California Institute of Technology An Asynchronous Optical Sampling THz-TDS Spectrometer and THz Frequency Comb Spectrometer Jacob Good, Dan Holland, Ian Finneran, Brandon Carroll, Marco Allodi, Geoff Blake California Institute of Technology

Gas-Phase Rotational Spectroscopy D2O 10 THz/100 kHz 10-8 precision 10 ppb Gas-Phase rotational spectroscopy of small molecules overlaps with the bulk of the THz region from 0.1-10 THz. In order to assign transitions using quantum theory, we need to measure transitions to a precision of 10 ppb in order to compare with theory. https://www.astro.uni-koeln.de/cgi-bin/cdmsinfo?file=e020502.cat

What does 10-8 Precision Look Like? In terms of length, measuring the diameter of a tennis ball in units of nm (width of a glucose molecule) would represent 10 ppb precision. Tunable CW sources have been able to achieve greater than 1 ppb precision. However, they require expensive electronics and require a prohibitively long time to scan across the entire THz region. 108 nm to 1 nm 10-8 precision 10 ppb publications.nigms.nih.gov/chemhealth/images/ch4_size.jpg

Part I: Building the Spectrometer Part I answers the question: Why do we need a unique spectrometer for this work? Part I: Building the Spectrometer

Time Domain Spectroscopy Fortunately, TDS has already developed a methodology for measuring units of time with extreme precision. An ultrafast laser emits extremely short pulses of light at an extremely precise interval. These pulses can be used to initiate an ultrafast process of interest, but the electronics to directly measure this process do not exist. If the ultrafast process is repetitive, we can circumvent this problem by splitting off a small amount of light from the laser and sending it to a delay stage to add a variable amount of delay. We can use these variably delayed pulses to sample the process of interest across the entire time interval, effectively slowing down time. coherent.com; Dan Holland, Caltech PhD Thesis, (2014)

Delay Line Scan Rate vs Scan Range Unfortunately, even obtaining 1 ppm precision is too demanding for a mechanical delay stage. More delay time requires a longer stage and precision mechanical motors can only move so quickly. To get to 1 ppm we need 100 ns of delay which would require a 30 m long delay stage, longer than most laser tables! 10 THz to 10 GHz 10-3 precision 1 ppm 10 THz 10 GHz Dan Holland, Caltech PhD Thesis, (2014)

ASynchronous OPtical Sampling Master Slave ASOPS is a method that uses a separate laser to emit a delayed probe pulse train, eliminating the need for a delay stage entirely. ASOPS requires extremely precise asynchronization of two lasers but with no moving parts ASOPS is far more precise than a delay stage based setup. coherent.com; Dan Holland, Caltech PhD Thesis, (2014)

ASOPS Sweep Rate vs Scan Range 80 THz With ASOPS, we can generate an arbitrarily long delay by choosing the repetition rate of our master laser. We choose our high-frequency resolution and scan speed by choosing the repetition rate offset between master and slave lasers. We can obtain plenty of high-frequency resolution and scan speed, however we will need to use data processing methods to improve our precision. 10 THz to 10 MHz 10-6 precision 1 ppm 10 MHz Dan Holland, Caltech PhD Thesis, (2014)

THz Electro-Optic Detection ASOPS-THz-TDS System THz Electro-Optic Detection Balanced PDs Personal Computer 180 MS/s Digitizer Wollaston Prism λ/4 ZnTe 10 MHz Rb Standard Lens Periscope Master Ti:Sapph ITO Slow PZT Scan Triggering Sample Cell 12.5 cm Frequency Counter 50% 90% Clock PD PMT BBO Slave Ti:Sapph Periscope Slave PD The electronic circuit that we just built doesn’t actually keep track of when the pulse trains of the Master and Slave lasers synchronize. In order to know when to start a new scan, we use a cross-correlator that generates pulses of blue light and converts them to an electronic trigger signal. Since we want to do THz spectroscopy we use a THz emitter to convert our pulses of optical light into pulses of THz light. Water vapor in the air strongly absorbs THz photons so we need to house the THz portion of our beam in a purge box. We route our THz beam through our sample cell and onto a ZnTe crystal. ZnTe changes its birefringence in response to THz pulses. We then route our optical probe beam onto this crystal where its polarization changes in response to the THz induced birefringence changes. We then extract the component polarizations of the beam and measure the magnitude of the polarization change on a pair of balanced photodiodes. We precisely digitize the resulting signal and can then use FFT data processing to extract its frequency content. THz Emitter Slow PZT Fast PZT 50% Lens Master PD PLL Electronics 50% Nitrogen Purge Box 90%

80 MHz ASOPS THz Pulse J.T. Good, et al., Rev. Sci. Instrum., In Prep We now have the raw data that we need to extract frequency content. This data is sampled in the time domain, so the rest of the talk will discuss Data processing to produce high resolution gas-phase spectra. J.T. Good, et al., Rev. Sci. Instrum., In Prep

80 MHz ASOPS Bandwidth J.T. Good, et al., Rev. Sci. Instrum., In Prep 1 hour integration J.T. Good, et al., Rev. Sci. Instrum., In Prep

Part II: Gas-Phase Spectroscopy

Water 1 Torr − 80 MHz ASOPS 1 hour integration J.T. Good, et al., Rev. Sci. Instrum., In Prep

Pressure Broadened Line Shape -2.67 MHz Observed - Calculated 114 MHz FWHM Lorentzian Line Shape 10 THz to 10 MHz 10-6 precision

CH3CN 2 Torr − 80 MHz ASOPS ν = 2B(J + 1) − 2DJKK2(J + 1) 1 hour integration J.T. Good, et al., Rev. Sci. Instrum., In Prep

CH3CN − Unresolved Structure ν = 2B(J + 1) − 2DJKK2(J + 1)

5 MHz FCS THz Pulse Train J.T. Good, et al., Rev. Sci. Instrum., In Prep

18 Pulse Frequency Comb I.A. Finneran, J.T. Good, et al., Phys. Rev. Lett. 114, 163902, (2015)

5 MHz FCS Bandwidth J.T. Good, et al., Rev. Sci. Instrum., In Prep 1 hour integration J.T. Good, et al., Rev. Sci. Instrum., In Prep

CH3CN 150 mTorr − 5 MHz FCS 1 hour integration J.T. Good, et al., Rev. Sci. Instrum., In Prep

5 MHz FCS vs 80 MHz ASOPS 3 J = 46 K = 6 2 1 4 5 J.T. Good, et al., Rev. Sci. Instrum., In Prep

Doppler Broadened Line Shape 0.086 MHz Observed - Calculated 4.7 MHz FWHM Gaussian Line Shape 10 THz to 100 kHz 10-8 precision

MeOH 140 mTorr FCS

MeOH 140 mTorr FCS

MeOH 140 mTorr FCS

MeOH 140 mTorr Peak Fitting 8.5 MHz FWHM Gaussian Line Shape

MeOH 140 mTorr Prediction (THz) Observed (THz) Log Intensity Error (MHz) 1.588594756 1.588595486 -2.2727 0.730 1.588597991 Fixed -2.6116 N/A 1.588597992 1.588608982 1.588608450 -2.2406 -0.532 1.588621485 1.588620710 -2.3539 -0.775 1.588638007 1.588637823 -2.2148 -0.184 1.588668167 1.588669835 -2.4023 1.668 1.588679307 1.588679276 -2.1959 -0.031 RMS 0.54 MHz

THz Electro-Optic Detection Long Path Double Pass Cell THz Electro-Optic Detection Balanced PDs Personal Computer 180 MS/s Digitizer Wollaston Prism λ/4 ZnTe 10 MHz Rb Standard Lens Periscope Master Ti:Sapph ITO Slow PZT Scan Triggering Frequency Counter 50% 90% Clock PD PMT BBO Slave Ti:Sapph Periscope Slave PD THz Emitter Slow PZT Fast PZT 1 hour integration 50% Lens Master PD PLL Electronics 50% 90% Sample Cell 2x 0.94 m 188 cm / 12.5 cm = 15x enhancement THz Polarizer Rooftop Reflector

21,470 Pulse Frequency Comb 10 THz to 10 kHz >10-9 precision I.A. Finneran, J.T. Good, et al., Phys. Rev. Lett. 114, 163902, (2015)

THz Bandwidth vs Pulse Width P.J. Hale, et al., Opt. Express, 22, 26358-26364, (2014)

Prism Compressor http://www.swampoptics.com/images/tutorial_compressor_fig2.gif

Acknowledgements Blake Group Prof. Jun Ye NSF Funding

THF 5 Torr − 80 MHz ASOPS 1 hour integration

THF − Pseudorotational Modes Q Q P R P R P R P R