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Virgina Tech, Blacksburg VA
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Research Subjects

Quantum Dot (QD) Solar Cells

  • In this research project InAs quantum dots (QD) solar cells are studied and investigated. The theory of operation of QDs acting as an intermediate band in a GaAs-based solar cell is the primary motivation. The main interest behind QD solar cells is that it has been shown theoretically that the calculated efficiency can reach as high as 63% [1].

  • MBE and MOCVD sample growth options are available. The device fabrication and characterization is carried out at CHTM. Measurements of interest:

    • Illuminated solar cell I-V characteristics

    • Solar cell spectral response

Solar cells

Figure 1. Processed solar cell sample

Char solar cells

Figure 2. Solar cell I-V characteristics under illumenation at 100.0mW/cm2

Relevant papers

  • [1] A. Luque, A. Marti, Phys. Rev. Let., 78, pp. 5014-5017, (1997).

  • R. B. Laghumavarapu, A. J. Moscho, A. Khoshakhlagh, M. El-Emawy, L. F. Lester and D. L. Huffaker, "GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response", Applied Physics Letters, 90(17), p. 173125, (2007).

  • R. B. Laghumavarapu, M. El-Emawy, N. Nuntawong, A. J. Moscho, L. F. Lester and D. L. Huffaker, "Improved device performance of InAs/GaAs quantum dot solar cells with GaP strain compensation layers", Applied Physics Letters, 91(24), p. 243115, (2007).

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InAs/InGaAs dots-in-a-well (DWELL) flexible thin film solar cell

    Lightweight, Flexible Thin Film QD Solar Cells for Micro-Autonomous Systems Technology (MAST) Applications. The goal here is to study thin film solar cells, which can be attached to a micro-autonomous flying robot (COMBAT) for power generation. Therefore, watt/g has become a parameter of interest because weight becomes an important factor during flight and most importantly at liftoff.

         Advantages for using DWELL flexible solar cells:  

               Increase efficiency 2.5X over existing thin-film amorphous silicon

               Increase power output by 3X

               Increase specific power (Watt/g) by more than 20X

               Much higher efficiency

               Ultra lightweight

Solar cells

 Animated drawing for the autonomous flying system (COMBAT), showing the solar cells on the wing.  

Char solar cells

Thin film solar technology comparison between amorphous silicon base solar cells and MAST GaAs based solar cells.

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Passively Mode-Locked Quantum Dot Lasers  Pdf

  • The compact size, low power consumption, direct electrical pumping of semiconductor mode-locked lasers make them best suited for inter-chip/intra-chip clocking as well as other applications including high bit-rate optical time division multiplexing, electro-optic sampling, and impulse response measurement of optical components. Although monolithic diode laser pulse sources offer such advantages, they have generally not had as good pulse quality. Pulse duration has been longer, stability impaired, pulse asymmetric, and peak power compromised. Thus, research is necessary to improve the characteristic of semiconductor mode-locked lasers.

  • Quantum dot (QD) lasers are ideal choice for semiconductor monolithic mode-locked lasers because of their unique characteristics, such as ultrabroad bandwidth, ultrafast gain dynamics, easily saturated absorption, strong inversion and wide gain bandwidth.


The device layout of an absorber-passive-gain mode-locked quantum dot laser


Schematic diagram of the mode-locked quantum dot laser measurement setup

Relevant papers

  • Y. C. Xin, C. Y. Lin, Y. Li, H. P. Bae, H. B. Yuen, M. A. Wistey, J. S. H. Jun, S. R. Bank and L. F. Lester, "Monolithic 1.55 mm GaInNAsSb quantum well passively modelocked lasers", Electronics Letters, 44(9), pp. 581-582, (2008).

  • Y. C. Xin, Y. Li, V. Kovanis, A. L. Gray, L. Zhang and L. F. Lester, "Reconfigurable quantum dot monolithic multisection passive mode-locked lasers", Optics Express, 15(12), pp. 7623-7633, (2007).

  • Y. C. Xin, Y. Li, A. Martinez, T. J. Rotter, H. Su, L. Zhang, A. L. Gray, S. Luong, K. Sun, Z. Zou, J. Zilko, P. M. Varangis and L. F. Lester, "Optical gain and absorption of quantum dots measured using an alternative segmented contact method", IEEE Journal of Quantum Electronics, 42, pp. 725-732, (2006).

  • L. Zhang, L. Cheng, A. L. Gray, S. Luong, J. Nagyvary, F. Nabulsi, L. Olona, K. Sun, T. Tumolillo, R. Wang, C. Wiggins, J. Zilko, Z. Zou, P. M. Varangis, H. Su and L. F. Lester, "Low timing jitter, 5 GHz optical pulses from monolithic two-section passively mode-locked 1250/1310 nm quantum dot lasers for high-speed optical interconnects", Optical Fiber Communication Conference, 2005. Technical Digest. OFC/NFOEC, vol.3, 6-11 March 2005.

  • X. D. Huang, A. Stintz, H. Li, L. F. Lester, J. Cheng and K. J. Malloy, "Passive mode-locking in 1.3 m two-section InAs quantum dot lasers", Applied Physics Letters, 78(19), pp. 2825-2827, (2001).

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Injection locking

  • Injection Locking is shown to enhance several parameters of free-running semiconductor lasers:

    • Single mode performance and improved side mode suppression

    • Enhanced modulation bandwidths and increased relaxation oscillation frequencies

    • Reduce relative intensity noise and chirp

  • Can be used to measure the linewidth enhancement factor of a semiconductor laser above threshold


  • When the frequencies of the two lasers are sufficiently close, a phase locking phenomenon occurs, and the frequency of the slave (fs) laser is locked to the master frequency (fm):

    • Slave laser's tested in Lester's Group: multi-mode quantum dash Fabry-Perot lasers, single mode quantum dash distributed feedback lasers, etc.

    • Master laser: high power (strong injection), narrow linewidth

    • Motivation is to enhance the modulation bandwidth of the slave laser and to study of how the linewidth enhancement factor varies above threshold

  • Goals include:

    • Develop a mathematical tool that can be used to simulate an injection locked system under modulation and allow operating parameters to be extracted

      • Calculate/predict operating conditions (injection strength, detuning frequency) to achieve maximum modulation bandwidths

  • Uniqueness of Lester's Group Injection Locking Research Efforts:

    • Collaboration with the Air Force Research Laboratory

    • Top of the line experimental equipment:

      • 1550 nm external cavity tunable diode laser

      • 1550 nm erbium doped fiber amplifer

      • High resolution optical spectrum analyzer (10 pm) (Ando AQ6317C)

      • Hewlett Packard HP 8722D Network Analyzer

      • Various DFB master lasers in butterfly package at 1310 and 1550 nm

      • Polarization maintaining circulator


Typical Experimental Setup

Demonstration of Single Mode Performance and Improved Side Mode Suppression:

Fabry Perot

Locked Fabry Perot Semiconductor Laser (top); Free Running Fabry Perot Semiconductor Laser (bottom)

Enhanced Modulation Bandwidths and Increased Relaxation Oscillation Frequencies:


Typical modulation response results for a given injection strength and varied frequency detuning.
An ~3x improvement of the modulation bandwidth of the coupled system is observed

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Optical feedback in semiconductor lasers  Pdf

  • The implementation of optics into the metropolitan network is today made difficult by the price of the optical components. One of the main source of cost originates from the need for optical isolation. It is well known that the performances of a semiconductor laser are strongly altered by any source of external optical feedback. The laser sensitivity can be such that even under a feedback level in the percent range, the laser becomes unstable and starts operating within the so-called coherence collapse regime. The main consequence of such a regime on the semiconductor laser is a drastic enhancement of the laser linewidth up to several GHz which is very detrimental to most applications.

  • The dramatic variation in the linewidth enhancement factor (alpha parameter) that has been reported for quantum dot lasers makes them an interesting subject for optical feedback studies. A low alpha parameter combined with a high damping factor is especially interesting because it should increase the tolerance to optical fee dback in these devices and may offer potential advantages for direct modulation. In the particular case of QD lasers, the carrier density and distribution are not clearly clamped at threshold. The lasing wavelength can switch from the ground state (GS) to the excited state (ES) as the current injection increases meaning that a carrier accumulation occurs in the ES even though lasing in the GS is still occurring. The filling of the ES inevitably enhances the alpha of the GS above threshold. Consequently, this strong variation of the GS alpha, which is much more important compared to QW devices, should theoretically produce a significant variation in the onset of coherence collapse due to feedback. Under specific conditions, i.e., in the case of a strong enhancement in the alpha parameter, the feedback sensitivity of the laser can strongly vary within the same device.


Schematics of the experimental apparatus for the feedback measurements

Coherence collapse

Coherence collapse thresholds as a function of the bias current for a quantum dash Fabry-Perot laser. After [1].

Relevant papers

  • F. Grillot, N. Naderi, M. Pochet, C.-Y. Lin and L. F. Lester, "Variation of the Feedback Sensitivity in a 1.55?m InAs/InP Quantum-Dash Fabry-Perot Semiconductor Laser", Applied Physics Letters, 93, (2008).

  • F. Grillot, B. Dagens, J. G. Provost, H. Su and L. F. Lester, "Gain compression and above-threshold linewidth enhancement factor in 1.3 m InAs-GaAs quantum dot lasers", IEEE Journal of Quantum Electronics, 44(10), pp. 946-951, (2008).

  • F. Grillot, G. H. Duan and B. Thedrez, "Feedback sensitivity and coherence collapse threshold of semiconductor DFB lasers with complex structures", IEEE Journal of Quantum Electronics, 40(3), pp. 1-11, (2004).

  • F. Grillot, B. Thedrez, V. Voiriot and J. L. Lafragette, "Coherence collapse threshold of 1.3 m semiconductor DFB lasers", IEEE Photonics Technology Letters, 1, pp. 1-3, (2003).

  • F. Grillot, B. Thedrez, J. Py, O. Gauthier-Lafaye, V. Voiriot and J. L. Lafragette, "2.5 Gb/s transmission characteristics of 1.3 m DFB lasers with external optical feedback", IEEE Photonics Technology Letters, 14, pp. 101-103, (2002).

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