Suppression of residual single-photon absorption (SPA) relative to two-photon absorption (TPA) in a high finesse GaAs planar microcavity is explored. The TPA photocurrent becomes larger than the SPA photocurrent as long as the incident continuous-wave optical power exceeds 0.09 mW. An optical power of 5 mW would be required if the relative SPA suppression did not exist

Barry, Liam P.

Bondarczuk, Krzysztof

Bradley, Ann Louise

Donegan, John Francis

Flood, Edward

Guo, Wei Hua

Lynch, Michael

Maguire, Paul J.

O'Dowd, John

Quinlan, Tom

English

DCU Online Research Access Service

1426 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 16, AUGUST 15, 2008Suppression of Residual Single-Photon AbsorptionRelative to Two-Photon Absorption in High FinessePlanar MicrocavitiesWei-Hua Guo, John O’Dowd, Edward Flood, Tom Quinlan, Michael Lynch, Ann Louise Bradley,John Francis Donegan, Senior Member, IEEE, Krzysztof Bondarczuk, Paul J. Maguire, Member, IEEE, andLiam P. Barry, Member, IEEEAbstract—Suppression of residual single-photon absorption(SPA) relative to two-photon absorption (TPA) in a high finesseGaAs planar microcavity is explored. The TPA photocurrentbecomes larger than the SPA photocurrent as long as the incidentcontinuous-wave optical power exceeds 0.09 mW. An opticalpower of 5 mW would be required if the relative SPA suppressiondid not exist.Index Terms—Finesse, planar microcavity, single-photon ab-sorption (SPA), two-photon absorption (TPA).I. INTRODUCTIONT WO-PHOTON absorption (TPA) in semiconductors isof strong interest in applications such as optical sam-pling [1], optical clock recovery [2], and optical performancemonitoring [3]. Although it is a weak third-order nonlinearprocess, TPA in semiconductors can be made very sensi-tive by employing absorption over a long distance [4], highgain in an avalanche photodiode [5], or high enhancementin a planar microcavity [6]. In TPA photodetectors, residualsingle-photon absorption (SPA) also exists due to deep defectsor the Franz–Keldysh effect [7]. For optical signals with verylow-repetition frequency and very high peak power, the SPAcurrent accounts for so small a portion of the total currentdetected it can be neglected. However, as the optical signal rep-etition frequency becomes high and the peak power decreases,the influence of SPA can no longer be neglected because it mayeven exceed the TPA current. So addressing residual SPA is avery important issue if these TPA detectors are to be used inoptical communications since optical signals used in this envi-ronment have relatively low peak powers ( mW). In [8], it hasbeen shown that the TPA efficiency can be enhanced greatly byManuscript received April 11, 2008; revised May 26, 2008. This work wassupported by SFI under its CSET Centre for Telecommunication Value DrivenResearch (CTVR), Grant 03/CE3/I405, and its RFP programme, Grant RFP/2006/ENE012.W.-H. Guo, J. O’Dowd, E. Flood, T. Quinlan, M. Lynch, A. L. Bradley, andJ. F. Donegan are with the Semiconductor Photonics Group, School of Physicsand Centre for Telecommunication Value Driven Research (CTVR), Trinity Col-lege, Dublin 2, Ireland (e-mail: guow@tcd.ie).K. Bondarczuk, P. J. Maguire, and L. P. Barry are with the Radio and OpticalCommunications Laboratory, School of Electronic Engineering, Dublin CityUniversity, Dublin 9, Ireland.Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2008.927886using a high finesse planar microcavity. In this paper, we showthat this high finesse microcavity not only improves the TPAefficiency but also suppresses the residual SPA relative to TPAso as to decrease the power requirement for TPA to dominate.II. THEORETICAL MODELWithout any cavity enhancement, the TPA current generatedin a TPA photodetector can be generally described by, where is a factor related to the de-tector collection efficiency, is the TPA coefficient, is the ab-sorption length, is the spot area where is the beamwaist of the incident beam assumed to be Gaussian, is the elec-tron charge, is the photon energy, and is the generalizedduty cycle of the incident optical signal [3]. The residual SPAcurrent can be expressed as , whereis the SPA coefficient. To have the TPA current larger than theSPA current requires . So reducing the spotsize or reducing the generalized duty cycle can reduce the powerrequirement for TPA to dominate. If the absorption layer of thedetector is placed between two highly reflective Bragg mirrors,the optical field can be greatly enhanced in the absorption layerso that TPA and SPA will both be enhanced. The TPA enhance-ment factor for the normally incident resonant wavelength is [9](1)where and are the top and bottom mirror reflectivity,respectively. The SPA enhancement factor is(2)To obtain these two formulae we have assumed that the mi-crocavity structure is simply a 1- -thick absorption layer sand-wiched by two Bragg mirrors, as in [8]. The TPA enhancementis larger than the SPA enhancement as it depends on the squaredoptical intensity inside the absorption layer. Although SPA isenhanced as compared with the noncavity case, relative to TPA,it is in fact relatively suppressed. So now the power required forthe TPA current to exceed the SPA current can be expressed as(3)1041-1135/$25.00 © 2008 IEEEAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 12, 2009 at 06:35 from IEEE Xplore.  Restrictions apply.GUO et al.: SUPPRESSION OF RESIDUAL SPA RELATIVE TO TPA IN HIGH FINESSE PLANAR MICROCAVITIES 1427Fig. 1. (a) Schematic diagram of the device structure; (b) Cavity spectrum mea-sured as the TPA and SPA dominated respectively and the incident beam wasnearly a parallel beam.where describes the relative suppression of the residual SPAdue to the high finesse microcavity. According to the followinganalysis, can reach a level around 100.III. EXPERIMENTThe TPA detector is a GaAs planar microcavity structure asshown in Fig. 1(a). The top mirror is p-doped and consists of13-pairs of GaAs–AlAs layers. The bottom mirror is n-dopedand consists of 23-pairs of GaAs–AlAs layers. The absorptionlayer is 0.46- m-thick (1 ) unintentionally doped GaAs. Theresonant wavelength is designed to be 1560 nm. The detector istop and bottom contacted with an input aperture, of 20 m diam-eter, on the top. The experimental setup is as follows: the con-tinuous-wave (CW) output from a widely tunable laser passesthrough an erbium-doped fiber amplifier, followed by an atten-uator and then a polarization controller. It is focused onto thedetector through a lensed fiber. The photocurrent is detected bya Keithley picoameter.Because TPA in GaAs is polarization-sensitive [10], the po-larization controller is used to adjust the incident polarizationso as to maximize the photocurrent. The optimized polariza-tion is kept unchanged throughout the measurement. The cavityspectrum is first measured using a defocused spot much largerthan the detector aperture. In this case, the input Gaussian beamcan be treated as a parallel beam. The spectrum is measured byscanning the tunable laser wavelength while keeping the inci-dent power constant. The incident power is selected to ensureeither TPA (5.84 mW) or SPA (0.2 mW) dominates the de-tector response. Fig. 1(b) shows the normalized spectrum. Thefull-width at half-maximum of the SPA spectrum is 1.88 nm,compared with 1.32 nm for the TPA spectrum. The TPA spec-tral width is exactly 1.4 times less than the SPA spectral width,which is expected because the TPA depends on the squared op-tical intensity inside the cavity while the SPA signal just dependson the optical intensity. The SPA spectral width is broader thanour design value, which could be caused by factors like absorp-tion in the doped mirrors, especially the p-doped mirrors, andalso layer thickness fluctuations. However, through simulationwe find that the penetration depth of the optical field into themirrors is not strongly influenced by these factors, so we assumethat the practical effective cavity length is still our design value,which is 2.04 m. From the SPA spectral width we can find theaveraged mirror reflectivity to be 0.965. Asseen from (1) and (2), even as the averaged mirror reflectivity isfixed, the distribution of the top and bottom mirror reflectivityFig. 2. Cavity spectrum measured and simulated with the TPA and SPA dom-inating respectively and a tightly focused incident beam.will still influence the final enhancement. A higher bottom re-flectivity tends to yield higher enhancement. In our case, thebottom mirror is designed to generate a reflectivity as close aspossible to one, so it has a much larger number of periods thanthe top mirror. Practically, because the absorption loss in thep-doped top mirror would be much higher than in the n-dopedbottom mirror, we would expect that the bottom mirror reflec-tivity is closer to our design value. So we estimate our bottommirror reflectivity would be between 0.98 and 0.99. We take avalue of 0.985 here, so the top mirror reflectivity would then beabout 0.945. Based on these estimations, the TPA enhancementis 11 600 and the SPA enhancement is 90 as calculated from (1)and (2). The SPA suppression factor would be around 130 ac-cording to (3).However practically we need to consider the limited accep-tance angle of the planar microcavity which is defined by theangle at which the intensity inside the cavity drops by half asthe incident beam is angled from normal incidence to greater an-gles. The incident wavelength is the resonant wavelength for thenormal incidence case. The acceptance angle which is directlyrelated to the cavity spectral width is estimated to be 12.7 . Thelensed fiber we use can generate a Gaussian beam with a beamwaist of approximately 3.5 m ( ) which corresponds to adivergence angle of 16.1 . We measured the spectrum againwith the beam tightly focused, giving a spot on the detectormuch smaller than the detector aperture. In this case, the inci-dent power used for the TPA and SPA dominance is 1.3 mWand 4.6 W, respectively. The result is shown in Fig. 2. A clearasymmetry can be seen which is due to the angular componentsof the focused Gaussian beam. A theoretical analysis similar to[9] taking account of the greater angular distribution is carriedout, with the result comparing well with the experimental resultas shown in Fig. 2. Because of the limited acceptance angle ofthe planar microcavity, the angled components of the incidentfocused Gaussian beam cannot be enhanced as much as the nor-mally incident component, so the overall enhancement woulddrop. From the same simulation, the decrease in the enhance-ment for TPA is found to be 71% compared with 32% for SPA.With the incident wavelength corresponding to the peakwavelength from the spectrum in Fig. 2, we measure the pho-tocurrent as a function of the CW power. The result is shownin Fig. 3. A fitting of the current-power curve yields a relationof (mW) which means forAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 12, 2009 at 06:35 from IEEE Xplore.  Restrictions apply.1428 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 16, AUGUST 15, 2008Fig. 3. Measured photocurrent as the incident CW power is varied. The curvewithout SPA suppression was simulated from a noncavity detector with the sameabsorption length.an input power equal to 0.09 mW, the TPA current is equalto the SPA current. Since reverse bias does not increase thephotocurrent detected, we do not apply any reverse bias inour measurement and the dark current is lower than 1 pA.Now using the formula (1) with the decrease in enhancementtaken into account, the TPA coefficient in our case would be14.6 cm/GW assuming the collection efficiency is equal toone. In [10], the measured TPA coefficient of (001)-GaAsat 950 nm is between 20 and 27 cm/GW, where 20 cm/GWis for linear polarization along the (100) direction which de-scribes the strength of the TPA in the isotropic limit [11], and27 cm/GW is for linear polarization along the (110) directionwhich represents an anisotropic factor of 0.76. The wave-length dependence of the TPA coefficient is described by thefunction [12] ,which was obtained based on an isotropic band structure. Usingthis relation we would estimate the TPA coefficient in theisotropic limit in (001)-GaAs at 1560 nm is 12.5 cm/GW. Theanisotropic factor at 1560 nm was estimated to be half of thevalue at 950 nm [11], so a value of 0.38 could be used. Thenthe TPA coefficient for polarized light along the (110) directionat 1560 nm would be 14.8 cm/GW which is also the highestTPA coefficient for any polarizations for (001)-GaAs. Thevalue estimated from our measurement agrees with this valuevery well. In [13], a value around 19 cm/GW was measured at1560 nm which agrees reasonably well with our result takingaccount of their data fluctuation around 1550 nm. From themeasured SPA current, we estimate the SPA coefficient in ourdetector to be cm , which is a very low value.However, if there was no relative suppression of this smallresidual SPA, the SPA current would exceed the TPA currentfor the incident CW power less than 5 mW as shown in Fig. 3where the simulated data is based on a noncavity detectorwith the same absorption length. If the acceptance angle of themicrocavity could be increased allowing the focused beam tobe enhanced as much as the nonfocused beam, the TPA signalwould become equal to the SPA signal for an incident CWpower of only 38 W.IV. CONCLUSIONA high-finesse microcavity not only improves the TPA effi-ciency but also relatively suppresses the residual SPA resultingin a TPA dominated signal at much lower incident power. TheTPA coefficient is estimated to be 15 cm/GW at 1560 nm in(001) GaAs and the residual SPA coefficient in our unintention-ally doped GaAs grown by metal–organic chemical vapor depo-sition is approximately cm .ACKNOWLEDGMENTThe authors would like to thank D. Kilper and M. Dinu ofBell Laboratories, Crawford Hill, for valuable discussions.REFERENCES[1] K. Kikuchi, “Optical sampling system at 1.5  m using two photon ab-sorption in Si avalanche photodiode,” Electron. Lett., vol. 34, no. 13,pp. 1354–1355, 1998.[2] R. Salem and T. E. 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Suppression of residual single-photon absorption relative to two-photon absorption in high finesse planar microcavities

O\u27Dowd, John

Irish Universities

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Suppression of residual single-photon absorption relative to two-photon absorption in high finesse planar microcavities

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