QM1G.2.pdf
CLEO Technical Digest © OSA 2012
1.4ps Superradiant Pulses from a GaN-based Laser V. F. Olle1, P. P. Vasil’ev1,2, A. Wonfor1, R. V. Penty1 and I. H. White1 1)
Centre for Photonic Systems, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA UK 2) PN Lebedev Physical Institute, 53 Leninsky Prospect, Moscow 119991, Russia
[email protected]
Abstract: The generation of picosecond superradiant pulses from 408nm a GaN/InGaN laser diode is demonstrated for the first time. Pulses with peak powers above 2.8W, pulse energy of 57pJ and durations of 1.4ps are generated. OCIS codes: 140.6630 Superradiance, superfluorescence; 230.5590 Quantum-well, -wire and -dot devices
1. Introduction Dicke superradiance is a promising technique for ultrafast pulse generation, unlike either mode-locking or gain/Qswitching, having enabled both picosecond and sub-picosecond optical pulses to be generated directly on-demand [1]. Until recently, the generation of ultrafast superradiant pulses has only been reported experimentally in GaAs/AlGaAs, InAs/InGaAs and InGaAsP/InP diode laser diodes, with emission wavelengths between 880nm and 1580nm [1-3]. However there has been much interest in generating ultrafast pulses from GaN-based devices in the blue/violet wavelength range for high density storage, sensing and biomedical applications [4,5]. For example, 10ps, 12W pulse generation has been demonstrated at a repetition rate of 100kHz from a gain switched InGaN laser with an emission wavelength of 405nm [4]. In addition, a post-amplification technique has been used to enable the generation of a 3ps, 100W pulse train at the repetition rate of 1GHz from an InGaN passively mode-locked master oscillator power amplifier system within an external cavity [5]. Here, we report the experimental demonstration of superradiant emission from a multisection GaN/InGaN semiconductor laser diode for the first time. The resultant 408nm optical pulses have peak powers in excess of 2.8W, maximum pulse energies of 57pJ, and durations as short as 1.4ps at repetition rates up to 10MHz. These pulses are shown to exhibit similar properties to superradiant pulses in other semiconductor material systems, including a characteristic spectral red shift, increasing with an increase of saturable absorption, an anomalously large timing jitter, and a reduced time bandwidth product compared with gain/Q-switching. 2. Experimental setup The laser diode used in this experiment is a 2-section device with the intercontact isolation created using focused ion beam etching. This etching results in a 100µm long absorber section located at the rear of the device and a 285µm long gain section at the front facet separated by a 10µm wide gap which provides intercontact resistance of 50kΩ. In operation, the absorber section is reverse biased up to 17V. The gain section is driven with 9ns long electrical pulses of up to 500mA peak current from an HP 214B pulse generator at repetition rates between 1-10MHz. It is this maximum drive pulse repetition rate which limits the SR repetition rate achievable. The output power is either coupled using a lensed fibre to a 40GHz visible photodiode (New Focus 1004) or collimated using free space optics into a single-shot streak camera with a temporal resolution of approximately 900fs. In addition, the optical spectrum of the laser diode is recorded with a spectrometer with a resolution bandwidth of 1.2nm. The threshold current of a 2-section device is 29mA with a typical slope efficiency of 1.7W/A measured with both the gain and absorber sections electrically connected. 3. Experimental demonstration of superradiance A comparison of optical waveforms in the Q-switching and superradiance regimes is shown in Fig. 1a and Fig.1b. At zero reverse bias applied to the absorber section and with the gain section biased with pulsed current of 170 mA, Q-switched pulses are generated (Fig. 1a). When the reverse bias is increased up to -3.7V (Fig. 1b), superradiance is achieved with the optical pulse being delayed by more than 2ns and having a peak optical power in excess of 2.5W. A comparison of the emission spectra in these regimes is provided in Fig. 1c. This shows that with increased reverse bias, the peak optical wavelength is red-shifted by 4.86THz (2.7nm), this value being similar to the red shifts reported in other material systems [1-3]. Fig. 2a presents a typical superradiant pulse detected by a single-shot streak camera with 0.9ps time resolution. Fig. 2b shows the jitter of gain-switched pulses versus the driving current amplitude from a single contact laser diode. As the pulsed current increases, the jitter drops from around 25ps down to below 10ps. These values include the intrinsic jitter of the pulse driver and sampling oscilloscope. By contrast, the measured jitter for superradiant pulses increases from 25ps to 200ps as the reverse bias is increased (Fig. 2c). The physical reason for enhanced
QM1G.2.pdf
CLEO Technical Digest © OSA 2012
timing jitter of superradiant pulses originates from fundamental quantum mechanical fluctuations of the decay of the coherent macroscopic cooperative e-h state [1]. Optical power (a.u.)
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Wavelength (nm) (a) (b) (c) Figure 1: Optical waveforms under (a) Q-switching (b) superradiance. A comparison of optical spectra (c) is plotted for Q-switching (black trace) and superradiance (red trace). 30 25 20 15 10 5 0
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The effects of absorber voltage and pulsed current on the laser operating regime are shown in Fig. 3a. At low absorber voltage Q-switched pulses (green area) are generated whilst increasing the absorber voltage results in the device emission switching from a long optical pulse to a series of superradiant pulses (blue area). The pulse energy of the superradiant pulses is shown as a series of contours of increased gain section current in Fig. 3b. Further increases in absorber voltage reduce the number of superradiant pulses hence pulse energy and can terminate superradiance, resulting in a regime of amplified spontaneous emission (red area). The pulse energy of a single superradiant pulse increases approximately linearly with increasing pulsed current and reaches maximum value of 57pJ for a pulsed current to the gain section of 280 mA. 1000 Q-switching
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Pulsed current (mA) (a) (b) (c) Figure 3: A schematic representation of (a) different regimes of operation as a function of absorber voltage and pulsed current, (b) pulse energy in superradiance regimes and (c) pulse energy of single superradiance pulse as a function of pulsed currents(with absorber voltage optimized at each current).
4. Summary In this paper we have demonstrated for the first time the generation of ultrafast superradiant pulses from a GaN/InGaN laser diode operating at 408nm. This 2-section laser diode produces short optical pulses on demand with peak powers in excess of 2.8W, pulse durations of 1.4ps, pulse energies up to 57pJ, and repetition rates up to 10MHz which are limited by the pulse driver. This flexible technique of pulse production is a candidate solution for biological imaging and high density storage applications. 5. References [1] P. P. Vasil’ev, “Femtosecond Superradiant Emission in Inorganic Semiconductors,” Reports on progress in Physics, 72, 076501 (2009). [2] M. Xia, R. V. Penty, I. H. White, P. P. Vasil'ev, “Superradiant Emission from a Tapered Quantum-Dot Semiconductor Diode Emitter,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMY2. [3] M. Xia, R. V. Penty, I. H. White, P. P. Vasil’ev, “Superradiant emission from AlInGaAs/InGaAsP quantum-well waveguides,” in Proceedings of European Conference Integrated Optics, (Cambridge, U.K., 2010), paper THD5 [4] S. Kono, T. Oki, T. Miyajima, M. Ikeda, H. Yokoyama, “12 W peak-power 10 ps duration optical pulse generation by gain switching of a single-transverse-mode GaInN blue laser diode,” Applied Physics Letters, 93, 131113 (2008). [5] R. Koda, T. Oki, T. Mayajima, H. Watanabe, M. Kuramoto, M. Ikeda, H. Yokoyama, “100W peak-power 1GHz repetition picoseconds optical pulse generation using blue-violet GaInN diode laser mode-locked oscillator and optical amplifier,” Applied Physics Letters, 97, 021101 (2010).
