A superconducting NbN detector for neutral nanoparticles
M. Marksteiner, A. Divochiy, M. Sclafani, P. Haslinger, H. Ulbricht, A. Korneev, A. Semenov, G. Goltsman, M. Arndt
aa r X i v : . [ phy s i c s . a t m - c l u s ] F e b published in: Nanotechnology 20, 455501 (2009). http://dx.doi.org/10.1088/0957-4484/20/45/455501 Superconducting NbN detector for neutral nanoparticles
Markus Marksteiner, Alexander Divochiy, Michele Sclafani, Philipp Haslinger, HendrikUlbricht, Alexander Korneev, Alexander Semenov, Gregory Gol’tsman, and Markus Arndt ∗ Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria Department of Physics, Moscow State Pedagogical University,M. Pirogovskaya Street 1, Moscow 119992, Russia School of Physics and Astronomy, University of Southampton,Highfield, Southampton, SO171BJ, United Kingdom (Dated: September 18, 2018)We present a proof-of-principle study of superconducting single photon detectors (SSPD) for thedetection of individual neutral molecules/nanoparticles at low energies. The new detector is appliedto characterize a laser desorption source for biomolecules and it allows to retrieve the arrival timedistribution of a pulsed molecular beam containing the amino acid tryptophan, the polypeptidegramicidin as well as insulin, myoglobin and hemoglobin. We discuss the experimental evidencethat the detector is actually sensitive to isolated neutral particles.
I. INTRODUCTION
The detection of isolated neutral molecules andnanoparticles in the gas phase is both a necessity anda challenge for many experiments that range from physi-cal chemistry over environmental monitoring [1] to thefoundations of physics. Our own work was originallymotivated by matter wave interferometry with mas-sive molecules [2–4] and applications in molecule metrol-ogy [5, 6]. The extension of such experiments to highermasses requires also improved methods for detecting neu-tral nanoparticles. While ionization techniques are rou-tinely used for particles up to about 2000 Da, postion-ization of organic molecules beyond that mass has re-mained a significant challenge [7, 8]. Recent experimentsobserved photoionization of tryptophan-metal complexesand nucleotide clusters up to 6000 Da [9, 10]. But for themajority of high-mass biomolecules this method seems tobe precluded.Hyperthermal surface ionization [12] was shown to al-low the detection of some neutral molecules, with insulincurrently setting the mass record [13].Modern nanofabrication technologies also allow tobuild nanoscale oscillators which change their resonancefrequency when their mass is augmented by even a singlemolecule. Such cantilever based detectors [14, 15] haverather a lower than an upper mass limit. They currentlyreach a sensitivity of below 200 Da [16]. First proof-ofprinciple mass spectrometer applications achieved the ca-pability to detect single proteins [17].Whenever mass cannot be measured directly, bolome-ter detectors [18–20] may convert molecular energy firstinto sensor temperature and then into an electrical sig-nal. However, the translational energy of a single aminoacid, such as tryptophan, does not exceed 0.3 eV, evenat a molecular velocity of 500 m/s. This is why the firstbolometers [21] still operated with a minimum detection ∗ threshold of about 10 molecules per second. Supercon-ductors were suggested as promising sensors [19, 22, 23]since their conductivity changes strongly with tempera-ture in the vicinity of the phase transition edge.The implementation of superconducting tunnelingjunctions (STJ) made it possible to detect charged in-dividual molecules [24–26]. This is interesting for massspectrometry because the STJ response depends on theparticle’s energy [27]. This allows to combine the massdiscrimination of a time-of-flight spectrometer with a de-tector whose efficiency remains constant over a wide massrange. Tunneling junction detectors require, however,cooling well below 4 K and up to now they were onlyused for recording either ensembles of slow neutral par-ticles or for detecting individual but energetic chargedparticles.In this letter we present our first experimental evidencethat a combination of both is feasible, i.e. a detector forsingle neutral molecules of low kinetic and low internalenergy. Our nanowire detector was originally fabricatedas a superconducting single photon detector. Its sensitiv-ity to single photons was demonstrated across the entirespectrum from UV to mid-IR, with quantum efficienciesup to 30 % [28–32]. Before we started the experiments,it was far from obvious that such a device would also besensitive to slow nanoparticles. In the following we dis-cuss the acquired evidence that this detector is capable ofrecording the incidence of isolated neutral biomolecules. II. EXPERIMENTAL SETUP
The entire experiment consists of a pulsed molecu-lar beam, a free flight trajectory in high vacuum andthe superconducting nanowire detector in a differentiallypumped helium cryostat, about 76 cm behind the source.
FIG. 1. (a) Electron microscopy image of the 10 µ m × µ msensitive SSPD element. The NbN film appears is colored ingrey. (b) Sketch (right) and SEM image (left) of the super-conducting NbN bolometer. The bolometer strips and gapsare about 2 µ m wide. A. Superconducting detectors
Two different types of detectors were tested: Super-conducting single photon counting devices (SSPDs) andsuperconducting bolometers (SBs).The SSPDs were fabricated by depositing a NbN filmof 3.5..4 nm thickness on a sapphire substrate. Wetested chips with an open area of either 10 × µ m or20 × µ m . The 100..120 nm wide superconducting wiremeanders on the surface with a filling factor of 60 %. Thecritical temperature of NbN is T c =10..11 K and the crit-ical current density amounts to j c =3..5 × A/cm .The SSPD fabrication process was described in de-tail in [33]. In brief, NbN superconducting films weredeposited on R-cut sapphire substrates by DC reactivemagnetron sputtering in an Ar and N mixture. Thefilm was patterned by direct electron beam lithographyand reactive ion etching. Gold contacts were added usingphotolithography and wet etching. In Figure 1 we showelectron microscopy images of the sensitive element. Weoperate the device in a liquid helium bath cryostat at4.2 K and apply a DC bias current slightly smaller thanthe critical current. The signal is capacitively coupledfrom the chip to the oscilloscope (see Figure 2).The detection mechanism may be understood as fol-lows: When a molecule hits the film surface, it cre-ates high-energy acoustic phonons. These phonons arerapidly absorbed by the electron subsystem of the filmdue to their short inelastic mean free path. Excess quasi-particles are then created which, in turn, dispose of theirenergy by the emission of second generation phonons. This triggers an avalanche multiplication cascade. Theprocess is similar to what happens during photon detec-tion. The distinctive difference lies in the first step: thephoton detection cascade starts from a single high-energyquasiparticle created by the photon. The dynamics of thesubsequent stages is determined only by the absorbed en-ergy.When the energy of the quasiparticles decreases to avalue around 10 K, the electron-electron interaction be-comes more efficient for the multiplication of the quasi-particles than the electron-phonon interaction. Due tothis fact, the main part of the energy, that is initiallydeposited in the film remains in the quasiparticle sub-system. At the end of the cascade, a hot spot of excessquasiparticles is formed and the supercurrent is forced toflow around the new normal-conducting area. If the hotspot is sufficiently large the current density in the ‘side-walks’ increases beyond the critical current density. Thisresults in a short breakdown of superconductivity acrossthe entire width of the nanostripe and in a voltage pulsethat can be easily detected [28–31]. For photons, a typi-cal pulse response lasts over 10 ns.The second detector type that we tested was a classi-cal superconducting bolometer. These chips were madefrom the same NbN film, again using photolithographyand wet etching. Figure 1(b) presents the sketch of itssensitive element and an SEM image. The bolometerchip is working at the critical temperature T c . The ad-ditional energy that is delivered to the surface by theimpacting molecules may heat the superconductor above T c and cause a voltage peak. The incident energy hasto sufficiently high to induce the required temperaturechange. Because of the large width of the stripes thiscondition can often not be met by a single moleculealone and the sensors responds only to many simulta-neously impacting particles. With this second detectortype we were not able to detect any molecular signal inour experiments. This type of superconducting wide-areabolometer is therefore not further discussed in the follow-ing where we rather focus on our explorations of nanos-tructured SSPD chips. B. Molecular beam source
The details of our laser desorption source have alreadybeen described elsewhere [9, 10]: Organic molecules werelaser desorbed by a Nd:YAG laser beam (355 nm, 5 ns,6..10 mJ), which was focused to a spot on a pressed pow-der sample (Figure 2). The desorbed molecules are cooledand entrained by a jet of helium gas that fills the mixingchannel before it exits through a 1 mm opening into avacuum of 10 − mbar. About 2 cm behind the mixingchannel the beam passes a skimmer of 1 mm diameter,which separates the source from a differential pumpingstage. In this second chamber the molecular beam is fil-tered by a copper mesh with 7.5 µ m openings. This mi-crostructure is used to reject grains of powder that might FIG. 2. A pulsed laser desorbs biomolecules, which are then entrained by a supersonically expanding noble gas jet. Aftertwo-fold mechanical filtering the molecular beam hits the superconducting NbN detector. The impact of molecules releases avoltage pulse which is stored by an oscilloscope and a timer card. be ejected during the ablation process.The stream of single molecules as well as possibly abackground of microscopic particles leaves the secondpumping stage through a 1 cm diameter opening into thedetection chamber where the SSPD chip is attached to aHelium bath cryostat. The overall distance from the des-orption spot to the superconducting chip is 76 cm. Forsome experiments we added a second mechanical filter: aSiN x line grating with a period of 266 nm and openingsas small as 90 nm was attached to the entrance windowof the cryostat. This addition further limited the trans-mitted particle size and also helped extending the lifetime of the SSPD chips, which was otherwise strongly af-fected by the accumulation of molecular material. Evenwith the additional filter in place, the active time of anindividual chip was limited to about 20.000 desorptionshots, time after which a layer of molecules had cov-ered the surface and made it insensitive. It is known [11]that SSPD chips can even be used to resolve the energyof incident photons. Similar energy dependent measure-ments with molecules were, however, still impeded in ourpresent proof-of-principle study by the time-varying sur-face coverage. III. RESULTS
In order to explore its detection capabilities the SSPDchip was placed into the laser desorbed biomolecularbeam. The first experiments were performed with amixture of several molecules. It contained 0.2 g myo-globin (17 kDa), 0.3 g β -carotene (537 Da), 0.3 g insulin(5.8 kDa), 0.25 g bovine serum albumin (BSA,66 kDa),and 0.5 g cellulose of unspecified chain length to mechan-ically bind the other components.In this first test we used a 20 × µ m SSPD chipand in Figure 3 we show a typical individual detectionevent from the desorbed molecule mixture. The peak isabout 10 ns wide (FWHM) and indicates a high temporaldetector resolution also for neutral nanoparticles. We performed several tests to corroborate the evidencefor the SSPD’s sensitivity to isolated molecules and toexclude possible other reasons for the observed signalssuch as for instance the co-propagating seed gas or co-desorbed cellulose:The influence of the rapidly expanding seed gas canbe tested by switching off the desorbing laser beam. Wesearched for signs of the expanding helium carrier gaspulse alone and the complete absence of any signal inthis setting indicates that the SSPD chip is not capableof detecting individual helium atoms. The same is alsotrue for all of the heavier noble gases such as neon, argon,krypton or xenon. None of them showed any detectablesignal under our experimental conditions.One might speculate that the higher kinetic energy ofthe more massive biomolecules could be outweighed bythe larger number of lighter noble gas atoms. This ar-gument would be supported by the fact that seed gasatoms must be much more abundant than the laser im-planted biomolecules – otherwise supersonic expansionwould never occur. However, since no signal was de-tected for the pure noble gas beams alone, a collectiveeffect by the dense gas jet can be excluded. Since thebiomolecular beam is more dilute than the carrier gas, acollective effect of organic particles is even less likely.This finding is in variance to that for classic bolome-ters where the incidence of many particles was actuallyrequired to trigger a signal [21]. It has, however, to benoted that these detectors were not nanostructured andthey were exploiting a different mechanism. Our resultgives thus first evidence that the SSPD chip is indeed notsensitive to the intense particle flux of atoms but ratherto the local energy density of single complex nanoparti-cles.As mentioned before, the molecules were always ad-mixed with cellulose to achieve mechanical stabilizationof the sample. In order to separate matrix signals fromanalyte signals we also performed a separate desorptionexperiment with pure cellulose powder alone. The com-plete absence of any measurable signal indicates again -50 -40 -30 -20 -10 0 10 20 30 40 50-0,004-0,0020,0000,0020,0040,0060,0080,010 S i gna l [ V ] Time [ns]
FIG. 3. A typical individual peak, that is attributed to theimpact of neutral molecules on the chip. The 20 × µm SSPDchip was biased with a current of 19.5 µA . The signal wasamplified by 20 dB. that the ablated matrix particles do not contribute anybackground in the SSPD counter. In order to enablea more quantitative evaluation we switched to a pulsecounting mode and recorded time-of-flight curves for var-ious experimental settings. The molecular velocity maydiffer from that of a free supersonic expansion, since themolecules can be delayed inside the gas mixing channelbefore they exit. This delay may lead to an underesti-mation of the actual velocity. The velocities and kineticenergies below are therefore reasonable lower limits.In all the following experiments the samples containedonly biomolecules from one species mixed 1:1 with cel-lulose. In Figure 4a we show the arrival time distribu-tion for a tryptophan (204 Da) and gramicidin (1.9 kDa)sample, respectively. Figure 4b depicts the distributionsfor insulin, myoglobin and porcine hemoglobin (66 kDa)sample. All curves in Figure 4 were recorded using thesame 20 × µm SSPD chip, the same discriminator level,a bias current of 20 ± µA and two particle filters in thebeam line, i.e. a 7.5 µ m mesh as well as the 90 nm SiNfilter. The opening time of the valve was set to 700 µ s forall recorded curves in Figure 4, except for the one of tryp-tophan as discussed below. The arrival time distributionsin Figure 4 show a double structure which is a result ofthe particular valve setting in these experiments.The agreement of the detected arrival times with theexpected flight times may already be interpreted as agood indication for the detection of neutral particles. Itis, however, desirable to corroborate this statement bycomplementary measurements which must rely on alter-native detectors. As they are not readily available in themass range beyond 2000 Da, where ionization detectorsstart to fail [7, 8] and nanomechanical detectors are notyet commercially available, it is also still open whetherhigh mass molecules (e.g. hemoglobin) can survive thedesorption process as intact particles or whether the ob-served signals are rather caused by smaller neutral frag-ments generated in the source.This is why our first checks were focused on charac-terizing the molecular beam source for tryptophan andgramicidin, where it is known [34, 35] that VUV laser ion-ization is soft and capable of detecting isolated molecules S i gna l [ a . u .] Arrival time [ms]
Tryptophan Gramicidin a ) Hemoglobin Myoglobin Insulin S i gna l [ a . u ] Arrival time [ms] b) FIG. 4. A) Arrival time distribution of tryptophan and gram-icidin. B) arrival time distribution of myoglobin, insulin andhemoglobin. All molecules were detected using the 20 × µm SSPD chip. as well as larger clusters [9, 10]. A F laser (157 nm,5 ns, up to 3 mJ) is here combined with time-of-flightmass spectrometry (TOF-MS) to reveal the arrival timeand mass distribution of all molecules emerging from thesource.Figure 5 shows a representative arrival time distribu-tion for tryptophan, measured using TOF-MS and thesame source settings as in the SSPD experiments. Onlythe flight distance between source and detector was short-ened to 0.5 m. This reduces also the molecular flighttimes.The arrival time distribution, recorded in photoioniza-tion TOF-MS at the mass of the single monomer (Fig-ure 4) has the same structure as the signals recorded bythe SSPD (Figure 5). Small differences in the arrivaltimes can be assigned to an accidental reduction of thevalve opening time which influences both the pressure in-side the mixing channel and the velocity of the expandingmolecules. This is supported by Figure 6 which depictsthe arrival time distribution for gramicidin for three dif-ferent valve opening times recorded by the SSPD chip. S i gn a l [ a . u . ] Time [ms]
FIG. 5. Arrival time distribution of tryptophan recorded vialaser postionization and time-of-flight (TOF) mass spectrom-etry. The shape of the TOF-distribution reproduces the dis-tribution recorded by the superconducting chip (see figure 4).
Arrival time [ms] 700(cid:181)s S i gna l [ a . u .] FIG. 6. Arrival time distribution of gramicidin for differentvalve opening times, using a 10 × µ m SSPD chip. A re-duction of the valve opening time leads to shifted arrival timedistributions (slower molecules). For short pulse times (Figure 6, top) the arrival time dis-tribution resembles the one recorded for tryptophan in4a, where the molecule arrival time was delayed. In-terestingly, valve times around 600 µ s do not show thedouble structure, as can be seen in Figure 6, center. Wechose a slightly higher opening time of 700 µ s (Figure 6,bottom) for our experiments. This causes a double peakin the arrival time but also adds to the signal.From these tests we gather the general insight that thearrival time distributions are rather identical for boththe SSPD and the ionization detector. They are bothconsistent with the arrival of individual molecules. Theabsence of any signal related to the individual carrier gasatoms is a hint to an energy threshold in the SSPD whichcan only be overcome by sufficiently massive and ener-getic molecules. In our experiments the least energeticmolecules detected by the SSPD were tryptophan par-ticles in the velocity band of 300..500 m/s, i.e. with a kinetic energy of 100..300 meV if we assume to see iso-lated molecules.Compared to that value, all rare gas beams have stilltoo little kinetic energy to be detected. Helium at800 m/s reaches only 10 meV and xenon at 250 m/s wouldonly attain 40 meV – still well below the value for tryp-tophan, which carries also internal energy in addition. IV. CONCLUSION
The present experiments are just a promising start ofan interesting journey into single neutral molecule detec-tion using superconducting single photon detectors. Ourexperiments give good first evidence that SSPDs can beused to register the incidence of neutral single nanopar-ticles.As of today there is no efficient easy-to-implement wayof detecting neutral large proteins to cross-check ourresults with individual, isolated insulin, hemoglobin ormyoglobin. But we see a good consistence between theSSPD results and photoionization mass spectrometry inthe flight times for tryptophan and gramicidin.One might furthermore ask whether a high internal ex-citation may also add sufficient energy to the chip, whichwill be tested in future experiments by systematicallyvarying the internal temperature of the molecules.The possibility of detecting more massive neutral andlabile molecules is promising for many applications inphysical chemistry and also for matter wave interferome-try. Even if the SSPD method, cannot (yet) discriminatebetween different masses, de Broglie interferometry itselfhas been shown to be capable of discriminating differentmolecular properties [5] and experiments with clean andmass selected sources would only require a good sensitiv-ity to the existence, not to the mass of the particle.The technology certainly requires further developmentand exploration but it may allow us to close a ’detectorloophole’ for particles which are too complex to be effi-ciently photoionized and yet too small to be well detectedby other means.
ACKNOWLEDGMENTS
This work was supported by the Austrian ScienceFunds FWF within the Wittgenstein program Z149 aswell as by the Italian Fondazione Angelo Della Ric-cia. We thank Sanofi-Aventis Inc. for the donation ofpure insulin powder. This work was also supported bythe Russian Agency of Science and Innovation (contract02.513.11.3446) and the Russian Foundation for BasicResearch (grant 09-02-12364).
REFERENCES [1] Christopher A Noble and Kimberly A Prather. Real-timesingle particle mass spectrometry: A historical reviewof a quarter century of the chemical analysis of aerosol.
Mass Spectrom. Rev. , 19:248–274, 2000.[2] W. Sch¨ollkopf and J.P. Toennies. The nondestructive de-tection of the helium dimer and trimer.
J. Chem. Phys. ,104:1155–1158, 1996.[3] M. Arndt, O. Nairz, J. Voss-Andreae, C. Keller,G. Van der Zouw and A. Zeilinger . Wave-particle dualityof C60 molecules.
Nature , 401:680–682, 1999.[4] L. Hackerm¨uller, S. Uttenthaler, K. Hornberger,E. Reiger, B.Brezgerand, A. Zeilinger and M. Arndt.Wave nature of biomolecules and fluorofullerenes.
Phys.Rev. Lett. , 91:90408, 2003.[5] Stefan Gerlich, Michael Gring, Hendrik Ulbricht, KlausHornberger, Jens T¨uxen, Marcel Mayor and MarkusArndt. Matter-wave metrology as a complementary toolfor mass spectrometry.
Angew. Chem. Int. Ed. Engl. ,47(33):6195–8, 2008.[6] Hendrik Ulbricht, Martin Berninger, Sarayut Deacha-punya, Andre Stefanov and Markus Arndt Gas phasesorting of fullerenes, polypeptides and carbon nanotubes.
Nanotechnology , 19:045502, 2008.[7] E. W. Schlag and J. Grotemeyer. Do large moleculesionize?
Chem. Phys. Lett. , 190:521–527, 1992.[8] C. H. Becker and K. J. Wu. On the photoionization oflarge molecules.
J. Am. Soc. Mass Spectrom. , 6:883 –888, 1995.[9] M. Marksteiner, P. Haslinger, M. Sclafani, H. Ulbrichtand M. Arndt. Gas-Phase Formation of Large NeutralAlkaline-Earth Metal Tryptophan Complexes.
J. Am.Soc. Mass Spectr. , 19(7):1021–1026, 2008.[10] M. Marksteiner, P. Haslinger, M. Sclafani,H. Ulbrichtand M. Arndt. UV and VUV ionization of organicmolecules, clusters and complexes.
J. Phys. Chem. A,DOI: 10.1021/jp905039f , 2009.[11] Elisabeth Reiger, Sander Dorenbos, Valery Zwiller,Alexander Korneev, Galina Chulkova, Irina Milostnaya,Olga Minaeva, Gregory Goltsman, Jennifer Kitaygorsky,Dong Pan, Wojtek Slysz, Arturas Jukna and RomanSobolewski. Spectroscopy with nanostructured super-conducting single photon detectors.
IEEE J. Sel. Top.Quant. Electr. , 13:934–943, 2007.[12] Aviv Amirav. Electron impact and hyperthermal sur-face ionization mass spectrometry insupersonicmolecularbeams.
Org. Mass Spectr. , 26:1–17, 1991.[13] C. Weickhardt, L. Draack and A. Amirav. Laser des-orption combined with hyperthermal surface ionizationtime-of-flightmassspectrometry.
Anal. Chem. , 75:5602,2003.[14] B. Ilic, H. G. Craighead, S. Krylov, W. Senaratne,C. Ober and P. Neuzil. Attogram detection using na-noelectromechanical oscillators.
J. Appl. Phys. , 95:3694–3703, 2004.[15] Mo Li, H. X. Tang and M. L. Roukes. Ultra-sensitivenems-based cantilevers for sensing, scanned probe andvery high-frequency applications.
Nature Nanotech. ,2:114 –120, 2007.[16] K. Jensen, K. Kim and A. Zettl. An atomic-resolution nanomechanical mass sensor.
Nature Nan-otech. , 3:533537, 2008. [17] A. K. Naik, M. S. Hanay, W. K. Hiebert, X. L. Feng andM. L. Roukes. Towards single-molecule nanomechanicalmass spectrometry.
Nature Nanotech. , Advanced online:21 June 2009:1–6, 2009.[18] Frank J. Low. Low-temperature germanium bolometer.
J. Opt. Soc. Am. , 51:1300–1304, 1961.[19] H Kraus. Superconductive bolometers and calorimeters.
Supercond. Sci. Technol. , 9:827842, 1996.[20] C. Enss and D. McCammon. Physical principles oflow temperature detectors: Ultimate performance limitsand current detector capabilities.
J. Low. Temp. Phys. ,151:524, 2008.[21] M. Cavallini, L. Meneghetti, G. Scoles and M. Yealland.Molecular beam scattering apparatus with low tempera-ture bolometer detector.
Rev. Sci. Instr. , 42:1759, 1971.[22] DH Andrews, RM Milton and W Desorbo. A fast su-perconducting bolometer.
J. Opt. Soc. Am. , 36:518–524,1946.[23] Norman E Booth and David J Goldie. Superconductingparticle detectors.
Supercond. Sci. Technol. , 9:493–516,1996.[24] M. Frank, C. A. Mean, Simon E. Labov, W. H. Benner,D. Horn, J. M. Jaklevic and A. T. Barfknecht. High-efficiency detection of 66 000 da protein molecules usinga cryogenic detector in a matrix-assisted laser desorp-tion/ionization time-of-flight mass spectrometer.
Rap.Comm. Mass Spectr. , 10:1946–1950, 1996.[25] Damian Twerenbold, Jean-Luc Vuilleumier, Daniel Ger-ber, Almut Tadsen, Ben van den Brandt and Patrick M.Gillevet. Detection of single macromolecules using acryogenic particle detector coupled to a biopolymer massspectrometer.
Appl. Phys. Lett. , 68:3503–3505, 1996.[26] E. Esposito, R. Cristiano, S. Pagano, D. Perez de Laraand D. Twerenbold. Fast josephson cryodetector for timeof flight mass spectrometry.
Physica C , 372–376:423 – –426, 2002.[27] Damian Twerenbold, Daniel Gerber, Dominique Gritti,Yvan Gonin, Alexandre Netuschill, Fr´ed´eric Rossel, Do-minique Schenker and Jean-Luc Vuilleumier. Singlemolecule detector for mass spectrometry with mass inde-pendent detection efficiency.
Proteomics , 1:66–69, 2001.[28] G. N. Goltsman, O. Okunev, G. Chulkova, A. Lipatov,A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov,C. Williams and Roman Sobolewski. Picosecond super-conducting single-photon optical detector.
Appl. Phys.Lett. , 79:705–707, 2001.[29] Alex D. Semenov, Gregory N. Goltsman and Alexan-der A. Korneev. Quantum detection by current carryingsuperconducting film.
Physica C , 351:349–356, 2001.[30] A. Verevkin, J. Zhang, Roman Sobolewski, A. Lipatov,O. Okunev, G. Chulkova, A. Korneev, K. Smirnov, ,G. N. Goltsman and A. Semenov. Detection efficiency oflarge-active-area nbn single-photon superconducting de-tectors in the ultraviolet to near-infrared range.
Appl.Phys. Lett. , 80:4687–4689, 2002.[31] A. Korneev, V. Matvienko, O. Minaeva, I. Milostnaya,I. Rubtsova, G. Chulkova, K. Smirnov, V. Voronov,G. Goltsman, W. Slysz, A. Pearlman, A. Verevkin andRoman Sobolewski. Quantum efficiency and noise equiv-alent power of nanostructured, nbn, single-photon de-tectors in the wavelength range from visible to infrared.
IEEE Transact. Appl. Supercond. , 15:571–574, 2005.[32] G. Goltsman, O. Minaeva, A. Korneev, M. Tarkhov,I. Rubtsova, A. Divochiy, I. Milostnaya, G. Chulkova,N. Kaurova, B. Voronov, D. Pan, A. Cross, A. Pearl-man, I. Komissarov, W. Slysz and R. Sobolewski. Middle-infrared to visible-light ultrafast superconducting single-photon detector.
IEEE Transact. Appl. Supercond. ,17(1):246–251, 2007.[33] G. Gol’tsman, K. Smirnov, P. Kouminov, B. Voronov,N. Kaurova, V. Drakinsky, J. Zhang, A. Verevkin and R. Sobolewski. Fabrication of nanostructured supercon-ducting single-photon detectors.
IEEE Transact. Appl.Supercond. , 13(2):192–195, June 2003.[34] Claus K¨oster and J¨urgen Grotemeyer. Single-photonand multi-photon ionization of infrared laser-desorbedbiomolecules.
Org, Mass Spectr. , 27:463–471, 1992.[35] Jh. Arps, Ch. Chen, Mp. McCann and I. Datskou.Ionization of organic-molecules using coherent vacuumultraviolet-light.