contours , but the contours at the field tails for t pd = 50 nm are significantly squeezed in by the pd coating , despite the very small modal overlap . the high sensitivity of the spatial - mode dependence on pd coating is , most likely , due to the optical cavity losses experienced by the guided - mode field in the presence of the thin film . given the dependence of the mode profile as a function of the adsorbing coating , a real - time measurement of changes in the spatial - mode pattern may , in fact , provide additional evidence regarding the presence of trace - gas species . these chip - scale eel - based sensors offer high sensitivity , wide dynamic range , inline integration with photodetectors and 2 - d scalability for drift compensation and for detection and identification of multiple species . for maximal sensitivity , a large spatial - mode field overlap with the coating is needed . this can be achieved with a narrow ridge , a thick coating layer , zero horizontal distance between ridge and coating , and a small vertical separation between coating and active region . for the 8 μm × 250 μm h 2 sensor with w = 2 μm , v = 100 nm , t = 50 nm , a lod of 1 ppm is predicted . returning to fig1 , the absorption layer ( s ) can be deposited at other locations on the eel and in various patterns , with the proviso that the optical parameters of the eel output and / or the cavity mode pattern , 250 , is measurably altered as a result of the adsorption of the desired gas specie ( s ). the eels can be configured using differently designed optical resonators in place of ( broadband ) fresnel reflective interfaces . examples of different eel resonators include those formed using distributed bragg reflecting ( dbr ) gratings as narrowband end mirrors or , distributed bragg feedback grating structures ( dfb ), as well as micro - toroidal and ring resonators . the dbr and dfb lasers can provide single longitudinal - mode operation , which may increase the sensitivity of the resultant eel gas sensor , as well as provide for enhanced performance detection via common - mode rejection techniques ( using closely situated pairs of coated and uncoated eels , on a common substrate ). a laser with a suitable output wavelength can be designed so that the quantum levels of the desired species ( or , isotope , thereof ) to be sensed can be resonant with the optical mode . in this case , a single eel can provide a sensor with a dual - detection modality : ( 1 ) simultaneous resonant optical detection of the desired trace species via direct optical interaction of the molecule of choice with the laser photons and ( 2 ) broadband detection of the trace species via state - selective adsorption of the molecule of choice , as manifested by non - resonant changes in the laser output characteristics , such as cavity mode , wavelength , polarization , etc . fig5 shows an embodiment of a trace - gas sensor using a specially coated vertical cavity surface optical amplifier ( vcsoa ), 600 . the basic vcsoa is similar in its configuration to that of a conventional vertical cavity surface emitting laser ( vcsel ), except for the fact that the vcsoa is not an oscillator , but , instead , a multi - pass optical amplifier . in essence , the vcsoa is a vcsel that operates below the laser threshold condition . typically , the vcsoa is fabricated using either a lower gain and / or a lower mirror reflectivity relative to a vcsel for a given bias . the vcsoa and the vcsel devices are well known in the art . in general , both devices are comprised of a substrate 610 , with an internally fabricated single or multi - layer reflecting mirror , 620 , whose reflectivity is 100 %. an internal gain medium , 630 , typically a semiconductor based multi - layer structure , is grown onto the mirror 620 . in most cases , an input / output mirror , in the form of a partially transmitting mirror , 640 , is subsequently grown above the gain medium , completing the basic structure ( for simplicity , electrical contacting layers and various buffer layers are not described , as they are well known in the art ). during operation , an optical interrogation beam , 650 , is incident upon the vcsoa . the incident beam interacts with the optical cavity , resulting in an amplified output beam , 660 . owing to the geometry of the device , the counter - propagating input and output beams , 655 , overlap in space . typically , an optical circulator , 670 , is employed to distinguish the input and output beams . in the present embodiment , the basic vcsoa is augmented through the use of a vertically displaced , deformable membrane , 680 , typically , sin x with a coating of al , upon which the input / output coupling mirror , 640 , is grown . one function of the deformable membrane / mirror is to vary the optical cavity length , and , therefore , to control the spectral properties of the vcsoa . for a given interrogation wavelength , the cavity can therefore be tuned so that the fabry perot mode ( s ) within the cavity are in resonance with the input probe beam for maximum sensitivity of the sensor . the presence of the vertically displaced output mirror , 640 , results in an air gap , 690 , formed between the upper surface of the basic structure and the inner - cavity surface of the output mirror . during operation , the gas sample to be analyzed is allowed to flow into the intra - cavity gap , 690 . the trace - gas sensor also consists of a metal oxide coating layer , 695 , grown onto the inner - cavity surface of the membrane ( 680 )- mirror ( 640 ) structure . this oxide layer is selected to modify the effective coupling - mirror reflectivity in the presence of the trace - species to be sensed . the change in reflectivity is due to a change in the refractive index and / or optical absorption in the coating layer as the trace - gas is adsorbed in the film . this change in the reflectivity , as a result of the presence of the trace - gas , can be detected by measuring changes in the vcsoa output beam , 660 , including changes in the optical power and / or shifts in the output spectrum . examples of adsorbing materials and coatings include wo 3 , sno 2 , pdo , zno , and porous si . the optical properties of these coatings are modified in the presence of a variety of trace gases , including no x , co , h 2 s and cl 2 . by changing the coating , different gases can be detected , with varying degrees of selectivity and specificity . turning now to fig6 , an embodiment using a vcsel , 700 , is shown , that combines two different , yet , complementary , trace - gas detection modalities into a single device : ( 1 ) sensing changes in the properties of an optical device via non - resonant adsorption of a given species within the cavity and ( 2 ) sensing resonant photon interactions of the intracavity optical beam directly with a specific molecular absorption feature of the trace - gas species . the first modality has been described above , whereby indirect optical measurements are performed to infer the presence of a given adsorbed species in an optical resonator . the second modality , which complements the first approach , is a form of nir absorption spectroscopy performed within a laser cavity . the basic configuration , as shown in fig6 , is that of a vcsel , with a vertically displaced , membrane - mounted output mirror , 740 , an intra - cavity air gap , 790 , and an internal adsorption oxide layer , 795 . hence , the basic configuration is similar to that of the vcsoa embodiment ( see , e . g ., fig5 ). as discussed above , the vcsel is an optical oscillator ( a laser ), and as such , generates an output optical beam , 760 . therefore , no external interrogation beam is required , as the optical oscillator provides an effective probe beam within the cavity . since the oscillating beam passes through the air gap multiple times , the presence of a trace gas can be sensed via changes in the output beam , as before . however , the output beam changes can be induced by either cavity perturbations resulting from changes in the internal optical coating , 795 , and / or by direct optical absorption with the gas sample itself . the presence of a trace - gas can therefore be inferred by changes in the vcsel laser output beam , including its output power and its output spectrum . furthermore , the vcsel operating wavelength can be tuned via changes in the vertical displacement of the moveable mirror . thus , the vcsel trace - gas sensor can be described as a wavelength tunable , multi - pass cell with optical gain . in operation , the laser is electrically driven above threshold , with the gas sample allowed to flow through the air gap , 790 . the presence of the desired trace - gas spoils the gain - loss balance necessary for lasing by increasing the absorption losses within the cavity , either by resonant coupling with the gas molecule within the air gap and / or by non - resonant optical cavity changes induced by adsorption of the gas into the internal oxide layer , 795 . in either case , the high - q of the vcsel structure enhances the sensing by adsorption and / or absorption , as the light is reflected multiple (& gt ; 100 ) times within the resonant cavity , formed on one end by the external membrane distributed bragg reflecting ( dbr ) mirror , 740 , and on the other end by the internal distributed dbr mirror , 720 . during operation , the lasing power and / or output spectrum can be monitored remotely by transmission through an optical fiber or directly by an integrated detector . a vcsel with a moveable membrane - based output coupler was fabricated and evaluated in terms of its spectral properties as well as its temporal response to transient changes in the optical cavity length . fig7 shows results of the measured dependence of the vcsel output spectrum for different values of a bias control voltage , as applied to the moveable mirror membrane . as the membrane is electrostatically displaced , the output spectrum is observed to change correspondingly , shifting to shorter wavelengths as the cavity length decreases ( increasing bias voltages ). the multiple spectral features for each measured spectral scan are attributed to the presence of the vcsel &# 39 ; s multiple longitudinal modes , given the cavity design parameters . figure a8 - 8 c show results of the temporal response time of the moveable membrane . fig8 a shows the transient control voltage , as applied to the membrane . the response of the membrane in the presence of this transient signal is shown in fig8 b and 8c , which , respectively , display measurements of the membrane velocity and displacement . the membrane response time is observed to be in the range of ≈ 10 μsec under these conditions . fig9 and 10 show two different systems implementations of the trace - 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