Design and construction of a mid-IR SPIDER apparatus 09/10/2012 Malte Christian Brahms Imperial College London 09/10/20121

Contents Purpose of SPIDER Working principle Design of mid-IR SPIDER User’s manual Outlook 09/10/20122

Purpose Pulse characterised completely by –Field as fct. of time or –Spectral amplitude and phase Oscillations in on the order of fs –Temporal resolution required too high Instead: Measure spectral amplitude and phase: S pectral P hase I nterferometry for D irect E lectric F ield R econstruction (SPIDER) 09/10/20123

Purpose SPIDERs available commercially Why build one? –Unusual wavelength in mid-IR –Specific variable wavelength SPIDER needed for TOPAS 09/10/20124

Contents Purpose of SPIDER Working principle Design of mid-IR SPIDER User’s manual Outlook 09/10/20125

Working principle Based on two replicas of the test pulse Displaced relative to each other –In time: Delay τ –In frequency: Shear Ω In frequency domain: 09/10/20126 Frequency Shear Time delay

Working principle Interferogram: 09/10/20127

Working principle 09/10/20128

Working principle 09/10/20129

Contents Purpose of SPIDER Working principle Design of mid-IR SPIDER User’s manual Outlook 09/10/201210

Design Two problems to solve: –Time delay –Frequency shear Solutions: –Delay: Split mirror Allows variable delay –Shear: SFG in BBO with chirped pulse 09/10/201211

Design – Time delay 09/10/ Split mirrors

Design – Frequency shearing Use SFG with chirped ancillary pulse replica –Chirp: Carrier frequency depends linearly on time: –SFG with same original frequency at two different times  different output frequencies: Obvious solution: Piece of glass –But: Most glass almost dispersion-free at ca. 1500nm 09/10/ Stretched ancilla Test pulse replicas

Design – Frequency shearing Chirping the ancillary pulse Grating Compressor: 09/10/ Image: 2010 J. Phys. B: At. Mol. Opt. Phys , p.5 Walmsley et al.: The role of Dispersion in ultrafast optics, Rev. Sci. Instrum., Vol. 27, No.1, Jan 2001, p. 7

Design – Recombination 09/10/ Sum-frequency generation Focus delayed pulse-pair and chirped ancillary pulse into BBO crystal Image: 2010 J. Phys. B: At. Mol. Opt. Phys p. 25 SHG + Original SFG

Design – Constraints On delay τ: Spectrometer resolution and shear –Fringe spacing 2π/τ must be sufficiently large   τ cannot be too large –τ must be sufficiently large resolve peaks in FT On shear Ω: –Small enough to satisfy the sampling theorem: –Satisfied in practice, usual: Ω=10% of bandwidth 09/10/ T: Reconstruction window

Design – Constraints On chirp : –Only SFG with monochromatic field conserves shape of spectrum: In time domain: In frequency domain (if E anc is monochromatic): –Need slow-varying, quasi-monochromatic frequency  lower limit on 09/10/201217

Design - Constraints Resolution: Shear size: Lower limit: 09/10/201218

Design – Constraints With numbers: At nm for a ca. 40fs pulse Both the constraints and the value are wavelength-dependent! Chirp determined by compressor angle and separation –Can vary angle easily, use mostly –If necessary, enough space to move one grating 09/10/201219

Design – Setup 09/10/ Spectromete r A1 A2 A3 f=100mm D-mirror BS ( nm)

Contents Purpose of SPIDER Working principle Design of mid-IR SPIDER User’s manual Outlook 09/10/201221

User’s Manual – Practical issues Calibration by simply adding ignores frequency-dependent behaviour (noise) in apparatus –Need to calibrate for this –Use signal without shear instead: Depending on wavelength and spectrometer: –Measure calibration either in SHG (blue) or original (red) –SPIDER in SFG For this SPIDER at nm: –Measure calibration and SPIDER signal in blue, first order 09/10/201222

User’s Manual – Practical issues Spectral amplitude: –Determined from calibration signal Take magnitude instead of phase angle: Determining shear Ω: –Measure compressor parameters or block one of sheared pair (spatially) –In practice: Use compressor parameters τ determined by software 09/10/201223

User’s manual – Procedure Good signal: –High fringe contrast –Well separated fringes –Near saturation Needed for this: –Phase-matching and crystal position –Spatial overlap (focus on crystal) –Temporal overlap (compressor translation) –Right amount of delay –SHG beams blocked (use aperture A3) 09/10/201224

User’s manual – Procedure 1.Take reading without ancillary pulse (calibration) –Block after beam splitter 2.Take SPIDER signal –Unblock ancillary, take trace 3.Analyse OR: Use Tobi’s TwinSPIDER (live operation) –Ask Tobi for more advice 09/10/201225

User’s Manual – Software 09/10/201226

Outlook This week or next: Use on TOPAS Later: Test on few-cycle pulses Intended for long-term use on TOPAS 09/10/201227

Thank you! Questions? 09/10/201228