The lasers

The main femtosecond laser at the heart of WCUS is an amplified Ti:sapphire laser -- a "Spitfire Ace PA" -- from the company Newport Spectra Physics. This consists of a Ti:Sapphire oscillator (MaiTai) which generates a train of low energy femtosecond pulses at 800nm. In the amplifier one "seed" pulse from this train is selected, then stretched in time, before being massively amplified in pulse energy by two amplification steps. Each amplification step involves a Ti:sapphire crystal pumped by intense nanosecond pulses (Ascend lasers). The MaiTai and two Ascend lasers are cooled with a closed-loop chiller system, kept in a separate annex room for greater condition stability within the lab.

MaiTai SP

The oscillator in our laser system is a Spectra Physics MaiTai SP Ti:sapphire laser. This generates a train of 800 nm pulses at an 84 MHz repetition rate with an average power of >400 mW. The bandwidth of the system can be set anywhere between 10 and 60 nm (corresponding to pulse durations of 100 - 25 fs) with a tunable central wavelength of 780-820 nm. The horizontally-polarised beam has a diameter of 1.5 mm and a divergence of <1 mrad. The MaiTai has a power stability of 0.5% drift over 2 hours.

Ascend 60

Each Ascend 60 pump laser outputs 527 nm light at a 1 kHz repetition rate with an average power of >35 W. They output a horizontally polarised, 3.5 mm beam with a pointing error of <10 urad/^oC. They have a power stability of <0.15% over 8 hours.

Spitfire Ace - PA - 40

The overall amplifier system produces laser pulses with a wavelength spanning 780-820nm, at a repetition rate of 1 kHz and a pulse duration of <40 fs. The average power of the beam is >13 W, giving >13 mJ per pulse and pre- and post-pulse ratios of 1000:1 and 100:1 respectively, with a stability of 0.75% over a 24 hour period. It outputs a horizontally polarised, 12 mm diameter beam, with a pointing error of <20 urad/^oC.

Wavelength Conversion

WCUS is equipped with several methods for converting the 800nm fundamental output of the Spitfire Ace into a wide range of wavelengths from the deep UV (200 nm) to the Far IR (20 microns) and into the THz regime.

TOPAS Prime / TOPAS C

WCUS possesses three TOPAS Prime optical parametric amplifiers (Light Conversion) which can generate wavelengths of i) 238 - 2500 nm, ii) 290 - 2500 nm, and ii) 2500-9000 nm, and one TOPAS C which generates light from 1160 - 15000 nm. Each outputs 1 kHz, 40 fs pulses with powers ranging from 2 - 900 mW (3% rms stability). Wavelength selection is fully automated, with the exception of moving from <2500 nm to >2500 nm on TOPAS Prime ii, which requires the addition of one mirror into the system.

White Light Generation

Transient electronic absorption spectroscopy is performed by the means of a white light probe. This is generated by focussing a small fraction (<0.05 mJ) of the 800 nm fundamental onto a calcium fluoride crystal. This induces high order non-linear optical process to produce a white light continuum from 330-730 nm, with an RMS noise of <1% across the majority of its spectrum. This continuum can be expanded into the UV by pumping the calcium fluoride crystal with 400 nm, instead of 800 nm.

Beta Barium Borate

UV light can also be produced via Third Harmonic Generation (THG) of the fundamental 800 nm beam. Here, 1 W of the fundamental is passed through Type I and Type II Beta Barium Borate (BBO) crystals. The first (Type I) causes Second Harmonic Generation (SHG) by mixing two photons on 800 nm to produce 400 nm light. The second harmonic and residual fundamental beams are then passed though a Type II BBO which mixes one photon of 400 nm and one photon of 800 nm light to produce 267 nm light, the third harmonic. Each of these processes has 10-20 % efficiency, resulting in a final 267 nm beam of ~10 mW, with a pulse duration of ~80 fs.

Sources of THz radiation

Optical rectification is one key source of THz radiation, whereby different wavelengths within the bandwidth of the intense 800nm femtosecond beam can be mixed to produce low-frequency (THz) light. Spintronic emitters are also highly efficient, convenient sources of THz radiation. We generate THz radiation either via optical rectification in <110> oriented GaP crystals or via spintronic emitters, for good bandwidth (>4THz) and reliable day-to-day operation. For high field (10kV/cm to 1.2MV/cm) THz generation we employ the tilted pulse wavefront method in LiNbO₃.