Narrowband Tunable THz generation in Orientation Patterned Gallium Phosphide

Thick hydride vapor phase epitaxially grown orientation-patterned gallium phosphide (OPGaP) is a leading material for quasi-phase matching (QPM) frequency conversion in the mid- and longwave infrared (IR). This is due to its negligible two-photon absorption (2PA) in the convenient pumping range 1 – 1.7 μm, compared with the 2PA of some traditional QPM materials, such as GaAs. In this paper, we describe homo- and heteroepitaxial growth techniques aimed to produce hundreds of microns thick OPGaP on: 1) OPGaAs templates fabricated using an improved wafer-fusion process; 2) OPGaAs templates fabricated by using a molecular beam epitaxy (MBE) for sublattice polarity inversion, but one with and one without MBE regrowth after the inversion. Some of the advantages of the heteroepitaxial growth of OPGaP on OPGaAs templates include: 1) achieving good domain fidelity as a result of the significantly higher OPGaAs template quality; 2) eliminating the needs of using the poor quality commercially available GaP in the production of thick OPGaP material, and 3) suppression of the additional absorption band between 2 – 4 μm (which is due to incorporation of n-type impurities) and, in general, improvement of the IR transmittance in the entire IR region. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, will accelerate the development of high power, broadly tunable laser sources in the IR which, in addition, will be offered with higher device quality and at a reasonably lower unit cost.

We report the generation of tunable high-repetition-rate picosecond radiation in the mid-infrared using the new quasi-phase-matched nonlinear material of orientation-patterned gallium phosphide (OP-GaP). The source is realized by single-pass difference-frequency-generation (DFG) between the output signal of a picosecond optical parametric oscillator (OPO) tunable across 1609-1637 nm with input pump pulses at 1064 nm in OP-GaP, resulting in tunable radiation across 3040-3132 nm. Using a 40-mm-long crystal, we have generated up to 57 mW of DFG average power at ~80 MHz repetition rate for a pump power of 5 W and signal power of 0.9 W, with >30 mW over >50% of the tuning range. The DFG source exhibits a passive power stability better than 3.2% rms over 1 hour in good spatial beam quality. To the best of our knowledge, this is the first picosecond frequency conversion source based on OP-GaP.

Recent decade brought a tremendous progress laser-based gas detection techniques. This progress would not be possible without the relentless devotion of the scientific community in developing state-of-the-art sources capable of targeting strong absorption lines of gases localized in the mid-infrared (mid-IR) wavelength region. Although narrowband, semiconductor-based mid-IR lasers offer non-complex implementation their tuning range is usually limited to several cm−1. Therefore designing a sensor capable of simultaneous monitoring of several gas species, even with absorption profiles clustered nearby, requires incorporating individual expensive quantum cascade laser (QCL) or interband cascade laser (ICL) sources into a single platform. Recent development of reliable mid-IR comb sources, combined with clever methods of signal processing allowed for designing gas sensors capable of simultaneous monitoring of several gas analytes in fractions of a second [1]. Obtaining broadband pulsed radiation in the mid-IR region is usually accomplished by utilizing nonlinear conversion or in recently presented on-chip QCL combs [2]. Although QCL combs offer compactness and efficiency they do not allow straightforward control of the repetition frequency (f rep ) or the carrier-envelope offset frequency (f CEo ), which are critical in some particular cavity-enhanced comb-based sensing techniques, e.g. Vernier spectroscopy [3]. Here we present a fully-fiberized, f CEO -free mid-IR optical frequency comb source capable of covering 6–9 μm wavelength region with tunable f rep . The setup of the source is presented in Fig 1. The heart of the source is a ring-cavity graphene mode-locked laser generating sub-250 fs pulses at 1560 nm and f rep of 100 MHz. The pulses are split using a 50/50 coupler and directed to separate sections of the source. In the blue section the source the pulses are amplified to 80 mW in an all-fiber-based chirped pulse amplifier (CPA) and recompressed in a 8 m-long section of PM1550 fiber to ∼200 fs. In the red section a custom manufactured nonlinear fiber with tailored dispersion (PCF) is utilized to soliton-shift the 1.55 μm pulses into the 1.9–2 μm wavelength region. Subsequently the pulses are amplified in a CPA and recompressed to ∼200 fs. Combined 1.55 and 2 μm pulses are focused on a 3 mm-long orientation patterned gallium phosphide (OP-GaP) crystal to generate mid-IR radiation via differential frequency generation (DFG). Since both pulses originated from the same ML source, the generated mid-IR frequency comb is inherently fceo-free and can be used in laser-based gas sensors requiring stable carrier-envelope offset. The source is built entirely from PM single-mode fibers.

We demonstrate the generation of broadband (6—12 μm) long-wave infrared frequency combs via intra-pulse difference-frequency generation in orientation-patterned gallium phosphide and use the source to perform dual-comb spectroscopy on methanol.

Here we report on recent advances in the development of orientation-patterned gallium phosphide (OP-GaP) - a low loss, non-critically phase-matched, 1- m-pumpable binary analog of ZGP with three times the thermal conductivity for high power applications.

6 - Mid-infrared frequency conversion in quasiphase matched semiconductors

We discuss the Cross-Reststrahlen band phasematching technique for narrow-band three-wave interactions. In GaP, assuming a maximum practical crystal length of 100 millimeters, the calculations predict that with a source wavelength of 0.965 micrometers, a tuning range of greater than 50 GHz is possible around the perfectly phasematched 3.0 THz center frequency. When GaP is pumped with a source wavelength of 1.000 micrometers, the coherence length is at least 100 mm for a frequency range of greater than 600 GHz around the perfectly phasematched 630 GHz center frequency. High Resistivity GaP is available in cylindrical boules that are larger than 50 millimeters in diameter and 75 millimeters in length. GaP has a bandgap cutoff wavelength in the Visible at 0.55 micrometers and a simple phonon absorption spectrum with a single fundamental absorption at approximately 27 micrometers, which indicates a potential for high transmission in both the NIR and FIR. Any remaining FIR absorption can be attributed to free electron absorption and two phonon absorption processes. In this paper, we report new measurements of GaP regarding FIR absorption, optical damage threshold and optical quality. These measurements indicate that undoped, high-resistivity GaP single crystal can be used to generate THz waves.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We demonstrate terahertz (THz) generation at megahertz repetition rate by optical rectification in GaP crystals, using excitation average power levels exceeding 100 W. The laser source is a state-of-the-art diode-pumped Yb:YAG SESAM-mode-locked thin-disk laser, capable of generating 580 fs pulses at an average power up to 120 W and a repetition rate of 13.4 MHz directly from a one-box oscillator, without the need for any extra amplification stages. In this first demonstration, we measure a maximum THz average power of 78 μW at a central frequency of 0.8 THz. Our results show that optical rectification of state-of-the-art high average power ultrafast sources in nonlinear crystals is within reach and paves the way toward high average power, ultrafast laser pumped THz sources.

We demonstrate THz generation by optical rectification in GaP crystals using excitation average power levels exceeding 100 $W$ . The laser source is a state-of-the-art diode-pumped Yb:YAG modelocked thin-disk laser, capable of generating 120W at 13.4 MHz directly from a one-box oscillator, without the need for any amplification stages. In this first demonstration, we measured a maximum average power of $78 \mu \mathrm{W}$ at a central frequency of 0.8THz. This first result shows that optical rectification of high average power (100 $W$ class) ultrafast sources is within reach, and paves the way towards Watt-level, ultrafast laser pumped THz sources.

The generation of THz pulses having tens of microjoules energy by tilted pulse-front excitation is reviewed. Possibilities of further up-scaling the THz energy as well as existing and future applications of these pulses are analyzed.

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Ultrashort pulses are capable of processing practically any material with negligible heat affected zone. Typical pulse durations for industrial applications are situated in the low picosecond-regime. Pulse durations of 5 ps or below are a well established compromise between the electron-phonon interaction time of most materials and the need for pulses long enough to suppress detrimental effects such as nonlinear interaction with the ablated plasma plume. However, sub-picosecond pulses can further increase the ablation efficiency for certain materials, depending on the available average power, pulse energy and peak fluence. Based on the well established TruMicro 5000 platform (first release in 2007, third generation in 2011) an Yb:YAG disk amplifier in combination with a broadband seed laser was used to scale the output power for industrial femtosecond-light sources: We report on a subpicosecond amplifier that delivers a maximum of 160 W of average output power at pulse durations of 750 fs. Optimizing the system for maximum peak power allowed for pulse energies of 850 μJ at pulse durations of 650 fs. Based on this study and the approved design of the TruMicro 5000 product-series, industrygrade, high average power femtosecond-light sources are now available for 24/7 operation. Since their release in May 2013 we were able to increase the average output power of the TruMicro 5000 FemtoEdition from 40 W to 80 W while maintaining pulse durations around 800 fs. First studies on metals reveal a drastic increase of processing speed for some micro processing applications.

Terahertz (THz) wave generation based on ultrafast laser are one of the most predominant and popular technologies. In particular, for THz generation by photoconductor antenna and optical rectification, higher optical to THz conversion efficiency usually requires ultrafast laser excitation into semiconductor or nonlinear crystal, therefore it is of great desire to develop ultrafast laser with high performance. CW Yb-doped lasers at 980 nm have been applied to generate THz wave range from 0.5 to 6 THz by DFG technique. To meet the demand of some THz wave generation (e.g. BNA crystal, gold nanoplasma), 980 nm ultrafast fiber laser are ideal pump sources and in desire to be developed. Here we demonstrate a 978 nm robust all-polarization-maintaining (PM) picosecond fiber laser mode- locked by phase-biased nonlinear amplifying loop mirror (PB- NALM). By introducing a nonreciprocal - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula> /2 phase shifter into the fiber loop, a self-starting stable mode-locking is obtained with the center wavelength of 978.14 nm and a 3-dB bandwidth of 0.43 nm. The pulse duration is 8.1 ps corresponding to the time bandwidth product (TBP) of 1.09. The maximum average output power is 14.5 mW with a repetition rate of 45.438 MHz. After a master oscillator power amplification system, the 978 nm average output power reaches 2.12 W, which is then injected into a PPLN crystal for second harmonic generation and 52 mW 489 nm ultrashort lasers is obtained. This is, to the best of our knowledge, the first report of PB-NALM-based mode-locked fiber laser at 978 nm. The 978 nm ultrafast fiber laser and SHG to 489 nm have great potential in terahertz emission by shorter NIR and blue light.

Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.

Attosecond science is one of the driving forces for developing the femtosecond amplifiers of high average power and ultrashort pulse duration. In this work, the regenerative amplification is studied experimentally and theoretically based on Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; crystal for the practical needs of high-repetition-rate attosecond light sources. In the theoretical study, a mode-tunable regenerative cavity with good thermal stability is designed based on the thermal lens calculations of Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; crystal; the amplified output energy and spectra of &lt;i&gt;π&lt;/i&gt; and &lt;i&gt;σ&lt;/i&gt; polarization of the crystal are calculated. In the experiment, the &lt;i&gt;π&lt;/i&gt;-axis of Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; crystal is parallel to the laser polarization, and the laser amplifier emits 1.61 mJ pulses with average power 16.1 W. Notably, the dip of the &lt;i&gt;π&lt;/i&gt;-polarization emission spectrum near 1025.1 nm compensates for the gain narrowing of the seed laser during amplification. Thus, the center wavelength and the spectral full width at a half maximum of the amplified laser are 1030 nm and 16 nm respectively. Using a grating-pair for compression, 149 fs pulses with peak power 9.5 GW are obtained. In comparison, the σ-polarization emission spectrum of Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; crystal is relatively flat in a range from 1000 to 1050 nm, but with a larger gain cross-section. When the laser polarization is parallel to the &lt;i&gt;σ&lt;/i&gt;-axis of Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; crystal, 2.87 mJ pulses at 10 kHz repetition rate are achieved, with an average power of 28.7 W. In this case, the center wavelength and the spectral full width at half maximum of the amplified laser are 1037 nm and 11 nm respectively. Using a grating-pair for compression, 178 fs pulses with peak power of 14.2 GW are obtained. The beam quality factor measured is 1.09 along the &lt;i&gt;x&lt;/i&gt;-axis of the amplified laser and 1.17 along the &lt;i&gt;y&lt;/i&gt;-axis. To the best of our knowledge, this is the highest average power and the maximum pulse energy obtained from the Yb:CaYAlO&lt;sub&gt;4&lt;/sub&gt; amplifier. For applications in high-repetition-rate attosecond light sources, terahertz generation and optical parametric amplification, subsequent laser outputs with average power 200 W, pulse energy 20 mJ and pulse duration less than 200 fs are expected to be achieved by adding two stages of traveling-wave amplification.