APPLIED PHYSICS LETTERS 102, 161102 (2013)

Optical control of individual carbon nanotube light emitters by spectral
double resonance in silicon microdisk resonators
S. Imamura, R. Watahiki, R. Miura, T. Shimada, and Y. K. Katoa)
Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan(Received 15 March 2013; accepted 10 April 2013; published online 22 April 2013)
We demonstrate integration of individual light-emitting carbon nanotubes with silicon microdisk
resonators. Photons emitted from nanotubes are efficiently coupled to whispering gallery modes,
circulating within the disks and lighting up their perimeters. Furthermore, we control such emission
by tuning the excitation wavelength in and out of resonance with higher order modes in the samedisk. Our results open up the possibilities of using nanotube emitters embedded in photonic circuits
C 2013 AIP Publishing LLC
that are individually addressable through spectral double resonance. V[http://dx.doi.org/10.1063/1.4802930]
Advances in silicon photonics have enabled on-chip
integration of various devices,1–4 expanding the capabilities
of monolithic photonic circuits. For further scaling andincreased functionality, however, integration of nanoscale
emitters is desirable. Unfortunately, partly due to the factthat silicon is an indirect gap semiconductor, coupling ofindividually addressable nanoscale emitters to silicon photonics devices has remained a challenge.
In this regard, carbon nanotubes (CNTs) are promisingbecause they are room-temperature telecom-band emitters5,6
that can be directly synthesized on silicon7 and be electrically driven.8–12 In particular, as-grown air-suspended CNTs
show excellent optical properties,9,10,13 making them ideal
for use as individual emitters compared to on-substrate11,12
or solution based materials.14,15
As a silicon photonics device for coupling with airsuspended CNTs, microdisk resonators16 have many desirable properties. They are a commonly used microcavity
structure, supporting whispering gallery modes (WGMs)
where optical waves are guided along the circumference ofthe disk by continuous internal reflection. Because they canhave ultra-high quality factors17,18 and small mode volumes,
WGMs are suited for coupling with nanoscale emitters suchas quantum dots.19–21 It is relatively easy to tune the cavity
resonance to the emitter wavelength by fabricating diskswith an appropriate diameter, and the three-dimensional
structure of post-supported microdisks16,19,20 is compatible
with air-suspended CNTs. Furthermore, they can be coupled
to waveguides with high efficiency because they are
traveling-wave resonators,18 making them a preferred choicecompared to planar cavities11 for integration in silicon
photonics.
Here, we report on the integration of individual CNT
emitters with silicon microdisks using fabrication processes
compatible with standard silicon photonics. When photons
emitted from nanotubes are coupled to the WGMs of the
microdisks, emission enhancement occurs in a narrow spectral windows corresponding to the modes. Spatial imaging of
the emission at the WGM wavelength shows the peripheralof the disk illuminated by the coupled CNT emission thata)Author to whom correspondence should be addressed. Electronic mail:ykato@sogo.t.u-tokyo.ac.jp0003-6951/2013/102(16)/161102/4/$30.00

circulates within the microdisk. In addition, we show that theexcitation laser can also be coupled to whispering gallery
modes in the same disk, allowing for non-local control overnanotube emission.
We fabricate microdisks from a silicon-on-insulator wafer with a 260-nm-thick top Si layer and a 2000-nm-thickburied oxide layer. Electron beam lithography and dryetching processes are used to form doughnut-shaped trenches
in the top Si layer. In order to create a post-supported disk
structure, the buried oxide layer is etched in buffered hydrofluoric acid. A scanning electron microscope image of a typical microdisk prior to nanotube growth is shown in Fig. 1(a).
The devices are characterized with a home-built laserscanning confocal photoluminescence (PL) microscopy
system.14,22,23 An output of a wavelength-tunable continuouswave Ti:sapphire laser is focused onto the sample by an objective lens to a spot size of 1 lm. PL is coupled to a 300-mm
spectrometer through a pinhole in a confocal configuration.
An InGaAs photodiode array is used for detection and PL
images are collected with a steering mirror. All measurements
are done in air at room temperature.
A PL spectrum of a microdisk taken with an excitation
wavelength kex ¼ 755 nm and a power P ¼ 1 mW is shown
as a red curve in Fig. 1(b). A forest of resonances consistingof WGMs as well as Fabry-Perot modes21 is visible within
the silicon bandgap emission.
An interesting observation is that the emission intensity
decreases by a factor of 5 when kex is tuned to 748 nm[green curve in Fig. 1(b)]. To investigate this strong excitation
wavelength dependence, PL excitation (PLE) spectroscopy is
performed [Figs. 1(c) and 1(d)]. We interpret the cross hatch
pattern in the PLE map [Fig. 1(d)] as coupling of cavity
modes at both excitation and emission wavelengths. The vertical streak-like features correspond to microdisk modes
coupled to silicon emission, and the horizontal streaks represent modes coupled to the excitation laser. The intersectionsof vertical and horizontal streaks indicate simultaneous resonance of the excitation and emission, which allows control
over PL intensity by about an order of magnitude [Fig. 1(b)].
The PLE spectrum [Fig. 1(c)] shows clear resonanceswith quality factors of 100, but precise mode assignment
is difficult at these wavelengths because of high modeorders and strong absorption. Nevertheless, examination of102, 161102-1

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FIG. 1. (a) Scanning electron micrograph of an as-fabricated silicon microdisk. (b) PL spectra of a microdisk with a diameter of 2.93 lm under resonant excitation (red, kex ¼ 755 nm) and off-resonant excitation (green,kex ¼ 748 nm). Blue curve is a PL spectrum of the unpatterned substrate(kex ¼ 755 nm). (c) PLE spectra of the microdisk (red) and unpatterned substrate (blue), obtained by integrating emission spectra over a spectral window of 1 nm centered around 1055 nm. (d) PLE map. In (b)–(d), PL counts
are normalized to P ¼ 1 mW to account for laser power changes with wavelength. (e) Reflectivity image of the microdisk measured in (b)–(g). (f) and(g) PL images of the microdisk with horizontally and vertically polarized excitation, respectively. Images are obtained by integrating PL over a spectral
window of 5 nm centered around 1055 nm. The polarization directions areschematically shown at the top-right corners of the images. The scale bars in(e)–(g) are 2 lm.

reflectivity and PL images shows that PL is enhanced at theedges of the disk when the polarization matches the radial
direction [Figs. 1(e)–1(g)]. Such a polarization dependence
is consistent with what one expects for WGMs withtransverse-electric (TE) polarization. The coupling of excitation laser to WGMs implies that the laser photons canpropagate within the disk at resonance, allowing for nonlocal excitation.
In order to couple single CNTs to microdisk WGMs, it isnecessary to choose a disk with an appropriate diameter suchthat a suitable WGM exists at the emission wavelength ofnanotubes. In addition, as CNTs interact primarily with lightpolarized along their axis,22,24,25 it is also important to consider the mode polarization. WGMs can have either TE ortransverse-magnetic (TM) polarization, and we expect TE
modes to have better coupling with the nanotubes since CNTs
suspended onto the microdisks will be parallel to the substrate.
We identify WGMs at CNT emission wavelengths byspin coating nanotube solution on the microdisks and performing PL measurements in a manner similar to experiments done on photonic crystal microcavities.14 Figure 2(a)shows spectra from a few microdisks with different diameters, in which the sharp WGMs show up as a result of broad

Appl. Phys. Lett. 102, 161102 (2013)

FIG. 2. (a) PL spectra of microdisks with diameters from 2.97 lm to
3.25 lm after nanotube deposition. (b) Assignments of microdisk resonatormodes. Lines show calculated wavelengths of fundamental TE modes using
an approximated analytic model with no adjustable parameters. Numbers
above the lines indicate mode index. Experimental data are obtained fromspectra including those shown in (a). Filled circles are resonances identified
as fundamental TE modes. Those assigned to fundamental TM modes aremarked by open circles. Unassigned modes, which include higher ordermodes, are indicated by crosses.

emission from nanotubes. The mode wavelengths shift withdiameter as expected.
The disk diameter dependence of WGMs is summarized
in Fig. 2(b). We assign the TE modes by comparing the data
to an analytical model with no adjustable parameters.17 The
TM modes are identified by a weaker dependence on the diameter, which is a result of their smaller effective index ofrefraction. Within the wavelength range of interest for coupling with CNTs (1300–1500 nm), we confirm the presenceof a few TE modes.
We now investigate CNT emitters integrated with silicon microdisks [Fig. 3(a)]. Another electron beam lithography step is performed to define catalyst windows at outeredges of the doughnut-shaped trenches. Single-walled CNTs
are grown by chemical vapor deposition using ethanol as acarbon source.26 Catalyst solution is prepared by ultrasonicating 5.0 mg of cobalt(II) acetate tetrahydrate and 50.0 mg

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FIG. 3. (a) Scanning electron micrograph of a suspended nanotube attached
to a microdisk. (b) Reflectivity image of a device measured in (c)–(f). The
disk diameter is 3.00 lm. kex ¼ 800 nm and P ¼ 0.2 mW are used. Blue andred dots indicate the positions at which the data in (c),(d) and (e),(f) aretaken, respectively. The white box shows the position of the suspended
nanotube. The scale bars in (a) and (b) are 2 lm. (c) Directly measured PL
spectrum of the suspended nanotube for kex ¼ 857 nm. (d) PLE map
obtained by direct measurement of the suspended nanotube. (e) PL spectrumof nanotube emission coupled to the microdisk for kex ¼ 857 nm (red solidline) and 827 nm (red broken line). (f) PLE map of nanotube emission
coupled to the microdisk. In (c),(d) and (e),(f), the PL counts are normalized
to laser powers of 10 lW and 500 lW, respectively. In (c),(d), laser is linearly polarized parallel to the nanotube, while in (e),(f), it is polarized along
the radial direction of the disk.

of fumed silica in 40.0 g of ethanol. We spin coat and lift off
the catalyst solution, and the samples are annealed in air for
5 min at 400  C prior to growth. The samples are then placedin a quartz tube furnace, and the temperature is elevated to800  C while flowing Ar with 3% H2. Ethanol is introducedby bubbling the carrier gas for 10 min. With some probability, nanotubes can get suspended across the trench onto themicrodisk. By controlling the size of the catalyst areas, weare able to obtain a reasonable yield of 10% for microdiskswith single suspended tubes. We note that the yield can beeasily increased by placing more catalyst, if devices withmultiple nanotubes are acceptable.
Figure 3(b) shows a reflectivity image of a device with a
CNT attached. Although the nanotube does not show up in
this image, the white box indicates the position at where thenanotube is suspended. PL spectrum taken with the laser
spot on the suspended tube is shown in Fig. 3(c). The chiralindex of this tube is determined to be (10,8) by comparing
the excitation map [Fig. 3(d)] to tabulated data.27
Surprisingly, the nanotube emission can also be
observed at the opposite side of the disk [Fig. 3(e)], although
it is spatially separated from the nanotube by 3 lm.

Appl. Phys. Lett. 102, 161102 (2013)

Notable is the sharp peak that appears on top of the nanotube
spectrum. We identify this peak to be the 15th-order WGM
with TE polarization [Fig. 2(b)]. As the mode wavelength
corresponds to the energy far below the bandgap of Si, we attribute the emergence of this peak to nanotube emission
coupled to a WGM.
The PLE map taken at the same position [Fig. 3(f)]resembles that of the directly measured nanotube [Fig. 3(d)],and, in particular, the narrow WGM peak and the broader
CNT emission peak are both maximized at the same excitation wavelength. This implies that the absorption characteristics are of the same origin for both WGM and CNT peaks,
further supporting that the emission from the suspended
nanotube is coupled to the WGM. We also note that faint Siemission is visible at wavelengths shorter than 1200 nm in
Fig. 3(f). The contrast with bright CNT emission demonstrates the advantage of using direct-gap nanotube emitters.
Different from the direct measurement of the suspended
CNT, the PLE map for coupled emission shows strong modulation of intensity with changes in the excitation wavelength. As mentioned previously, there exist WGMs at thesewavelengths as well, and they can cause enhancements ofthe excitation efficiency when excitation is resonant with a
WGM. When both the excitation and emission are in resonance with WGMs, we meet the doubly resonant conditionof the cavity. Similar to the case of Si PL, the emission isenhanced by a factor of 10 at double resonance compared
to off-resonant conditions [Fig. 3(e)], allowing non-local
control over nanotube emission through spectral tuning.
The photons that couple to the WGM circulate within
the microdisk, and this can be visualized through spectrally
resolved PL imaging as in the case of WGM at Si emission
wavelengths [Fig. 4(a)]. Using a higher resolution grating,
we collect PL spectra as we raster the laser spot over thescan area, and fit them with two Lorentzian functions corresponding to the WGM peak and the direct CNT emission
peak [Fig. 4(b)]. We find that the measured quality factor of
the WGM is limited by spectrometer resolution to be 3000,
which is already two orders of magnitude larger compared toa simple planar cavity.11
In Figs. 4(c) and 4(d), spectrally resolved PL images
corresponding to the WGM and direct CNT emission arepresented. It is clear that nanotube emission coupled to the
WGM circulates within the disk and illuminates the circumference [Fig. 4(c)], showing that CNT emission is efficiently
guided into the silicon photonic structure. We note that the
coupling occurs through the evanescent fields of the WGM,
as the nanotube is located outside of the resonator. This is incontrast to the case of quantum dots that are embedded
within the microdisks, and our results show that the evanescent fields are strong enough to couple the nanotube emission into the WGM.
The direct emission from the CNT is centered on the
nanotube itself as expected [Fig. 4(d)], although one canobserve a lower intensity spot at the opposite side of the
disk. We interpret the existence of such a remote spot interms of in-plane Fabry-Perot mode within the microdisk.21
Such an interpretation is also consistent with the image ofthe WGM [Fig. 4(c)], which shows larger intensity nearthat spot.

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Appl. Phys. Lett. 102, 161102 (2013)

were fabricated at the Center for Nano Lithography &Analysis at The University of Tokyo.
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2

FIG. 4. (a) A PL image of Si emission coupled to a WGM, taken withkex ¼ 771 nm and P ¼ 1.5 mW. To construct this image, the spectra havebeen integrated from 1171 nm to 1175 nm. (b) High resolution PL spectrumof nanotube emission coupled to a WGM. Blue dots are data and lines are
Lorentzian fits as explained in the text. The peak values obtained from thefits are plotted in (c) and (d). (c) An image of CNT emission coupled
to a WGM. (d) An image of direct CNT emission. For these images,
kex ¼ 857 nm and P ¼ 0.3 mW with circular polarization are used. The scalebars in the images are 2 lm.

Our results demonstrate the feasibility of integrating
telecom-wavelength nanotube emitters in silicon photonics,
opening up further possibilities for scaling down monolithic
photonic circuits. By fabricating arrays of microdisks withdifferent diameters and thus different resonant wavelengths,
it should be possible to address each microdisk by tuning theexcitation wavelength. Furthermore, the WGMs that couple
to emission will also differ, which would be useful for
implementing on-chip wavelength-division multiplexing.
Integration of single photon emitting CNTs28,29 with silicon
photonics may open up new opportunities for quantum optics
on a chip.30
The authors acknowledge the support from SCOPE,
KAKENHI (21684016, 23104704, 24340066, 24654084),
Asahi Glass Foundation, KDDI Foundation, and the Photon
Frontier Network Program of MEXT, Japan. The devices