- Nano Express
- Open Access
Optical characterization of In-flushed InAs/GaAs quantum dots emitting a broadband spectrum with multiple peaks at ~1 μm
© Kitamura et al.; licensee Springer. 2015
- Received: 22 January 2015
- Accepted: 15 May 2015
- Published: 27 May 2015
We investigated optical properties of In-flushed InAs quantum dots (QDs) grown on a GaAs substrate by molecular beam epitaxy. By using the In-flush technique for setting the height of self-assembled InAs QDs, we have tuned the emission wavelength of InAs QDs to the ~1 μm regime, which can be utilized as a non-invasive and deeply penetrative probe for biological and medical imaging systems. The controlled emission exhibited a broadband spectrum comprising multiple peaks with an interval of approximately 30 meV. We examined the origin of the multiple peaks using spectral and time-resolved photoluminescence, and concluded that it is attributed to monolayer step fluctuations in the height of the In-flushed QDs. This feature can be advantageous for realizing a broadband light source centered at the ~1 μm regime, which is especially suitable for the non-invasive cross-sectional biological and medical imaging system known as optical coherence tomography.
- Quantum dot
- Time-resolved PL
- Broadband light source
Optical biological/medical imaging system that uses near-infrared (NIR) light has been developed as a non-invasive and deeply penetrative diagnosis method. In particular, the NIR light of ~1 μm wavelength has a relatively long penetration depth in living aqueous tissues, since the absorption of light by the main ingredients of tissue, oxyhemoglobin (HbO2) and water (H2O), is minimized at 1.05 μm . Among the imaging systems, optical coherence tomography (OCT)  has been extensively developed and widespread in various medical fields over the last few decades . OCT is a non-invasive cross-sectional imaging system based on a low-coherence interferometer. The axial resolution of OCT is governed by the center wavelength and bandwidth of the light source, e.g., the axial resolution given by the expression 0.44 × λ 0 2/Δλ for a light with a Gaussian spectral shape centered at λ 0 and a bandwidth of Δλ . Thus, the development of a broadband light source with a 1-μm-centered wavelength is crucial for obtaining OCT images with a high-resolution and a large penetration depth in biological and medical samples. For instance, light with a bandwidth of 100 nm centered at an emission wavelength of 1.05 μm can be expected to have an axial resolution of 4.85 μm, which is comparable with cell sizes in biological samples.
So far, we have developed the NIR broadband light source based on self-assembled InAs quantum dots (QDs) for OCT light source [5–7]. The InAs QDs are derived from strain induced by a lattice mismatch between the deposited InAs and the GaAs substrate, and the ensemble of QDs has distributions of size and composition. Thus, the ensemble of InAs QDs emits a broadband spectrum because of inhomogeneous broadening and has been recognized as a good material for OCT light sources . Recently, a broadband light source based on InAs QDs was developed and achieved a bandwidth of over 200 nm [9–12]. However, the InAs/GaAs QDs typically emit light of approximately 1.2–1.3 μm wavelength; therefore, we have developed various methods to control the emission wavelength of InAs QDs . For the adjustment of the emission wavelength to the 1 μm regime, we have utilized the In-flush technique [14, 15]. The In-flush technique was previously developed for setting the QD height to a constant value and obtaining a highly homogeneous QD emission wavelength . This method can also be applied to the control of the InAs QD emission wavelength and enables the creation of a broadband light source by combining the emission–wavelength-controlled QDs [14, 15, 17–19]. In our previous work regarding the control of the emission wavelength via the In-flush method, we have found that the In-flushed QDs exhibit multiple emission peaks in the emission spectrum at the ~1 μm regime . This result implies a unique optical feature of the In-flushed QDs, which differ from the conventional InAs QDs, and the emission could be suitable for a broadband light source emitting at the 1 μm regime. In this work, we investigate the origin of the multiple emission peaks of the In-flushed QDs through spectral and temporal photoluminescence (PL) measurements, and discuss the potential of the In-flushed QDs for applications to the OCT light source.
On the other hand, the In-flushed QDs emit a PL spectrum centered at approximately 1.17 eV (λ = 1.06 μm) with multiple peaks, which can be fitted with a Gaussian function centered at 1.11, 1.14, 1.17, and 1.20 eV with a FWHM of 29 meV, as labeled A–D in Fig. 3b. The fitting line is indicated by a gray dashed line. The weak emission peak at approximately 1.31 eV (λ = 0.950 μm) is due to the InAs wetting layer (WL). The emission wavelength, blue-shifted to the 1.05 μm regime, indicates the controllability of the In-flush method. The multiple peaks with intervals of approximately 30 meV, which were also seen in our previous report , can be attributed to the step-like height fluctuation of the In-flushed QDs. As schematically shown in Fig. 3c, the height of the In-flushed QDs decreases to d cap, which is approximately 2 nm (6 ML), and they have a truncated pyramid (disk-like) shape. The decrease in the ratio of the height to base diameter (approximately 48 nm) should result in a change in the quantum confinement of electrons and holes from a three-dimensional confinement to a quasi-one-dimensional strong confinement in the height direction. This results in a large change in the bandgap energy of the In-flushed QDs with a variation of even 1 ML in height. Thus, the emission peak energy should be divided with step values corresponding to the ML-step height. Bimberg et al. have also reported the similar multi-peak emission due to the ML-step height fluctuation in a shell-like formation of InAs QDs by the metal–organic chemical vapor deposition method [21, 22]. They reported the interval of emission energy for one ML of the truncated QD as 29–51 meV and the energy shift between the truncated QDs with heights of 6–7 ML as 32 meV. This value is close to our observed interval energy between the peaks. Moreover, the reported FWHM of the peak was 29 meV, which is almost the same as our measured value. The decrease in the FWHM from that of as-grown QDs implies that the transformation of the QD structure to a quasi-one-dimensional confinement structure resulted in a reduction of the inhomogeneous broadening. The remaining inhomogeneous broadening may be due to the distribution of In compositions and the base diameters of the QDs, causing lateral quantum confinement.
From the aforementioned spectral and time-resolved PL results, we conclude that the multiple emission peaks that appear in the PL spectra obtained from the In-flushed QDs originate from multiple emission centers with ML-step fluctuations of the height of the truncated QDs. These multiple centers result in emission broadening and are useful for realizing a broadband light source, which is especially applicable to the OCT. The bandwidth of the In-flushed QDs is approximately 75 nm with contributions from only the GS emissions and increases up to 100 nm with the addition of ES emissions. This indicates that an axial resolution that is less than 5 μm in air can be achieved using this as a light source in OCT system. Furthermore, precise control of the emission wavelength can be executed by the variation of d cap, as previously reported [14, 15], and further broadening and control of the center wavelength can be expected. Thus, the In-flushed QDs offer a novel approach for developing a broadband light source centered at 1.05 μm. In addition, this approach using the In-flushed method might be effective for other In-incorporated QD systems.
The optical properties of In-flushed QDs have been investigated using spectral and time-resolved PL measurements. The RT–PL spectrum shows multi-peak emissions from the In-flushed QDs. The emission energy intervals and the excitation power dependence of the multi-peak emissions indicate that the peaks originate from the GS emissions of In-flushed QDs with ML-step height fluctuation. The time-resolved PL measurements also demonstrate that the multi-peak emissions have almost identical decay times and are similar to the typical decay time of the GS of QDs. This feature of the In-flushed QDs can be suitable for broadband emission centered at the ~1 μm range.
This study was partly supported by Grants-in-Aid for Scientific Research (KAKENHI) (Grant Number 25286052), the Canon Foundation, the CASIO Science Promotion Foundation.
- Patterson MS, Wilson BC, Wyman DR. The propagation of optical radiation in tissue. II: optical properties of tissues and resulting fluence distributions. Lasers Med Sci. 1991;6:379–90.Google Scholar
- Huang D, Swanson EA, Lin CP, Schuman JS, Stinson WG, Chang W, et al. Optical coherence tomography. Science. 1991;254:1178–81.View ArticleGoogle Scholar
- Drexler W, Liu M, Kumar A, Kamali T, Unterhuber A, Leitgeb RA. Optical coherence tomography today: speed, contrast, and multimodality. J Biomed Opt. 2014;19(071412):1–34.Google Scholar
- Brezinski ME. Optical coherence tomography: principles and applications. Burlington: Academic Press; 2006.Google Scholar
- Ozaki N, Takeuchi K, Ohkouchi S, Ikeda N, Sugimoto Y, Oda H, et al. Monolithically grown multi-color InAs quantum dots as a spectral-shape-controllable near-infrared broadband light source. Appl Phys Lett. 2013;103(051121):1–4.Google Scholar
- Ozaki N, Yasuda T, Ohkouchi S, Watanabe E, Ikeda N, Sugimoto Y, et al. Near-infrared superluminescent diode using stacked self-assembled InAs quantum dots with controlled emission wavelengths. Jpn J Appl Phys. 2014;53(04EG10):1–4.Google Scholar
- Shibata H, Ozaki N, Yasuda T, Ohkouchi S, Ikeda N, Ohsato H, et al. Imaging of spectral-domain optical coherence tomography using a superluminescent diode based on InAs quantum dots emitting broadband spectrum with Gaussian-like shape. Jpn J Appl Phys. 2015;54(04DG07):1–5.Google Scholar
- Sun Z, Ding D, Gong Q, Zhou W, Xu B, Wang ZG. Quantum-dot superluminescent diode: a proposal for an ultra-wide output spectrum. Opt Quant Electron. 1999;31:1235–46.Google Scholar
- Chen SM, Zhou KJ, Zhang ZY, Childs DTD, Hugues M, Ramsay AJ, et al. Ultra-broad spontaneous emission and modal gain spectrum from a hybrid quantum well/quantum dot laser structure. Appl Phys Lett. 2012;100(041118):1–3.Google Scholar
- Chen S, Zhou K, Zhang Z, Orchard JR, Childs DTD, Hugues M, et al. Hybrid quantum well/quantum dot structure for broad spectral bandwidth emitters. IEEE J Sel Top Quant Electron. 2013;19(1900209):1–9.Google Scholar
- Zhou KJ, Jiang Q, Zhang ZY, Chen SM, Liu HY, Lu ZH, et al. Quantum dot selective area intermixing for broadband light sources. Opt Exp. 2012;20:26950–7.Google Scholar
- Lv X, Jin P, Chen H, Wu Y, Wang F, Wang Z. Broadband light emission from chirped multiple InAs quantum dot structure. Chin Phys Lett. 2013;30(118102):1–4.Google Scholar
- Ozaki N, Takeuchi K, Hino Y, Nakatani Y, Yasuda T, Ohkouchi S, et al. Integration of emission-wavelength controlled InAs quantum dots for ultrabroadband near-infrared light source. Nanomater Nanotechnol. 2014;4(26):1–17.Google Scholar
- Hino Y, Ozaki N, Ohkouchi S, Ikeda N, Sugimoto Y. Growth of InAs/GaAs quantum dots with central emission wavelength of 1.05 μm using In-flush technique for broadband near-infrared light source. J Cryst Growth. 2013;378:01–505.Google Scholar
- Ozaki N, Hino Y, Ohkouchi S, Ikeda N, Sugimoto Y. Broadband emission centered at ~1μm with a Gaussian-like spectrum by stacking In-flushed QD layers. Phys Status Solidi C. 2013;10:1361–4.Google Scholar
- Wasilewski ZR, Fafard S, McCaffrey JP. Size and shape engineering of vertically stacked self-assembled quantum dots. J Cryst Growth. 1999;201/202:1131–5.Google Scholar
- Haffouz S, Raymond S, Lu ZG, Barrios PJ, Roy-Guay D, Wu X, et al. Growth and fabrication of quantum dots superluminescent diodes using the indium-flush technique: a new approach in controlling the bandwidth. J Cryst Growth. 2009;311:1803–6.Google Scholar
- Haffouz S, Rodermans M, Barrios PJ, Lapointe J, Raymond S, Lu Z, et al. Broadband superluminescent diodes with height-engineered InAs-GaAs quantum dots. Electron Lett. 2010;46:1144–5.Google Scholar
- Haffouz S, Barrios PJ, Normandin R, Poitras D, Lu Z. Ultrawide-bandwidth, superluminescent light emitting diodes using InAs quantum dots of tuned height. Opt Lett. 2012;37:1103–5.Google Scholar
- Leonardo D, Krishnamurthy M, Reaves CM, Denbaars SP, Petroff PM. Direct formation of quantum-sized dots from uniform coherent islands of InGaAs on GaAs surfaces. Appl Phys Lett. 1993;63:3203–5.Google Scholar
- Guffarth F, Heitz R, Schliwa A, Pötschke K, Bimberg D. Observation of monolayer-splitting for InAs/GaAs quantum dots. Phys E. 2004;21:326–30.Google Scholar
- Heitz R, Guffarth F, Pötschke K, Schliwa A, Bimberg D, Zakharov ND, et al. Shell-like formation of self-organized InAs/GaAs quantum dots. Phys Rev B. 2005;71(045325):1–7.Google Scholar
- Buckle PD, Dawson P, Hall SA, Chen X, Steer MJ, Mowbray DJ, et al. Photoluminescence decay time measurements from self-organized InAs/GaAs quantum dots. J Appl Phys. 1999;86:2555–61.Google Scholar
- Heitz R, Veit M, Ledentsov NN, Hoffmann A, Bimberg D, Ustinov VM, et al. Energy relaxation by multiphonon processes in InAs/GaAs quantum dots. Phys Rev B. 1997;56:10435–45.Google Scholar
- Alder F, Geiger M, Bauknecht A, Scholz F, Schweizer H, Pikuhn MH, et al. Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots. J Appl Phys. 1996;80:4019–26.Google Scholar
- Siegert J, Marcinkevičius S, Zhao QX. Carrier dynamics in modulation-doped InAs/GaAs quantum dots. Phys Rev B. 2005;72(085316):1–7.Google Scholar
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