Ultralow Threshold On Chip Toroidal Microcavity Nanocrystal Quantum Dot Lasers

Transcription

1 Ultralow Threshold On Chip Toroidal Microcavity Nanocrystal Quantum Dot Lasers Bumki Min, Sungjee Kim, Koichi Okamoto, Lan Yang, Axel Scherer, Harry Atwater, and Kerry Vahala California Institute of Technology

2 Outline 1 High Q Toroidal Microcavity on a Chip 2 CdSe/ZnS (core/shell) Nanocrystal Quantum Dots 3 Free Space Pulsed Excitation Low efficiency lasing operation 4 Tapered Fiber Pulsed Excitation Transparency limited lasing operation

3 Tapered Fiber Coupled Microtoroid Resonators Ultrahigh Q factor Temporal confinement Q up to nm Small mode volume V Spatial confinement V ~ 500 μm 3 Efficient tapered fiber coupling Critical coupling High ideality Low insertion loss Tapered fiber

4 Motivations: Nanocrystal quantum dot lasers Observation of gain/ase in CdSe nanocrystal [V. I. Klimov et al. Science, 290 (2000)] Klimov et al. CdSe nanocrystal lasers Distributed Feedback [H. J. Eisler et al. Appl. Phys. Lett. 80, (2002)] Microcapillary Tubes [A. V. Malko et al. Appl. Phys. Lett. 81, (2002)] Microspheres [P. T. Snee et al. Adv. Mater. 17 (2005)] Snee et al.

5 Colloidal Nanocrystal Quantum Dots (QDs( QDs) Image taken under UV illumination Nanocrystal QDs with Peak wavelengths of nm are chosen for the reliable collection of emission (this will be explained later) Absorption/Emission spectra (CdSe/ZnS (core/shell) in hexane) Measured by spectrophotometer and spectrometer (GaN laser nm)

6 SEM Micrograph of QD cast microcavity Roughness/blemishes were not observed in SEM micrograph AFM (σ ~ nm) Spin casting (speed = 2,500 3,200 rpm) : QD solgel coating on the toroidal microcavity (Optimization still in progress)

7 Free space pulsed excitation of NQD laser Pumping Collecting (liquid nitrogen) Pumping efficiency Ü (λ/r) 2 ~0.05 % (Optimistic limit) CdSe/ZnS coated toroidal microcavity Threshold energy ~ 2.6 nj Threshold insensitive to temperature change Peak wavelength is red shifted at the time of excitation

8 Rendering of toroidal microcavity QD laser QD cast toroidal microcavity Pump pulse Lasing CW excitation Optical micrograph Lasing Rendering of the device Pump nm Repetition = 80 MHz Tapered fiber is in contact with the toroid FEM simulation (cross sectional view)

9 Q factor factor Analysis of QD lasers Absorption limited Q AFM image of QD film Scattering limited Q Rms roughness = nm Q factor estimation based on measured parameters Q ss from AFM roughness measuremnts Q abs from absorption measurements High Q in the lasing band / Relatively low Q in the pump band

10 Experimental Setup Ti: Sapphire laser CCD camera Frequency doubling crystal Fiber optic illuminator Neutral density filter Band pass filter Detector To tapered fiber Streak camera Flip mirror Sample & tapered fiber holder Triggering for a streak camera

11 Lasing spectrum Single mode lasing Multimode lasing Tapered fiber emission Time resolved spectrum Lasing spectrum Single mode lasing Multimode lasing Tapered fiber emission background ~5nm spacing

12 LL curve Tapered fiber coupling Integrated over 50 ps window Fast component : 70 ps lifetime Saturation LL curve with tapered fiber coupling (E th ~ 1 pj) ~2,600 improvement from free space excitation Selective excitation of QDs in the active region

13 Advantages of using tapered fiber 1 Average # of initially excited e hpairs per NC For free space, A = ~3,000 μm 2 For tapered fiber coupling, A = ~1.2 μm 400 nm ~2,400 enhancement can be achieved 2 Pump energy reuse with tapered fiber coupling Average # of pump pulse circulation around the cavity For Q = 10 5, N circ 26 Tapered fiber coupler is extremely effective in pumping NQD lasers

14 Effect of decreased QD number ~ (cm -3 ) ~ (cm -3 ) ~ (cm -3 ) LL curves with tapered fiber coupling 9.9 fj threshold achieved Transparency dominated threshold High Q

15 Conclusions Demonstration of tapered fiber coupled on chip toroidal microcavity NQD lasers Minimum threshold of 9.9 fj Ultralow threshold operation due to tapered fiber coupling and QD density control Transparency limited laser threshold operation