Broadband High-resolution Spectroscopy with Fabry-Perot Quantum Cascade Lasers Yin Wang and Gerard Wysocki Department of Electrical Engineering Princeton University, Princeton, NJ ISMS 2014 June 17, 2014 University of Illinois at Urbana-Champaign

Outline Motivation Operation principle of the developed spectrometer Experimental results Summary and future directions 2

Motivation Detecting large molecules using Mid-IR spectroscopy Distinguishing narrow spectral lines from broadband High spectral resolution Wide spectral range TNT Wide spectral range R. Furstenberg et al. Appl. Phys. Lett. 93, Source:

Fourier Transform Infra-red Spectrometer - FTIR Mid-IR Wide spectral range: cm -1 ( μm) High spectral resolution  Scales with size  Large, slow, expensive Resolution: cm -1 Resolution: cm -1 4

Nonlinear Optical Conversion Sources  OPO, DFG + Broadband + High spectral resolution - Complex tuning - Large, expensive (OPO) - Low power (DFG) Frequency Combs + Ultra-broadband + High spectral resolution - Usually nonlinear conv. Based - Limited availability QCLs, ICLs +Robust, small +High spectral resolution - Narrow/moderate tuning range EXAMPLE QCLs:  DFB-QCLs  t hermal tuning range (~10 cm -1 )  EC-QCLs  broadband (>300cm -1 ) but vibration sensitive  DFB-QCL arrays  Difficult beam-combining Capabilities and limitations of laser-based mid-IR spectrometers D. Hofstetter et al., Appl. Phys. Lett. 75, 665 (1999) 5

How about FP-QCLs ? 6 A. Gordon, F. Capasso et al, “Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning”, Phys Rev. A 77, (2008) Nonlinear processes promote multi-mode operation in FP-QCLs CW lasers show multi- mode lasing high above threshold Broad Spectral coverage Inter-mode phase-locking mechanism due to four wave mixing exists

Special - dispersion compensated FP-QCLs Mid-IR frequency comb in a specially designed FP-QCL realized by Hugi et al.  Intermode-beat frequency <10Hz  Control over mode-spacing and f o via RF injection Special gain design required  low dispersion waveguide Can we use conventional FP-QCLs?  simple and inexpensive !! Hugi et al Nature 492, 229 (2012) 6

Multi-heterodyne spectroscopy set-up Two conventional FP-QCLs Same gain material with different ridge-widths  different I th and FSRs Output beams separated in space 1 GHz MCT photodetector 7 RF freq. Amplitude (absorption) Phase info. Y. Wang, G. Wysocki et al APL 104, (2014)

FP-QCL mode structure Signal QCL LO QCL Difference in FSRs is not resolvable using FTIR Adjust bias current/laser temperature to achieve mode beatnotes within the detector bandwidth (1GHz) 8

Multi-heterodyne down-conversion results Faster detectors (e.g. QWIP) or smaller f  increase optical frq. range Strong phase-lock between the different longitudinal modes in FP-QCLs ~15MHz beatnote linewidth for free running FP-QCLs  optical resolution 1 min RF average spectrum Instantaneous RF spectrum 9

NH 3 broad absorption envelope retrieval Broad-band mid-IR spectroscopy using all four beat notes (no fine-tuning) Frequency coverage can be increased (faster detector, smaller f ) Frequency spacing can be decreased with longer devices 10

High resolution N 2 O absorption retrieval Signal FP-QCL modes tuned by current, LO FP-QCL current kept constant Spectral resolution ~ FWHM 15 MHz ~ cm -1 All beatnotes can be measured simultaneously  broadband, high- resolution spectroscopy No-moving parts, all electronic tuning 11

Summary Demonstrated a robust dual-FP-QCL multi-heterodyne technique for broadband, high-resolution Mid-IR spectroscopy All solid-state design High spectral resolution (~ cm -1 ) demonstrated Studies of amplitude and phase noise in multi- heterodyne FP-QCL technique Use high bandwidth detector such as QWIP Proper selection of FP-QCLs with smaller FSR Parallel processing of all available beat notes Future directions: 12

NSF Research Center MIRTHE U.S. Environmental Protection Agency grant No. RD Dr. Yamac Dikmelik ( Johns Hopkins University) for helpful discussions Prof. Federico Capasso (Harvard University), Dr. Laurent Diehl, Dr. Mariano Troccoli for providing FP chips for this study Acknowledgement 13