Three-dimensional Bandgap Confinement

Latest Research News, July 2007

To form functional active devices, various emitters have been incorporated into 3D photonic crystals (PCs). In this project quantum dots (QDs) with emission in telecommunication wavelength region were, for the first time, successfully infiltrated into 3D polymeric woodpile PCs. Time-resolved measurements revealed that the spontaneous emission from QDs were efficiently engineered by the bandgap effect of 3D PCs. Wavelength-dependent time decays have been observed for QDs both inside and outside the photonic crystals (PCs), i.e., the longer the wavelength, the slower the decay. Lifetime measurements have been conducted for QDs at different depths inside the PC (Fig. 1) and photonic band gap (PBG) effects on QD emission properties have been resolved in such a way. It has been indicated that at the midgap wavelength of the PBG (1580 nm in Fig. 1), the radiation is significantly inhibited (by 20%) for QDs in the centre part of the PC compared with QDs near the surface of the PC, i.e., the former showing a longer lifetime compared with the latter. Further investigations will be performed by varying the band gaps of the PC and by varying the detection positions and angles. This work has been accepted for publication in Advanced Materials [1].

July 07 Update Figure

Different lifetimes of infrared emission of PbSe QDs at different positions inside woodpile PC.

Reference
[1] J. Li, B. Jia, G. Zhou, C. Bullen, J. Serbin, and M.Gu, Spectral redistribution in spontaneous emission from quantum-dot-infiltrated 3D woodpile photonic crystals for telecommunications,” Advanced Materials, Accepted.

About the Project

Baohua Jia Min Gu
Project Manager: Baohua Jia Science Leader: Min Gu

Contributing staff: Yuri Kivshar (ANU), Lindsay Botten, Ara Asatryan, Michael Byrne, Andrew Norton (UTS), Judith Dawes, Mick Withford (Macquarie), Martijn de Sterke, Ross McPhedran, Mike Steel (Sydney), Jesper Serbin, Guangyong Zhou, Baohua Jia, Craig Bullen, Shuhui Wu (Swinburne)
Students: Michael Ventura, Jiafang Li (Swinburne), Aaron Matthews (ANU), Sam Myers (Macquarie)

Project Goals and Motivation
The aim of this project is to design, fabricate and characterise three dimensional photonic crystals (PCs) and related all-optical devices in a range of materials. The CUDOS team applies several direct laser writing methods for the fabrication of three-dimensional PCs with bandgaps in the near infrared wavelength regime. In particular, we have focused on three key developments:
(1) Developing 3D photonic bandgap material with full or partial gaps;
(2) Incorporating the Quantum dots (QDs) into the PCs;
(3) Demonstrating a significant change in radiation dynamics and/or non-linear effects.

Photonic bandgap materials are believed to be the basic materials in the next generation optical signal processing and communication systems because they can control and manipulate the behavior of light. By replacing electrons with photons as the carriers of information, the speed and bandwidth of the advanced communication systems will be dramatically increased. Such all-optics chips consist of various devices such as photonic crystal waveguides, superprisms, low threshold and directional emitters, as schematically shown in Figure 1.

Figure 1

Figure 1. A schematic diagram for an all-optical chip consisting of photonic devices that may be fabricated by the direct laser writing methods.

CUDOS strategy
CUDOS possesses unique expertise across its participating universities to conduct the cutting-edge research toward the arm of this flagship project. At Swinburne University two elegant and internationally unique experimental approaches, two-photon polymerisation and micro-explosion, have been developed to fabricate high quality 3D PCs with bandgaps in the near infrared region in polymer and lithium niobate. With the two-photon polymerisation method, a superprism based on the 3D photonic crystal has been achieved [1], showing a negative refraction effect [2].

Recent Achievements and Highlights
We have chosen PbS and PbSe quantum dots as the infrared radiation source because they can be conveniently introduced into the crystal matrix, and because they have strong emission which can be spectrally tuned by changing the dot size. Two different methods, doping and infiltration, have been used to incorporate the dots into the crystal matrix. In the doping approach, dots are combined with resin in its liquid phase to form a composite. 3D PCs are formed later by the two-photon polymerisation technique. In the infiltration process 3D PCs are developed first using the same approach, but the QDs are incorporated into the structures afterwards leading to a thin layer of QDs on the outside of each rod. In both methods the emission spectrum of the QDs matches the band gaps of the 3D PCs.

Figure 2

Figure 2. Illustration of the approach to the investigation into the radiation property from 3D PCs in 2006.

Another approach to incorporate radiation sources into two-dimensional photonic structures such as photonic crystal fibres was explored at Macquarie University, in which tapered photonic crystal fibres were filled with dye solution, excited and probed transversely to examine the angular, polarisation and spectral dependence of the emission [3].

Experimental efforts at both universities are closely linked to the theory groups at the University of Sydney, University of Technology Sydney, and RSoft Pty Ltd, an international leading software developer for photonics research. The modeling approach involves calculation of the local density of states in the woodpile using a combination of the commercially available RSoft FullWave for finite difference time-domain (FDTD) calculations, and a number of Mathematica packages developed by the UTS group and are looking at both time resolved and spatially resolved emission problems.

Uniform lead-salt quantum dots as the near infrared emission source: Lead-based QDs have strong emission in the near infrared region and the emission can be readily controlled by changing their sizes, but they are not commercially available. PbSe and PbS quantum dots were synthesised by Craig Bullen, Michael Ventura [4, 5] and Jiafang Li [6-8] with emission in a narrow band in the near infrared, indicating the narrow distribution of the size (see Figure 3). Polymer doping or infiltration is straightforward after surface modification. The high brightness and controllable emission spectroscopic property make these quantum dots an ideal emission source to study the radiation dynamics in PCs.

Figure 3

Figure 3. TEM image of PbS QDs synthesised by Michael Ventura as the near infrared emission source.

Radiation properties of PbSe QDs within the fundamental bandgap: The fundamental bandgap is normally wider than the higher order ones (if present) and should have a pronounced influence on the emission of QDs in that spectral range. Using the two-photon polymerization method, we fabricated 3D woodpile PCs of polymer with a fundamental bandgap covering the communication region of 1.3 to 1.6 µm. Jiafang Li successfully incorporated PbSe dots into these PCs through infiltration and doping. For the infiltration method, the dots can be added and removed flexibly, leading to a control¬lable shift in the band gap.

A change in the spectrum of the spontaneous emission from the dots was observed when they were infiltrated within the 3D PC (see the left column in Figure 4) [6, 7]. Evidence for partial bandgaps was found in the transmission spectrum, with up to 50% suppression ratio observed in structure fabricated in QD-doped resin (see the right column in Fig. 4) [8]. Angular-resolved and time-resolved measurements are currently being performed.

Figure 4

Figure 4. Left column: Normalised photoluminescence spectra from quantum dots inside PC and outside PC at the same excitation condition. The arrow indicates the spectral redistribution in the shorter wavelength range. Right column: Transmission spectra of 3D PCs with different lattice constants d. R is the suppression ratio.

Control of PbS QDs emission by using higher order photonic bandgaps: Higher order gaps are normally much narrower that the fundamental one and more sensitive to the crystal directions. By adjusting the lattice spacing, higher order gaps can be enhanced and controlled. Michael Ventura successfully developed a photopolymerisable PbS quantum dot-doped nanocomposite without losing emission efficiency. Homogeneous materials with a film thickness of up to a few hundreds of micrometres were formed, sufficient to fabricate 3D PCs [4, 5]. Woodpile PCs were fabricated within these nano-composites with higher order gaps overlapping the emission range of the dots. A highly sensitive infrared measurement system was set up to measure fluorescence lifetimes of the woodpile structures (Figure 5).

Figure 5

Figure 5. Michael Ventura measuring the lifetime of PbS QDs inside a 3D photonic crystal.

Characterisation of emission from 3D PCs with scanning near field optical microscopy: Dr. Baohua Jia has characterised 3D woodpile PCs using a scanning near-field optical microscope (SNOM) to detect the evanescent signals in the near-field region (~10 nm from the sample). She examined different photonic crystal structures and optimised the fabrication process using both the topographic and optical signals from the SNOM, as shown in Figure 6 [8]. Her next step is to combine this technique with lifetime measurement systems to characterise the radiation dynamics of QDs in the near field region.

Figure 6

Figure 6. Topography signals (a, c, e) and optical signals (b, d, f) recorded simultaneously with a SNOM from 3D woodpile structures fabricated with different conditions. (a, b): The voids between the rods were totally filled with resin. (c, d): The voids between the rods were partially filled with resin. (e, f): A well-developed woodpile PC with a high quality PBG.

Theoretical modeling of radiation dynamics: The effect of photonic crystals and other nano-structures on spontaneous emission rates can be inferred from calculations of the Local Density of States (LDOS), a classical quantity measuring the number of available electromagnetic modes into which the created photon can be emitted. As photonic structures become more complicated the numerical calculations become more lengthy and so efficient algorithms and procedures with generally available tools are required. In collaboration with Marc Dignam of Queen's University, we developed new techniques for the calculation of LDOS by finite-difference time-domain (FDTD) simulation.

This work [9], published in Physical Review Letters, contains two strands. In the first, we formalized existing techniques for LDOS calculation by FDTD into a more rigorous theory. This provided a basis for studying the effect that the initial conditions have on the calculated LDOS. The second strand examined LDOS in coupled cavities and showed that by introduction of a "reduced-LDOS", the system can be projected onto a basis of small number of modes, massively improving the efficiency of calculations. These techniques will be important in calculating radiation effects in 3D crystals where the computational load of the complete problem is extreme.

Radiation dynamics from two-dimensional photonic crystal fibre: Emission from 2D photonic crystals was also studied. In the 2D case, tapered photonic crystal fibres were filled with dye solution, excited and probed transversely to examine the angular, polarisation and spectral dependence of the emission. Researchers at University of Sydney and Macquarie University collaborated to create tapered hollow-core photonic crystal fibres. Theorists from the University of Sydney and Queens University, Canada modeled the local density of states to explain both enhancements and suppression of emission in various directions and for various wavelengths. This work was published in Optics Express [3].

References
[1] Serbin J, Gu M, Experimental Evidence for Superprism Effects in Three-Dimensional Polymer Photonic Crystals ADVANCED MATERIALS 18, 221-224, 2006
[2] Serbin J, Gu M Superprism phenomena in waveguide-coupled woodpile structures fabricated by two-photon polymerization OPTICS EXPRESS 14, 3563-3568 2006
[3] Myers SJ, Fussell D, Dawes JM, Mägi E, Eggleton BJ, McPhedran RC, de Sterke CM Manipulation of spontaneous emission in a tapered photonic crystal fibre OPTICS EXPRESS 14, 12439-12444 2006 [4] Michael J. Ventura, Craig Bullen and Min Gu, PbSe quantum dots embedded in three-dimensional photonic crystals, SPIE Photonics Conferences Europe, 4-6 April 2006, Strasbourg, France.
[5] Ventura MJ, Bullen C, Gu M Direct laser writing of three-dimensional photonic crystal lattices within PbS quantum-dot-doped polymer material OPTICS EXPRESS 15, 1817-1822 2007
[6] Jiafang Li, Jesper Serbin, Craig Bullen, Min Gu Incorporation of quantum dots into 3D photonic crystals for emission control, International Conference on Nanoscience and Nanotechnology, Brisbane, Australia, July 3-7, 2006.
[7] Jiafang Li, Baohua Jia, Craig Bullen, Jesper Serbin, and Min Gu, “Spectral redistribution and optical gain in spontaneous emission from quantum-dot-infiltrated 3D woodpile photonic crystals for telecommunications," Advanced Materials, 2006
[8] Li J, Jia B, Zhou G, Gu M Fabrication of three-dimensional woodpile photonic crystals in a PbSe quantum dot composite material OPTICS EXPRESS 14, 10740-10745, 2006
[9] Dignam MM, Fussell DP, Steel MJ, de Sterke CM, McPhedran RC Spontaneous Emission Suppression via Quantum Path Interference in Coupled Microcavities PHYSICAL REVIEW LETTERS 96, 103902, 2006