Harmonic Frequency Combs of Quantum Cascade Lasers: Origin, Control, and Prospective Applications. Presented by:

Transcription

1 Harmonic Frequency Combs of Quantum Cascade Lasers: Origin, Control, and Prospective Applications Presented by:

2 Technical Group Leadership APPLIED SPECTROSCOPY TECHNICAL GROUP Chair: Elina A. Vitol, Staff Scientist, ECOLAB Advisor: Samuel Achilefu, Professor, Washington University in St. Louis Education Lead: Matthias Fischer, Project Manager, Analytic Jena - AG Publications Lead: Frank Kuo, Project Manager, Mettler Toledo AutoChem Technical Meetings Lead: Prasoon Diwakar, Sr. Research Associate, Purdue University This Executive Committee is finishing its 3-year service term in June New leadership team will be announced soon! Thank you for your support. It has been a pleasure to serve the Applied Spectroscopy group!

3 Technical Group Activities APPLIED SPECTROSCOPY TECHNICAL GROUP Webinars Organized webinars on a variety of topics concerning latest advances in applied spectroscopy. Information dissemination Monthly collection of papers concerning various topics in applied spectroscopy from OSA suite of journals. Sent out to all group members by . Interested in presenting your research? Have ideas for technical group events? Contact us at OSA.AppliedSpectroscopy.TG@gmail.com. After mid-june 2018 we will redirect your questions to the new leadership team.

4 Welcome to today s webinar! APPLIED SPECTROSCOPY TECHNICAL GROUP Dr. Marco Piccardo School of Engineering and Applied Sciences, Harvard University Harmonic frequency combs of quantum cascade lasers: origin, control, and prospective applications May 29, 2018

5 Harmonic Frequency Combs of Quantum Cascade Lasers: Origin, Control, and Prospective Applications Marco Piccardo Capasso Group, SEAS, Harvard University Frei, V. Kandinsky

6 Outline

7 Outline Basic elements: quantum cascade lasers and multimode states

8 Outline Basic elements: quantum cascade lasers and multimode states The harmonic state

9 Outline Basic elements: quantum cascade lasers and multimode states Applications The harmonic state

10 Outline Basic elements: quantum cascade lasers and multimode states

11 What does a QCL look like? Laser = Gain Medium + Optical Cavity 1. Inject current 2. Light comes out

12 Interband vs. Intersubband InAlAs InGaAs InAlAs InAlAs InGaAs InAlAs λ depends on width! λ=1.65µm Intersubband: wavelength determined by layer thickness, not by the bandgap of the material! QCLs can be designed to emit from 3 to 300 μm M. Razeghi et al., SPIE Newsroom (2006)

13 It s a little more complicated, but keep it simple. Typically: τ 32 1 ps, τ ps These picosecond time-scales make the QCL very different from other lasers.

14 A zoology of spectra 2. Nothing forces the modes to be equally spaced, so it s not a frequency comb. νν mm = cc nn gg νν 2LL mm 1. QCLs emit many modes due to a lack of carrier diffusion. State of the understanding of QCL spectra in 2012 A. Gordon et al., Phys. Rev. A 77, (2008)

15 Dispersion engineering enables the formation of a frequency comb. A. Hugi et al., Nature, 492 (2012) J. Faist et al., Nanophotonics 5, 272 (2016) G. Villares et al., Optica 3, 252 (2016) Y. Bidaux et al., Opt. Lett. 42, 1604, (2017) Y. Bidaux et al., Laser & Photonics Reviews 12, (2018)

16 Dispersion engineering enables the formation of a frequency comb. Spatial hole burning + Resonant χχ (33) A. Hugi et al., Nature, 492 (2012) J. Faist et al., Nanophotonics 5, 272 (2016) G. Villares et al., Optica 3, 252 (2016) Y. Bidaux et al., Opt. Lett. 42, 1604, (2017) Y. Bidaux et al., Laser & Photonics Reviews 12, (2018)

17 A quantum cascade laser renaissance New physics that was hidden in plain sight Quantum Cascade Lasers (1994-present) Microresonators (2004-present) Laser Instabilities ( s) Nonlinearity Parametric interactions δω δω ω - ω ω +

18 Outline Basic elements: quantum cascade lasers and multimode states The harmonic state

19 A zoology of spectra 2. Nothing forces the modes to be equally spaced, so it s not a frequency comb. νν mm = cc nn gg νν 2LL mm 1. QCLs emit many modes due to a lack of carrier diffusion. State of the understanding of QCL spectra in 2012 A. Gordon et al., Phys. Rev. A 77, (2008)

20 What is the second mode to lase? Increase slowly the DC current in the laser T. S. Mansuripur et al., International Quantum Cascade Laser School and Workshop, Policoro, (2014)

21 The discovery of the harmonic state Dense state Harmonic state Single mode Intensity (10 db/div) 1220 ma 1165 ma 1160 ma 1050 ma 1000 ma 950 ma 900 ma 885 ma 880 ma 875 ma 735 ma λ =4.6 µm λ =3.8 µm λ =9.8 µm 46 FSR J/Jth= ma 502 ma 500 ma 480 ma 480 ma (transient) 470 ma 424 ma 416 ma 408 ma 407 ma 380 ma FSR FSR 20 FSR J/Jth= ma 1440 ma 1414 ma 1380 ma 1360 ma 1280 ma 1180 ma 1140 ma 1116 ma 1115 ma 983 ma 7 FSR J/Jth= Wavenumber (cm -1 ) Wavenumber (cm -1 ) Wavenumber (cm ) provided by: T. S. Mansuripur et al., Phys. Rev. A, 94, (2016) M. Piccardo, P. Chevalier et al., Opt. Express 26, 9464 (2018)

22 Observations in other groups Q.Y. Lu,, C. Sirtori, Appl. Phys. Lett. 106, (2015) H. Li,, S. Barbieri, Opt. Express 23, (2015) R. L. Tober,, G. Belenky, JOSA B 31, 2399 (2014)

23 The origin of the harmonic state 1. Population grating Standing-wave cavity λ/2 e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - <E 2 (z)> ω T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

24 The origin of the harmonic state 1. Population grating Standing-wave cavity λ/2 e - e - e - e - e - e - e - e - e - e - e - e - e - <E 2 (z)> ω Population grating z T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

25 The origin of the harmonic state 1. Population grating Standing-wave cavity λ/2 e - e - e - e - e - e - e - e - e - e - e - e - e - <E 2 (z)> ω - ω ω + Population grating Can electron diffusion wash out the grating? <E 2 (z)> 0 - sideband primary mode + sideband L z z few mm In 1 ps, an electron can diffuse µ kt τ 32 /q = 150 nm at 300K in InGaAs That s nowhere near enough to traverse a λ/4 from node to anti-node T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

26 The origin of the harmonic state 2. Population Pulsations Single-mode ω t T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

27 The origin of the harmonic state 2. Population Pulsations Single-mode t e - ω Spontaneous emission at another frequency ω ω + t Time varying intensity Time-varying transition rate τ 32 ~1 ps Population pulsations t T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

28 The origin of the harmonic state 2. Population Pulsations Single-mode t e - ω Spontaneous emission at another frequency ω ω + t Time varying intensity Time-varying transition rate τ 32 ~1 ps Population pulsations t Inversion responds to modulation up to some limit T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

29 Population pulsations and population grating work in tandem to create the harmonic state. Lorentzian gain Parametric gain 1 Total gain 0.9 Gain (a.u.) + Real[χ (3) ] = Gain (a.u.) FP modes T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

30 Linear analysis of the instability: a perturbative approach? ωω? ωω 0 ωω + ωω Find the linearized solution for weak sidebands in the presence of a strong central mode. Maxwell-Bloch equations This gives the gain spectrum for sideband generation. Y. Wang, A. Belyanin (Texas A&M) T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

31 Linear analysis of the instability: a perturbative approach?? ωω ωω ωω 0 ωω + T 1 = 1.0 ps, T 2 = 0.05 ps Parametric contribution becomes significant at a pumping level that is only fractionally higher than the lasing threshold Y. Wang, A. Belyanin (Texas A&M) T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

32 Linear analysis of the instability: a perturbative approach?? ωω ωω ωω 0 ωω + Detuning corresponding to maximum gain: Greater sideband gain with wider sideband separation for shorter dephasing time T 2 (broader laser gain spectrum). Y. Wang, A. Belyanin (Texas A&M) T. S. Mansuripur et al., Phys. Rev. A, 94, (2016)

33 Space- and time-domain QCL simulator Small seed Y. Wang, A. Belyanin, Opt. Exp. 23, 4173 (2015)

34 Space- and time-domain QCL simulator Small seed Harmonic state exists in a finite frequency range Y. Wang, A. Belyanin (Texas A&M)

35 Space- and time-domain QCL simulator Small seed Harmonic state exists in a finite frequency range and a finite range of pumping levels. Y. Wang, A. Belyanin (Texas A&M)

36 Space- and time-domain QCL simulator Experiment 1220 ma 1165 ma Intensity (10 db/div) 1160 ma 1050 ma 1000 ma 950 ma 900 ma 885 ma 880 ma 875 ma 735 ma FSR J/Jth= Wavenumber (cm -1 ) provided by: Y. Wang, A. Belyanin (Texas A&M)

37 Mode skipping does not imply mode locking.

38 Mode skipping does not imply mode locking.

39 Mode skipping does not imply mode locking. This is not a comb. A. E. Siegman, Lasers, University Science Books (1986)

40 Are the modes locked? 10 0 Locking Optical power (W) FSR Strategy: A reference comb can be used to create a downconverted replica of the sample comb Wavenumber (cm -1 ) FSR 1 mod FSR 2 Multiheterodyne beating FSR 1 FSR 2 0 RF domain optical domain P. Del Haye et al., Nature 450, 1214 (2007)

41 Multi-heterodyne detection Down-converted replica of the harmonic state D. Kazakov, M. Piccardo et al., Nat. Photonics 11, (2017)

42 Spacing uniformity of the harmonic comb 55 Frequency counting Multiheterodyne f 1 (MHz) f 2 (MHz) ff 3 ff 2 ff 2 ff 1 f 3 (MHz) µ = -27 Hz σ = 329 Hz Time (s) Counts 50 f 1 f 2 f Deviation from equidistant spacing (Hz) Normalized to the optical carrier frequency (66.7 THz), the spacing uniformity of the harmonic comb is verified with an accuracy of D. Kazakov, M. Piccardo et al., Nat. Photonics 11, (2017)

43 GHz Typically 5-15 GHz Harmonic frequency comb Fundamental frequency comb 39

44 GHz Gigahertz-repetition-rate microcomb Typically 5-15 GHz Harmonic frequency comb Fundamental frequency comb M-G. Suh and K. Vahala, Optica 5, 65 (2018) 40

45 Outline Basic elements: quantum cascade lasers and multimode states Applications The harmonic state

46 Prospective applications of the harmonic state Microwave and THz generation Pump-probe spectroscopy Broadband spectroscopy M. Piccardo, P. Chevalier et al., Opt. Express 26, 9464 (2018)

47 A new route in QCLs M. Piccardo, D. Kazakov, F. Capasso, Provisional patent submitted (US Patent Application # )

48 THz wireless communication Base station connection, device-to-device communication THz band: THz High-speed internet access in remote rural areas Broadcasting of multichannel HDTV data in sport events S. Koenig et al., Nat Photon 7, 977 (2013) T. Nagatsuma et al., Nat Photon 10, 371 (2016) T. Kleine-Ostmann et al., J. Infrared Milli Terahz Waves 32, 143 (2011)

49 THz wireless communication Increasing the bandwidth W increases the communication capacity: THz band: THz Why THz? C (bit s-1)=w log2(1+s/n) 0 khz 300 khz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

50 Can we tune the spacing of the harmonic comb?

51 Can we tune the spacing of the harmonic comb? Many free spectral ranges Fabry-Perot cavity modes

52 Can we tune the spacing of the harmonic comb?

53 Can we tune the spacing of the harmonic comb?

54 Can we tune the spacing of the harmonic comb?

55 Can we tune the spacing of the harmonic comb?

56 Can we tune the spacing of the harmonic comb? The spacing of self-starting harmonic combs is fixed by fundamental laser parameters: dipole moment grating lifetime dephasing time

57 Introducing an optical seed in the cavity EC-QCL HR FP-QCL pulled single-mode Optical seed AR OAP OAP Beam stopper FTIR Normalized intensity (10 db/div) f seed Wavenumber (cm harmonic -1 ) single-mode Two different scenarios Frequency (THz) P. Chevalier et al., Appl. Phys. Lett. 112, (2018)

58 E = 0.2 kv/cm Harmonic state Pulled single-mode E = 0.7 kv/cm Injection locking Seed amplification and mixing Simulations by Y. Wang, A. Belyanin (Texas A&M)

59 One seed to rule them all 0.64 THz 1.32 THz Normalized intensity (10 db/div) THz Wavenumber (cm 0.41 THz Wavenumber (cm 0.34 THz Wavenumber (cm dense Wavenumber (cm -1 ) -1 ) Seed -1 frequency ) -1 ) Normalized intensity (10 db/div) 1.25 THz Wavenumber (cm 1.01 THz Wavenumber (cm 0.93 THz Wavenumber (cm 0.82 THz Wavenumber (cm -1 ) -1 ) -1 ) -1 ) -1 ) Atten. (db km Frequency (THz) Frequency (THz) Frequency (THz) This corresponds to skipping between 44 and 171 longitudinal modes

60 How? ΔΔf ΔΔf

61 Back to basics The first lasing mode in a standing-wave laser induces a static population grating: m N S (x) Intensity burnt hole Cavity position

62 Back to basics Adjacent cavity modes will be able to extract gain from the medium and start lasing: m Intensity m+1 N S (x) burnt hole Cavity position A time-dependent population inversion grating oscillating at the beat frequency is then produced

63 Probing dynamic population gratings in a QCL Reference probe L Scanning probe Electrode Substrate Waveguide Substrate OSC SA Quantum cascade laser: the ideal platform Short picosecond gain recovery time: High-frequency (THz) oscillations of the pop. inv. are allowed Excited carriers diffuse within sub-wavelength distances (few 100 nm) SHB is not washed out M. Piccardo et al., Optica 5, 475 (2018)

64 Probing dynamic population gratings in a QCL Reference probe Scanning probe Electrode Substrate Waveguide OSC SA L Substrate QCL 1 Beatnote power (20 db/div) f B f (khz) - f 2f B 0 Beatnote power (arb. unit) theory exp. sim Cavity position (mm) Beatnote phase (rad) /2 0 / Cavity position (mm) Beatnote power (arb. unit) Norm. cavity position λ (μm) L (mm) QCL QCL QCL QCL M. Piccardo et al., Optica 5, 475 (2018)

65 Analytical model of the dynamic gratings R 1 Ingredients: EE EE + Counter propagating waves R 2 Cavity modes ( + + Total field intensity )2 ff BB Filter beatnote (Fourier series) M. Piccardo et al., Optica 5, 475 (2018)

66 Analytical model of the dynamic gratings R 1 Ingredients: EE EE + Counter propagating waves R 2 Cavity modes ( + + Total field intensity )2 ff BB Filter beatnote (Fourier series) This term is filtered out in the experiments: spatial oscillations of few microns << RF probe size M. Piccardo et al., Optica 5, 475 (2018)

67 M. Piccardo et al., Optica 5, 475 (2018)

68 In a device operating at optical frequencies new microwave applications are enabled M. Piccardo et al., Optica 5, 475 (2018)

69 The QCL as a microwave quadrature modulator Injection of two microwave signals Modulation of coherent QCL beatnotes dephased by 90 Extraction of the modulated orthogonal signals Transmission via a single channel Original signals retrieved by coherent demodulation M. Piccardo et al., Optica 5, 475 (2018)

70 A microwave engineer s perspective applications The dynamic grating can be seen as two internal radio frequency generators connected in series N B (x,t) Cavity position, x The continuity of the top electrode bounds microwave radiation inside the device 66

71 Towards wireless emission Introducing a gap in the top contact opens the possibility of radio wave emission N B (x,t) - + Gap + - Cavity position, x 67

72 Towards wireless emission Introducing a gap in the top contact opens the possibility of radio wave emission N B (x,t) - + Gap Cavity position, x + -! 68

73 Federico Capasso Tobias Mansuripur Marco Piccardo Paul Chevalier Dmitry Kazakov Noah Rubin Alexey Belyanin Yongrui Wang Benedikt Schwarz Frei, V. Kandinsky