The appeal of lasers can be attributed to both their ubiquitous applications and their role as model systems for elucidating nonequilibrium and cooperative phenomena [1]. Introducing novel concepts in lasers thus has a potential for both applied and fundamental implications [2]. Here we experimentally demonstrate that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 GHz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both commonly viewed as detrimental in spintronics [3] and conventional lasers [4]. 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.The global internet traffic will continue its dramatic increase in the near future [5]. Short-range and energy-efficient optical communication networks provide most of the communication bandwidth to secure the digital revolution. Key devices for high-speed optical interconnects, in particular in server farms, are current-driven intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) [4]. Analogous to a driven damped harmonic oscillator, modulated lasers have a resonance frequency f R for the relaxation oscillations of the light intensity [6]. For higher frequencies the response decays and reaches half of its low-frequency value at f 3dB ≈ 1 + √ 2f R , which quantifies the usable frequency range [4]. In conventional VCSELs the modulation bandwidth is limited by the dynamics of the coupled carrier-photon system and parasitic as well as thermal effects. The current record is f 3dB = 34 GHz [7]. Common approaches to enhance the bandwidth rely on the expression f R = v g aS/τ p /(2π), where v g is the group velocity, a the differential gain, S the photon density, and τ p the * markus.lindemann@rub.de † nils.gerhardt@rub.de arXiv:1807.02820v1 [cond-mat.mes-hall]

Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) offer a high potential to overcome several limitations of conventional purely charged-based laser devices. Presumably, the highest potential of spin-VCSELs lies in their ultrafast spin and polarization dynamics, which can be significantly faster than the intensity dynamics in conventional devices. Here, we experimentally demonstrate polarization oscillations in spin-VCSELs with frequencies up to 44 GHz. The results show that the oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. A tuning range of about 34 GHz is demonstrated. By measuring the polarization oscillation frequency and the birefringence governed mode splitting as a function of the applied strain simultaneously, we are able to investigate the correlation between birefringence and polarization oscillations in detail. The experimental findings are compared to numerical calculations based on the spin-flip model.

Using the elasto-optic effect, increase of the frequency difference between the two orthogonally polarised modes, the so-called birefringence splitting, in standard single-mode oxide-confined AlGaAs-based vertical-cavity surface-emitting lasers is achieved to values beyond 250 GHz. A large birefringence is required for the generation of ultra-fast polarisation oscillations for potential future high-speed communication applications.

Increasing the birefringence splitting in single-mode vertical-cavity surface-emitting lasers (VCSELs) enables high-speed polarisation dynamics which can be the basis to overcome the current bandwidth limitations in short-haul data transmission. The authors observe large birefringence splittings of up to 98 GHz in an oxide-confined AlGaAs-based VCSEL with a tailored integrated surface grating. Since surface gratings are routinely used in VCSEL production, there is a great potential of this technique to realise spin-VCSELs for ultrafast optical communication.

The birefringence splitting in vertical-cavity surface-emitting lasers offers an opportunity for spintronic-based high-frequency operation. By means of coupling of the carrier spin in the active region with the photons of the laser mode, the device can be excited to oscillations in the degree of circular polarization with a frequency corresponding to the birefringence splitting. On-chip frequency tunability of those oscillations is desirable for future applications. By asymmetric current-induced heating using the elasto-optic effect, we demonstrate a reversible tuning of the birefringence splitting of 45 GHz with less than 3 dB output power penalty.

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We investigate and compare the intensity and polarization dynamics in a vertical-cavity surface-emitting laser (VCSEL) with a monolithically integrated, electrically controlled birefringence tuning mechanism. The influence of the bias current on the polarization dynamics is investigated over a large range of birefringence values. Bias current tuning toward low values and simultaneous maximization of the resonance frequency is an important strategy to optimize the spin-VCSEL toward energy-efficient operation. A polarization dynamics resonance tuning range from a few GHz up to the maximum frequency of 36 GHz was achieved, and polarization dynamics at maximum frequency are demonstrated at minimum bias current and at high temperatures of approximately 70 °C. We propose a strategy for data communication with low energy consumption and low cooling effort.

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