Letter | Published:

Ultrafast spin-lasers

Abstract

Lasers have both ubiquitous applications and roles as model systems in which non-equilibrium and cooperative phenomena can be elucidated1. The introduction of novel concepts in laser operation thus has potential to lead to both new applications and fundamental insights2. Spintronics3, in which both the spin and the charge of the electron are used, has led to the development of spin-lasers, in which charge-carrier spin and photon spin are exploited. Here we show experimentally that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 gigahertz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation of the resultant spin-laser relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both of which are commonly viewed as detrimental in spintronics3 and conventional lasers4. Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication.

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The data sets generated and analysed in this work are available from the corresponding author on reasonable request.

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Acknowledgements

For supporting this work the authors thank the German Research Foundation (grant nos GE1231/2-2 and MI607/9-2), the US National Science Foundation (grant nos ECCS-1508873 and ECCS-1810266) and the US Office of Naval Research (grant no. 000141712793).

Reviewer information

Nature thanks Daniel Dolfi, Hiro Munekata and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

N.C.G. and M.R.H. conceived the project idea. M.L. planned and conducted the experiments and experimental analysis, and characterized the measurement system. T.P. developed the advanced laser mount and performed the device processing. R.M. devised methods of birefringence tuning of VCSELs and was involved in the device optimization. G.X. and I.Ž. developed the generalized model for simulation. M.L., G.X. and N.C.G. performed the numerical simulations. M.L., G.X., I.Ž. and N.C.G. wrote the manuscript. All authors discussed the results and revised the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Markus Lindemann or Nils C. Gerhardt.

Extended data figures and tables

  1. Extended Data Fig. 1 Strain-dependent behaviour.

    a, Strain Δl/l and the resulting mode splitting Δf. Points show determined values, lines are their direct connections and are included for improved visualization. b, Spectrum without applied external strain. The vertical lines mark the maxima of the two orthogonal modes. The arrow indicates the resulting Δf. The solid line is a smoothed curve through the raw data (points). c, Corresponding polarization oscillation in the circular polarization degree PC with a frequency of 9.2 GHz. The solid line is a smoothed curve through the raw data (points).

  2. Extended Data Fig. 2 Acquisition timing.

    Shown is the trace of the intensity obtained with the streak camera for a long measurement window (grey) and the trace of the circular polarization degree for a short measurement window with higher temporal resolution (black). The positioning of the shorter measurement window for acquisition of fast polarization oscillations over the full scale of the dynamic process caused by the spin injection pulse is evident.

  3. Extended Data Fig. 3 Modulation efficiency.

    The figure shows how polarization oscillation amplitudes depend on the birefringence-induced mode splitting. Grey trace, calculated amplitude APO,d derived from the spectrum by relating the intensities of lasing and non-lasing modes. Black trace, actual polarization oscillation amplitude APO,s from the temporal traces. Points mark raw data, lines show exponential fits. For splittings between 130 and 200 GHz no data are shown as the amplitude is quite close to the noise level of the streak camera. The data points in Fig. 2c were obtained by Fourier transform from these data. At 214 GHz the streak camera was used in another mode of measurement offering a better signal-to-noise ratio. For constant applied spin-injection, the resulting polarization oscillation amplitude and thus the modulation efficiency decreases with increasing mode splitting.

  4. Extended Data Fig. 4 Dichroism dependent polarization modulation response curves.

    Plots show simulated modulation (Mod.) response as a function of polarization modulation frequency for different values of dichroism γa (see key). The dashed green line marks the −3dB level as a measure for the lower limit of the desired modulation response.

  5. Extended Data Fig. 5 Experimental polarization modulation and its birefringence dependence.

    a, Results for several values of the birefringence-induced mode splitting, Δf (see key). Points mark raw data, lines are smoothed data. b, Numerical verification of results in a using the parameter set obtained in this work for the appropriate values of γp (see key).

  6. Extended Data Table 1 List of symbols used in the simulations
  7. Extended Data Table 2 Spin-flip model parameters for Fig. 2
  8. Extended Data Table 3 Spin-flip model parameters for Figs. 3a, b, 4a, b
  9. Extended Data Table 4 Spin-flip model parameters for Fig. 3c, d
  10. Extended Data Table 5 Spin-flip model parameters for Extended Data Fig. 4

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About this article

Fig. 1: Birefringent VCSEL and measurement set-up.
Fig. 2: Polarization behaviour of spin-VCSELs.
Fig. 3: Advantage of polarization modulation in dynamic performance.
Fig. 4: Influences on modulation bandwidth.
Extended Data Fig. 1: Strain-dependent behaviour.
Extended Data Fig. 2: Acquisition timing.
Extended Data Fig. 3: Modulation efficiency.
Extended Data Fig. 4: Dichroism dependent polarization modulation response curves.
Extended Data Fig. 5: Experimental polarization modulation and its birefringence dependence.

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