- Email: [email protected]
A new microcavity design for single molecule detection
ARTICLE IN PRESS
Journal of Luminescence 119–120 (2006) 167–172 www.elsevier.com/locate/jlumin
A new microcavity design for single molecule detection M. Steiner,1, F. Schleifenbaum1, C. Stupperich, A.V. Failla1, A. Hartschuh1, A.J. Meixner1 Universita¨t Siegen, Center for Micro- and Nanochemistry and Physikalische Chemie, 57068 Siegen, Germany Available online 28 February 2006
Abstract We present a new microcavity design which allows for efficient detection of single molecules by measuring the molecular fluorescence emission coupled into a resonant cavity mode. The Fabry–Perot-type microresonator consists of two silver mirrors separated by a thin polymer film doped with dye molecules in ultralow concenctration. By slightly tilting one of the mirrors different cavity lengths can be selected within the same sample. Locally, on a mm scale, the microcavity still acts as a planar Fabry–Perot resonator. Using scanning confocal fluorescence microscopy, single emitters on resonance with a single mode of the microresonator can be spatially addressed. Our microcavity is demonstrated to be well-suited for investigating the coupling mechanism between single quantum emitters and single modes of the electromagnetic field. The microcavity layout could be integrated in a lab-on-a-microchip design for ultrasensitive microfluidic analytics and can be considered as an important improvement for single photon sources based on single molecules operating at room temperature. r 2006 Elsevier B.V. All rights reserved. Keywords: Microcavity; Single molecules; Fluorescence microscopy
1. Introduction Optical microcavities, i.e. optical resonators with confinement dimensions in the range of one Corresponding author. Universita¨t Tubingen, Institut fur ¨ ¨ Physikalische und Theoretische Chemie, Auf der Morgenstelle 8, 72076 Tu¨bingen, Germany. Tel.: +49 7071 29 76171; fax: +49 7071 29 5490. E-mail address: [email protected] (M. Steiner). 1 Present address: Universita¨t Tu¨bingen, Institut fu¨r Physikalische und Theoretische Chemie, 72076 Tu¨bingen, Germany.
wavelength of the enclosed electromagnetic field, can be used to control the spatial, spectral, and temporal properties of radiation from embedded quantum emitters [1]. This control is essential for improving existing true single photon sources [2] and devices for ultrasensitive analytics [3,4]. Ultimately, it would be desirable to couple the fluorescence emission of a single molecule to a single resonant cavity mode. Up to now, evidence for the observation of single molecules embedded in optical microcavities has only been derived from the measured sub-poissonian photon statistics of
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.12.063
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
168
doped with dye molecules in ultralow concenctration. Silver mirrors were evaporated onto commercially available and cleaned microscope glass coverslips (thickness 170 mm) by an electron beam source (EB3, Edwards) under high vacuum conditions (106 mbar). The film thickness was monitored during the evaporation process using an oscillating quartz unit (FTM7, Edwards) and verified individually by AFM measurements. The thickness of the silver mirrors was chosen to be 60 (30) nm for the back mirror S2 and 30 (20) nm for the input/output mirror S1 to optimize the performance, i.e. excitation and fluorescence light for single molecule detection pass the same mirror (see Fig. 1(b)). Well-controlled concentrations of the perylen-derivative n (2,6-diisopropylphenyl)perylen-3,4-dicarboxymid (abbreviated PI in the
the fluorescence emission from highly diluted dye solutions enclosed within planar microresonators [5,6] and single molecule fluorescence bursts from streams of dye-doped microdroplets [7]. However, to study the interaction between single fluorescent emitters and a microcavity with well-defined coupling conditions, it is necessary to immobilize and spatially isolate them. In this case, the transition dipole is fixed with respect to the cavity mirrors and the cavity-mediated long-range dipole coupling [8] is prevented.
2. Sample preparation and characterization The Fabry–Perot type microcavity consists of two silver mirrors separated by a thin polymer film
Polychromatic light source for transmission microscopy
PI
F
y Ag-Layer (S2)
nglass
x
L(x)
z
npol
Microscope objective
nglass
Excitation laser light for fluorescence microscopy
Beamsplitter
Ag-Layer (S1)
UV-radiation
Detector (a)
(b)
Fig. 1. (a) Manufacturing of a microcavity for single molecule detection consisting of two silver coated glass coverslips separated by liquid monomers of an optical glue offering dye molecules (PI) in ultralow concentration (cPI1010 M). The microcavity length L is tuned applying a punctual force in the center of the microcavity and fixed by polymerization of the optical glue using UV-light. The procedure is controlled by observing the resulting interference pattern (see Fig. 2(a)) using a measuring microscope. (b) Experimental setup for single molecule detection and measurement of the local cavity length L(x) as described in text.
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
169
570 nmÞ and the positions x corresponding to distinct interference orders were measured using a division scale (see Fig. 2(a)). The transmission of the microcavity obeys the resonance condition
following) were embedded into an UV-polymerizing optical adhesive with refractive index npol ¼ 1:56 acting as a dielectric medium between the microcavity mirrors (see Fig. 1(a)). Single molecule experiments were carried out at concentrations of cPI10–10 M. For the preparation both PI molecules and the monomers of the adhesive were dissolved in dichloroethane. Evaporation of the solvent resulted in an active medium composed of randomly oriented dye molecules with controlled emitter concentration. UV-illumination of the microcavity leads to the polymerization of the optical adhesive effectively gluing together the two silver mirrors. To get access to various cavity lengths, one of the cavity mirrors is slightly inclined with respect to the other by applying a punctual force before the curing process is initialized (see Fig. 1(a)). This leads to a welldefined dependence of the cavity length L on the position within the microcavity x–y plane. The optical quality and uniformity of the resonators was controlled by observing the white light interference pattern, i.e. Newton rings of equal optical thickness. To determine the exact relation between L and x, the microcavity was simultaneously illuminated with parallel laser light ðl ¼
npol LðxÞ cos Y DF1 ðd 1 ; Y; lÞ þ DF2 ðd 2 ; Y; lÞ lðxÞ , ¼ m 2p 2 m ¼ 1; 2; 3; . . . ,
ð1Þ
where L denotes the thickness of the polymer film which deviates from the effective optical resonator length in the case of real cavity mirrors S1,2 with phase shifts DF1;2 ap:npol is the refractive index of the cured polymer, Y is the angle of incidence of an incoming parallel light beam with wavelength l, m is the interference order and d1,2 the thickness of the silver films. The local cavity length L(x) and hence the cavity length profile depicted in Fig. 2(b) was obtained from the optical image in Fig. 2(a) with the parameters npol ¼ 1:56; Y ¼ 01 and m increasing from 1 for the innermost ring up to m ¼ 12. For small values of m, L(x) is welldescribed by a second-order polynomial (dotted line in Fig. 2(b)) as it is expected for Newton rings. The DFi were taken as a result from computer
2.5 interference order m = 4 - 12 interference order m = 1 - 3 polynomial fit to m = 1 - 3
x
cavity length L (µm)
2.0
1.5
1.0
0.5
0.0 0.0 (a)
0.2
0.4
0.6
0.8 1.0 1.2 1.4 position x (mm)
1.6
1.8
2.0
(b)
Fig. 2. (a) Transmission widefield microscopy image (50 fold magnification) of the central part of the microcavity. Concentric laser ðl ¼ 570 nmÞ and white light interference fringes (Newton rings) are visible surrounding the dark center. (b) Cavity length profile L(x) calculated by Eq. (1) using the positions of the sharp laser fringes in the optical image (a). For m ¼ 123 (dots), L(x) is well-described by a second-order polynomial (dotted line).
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
simulations which were verified by reproducing comparable results for silver mirrors and microcavities reported elsewhere [9]. For the film thickness d1,2 considered here the phase shifts DFi lead to a nearly constant offset which reduces the real cavity length L by 50 nm as compared to the results for ideal metal mirrors ðDFi ¼ pÞ.
cavity length L (nm)
170
3. Experimental
4. Results and discussion For single molecule detection within the first resonator mode (i.e. the l/2-condition), we addressed an area of 100 mm 100 mm in the innermost interference fringe ðm ¼ 1Þ. Transmission wide field microscopy (see Fig. 1(b)) was used to determine the two relevant cavity parameters: The local cavity length L(x) (see Fig. 3(a)) and the Qfactor. A typical local transmission spectrum of an Ag-microcavity (d 1 ¼ 30 nm, d 2 ¼ 60 nm) is presented in Fig. 3(b). L(x) can be obtained from the wavelength at the transmission maximum l(x) using Eq. (1) with the parameters npol ¼ 1:56; Y ¼ 0 , m ¼ 1 and the corresponding DFi . A systematic non-linear increase of the cavity length
0 (a)
10 20 30 40 50 60 70 80 90 100 position x (µm)
1.0
cavity-Q = 44 @ x = 55 µm
0.8 intensity (a. u.)
All optical measurements were performed with an inverted confocal microscope (based on an Axiovert 135 TV, Zeiss) with a high numerical aperture objective (Planeoflex 100x=NA ¼ 1:3, Zeiss) as shown schematically in Fig. 1(b). Raster scanning the microcavity through the diffractionlimited spot of the microscope objective was done using a feedback-controlled sample stage (P517.K008, Physik Instrumente). A single line argon-ion-laser (60X-200, American Laser Corporation) at lexc ¼ 488 nm served as excitation source. The collected fluorescence signal was separated from the reflected laser light by a holographic notch filter (Kaiser Optical Systems) and detected by a single photon counting avalanche photo diode (SPCM-AQR-14, Perkin Elmer). For transmission wide field microscopy, the microcavity sample was illuminated from the top with a polychromatic light source.
170 160 150 140 130 120 110 100 90 80
0.6
δω
0.4 ωo 0.2 0.0 3.0
(b)
3.2
3.4 3.6 ω (1015 Hz)
3.8
4.0
Fig. 3. (a) Variation of the cavity length L(x) determined from a series of local transmission spectra (see (b)) measured by moving the microcavity stepwise in x direction with respect to the fixed microscope objective (see also Fig. 1 (b)). The peak wavelength l(x) is used to calculate the local cavity length L(x) (circles) according to Eq. (1). The solid line is a second-order polynomial fit. (b) Measured local microcavity transmission spectrum (dots) fitted by a Lorentzian line shape function (full line) giving a cavity quality factor Q ¼ 44.
with increasing distance from the cavity center is observed as shown in Fig. 3(a) and can be understood as a magnified section of the cavity length profile around m ¼ 1 as depicted in Fig. 2(b). From the data shown in Fig. 3(a) L0 ðxÞ ¼ DL=Dx103 is obtained which means that a displacement Dx of 500 nm results in a cavity length change DL of about 0.5 nm. This is one order of magnitude smaller than the surface roughness of our silver mirrors as determined by AFM measurements. This demonstrates that our microcavities act as planar resonators in a very
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
good approximation within the 500 nm focal spot diameter of our microscope. The measured transmission spectrum shown in Fig. 3(b) can be modeled using a Lorentzian line shape function and reveals the empty cavity linewidth do at the center frequency o0 from which the cavity-Q is determined according to Q ¼ o0 =do. For cavity lengths L between 120 and 150 nm which are typical for our measurements the cavity-Q was found to increase with increasing cavity length L and had typical values around 50. As a result, we can assign for every sample position the cavity parameters L and Q. Using scanning confocal fluorescence microscopy we were able to detect isolated bright spots that originate from single molecules on resonance with the first longitudinal mode of the microcavity. The section of the fluorescent ring shown in Fig. 4 corresponds to a section of the innermost interference fringe in Fig. 2(a). Time traces (Inset Fig. 4) of the emitted fluorescence intensity reveal blinking in the millisecond regime and sudden
100
intensity (counts / 10 ms)
100
50
0
150
200
80
L
171
bleaching which is typical for single quantum systems. In addition, we observed photon antibunching in the fluorescence emission of single terrylene molecules embedded in our microresonator at cryogenic temperatures (data not shown) clearly demonstrating that our cavity system is well-suited to produce single photons. Note that for cavity lengths L matching the resonance conditions of the fluorescence wavelength according to Eq. (1), single dye molecules were observed through silver mirrors with a thickness up to 30 nm. Off resonance, i.e. at other cavity lengths, the fluorescence signal was suppressed effectively and single molecule detection was not feasible as expected. Our microcavity was found to provide long-term stability over months without noticeable reduction of the cavity-Q. An extensive investigation of the spectral and temporal characteristics of single molecule fluorescence controlled by the microcavity is presented elsewhere [10]. We observed a spectral narrowing of the molecular fluorescence emission controlled by the microresonator as well as spontaneous emission rate enhancement due to Purcell effect [11]. Clearly, the single molecule experiments are more difficult to perform than those on ensembles. However, in the latter case cavity-assisted long-range dipole coupling [8] results in far more complex excited state dynamics.
y (µm)
60
5. Summary
int (cts / 10 ms)
40
20
200 150 100 50 4
0 0
20
5
40
6
60
t (s)
7
8
80
9
100
x (µm) Fig. 4. Scanning confocal fluorescence microscopy image of the first-order interference region ðm ¼ 1Þ of a Fabry–Perot type Ag-microcavity with a dye concentration cPI1010 M. Bright spots result from the fluorescence of individual molecules as can be seen from their characteristic blinking behavior in the time trace of the fluorescence intensity with time intervals of 10 ms (inset). The white arrow indicates the direction of increasing optical path difference, i.e. increasing cavity length L.
In conclusion, we presented a new microcavity design which provides long-term stability and single molecule sensitivity over months. It allows for studying single molecule dynamics in microresonators under ambient conditions [10] as well as at cryogenic temperatures. Benefit could be taken from this work by integrating our microcavity layout in a lab-on-a-microchip design and combining it with microcapillaries or sheath flow cuvettes to improve the signal-to-noise ratio and spectral selectivity of ultrasensitive microfluidic analytical devices [3,4]. Moreover, it can be considered as an important improvement of single photon sources based on single molecules operating at room temperature [2].
ARTICLE IN PRESS 172
M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
Acknowledgments The authors thank Huihong Qian and Hui Qian for performing the AFM-measurements on the silver films and G. Schulte (Universita¨t Siegen) for valuable technical support. Financial support by the Deutsche Forschungsgemeinschaft (Me 1600/ 6-1/2) is acknowledged.
References [1] H. Yokoyama, K. Ujihara (Eds.), Spontaneous Emission and Laser Oscillation in Microcavities, CRC Press, Boca Raton, 1995. [2] B. Lounis, W.E. Moerner, Nature 407 (2000) 491.
[3] W. Becker, H. Hickl, C. Zander, K.H. Drexhage, M. Sauer, S. Siebert, J. Wolfrum, Rev. Sci. Instrum. 70 (1999) 1835. [4] L.-Q. Li, L.M. Davis, Appl. Opt. 34 (1995) 3208. [5] F. De Martini, G. Di Giuseppe, M. Marrocco, Phys. Rev. Lett. 76 (1996) 900. [6] S.C. Kitson, P. Jonsson, J.G. Rarity, P.R. Tapster, Phys. Rev. A 58 (1998) 620. [7] M.D. Barnes, N. Lermer, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, Opt. Lett. 22 (1997) 1265. [8] F. De Martini, G.R. Jacobovitz, Phys. Rev. Lett. 60 (1988) 1711. [9] H. Becker, S.E. Burns, N. Tessler, R.H. Friend, J. Appl. Phys. 81 (1997) 2825. [10] M. Steiner, F. Schleifenbaum, C. Stupperich, A.V. Failla, A. Hartschuh, A.J. Meixner, Chem. Phys. Chem. 6 (2005) 2190. [11] E.M. Purcell, Phys. Rev. 69 (1946) 681.
Journal of Luminescence 119–120 (2006) 167–172 www.elsevier.com/locate/jlumin
A new microcavity design for single molecule detection M. Steiner,1, F. Schleifenbaum1, C. Stupperich, A.V. Failla1, A. Hartschuh1, A.J. Meixner1 Universita¨t Siegen, Center for Micro- and Nanochemistry and Physikalische Chemie, 57068 Siegen, Germany Available online 28 February 2006
Abstract We present a new microcavity design which allows for efficient detection of single molecules by measuring the molecular fluorescence emission coupled into a resonant cavity mode. The Fabry–Perot-type microresonator consists of two silver mirrors separated by a thin polymer film doped with dye molecules in ultralow concenctration. By slightly tilting one of the mirrors different cavity lengths can be selected within the same sample. Locally, on a mm scale, the microcavity still acts as a planar Fabry–Perot resonator. Using scanning confocal fluorescence microscopy, single emitters on resonance with a single mode of the microresonator can be spatially addressed. Our microcavity is demonstrated to be well-suited for investigating the coupling mechanism between single quantum emitters and single modes of the electromagnetic field. The microcavity layout could be integrated in a lab-on-a-microchip design for ultrasensitive microfluidic analytics and can be considered as an important improvement for single photon sources based on single molecules operating at room temperature. r 2006 Elsevier B.V. All rights reserved. Keywords: Microcavity; Single molecules; Fluorescence microscopy
1. Introduction Optical microcavities, i.e. optical resonators with confinement dimensions in the range of one Corresponding author. Universita¨t Tubingen, Institut fur ¨ ¨ Physikalische und Theoretische Chemie, Auf der Morgenstelle 8, 72076 Tu¨bingen, Germany. Tel.: +49 7071 29 76171; fax: +49 7071 29 5490. E-mail address: [email protected] (M. Steiner). 1 Present address: Universita¨t Tu¨bingen, Institut fu¨r Physikalische und Theoretische Chemie, 72076 Tu¨bingen, Germany.
wavelength of the enclosed electromagnetic field, can be used to control the spatial, spectral, and temporal properties of radiation from embedded quantum emitters [1]. This control is essential for improving existing true single photon sources [2] and devices for ultrasensitive analytics [3,4]. Ultimately, it would be desirable to couple the fluorescence emission of a single molecule to a single resonant cavity mode. Up to now, evidence for the observation of single molecules embedded in optical microcavities has only been derived from the measured sub-poissonian photon statistics of
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.12.063
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
168
doped with dye molecules in ultralow concenctration. Silver mirrors were evaporated onto commercially available and cleaned microscope glass coverslips (thickness 170 mm) by an electron beam source (EB3, Edwards) under high vacuum conditions (106 mbar). The film thickness was monitored during the evaporation process using an oscillating quartz unit (FTM7, Edwards) and verified individually by AFM measurements. The thickness of the silver mirrors was chosen to be 60 (30) nm for the back mirror S2 and 30 (20) nm for the input/output mirror S1 to optimize the performance, i.e. excitation and fluorescence light for single molecule detection pass the same mirror (see Fig. 1(b)). Well-controlled concentrations of the perylen-derivative n (2,6-diisopropylphenyl)perylen-3,4-dicarboxymid (abbreviated PI in the
the fluorescence emission from highly diluted dye solutions enclosed within planar microresonators [5,6] and single molecule fluorescence bursts from streams of dye-doped microdroplets [7]. However, to study the interaction between single fluorescent emitters and a microcavity with well-defined coupling conditions, it is necessary to immobilize and spatially isolate them. In this case, the transition dipole is fixed with respect to the cavity mirrors and the cavity-mediated long-range dipole coupling [8] is prevented.
2. Sample preparation and characterization The Fabry–Perot type microcavity consists of two silver mirrors separated by a thin polymer film
Polychromatic light source for transmission microscopy
PI
F
y Ag-Layer (S2)
nglass
x
L(x)
z
npol
Microscope objective
nglass
Excitation laser light for fluorescence microscopy
Beamsplitter
Ag-Layer (S1)
UV-radiation
Detector (a)
(b)
Fig. 1. (a) Manufacturing of a microcavity for single molecule detection consisting of two silver coated glass coverslips separated by liquid monomers of an optical glue offering dye molecules (PI) in ultralow concentration (cPI1010 M). The microcavity length L is tuned applying a punctual force in the center of the microcavity and fixed by polymerization of the optical glue using UV-light. The procedure is controlled by observing the resulting interference pattern (see Fig. 2(a)) using a measuring microscope. (b) Experimental setup for single molecule detection and measurement of the local cavity length L(x) as described in text.
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
169
570 nmÞ and the positions x corresponding to distinct interference orders were measured using a division scale (see Fig. 2(a)). The transmission of the microcavity obeys the resonance condition
following) were embedded into an UV-polymerizing optical adhesive with refractive index npol ¼ 1:56 acting as a dielectric medium between the microcavity mirrors (see Fig. 1(a)). Single molecule experiments were carried out at concentrations of cPI10–10 M. For the preparation both PI molecules and the monomers of the adhesive were dissolved in dichloroethane. Evaporation of the solvent resulted in an active medium composed of randomly oriented dye molecules with controlled emitter concentration. UV-illumination of the microcavity leads to the polymerization of the optical adhesive effectively gluing together the two silver mirrors. To get access to various cavity lengths, one of the cavity mirrors is slightly inclined with respect to the other by applying a punctual force before the curing process is initialized (see Fig. 1(a)). This leads to a welldefined dependence of the cavity length L on the position within the microcavity x–y plane. The optical quality and uniformity of the resonators was controlled by observing the white light interference pattern, i.e. Newton rings of equal optical thickness. To determine the exact relation between L and x, the microcavity was simultaneously illuminated with parallel laser light ðl ¼
npol LðxÞ cos Y DF1 ðd 1 ; Y; lÞ þ DF2 ðd 2 ; Y; lÞ lðxÞ , ¼ m 2p 2 m ¼ 1; 2; 3; . . . ,
ð1Þ
where L denotes the thickness of the polymer film which deviates from the effective optical resonator length in the case of real cavity mirrors S1,2 with phase shifts DF1;2 ap:npol is the refractive index of the cured polymer, Y is the angle of incidence of an incoming parallel light beam with wavelength l, m is the interference order and d1,2 the thickness of the silver films. The local cavity length L(x) and hence the cavity length profile depicted in Fig. 2(b) was obtained from the optical image in Fig. 2(a) with the parameters npol ¼ 1:56; Y ¼ 01 and m increasing from 1 for the innermost ring up to m ¼ 12. For small values of m, L(x) is welldescribed by a second-order polynomial (dotted line in Fig. 2(b)) as it is expected for Newton rings. The DFi were taken as a result from computer
2.5 interference order m = 4 - 12 interference order m = 1 - 3 polynomial fit to m = 1 - 3
x
cavity length L (µm)
2.0
1.5
1.0
0.5
0.0 0.0 (a)
0.2
0.4
0.6
0.8 1.0 1.2 1.4 position x (mm)
1.6
1.8
2.0
(b)
Fig. 2. (a) Transmission widefield microscopy image (50 fold magnification) of the central part of the microcavity. Concentric laser ðl ¼ 570 nmÞ and white light interference fringes (Newton rings) are visible surrounding the dark center. (b) Cavity length profile L(x) calculated by Eq. (1) using the positions of the sharp laser fringes in the optical image (a). For m ¼ 123 (dots), L(x) is well-described by a second-order polynomial (dotted line).
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
simulations which were verified by reproducing comparable results for silver mirrors and microcavities reported elsewhere [9]. For the film thickness d1,2 considered here the phase shifts DFi lead to a nearly constant offset which reduces the real cavity length L by 50 nm as compared to the results for ideal metal mirrors ðDFi ¼ pÞ.
cavity length L (nm)
170
3. Experimental
4. Results and discussion For single molecule detection within the first resonator mode (i.e. the l/2-condition), we addressed an area of 100 mm 100 mm in the innermost interference fringe ðm ¼ 1Þ. Transmission wide field microscopy (see Fig. 1(b)) was used to determine the two relevant cavity parameters: The local cavity length L(x) (see Fig. 3(a)) and the Qfactor. A typical local transmission spectrum of an Ag-microcavity (d 1 ¼ 30 nm, d 2 ¼ 60 nm) is presented in Fig. 3(b). L(x) can be obtained from the wavelength at the transmission maximum l(x) using Eq. (1) with the parameters npol ¼ 1:56; Y ¼ 0 , m ¼ 1 and the corresponding DFi . A systematic non-linear increase of the cavity length
0 (a)
10 20 30 40 50 60 70 80 90 100 position x (µm)
1.0
cavity-Q = 44 @ x = 55 µm
0.8 intensity (a. u.)
All optical measurements were performed with an inverted confocal microscope (based on an Axiovert 135 TV, Zeiss) with a high numerical aperture objective (Planeoflex 100x=NA ¼ 1:3, Zeiss) as shown schematically in Fig. 1(b). Raster scanning the microcavity through the diffractionlimited spot of the microscope objective was done using a feedback-controlled sample stage (P517.K008, Physik Instrumente). A single line argon-ion-laser (60X-200, American Laser Corporation) at lexc ¼ 488 nm served as excitation source. The collected fluorescence signal was separated from the reflected laser light by a holographic notch filter (Kaiser Optical Systems) and detected by a single photon counting avalanche photo diode (SPCM-AQR-14, Perkin Elmer). For transmission wide field microscopy, the microcavity sample was illuminated from the top with a polychromatic light source.
170 160 150 140 130 120 110 100 90 80
0.6
δω
0.4 ωo 0.2 0.0 3.0
(b)
3.2
3.4 3.6 ω (1015 Hz)
3.8
4.0
Fig. 3. (a) Variation of the cavity length L(x) determined from a series of local transmission spectra (see (b)) measured by moving the microcavity stepwise in x direction with respect to the fixed microscope objective (see also Fig. 1 (b)). The peak wavelength l(x) is used to calculate the local cavity length L(x) (circles) according to Eq. (1). The solid line is a second-order polynomial fit. (b) Measured local microcavity transmission spectrum (dots) fitted by a Lorentzian line shape function (full line) giving a cavity quality factor Q ¼ 44.
with increasing distance from the cavity center is observed as shown in Fig. 3(a) and can be understood as a magnified section of the cavity length profile around m ¼ 1 as depicted in Fig. 2(b). From the data shown in Fig. 3(a) L0 ðxÞ ¼ DL=Dx103 is obtained which means that a displacement Dx of 500 nm results in a cavity length change DL of about 0.5 nm. This is one order of magnitude smaller than the surface roughness of our silver mirrors as determined by AFM measurements. This demonstrates that our microcavities act as planar resonators in a very
ARTICLE IN PRESS M. Steiner et al. / Journal of Luminescence 119– 120 (2006) 167–172
good approximation within the 500 nm focal spot diameter of our microscope. The measured transmission spectrum shown in Fig. 3(b) can be modeled using a Lorentzian line shape function and reveals the empty cavity linewidth do at the center frequency o0 from which the cavity-Q is determined according to Q ¼ o0 =do. For cavity lengths L between 120 and 150 nm which are typical for our measurements the cavity-Q was found to increase with increasing cavity length L and had typical values around 50. As a result, we can assign for every sample position the cavity parameters L and Q. Using scanning confocal fluorescence microscopy we were able to detect isolated bright spots that originate from single molecules on resonance with the first longitudinal mode of the microcavity. The section of the fluorescent ring shown in Fig. 4 corresponds to a section of the innermost interference fringe in Fig. 2(a). Time traces (Inset Fig. 4) of the emitted fluorescence intensity reveal blinking in the millisecond regime and sudden
100
intensity (counts / 10 ms)
100
50
0
150
200
80
L
171
bleaching which is typical for single quantum systems. In addition, we observed photon antibunching in the fluorescence emission of single terrylene molecules embedded in our microresonator at cryogenic temperatures (data not shown) clearly demonstrating that our cavity system is well-suited to produce single photons. Note that for cavity lengths L matching the resonance conditions of the fluorescence wavelength according to Eq. (1), single dye molecules were observed through silver mirrors with a thickness up to 30 nm. Off resonance, i.e. at other cavity lengths, the fluorescence signal was suppressed effectively and single molecule detection was not feasible as expected. Our microcavity was found to provide long-term stability over months without noticeable reduction of the cavity-Q. An extensive investigation of the spectral and temporal characteristics of single molecule fluorescence controlled by the microcavity is presented elsewhere [10]. We observed a spectral narrowing of the molecular fluorescence emission controlled by the microresonator as well as spontaneous emission rate enhancement due to Purcell effect [11]. Clearly, the single molecule experiments are more difficult to perform than those on ensembles. However, in the latter case cavity-assisted long-range dipole coupling [8] results in far more complex excited state dynamics.
y (µm)
60
5. Summary
int (cts / 10 ms)
40
20
200 150 100 50 4
0 0
20
5
40
6
60
t (s)
7
8
80
9
100
x (µm) Fig. 4. Scanning confocal fluorescence microscopy image of the first-order interference region ðm ¼ 1Þ of a Fabry–Perot type Ag-microcavity with a dye concentration cPI1010 M. Bright spots result from the fluorescence of individual molecules as can be seen from their characteristic blinking behavior in the time trace of the fluorescence intensity with time intervals of 10 ms (inset). The white arrow indicates the direction of increasing optical path difference, i.e. increasing cavity length L.
In conclusion, we presented a new microcavity design which provides long-term stability and single molecule sensitivity over months. It allows for studying single molecule dynamics in microresonators under ambient conditions [10] as well as at cryogenic temperatures. Benefit could be taken from this work by integrating our microcavity layout in a lab-on-a-microchip design and combining it with microcapillaries or sheath flow cuvettes to improve the signal-to-noise ratio and spectral selectivity of ultrasensitive microfluidic analytical devices [3,4]. Moreover, it can be considered as an important improvement of single photon sources based on single molecules operating at room temperature [2].
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Acknowledgments The authors thank Huihong Qian and Hui Qian for performing the AFM-measurements on the silver films and G. Schulte (Universita¨t Siegen) for valuable technical support. Financial support by the Deutsche Forschungsgemeinschaft (Me 1600/ 6-1/2) is acknowledged.
References [1] H. Yokoyama, K. Ujihara (Eds.), Spontaneous Emission and Laser Oscillation in Microcavities, CRC Press, Boca Raton, 1995. [2] B. Lounis, W.E. Moerner, Nature 407 (2000) 491.
[3] W. Becker, H. Hickl, C. Zander, K.H. Drexhage, M. Sauer, S. Siebert, J. Wolfrum, Rev. Sci. Instrum. 70 (1999) 1835. [4] L.-Q. Li, L.M. Davis, Appl. Opt. 34 (1995) 3208. [5] F. De Martini, G. Di Giuseppe, M. Marrocco, Phys. Rev. Lett. 76 (1996) 900. [6] S.C. Kitson, P. Jonsson, J.G. Rarity, P.R. Tapster, Phys. Rev. A 58 (1998) 620. [7] M.D. Barnes, N. Lermer, C.-Y. Kung, W.B. Whitten, J.M. Ramsey, Opt. Lett. 22 (1997) 1265. [8] F. De Martini, G.R. Jacobovitz, Phys. Rev. Lett. 60 (1988) 1711. [9] H. Becker, S.E. Burns, N. Tessler, R.H. Friend, J. Appl. Phys. 81 (1997) 2825. [10] M. Steiner, F. Schleifenbaum, C. Stupperich, A.V. Failla, A. Hartschuh, A.J. Meixner, Chem. Phys. Chem. 6 (2005) 2190. [11] E.M. Purcell, Phys. Rev. 69 (1946) 681.