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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 19 — Sep. 12, 2011
  • pp: 18260–18271
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Demonstration of modulatable optical near-field interactions between dispersed resonant quantum dots

Naoya Tate, Makoto Naruse, Wataru Nomura, Tadashi Kawazoe, Takashi Yatsui, Morihisa Hoga, Yasuyuki Ohyagi, Yoko Sekine, Hiroshi Fujita, and Motoichi Ohtsu  »View Author Affiliations


Optics Express, Vol. 19, Issue 19, pp. 18260-18271 (2011)
http://dx.doi.org/10.1364/OE.19.018260


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Abstract

We experimentally demonstrated the basic concept of modulatable optical near-field interactions by utilizing energy transfer between closely positioned resonant CdSe/ZnS quantum dot (QD) pairs dispersed on a flexible substrate. Modulation by physical flexion of the substrate changes the distances between quantum dots to control the magnitude of the coupling strength. The modulation capability was qualitatively confirmed as a change of the emission spectrum. We defined two kinds of modulatability for quantitative evaluation of the capability, and an evident difference was revealed between resonant and non-resonant QDs.

© 2011 OSA

1. Introduction

Nanophotonics, which utilizes the local interaction between nanometric materials via optical near-fields, has realized novel photonic devices, fabrication techniques, and systems which will help to meet the requirements of future optical technology [1

1. M. Ohtsu, K. Kobayashi, T. Kawazoe, T. Yatsui, and M. Naruse, eds., Principles of Nanophotonics, (Taylor and Francis, 2008).

]. Optical near fields can be viewed as the elementary surface excitations on nanometric materials, which can mediate the local interaction between closely spaced nanometric materials. The interaction potential is expressed by a Yukawa function, which represents the localization of the optical near-field energy around the nanometric particles [2

2. M. Ohtsu and K. Kobayashi, Optical Near Fields (Springer-Verlag, 2003), pp. 109–150.

]. Recently, several applications of nanometric, and energy-efficient optical functions have been actively developed by utilizing local energy transfer via optical near-fields and its subsequent dissipation [3

3. K. Kobayashi, S. Sangu, T. Kawazoe, and M. Ohtsu, “Exciton dynamics and logic operations in a near-field optically coupled quantum-dot system,” J. Lumin. 112(1-4), 117–121 (2005). [CrossRef]

5

5. T. Kawazoe, M. Ohtsu, S. Aso, Y. Sawado, Y. Hosoda, K. Yoshizawa, K. Akahane, N. Yamamoto, and M. Naruse, “Two-dimensional array of room-temperature nanophotonic logic gates using InAs quantum dots in mesa structures,” Appl. Phys. B 103(3), 537–546 (2011). [CrossRef]

]. These energy transfer in nanometric materials have been observed in various materials such as [6

6. C. R. Kagan, C. B. Murray, and M. G. Bawendi, “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B Condens. Matter 54(12), 8633–8643 (1996). [CrossRef] [PubMed]

10

10. T. Franzl, T. A. Klar, S. Schietinger, A. L. Rogach, and J. Feldmann, “Exciton recycling in graded gap nanocrystal structures,” Nano Lett. 4(9), 1599–1603 (2004). [CrossRef]

].

The operation of nanophotonic devices described above exhibits a one-to-one correspondence with respect to input signals, because the physical properties, size, shape, and alignment of the components for the characteristic optical near-field interactions are built into the substrate and spatially fixed. In order to realize a one-to-many correspondence with a single nanophotonic device, it is necessary to implement modulatable optical near-field interactions and associated modulatable optical functions. Here, we propose a novel concept of Modulatable Nanophotonics to realize such a system. It is realized by providing appropriate external controls to modulate the parameters of the components. In our concept, optical far-field retrieval of induced optical near-field interactions is achieved via modulation of the intensity, polarization, and spectrum of the subsequent irradiation. This is an important step toward further exploiting the possibilities of light–matter interactions on the nanometer scale [11

11. N. Tate, W. Nomura, T. Yatsui, T. Kawazoe, M. Naruse, and M. Ohtsu, “Parallel retrieval of nanometer-scale light-matter interactions for nanophotonic systems,” Nat. Comput. 2, 298–307 (2010). [CrossRef]

].

In this paper, we demonstrate our concept by controlling the magnitude of the optical near-field coupling strength among multiple quantum dots (QDs) randomly dispersed on a flexible substrate. External control is provided by physical flexion of the substrate, whose response can be acquired as a change of the emission spectra in the optical far-field. In Section 2, we describe the basics of local energy transfer via optical near-fields and our experimental concept using QDs on a flexible substrate. Sections 3 and 4 show the concepts and results of numerical and experimental demonstrations, respectively. Quantitative evaluation of each result was performed based on specially defined figure-of-merits. We conclude in Section 5 with a brief summary.

2. Modulation of emission spectra of resonant quantum dot pairs

Here, we consider the case of similar QDs dispersed on a substrate having sufficient physical flexibility. Flexing the substrate can vary the distance between the QDs. Figure 2
Fig. 2 (a) Schematic diagram of modulatable optical near-field interactions between resonant QD pairs dispersed on flexible substrates. (b) Definition of sampled optical intensities on emission spectra I and I for quantitative evaluation of systems based on modulatability Msp, by using resonant QD pairs.
shows schematic diagrams comparing the emission processes in resonant and non-resonant QD pairs. In the case of a resonant QDs pair, denoted QDS-R and QDL-R in Fig. 2(a), energy transfers occur when the QDs are sufficiently close, because the magnitude of the optical near-field interaction, i.e., the coupling strength between QDS-R and QDL-R, depends on the distance r between them, as represented by the Yukawa function:
U=Aexp(μr)r,
(1)
where μ and A represent distribution of optical near-fields which determined by exciton energy and amount proportional to the dipole moment, respectively [2

2. M. Ohtsu and K. Kobayashi, Optical Near Fields (Springer-Verlag, 2003), pp. 109–150.

]. Thus, emission is preferentially obtained from E 1L of QDL-R. In contrast, if the separation between QDS-R and QDL-R is increased by flexion of the substrate in order to significantly decrease the coupling strength, both QDS-R and QDL-R emit independently. This means that the spectral intensities of QDS-R and QDL-R, as well as the relative spectral intensity ratio from them, can be modulated by the flexion, i.e., by modulating the coupling strength. Thus, the spectral intensity ratios from QDS-R and QDL-R with and without the flexion are evidently different, as shown in Fig. 2(b). On the other hand, in the case of non-resonant QDs, denoted QDS-NR and QDL-NR in Fig. 3(a)
Fig. 3 (a) Schematic diagram of independent emissions from dispersed non-resonant QD pairs. (b) Definition of sampled optical intensities on emission spectra I and I for quantitative evaluation of systems based on modulatability Msp, by using non-resonant QD pairs.
, energy transfer never occurs between the two. Each QD emits individually regardless of whether the substrate is flexed. In this case, only a change in spectral intensity that depends on the areal density of the QDs is obtained. Therefore, the spectral intensity ratios from QDS-NR and QDL-NR with and without flexion are equal, as shown in Fig. 3(b).

Here, we define modulatability Msp of the emission by using the spectral intensities I(λS) and I(λL) from the two QDs:
Msp=|I(λS)I(λS)I(λL)I(λL)|,
(2)
where I and I represent the peak emission intensities of the spectra without and with the flexion, respectively. Wavelengths of peak emission intensities of the spectra of QDS and QDL are represented as λS and λL, which are the emissions from E 1S and E 1L, respectively. In the case of the resonant QDs, as shown in Fig. 2(b), the value of I/I depends on the magnitude of the coupling strength, governed by the flexion, which means that the modulatability can take a non-zero value (Msp0). On the other hand, in the case of the non-resonant QDs, as shown in Fig. 3(b), the value of I/I remains unchanged for all wavelengths because the area density of the QDs is homogeneously modulated by the flexion. Therefore, Msp is calculated to be nearly zero (Msp0). One remark regarding Eq. (2) is that singularity may be a problem when denominators therein result in zero. However, as described below in Sections 3 and 4, we can assume that the denominators are non-zero in realistic physical situations, which guarantees the effectiveness as the representation of Eq. (2).

3. Numerical demonstration

To numerically estimate the spectral intensity modulation, we assumed a calculation model consisting of four QDs configured as resonant QD pairs, i.e., two small QDs (QDS-A and QDS-C) and two large QDs (QDL-B and QDL-D), representing many QDs dispersed on the substrate, as shown in Fig. 4
Fig. 4 Schematic diagrams of four-QD calculation model for numerical demonstration of multiple-QD system, showing (a) their geometrical alignment and (b) the conditions of optical near-field interactions between each QD. In our model, each parameter is variable, and their variations are assumed to be imposed by flexion of the substrate.
. We simulated the temporal evolution of exciton populations of the relevant excited states in these QDs by quantum master equations [15

15. H. J. Calmichael, Statistical Methods in Quantum Optics 1. (Springer-Verlag, 1999).

]. The exciton populations correspond to the intensity of radiation from each QD, and their evolutions are modulated by modulating the magnitude of the optical near-field coupling strength between QDs, by flexion of the substrate. As previously reported in [16

16. M. Naruse, H. Hori, K. Kobayashi, P. Holmström, L. Thylén, and M. Ohtsu, “Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions,” Opt. Express 18(Suppl 4), A544–A553 (2010). [CrossRef] [PubMed]

], the initial condition is set as a vacuum state, and the duration and the amplitude of the incident light are given by the Hamiltonian representing interactions between the incident light and the QDs. Here, the magnitude of the optical near-field interaction, i.e., the coupling strength, between the two QDs is denoted by the Yukawa function of Eq. (1).

First, we assumed stretching of the substrate in the x and y directions, as shown in Fig. 5(a)
Fig. 5 Schematic diagram of (a) stretch model and (c) shear model, and evolutions of the population of the radiation from four QDs by (b) stretching and (d) shearing the substrate.
, and calculated the exciton populations with various stretch lengths δr. In the case of the x-directional stretching, rAC and rBD are constant, as the first-order approximation, and rAB, rCD, rAD and rBC are varied. On the other hand, in the case of the y-directional stretching, rAB and rCD are constant, and rAC, rBD, rAD and rBC are varied. Figure 5(b) shows the calculated results. The horizontal axis represents the relative stretch length, which is defined as δr/L.

Evident changes in the exciton populations are obtained by the x-directional stretching, because the excitons always preferentially transfer from smaller QDs to larger QDs (from QDS-A to QDL-D). This result indicates that the coupling strength between the smaller QDs and the larger QDs is decreased by the x-directional stretching, and they emit independently.

Here we assume that the emission intensity from each QD is proportional to the exciton population at the corresponding QD energy level. Therefore, in the case of the x-directional stretching, as shown in Fig. 5(a), Msp is calculated as 9.26 at a stretch length of δr=500 nm. The population in the lowest excited state of the larger QDs finally approaches zero, and Msp is calculated as infinity. On the other hand, y-directional stretching gives Msp0 at the same stretch length. Therefore, only the former model can be said to be a modulatable nanophotonic system.

Next, a shear model is assumed for the x-directional shift of QDS-C and QDL-D. Its schematic diagram is shown in Fig. 5(c). In the case of the model, rAB and rCD are constant, and rAC and rBD are increased. Figure 5(d) shows the results of calculations. By increasing the sheared length δr, which is defined by the shifted values of QDS-C and QDL-D, only the exciton population at QDL-B increases, because the magnitude of energy transfer toward QDL-B increases not only from QDS-A but also from QDS-C. In contrast, that to QDL-D decreases. This is also because of the increase in the energy transferred from QDS-C not only to QDL-D but also to QDL-B. As a result, the population at QDS-C is decreased, whereas that at QDS-A remains unchanged, because the main route of energy transfer is from QDS-A to QDL-B, regardless of the stretching. Here, by these contributions of the exciton populations at the smaller QDs (QDS-A and QDS-C) and the larger QDs (QDL-B and QDL-D), the calculated M in the shear model shows a non-zero value (Msp = 0.11 for a sheared length of δr=100 nm).

4. Experimental demonstration

We used commercially available CdSe/ZnS spherical QDs (Evident Technologies) as a test specimen. QDs are uniformly dispersed in toluene solvents with 10 mg/mL of concentration. Their exciton energy transfer has already been studied [7

7. S. A. Crooker, J. A. Hollingsworth, S. Tretiak, and V. I. Klimov, “Spectrally resolved dynamics of energy transfer in quantum-dot assemblies: towards engineered energy flows in artificial materials,” Phys. Rev. Lett. 89(18), 186802 (2002). [CrossRef] [PubMed]

,19

19. M. Han, X. Gao, J. Z. Su, and S. Nie, “Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules,” Nat. Biotechnol. 19(7), 631–635 (2001). [CrossRef] [PubMed]

,20

20. M. Strassburg, M. Dworzak, H. Born, R. Heitz, A. Hoffmann, M. Bartels, K. Lischka, D. Schikora, and J. Christen, “Lateral redistribution of excitons in CdSe/ZnSe quantum dots,” Appl. Phys. Lett. 80(3), 473–475 (2002). [CrossRef]

], and resonant and non-resonant conditions via optical near-field interactions have been experimentally verified [17

17. W. Nomura, T. Yatsui, T. Kawazoe, and M. Ohtsu, “The observation of dissipated optical energy transfer between CdSe quantum dots,” J. Nanophotonics 1, 011591 (2007). [CrossRef]

]. The same types of QDs were adopted as QDL-R and QDL-NR. Their peak absorption wavelength λAB and peak emission wavelength λEM were as given in their technical specifications from the manufacturer, 583 nm and 605 nm, respectively, the latter corresponding to the wavelength of emission from E 1L in Fig. 1. The respective diameters D of the resonant pair, QDS-R and QDL-R, were assumed to be 8.2 nm and 8.7 nm, and those of the non-resonant pair, QDS-NR and QDL-NR, were assumed to be 7.7 nm and 8.7 nm. The QDs adopted as QDS-R have λAB = 523 nm and λEM = 546 nm, and those adopted as QDS-NR have λAB = 565 nm and λEM = 578 nm. These emission wavelengths λEM correspond to the wavelengths of emission from E 1L. As the flexible substrate, we used polydimethylsiloxane (PDMS), which is particularly known for its obvious rhelogical properties. We mixed 5 mL of each QD solution as resonant and non-resonant QD pairs. The mixed QD solutions were dispersed on a 2 cm × 2 cm square of PDMS substrate and allowed to dry naturally. In our experiments, although evident heterogeneity of the distributions was observed, the average r was assumed to be approximately 5–10 nm from the thickness of the ZnS shell and the length of the modified ligand to each QD.

Here we consider only 2-D flexion of the substrate. The QDs were excited by a He-Cd laser (wavelength 325 nm) with 5 mW/cm2 power density. In our experiment, as shown in Fig. 6
Fig. 6 Schematic diagram of experimental setup for demonstration of modulatable nanophotonic system using a flexible substrate on which resonant QD pairs are randomly dispersed. Flexion of the sample substrate is achieved by vacuum evacuation.
, the PDMS substrate was set on an aperture formed at the side of a vacuum desiccator and was flexed by evacuation. The flexion brings dispersed QDs close to each other, as represented by δr<0. The air pressure was decreased and fixed to ~0.07 MPa to achieve a 20% in-plane compression ratio of the substrate, which was geometrically determined from the size of the aperture and the depth of the flexed substrate.

We experimentally observed the emission spectra of the resonant and non-resonant QDs without and with flexion by using a spectrometer (JASCO; CT-25TP), as shown in Figs. 7(a) and (b)
Fig. 7 Obtained emission spectra with (a) resonant and (b) non-resonant samples before (blue curves) and after (red curves) flexion of the substrate. Dashed curves represent results of Gaussian fitting of each spectrum.
, respectively. The fractional ratio of the numbers of small QDs to large QDs was 1:1. The dashed curves in these figures represent Gaussian curves fitted to the measured spectral profiles. The differences in peak wavelength from those shown in the technical specifications depend on the variability of each particle's size and the inhomogeneous dispersion of the size distribution. In addition, because all peak wavelengths in Figs. 7 are shifted to red from 5 to 10 nm, the difference can also be explained by re-absorption effect in solid-state QDs on PDMS substrate.

As shown, the intensities of the spectra are increased as a whole because the numbers of QDs per unit area are increased by the flexion of each substrate. However, only an increase of the emission intensity from QDS-R was suppressed. This is because the average distance between QDS-R to QDL-R is shorten by the flexion, which induces energy transfer from QDS-R to QDL-R. Similar behavior was predicted by our numerical demonstrations described above. For quantitative evaluation of this behavior, we evaluated the intensities at the spectral peak from the fitted curves and calculated the values of I(λL)/I(λL) and I(λS)/I(λS), as shown in Fig. 7. By taking the difference, the modulatabilities Msp obtained with the resonant and non-resonant QDs were 0.45 and 0.02, respectively. Therefore, as in the numerical demonstration in Sec. 3, it was confirmed that the resonant QDs showed a much larger Msp compared with that of the non-resonant QDs.

5. Summary

In conclusion, we have described the basic concept of Modulatable Nanophotonics and numerically and experimentally demonstrated the concept by utilizing resonant and non-resonant QD pairs dispersed on flexible substrates. The modulatability was qualitatively evaluated by introducing modulatabilities Msp and Mch. Resonant QD pairs exhibited unique modulatability of their emission spectra, which depends on controlling the magnitude of the coupling strength between the QDs via optical near-field interactions. Such modulatability cannot be obtained with non-resonant QD pairs.

From the viewpoint of information retrieval, the results of our demonstration indicate that this method can retrieve the effects of optical near-field interactions as a modulation of the optical far-field response. As we demonstrated in this paper, selecting appropriate resonant QD pairs can realize various modulatabilities of the emission spectrum based on nanophotonics. By developing the concept further and experimentally examining various implementations, our idea can be applied to a modulatable multi-spectrum emitting element whose emission spectrum can be freely switched by applying external modulation.

Acknowledgments

References and links

1.

M. Ohtsu, K. Kobayashi, T. Kawazoe, T. Yatsui, and M. Naruse, eds., Principles of Nanophotonics, (Taylor and Francis, 2008).

2.

M. Ohtsu and K. Kobayashi, Optical Near Fields (Springer-Verlag, 2003), pp. 109–150.

3.

K. Kobayashi, S. Sangu, T. Kawazoe, and M. Ohtsu, “Exciton dynamics and logic operations in a near-field optically coupled quantum-dot system,” J. Lumin. 112(1-4), 117–121 (2005). [CrossRef]

4.

M. Ohtsu, T. Kawazoe, T. Yatsui, and M. Naruse, “Nanophotonics: application of dressed photons to novel photonic devices and systems,” IEEE J. Sel. Top. Quantum Electron. 14(6), 1404–1417 (2008). [CrossRef]

5.

T. Kawazoe, M. Ohtsu, S. Aso, Y. Sawado, Y. Hosoda, K. Yoshizawa, K. Akahane, N. Yamamoto, and M. Naruse, “Two-dimensional array of room-temperature nanophotonic logic gates using InAs quantum dots in mesa structures,” Appl. Phys. B 103(3), 537–546 (2011). [CrossRef]

6.

C. R. Kagan, C. B. Murray, and M. G. Bawendi, “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B Condens. Matter 54(12), 8633–8643 (1996). [CrossRef] [PubMed]

7.

S. A. Crooker, J. A. Hollingsworth, S. Tretiak, and V. I. Klimov, “Spectrally resolved dynamics of energy transfer in quantum-dot assemblies: towards engineered energy flows in artificial materials,” Phys. Rev. Lett. 89(18), 186802 (2002). [CrossRef] [PubMed]

8.

M. Achermann, M. A. Petruska, S. A. Crooker, and V. I. Klimov, “Picosecond energy transfer in quantum dot Langmuir-Blodgett nanoassemblies,” J. Phys. Chem. B 107(50), 13782–13787 (2003). [CrossRef]

9.

T. Franzl, D. S. Koktysh, T. A. Klar, A. L. Rogach, J. Feldmann, and N. Gaponik, “Fast energy transfer in layer-by-layer aAssembled CdTe nanocrystal bilayers,” Appl. Phys. Lett. 84(15), 2904–2906 (2004). [CrossRef]

10.

T. Franzl, T. A. Klar, S. Schietinger, A. L. Rogach, and J. Feldmann, “Exciton recycling in graded gap nanocrystal structures,” Nano Lett. 4(9), 1599–1603 (2004). [CrossRef]

11.

N. Tate, W. Nomura, T. Yatsui, T. Kawazoe, M. Naruse, and M. Ohtsu, “Parallel retrieval of nanometer-scale light-matter interactions for nanophotonic systems,” Nat. Comput. 2, 298–307 (2010). [CrossRef]

12.

N. Sakakura and Y. Masumoto, “Persistent spectral-hole-burning spectroscopy of CuCl quantum cubes,” Phys. Rev. B 56(7), 4051–4055 (1997). [CrossRef]

13.

Z. K. Tang, A. Yanase, T. Yasui, Y. Segawa, and K. Cho, “Optical selection rule and oscillator strength of confined exciton system in CuCl thin films,” Phys. Rev. Lett. 71(9), 1431–1434 (1993). [CrossRef] [PubMed]

14.

T. Kawazoe, K. Kobayashi, J. Lim, Y. Narita, and M. Ohtsu, “Direct observation of optically forbidden energy transfer between CuCl quantum cubes via near-field optical spectroscopy,” Phys. Rev. Lett. 88(6), 067404 (2002). [CrossRef] [PubMed]

15.

H. J. Calmichael, Statistical Methods in Quantum Optics 1. (Springer-Verlag, 1999).

16.

M. Naruse, H. Hori, K. Kobayashi, P. Holmström, L. Thylén, and M. Ohtsu, “Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions,” Opt. Express 18(Suppl 4), A544–A553 (2010). [CrossRef] [PubMed]

17.

W. Nomura, T. Yatsui, T. Kawazoe, and M. Ohtsu, “The observation of dissipated optical energy transfer between CdSe quantum dots,” J. Nanophotonics 1, 011591 (2007). [CrossRef]

18.

K. Kobayashi, S. Sangu, H. Ito, and M. Ohtsu, “Near-field optical potential for a neutral atom,” Phys. Rev. A 63, 013806 (2000). [CrossRef]

19.

M. Han, X. Gao, J. Z. Su, and S. Nie, “Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules,” Nat. Biotechnol. 19(7), 631–635 (2001). [CrossRef] [PubMed]

20.

M. Strassburg, M. Dworzak, H. Born, R. Heitz, A. Hoffmann, M. Bartels, K. Lischka, D. Schikora, and J. Christen, “Lateral redistribution of excitons in CdSe/ZnSe quantum dots,” Appl. Phys. Lett. 80(3), 473–475 (2002). [CrossRef]

21.

M. Naruse, T. Kawazoe, R. Ohta, W. Nomura, and M. Ohtsu, “Optimal mixture of randomly dispersed quantum dots for optical excitation transfer via optical near-field interactions,” Phys. Rev. B 80, 125325 (2009). [CrossRef]

OCIS Codes
(200.3050) Optics in computing : Information processing
(200.4740) Optics in computing : Optical processing
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(350.4238) Other areas of optics : Nanophotonics and photonic crystals

ToC Category:
Optics in Computing

History
Original Manuscript: June 7, 2011
Revised Manuscript: July 31, 2011
Manuscript Accepted: August 25, 2011
Published: September 2, 2011

Citation
Naoya Tate, Makoto Naruse, Wataru Nomura, Tadashi Kawazoe, Takashi Yatsui, Morihisa Hoga, Yasuyuki Ohyagi, Yoko Sekine, Hiroshi Fujita, and Motoichi Ohtsu, "Demonstration of modulatable optical near-field interactions between dispersed resonant quantum dots," Opt. Express 19, 18260-18271 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-19-18260


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References

  1. M. Ohtsu, K. Kobayashi, T. Kawazoe, T. Yatsui, and M. Naruse, eds., Principles of Nanophotonics, (Taylor and Francis, 2008).
  2. M. Ohtsu and K. Kobayashi, Optical Near Fields (Springer-Verlag, 2003), pp. 109–150.
  3. K. Kobayashi, S. Sangu, T. Kawazoe, and M. Ohtsu, “Exciton dynamics and logic operations in a near-field optically coupled quantum-dot system,” J. Lumin.112(1-4), 117–121 (2005). [CrossRef]
  4. M. Ohtsu, T. Kawazoe, T. Yatsui, and M. Naruse, “Nanophotonics: application of dressed photons to novel photonic devices and systems,” IEEE J. Sel. Top. Quantum Electron.14(6), 1404–1417 (2008). [CrossRef]
  5. T. Kawazoe, M. Ohtsu, S. Aso, Y. Sawado, Y. Hosoda, K. Yoshizawa, K. Akahane, N. Yamamoto, and M. Naruse, “Two-dimensional array of room-temperature nanophotonic logic gates using InAs quantum dots in mesa structures,” Appl. Phys. B103(3), 537–546 (2011). [CrossRef]
  6. C. R. Kagan, C. B. Murray, and M. G. Bawendi, “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B Condens. Matter54(12), 8633–8643 (1996). [CrossRef] [PubMed]
  7. S. A. Crooker, J. A. Hollingsworth, S. Tretiak, and V. I. Klimov, “Spectrally resolved dynamics of energy transfer in quantum-dot assemblies: towards engineered energy flows in artificial materials,” Phys. Rev. Lett.89(18), 186802 (2002). [CrossRef] [PubMed]
  8. M. Achermann, M. A. Petruska, S. A. Crooker, and V. I. Klimov, “Picosecond energy transfer in quantum dot Langmuir-Blodgett nanoassemblies,” J. Phys. Chem. B107(50), 13782–13787 (2003). [CrossRef]
  9. T. Franzl, D. S. Koktysh, T. A. Klar, A. L. Rogach, J. Feldmann, and N. Gaponik, “Fast energy transfer in layer-by-layer aAssembled CdTe nanocrystal bilayers,” Appl. Phys. Lett.84(15), 2904–2906 (2004). [CrossRef]
  10. T. Franzl, T. A. Klar, S. Schietinger, A. L. Rogach, and J. Feldmann, “Exciton recycling in graded gap nanocrystal structures,” Nano Lett.4(9), 1599–1603 (2004). [CrossRef]
  11. N. Tate, W. Nomura, T. Yatsui, T. Kawazoe, M. Naruse, and M. Ohtsu, “Parallel retrieval of nanometer-scale light-matter interactions for nanophotonic systems,” Nat. Comput.2, 298–307 (2010). [CrossRef]
  12. N. Sakakura and Y. Masumoto, “Persistent spectral-hole-burning spectroscopy of CuCl quantum cubes,” Phys. Rev. B56(7), 4051–4055 (1997). [CrossRef]
  13. Z. K. Tang, A. Yanase, T. Yasui, Y. Segawa, and K. Cho, “Optical selection rule and oscillator strength of confined exciton system in CuCl thin films,” Phys. Rev. Lett.71(9), 1431–1434 (1993). [CrossRef] [PubMed]
  14. T. Kawazoe, K. Kobayashi, J. Lim, Y. Narita, and M. Ohtsu, “Direct observation of optically forbidden energy transfer between CuCl quantum cubes via near-field optical spectroscopy,” Phys. Rev. Lett.88(6), 067404 (2002). [CrossRef] [PubMed]
  15. H. J. Calmichael, Statistical Methods in Quantum Optics 1. (Springer-Verlag, 1999).
  16. M. Naruse, H. Hori, K. Kobayashi, P. Holmström, L. Thylén, and M. Ohtsu, “Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions,” Opt. Express18(Suppl 4), A544–A553 (2010). [CrossRef] [PubMed]
  17. W. Nomura, T. Yatsui, T. Kawazoe, and M. Ohtsu, “The observation of dissipated optical energy transfer between CdSe quantum dots,” J. Nanophotonics1, 011591 (2007). [CrossRef]
  18. K. Kobayashi, S. Sangu, H. Ito, and M. Ohtsu, “Near-field optical potential for a neutral atom,” Phys. Rev. A63, 013806 (2000). [CrossRef]
  19. M. Han, X. Gao, J. Z. Su, and S. Nie, “Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules,” Nat. Biotechnol.19(7), 631–635 (2001). [CrossRef] [PubMed]
  20. M. Strassburg, M. Dworzak, H. Born, R. Heitz, A. Hoffmann, M. Bartels, K. Lischka, D. Schikora, and J. Christen, “Lateral redistribution of excitons in CdSe/ZnSe quantum dots,” Appl. Phys. Lett.80(3), 473–475 (2002). [CrossRef]
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