New directions in QCL applications: from optofluidic lasers to plasmonic laser antennas

Transcription

1 New directions in QCL applications: from optofluidic lasers to plasmonic laser antennas MIKHAIL BELKIN Capasso group Division of Engineering and Applied Sciences Harvard University

2 Optofluidic QCL research Electricallypumped QCL Various laser structures (photonic crystal, DFB, etc) MOCVD quantum cascade lasers with record performance Optofluidic QCL-based lab-on-a-chip, optofluidic tuning Novel designs for large tuning, sensing, spectroscopy, etc Microfluidics delivery Encapsulation of quantum cascade lasers with PDMS Goal: QCL fluid tuning and sensing for lab-on-a-chip

3 Tuning QCL with liquids Distributed feedback QCL + Microfluidic encapsulation D. Hofstetter et al, IEEE PTL, 1, 161 () "laser =! "grating! neff nm & '( # $! = 16.6 REU % 'n " Submitted to Optics Express

4 Sensing liquids with QCL J th! ~! waveguide waveguide $! +! no _ liquid mirror + # out "! liquid Laser mode losses, -1 cm Liquid absorbance at 7.1µ, cm Sample liquid with Re(n)=1.3 and different absorbance;!=7.1µ µ ridge laser µ ridge laser 6µ ridge laser Laser threshold with no liquid Liquid absorbance at 7.1µ, Im(n) Wavelength of DFB laser, µ µ ridge width µ ridge width µ ridge width 1.5µ ridge width Refractive index of a liquid Absorbtion spectra of water and isopropanol Absorbance, Im(n) Wavelength, cm -1 DI water Isopropanol QCL emission wavelength L-I of λ = 7.1 µm, 6µm ridge device Intensity, a.u. Absorbance, cm DI water Isopropanol λ = 7.1 µm 6µm ridge Current, A

5 More sensitivity and tuning? Holey QCL structures nm & '( # > 1 $! REU % 'n " theory nm ' () $! 1, % " REU & (n #theory Problem: Ga implantation from FIB leads to QCL shorting

6 Plasmonic laser antennas Generates subwavelength intense optical spot APPLICATIONS High-spatial-resolution imaging and spectroscopy Nano-optical tweezers Nano-optical lithography High-density optical data storage

7 Antenna results for QCL Signal at 1xΩ Signal at xω Intensity profile at xω 6 E Intensity/a.u. 5 Laser mode Near-field 3 We demonstrated similar results for 8nm laser diode, see E. Cubukcu et al., Appl. Phys Lett. ( 6) 1 3 Y, nm

8 Terahertz Current 35 3 ν 1 ν Intensity, (a.u.) ν THz = ν 1 -ν Wavenumbers, (cm -1 ) Requirements for good THz DFG device High-power dual wavelength QCL High nonlinearity for DFG THz waveguide with low-loss and high confinement Phase matching Efficient out-coupling of THz radiation

9 Surface emission in double-metal THz QCL Material: Grating design Frequency, cm -1 Symmetric and antisymmetric eigenmode frequencies VS grating duty cycle Antisymmetric mode curve Symmetric mode curve Duty cycle Bound-to-Continuum design, 3THz [Barbieri et al., APL 85, 167 ()]. Material is grown in the group of Edmund Linfield, University of Leeds. Processing: 15µ-wide double-metal waveguides; gratings with different periodicities. Reference devices have no grating.

10 SE in THz QCL: spectra and power 6 Collected peak power, µw Collected peak power, µw K 65K 8K 85K 3.µ grating 5K 3K 5K 6K 63K Current, A.16mW/A Current, A Without grating (15µ-wide,.8mm-long ridge).3mw/a Voltage, V Voltage (V) 5 5 Inensity, a.u Intensity, a.u Wavenumbers, cm -1 With grating (15µ-wide ridge, 1.5mm-long grating with absorbing edges).1.1 1E Wavenumber (cm -1 ) µ grating 9.8µ grating Note: spectra are scaled to appear at similar intensity

11 Far field profile Far field profile along the laser ridge for 3.µ grating 7 Intensity, a.u degrees Degrees Optical microscope image of the processed devices

12 Acknowledgments Harvard University: L. Diehl, M. Loncar, J. Fan, B. Lee, E. Cubukcu, N. Yu, M. Troccoli, K. Crozier, E. Kort University of Leeds: M. Lachab, S.Khanna, E. Linfield University of Neuchatel: T. Aellen, D. Hofstetter, J. Faist (Former) Agilent Technologies: D. Bour, S. Corzine, G. Hofler

13 Results for 8nm laser diode Schematics of measurement setup Simulations Measurements to detector E. Cubukcu et al., Appl. Phys Lett. ( 6)

14 Antenna design E Intensity Enhancement Au SiO L = 55 nm 3 L 1 = 15 nm L λ=83 nm 1! eff 3! eff Antenna Length (nm) nm Normalized Intensity Enhancement Antenna Length (nm)

15 Sensing liquids with QCL Preliminary results for narrow-ridge QCL sensing 1 1 Intenisty, a.u Laser emission spectra, I=15mA Wavelength, cm -1 No liquid Isopropanol Absorbtion spectra of water and isopropanol Absorbance, Im(n) Wavelength, cm -1 DI water Isopropanol QCL emission wavelength Voltage, V DI water Isopropanol Intensity, a.u. 3 DI water Isopropanol λ = 7.1 µm 6µm ridge Current, A Current, A

16 Sensing liquids with QCL SEM images of narrow ridge QCLs (T-shape design)

17 IV and LI for different gratings 6 5 Power, µw LI and IV for surface emitting structures with 9.8, 3., and 31.µ gratings 3 9.8µ grating periodicity 3.µ grating periodicity 31.µ grating periodicity Current density, A/cm

18 Absorbing edges 8 7 Peak power, µw no absorbing edges one absorbing edge T=5K Current density, A/cm Voltage, V 1 Inensity, a.u. 8 6 no absorbing edges one absorbing edge Wavenumbers, cm -1

19 Antenna fabrication Active region Original diode laser facet SiO deposition for electrical isolation and surface passivation Evaporation of gold layer Optical Antenna is defined with FIB 1 nm