A highly stable monolithic enhancement cavity for second harmonic generation in the ultraviolet

High power UV lasers employing elliptically focusing enhancement cavities

Continuous wave UV lasers have a plethora of applications in many fields, among others, laser spectroscopy, atom trapping, and other related areas. We present a high power continuous wave UV laser system based on twofold second harmonic generation of IR radiation for cooling bunched relativistic ion beams and laser spectroscopy with an output power exceeding 2.4 W in the UV range. Laser operation in the UV regime is often limited by degradation of β-barium borate (BBO) due to two-photon absorption (TPA). We were able to overcome this issue with a novel design of an elliptically focusing enhancement cavity employing commercially available BBO as the nonlinear medium. Building on previous results, we implemented a novel elliptically focusing enhancement cavity featuring an extreme waist ratio of w0,sag/w0,tan = 27.5 at the center of the BBO crystal, aiming to further reduce the peak intensity. This paper details the results as well as our advanced locking scheme for the enhancement cavities for both second harmonic generation steps.

Characterization of organic crystals for second-harmonic generation

Second-harmonic generation (SHG) is a common technique with many applications. Common inorganic single-crystalline materials used to produce SHG light are effective using short IR/visible wavelengths but generally do not perform well at longer, technologically relevant IR wavelengths such as 1300, 1550, and 2000 nm. Efficient SHG materials possess many of the same key material properties as terahertz (THz) generators, and certain single-crystalline organic THz generation materials have been reported to perform at longer IR wavelengths. Consequently, this work focuses on characterizing three efficient organic THz generators for SHG, namely, DAST (trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate), DSTMS (4-N,N-dimethylamino-4'-N'-methylstilbazolium 2,4,6-trimethylbenzenesulfonate), and the recently discovered generator PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). All three of these crystals outperform the beta-barium borate (BBO), an inorganic material commonly used for SHG, using IR pump wavelengths (1200-2000 nm).

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE₅ (C₃ N₃ O₃ )(OH)₁₂ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH₂ PO₄ ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C₃ N₃ O₃ )^3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Phase-stabilized UV light at 267 nm through twofold second harmonic generation

Providing phase stable laser light is important to extend the interrogation time of optical clocks towards many seconds and thus achieve small statistical uncertainties. We report a laser system providing more than 50 µW phase-stabilized UV light at 267.4 nm for an aluminium ion optical clock. The light is generated by frequency-quadrupling a fibre laser at 1069.6 nm in two cascaded non-linear crystals, both in single-pass configuration. In the first stage, a 10 mm long PPLN waveguide crystal converts 1 W fundamental light to more than 0.2 W at 534.8 nm. In the following 50 mm long DKDP crystal, more than 50 µW of light at 267.4 nm are generated. An upper limit for the passive short-term phase stability has been measured by a beat-node measurement with an existing phase-stabilized quadrupling system employing the same source laser. The resulting fractional frequency instability of less than 5×10^-17 after 1 s supports lifetime-limited probing of the ²⁷Al⁺ clock transition, given a sufficiently stable laser source. A further improved stability of the fourth harmonic light is expected through interferometric path length stabilisation of the pump light by back-reflecting it through the entire setup and correcting for frequency deviations. The in-loop error signal indicates an electronically limited instability of 1 × 10^-18 at 1 s.

Linien: A versatile, user-friendly, open-source FPGA-based tool for frequency stabilization and spectroscopy parameter optimization

We present a user-friendly and versatile tool for laser frequency stabilization. Its main focus is spectroscopy locking, but the software is suitable for lock-in techniques in general as well as bare proportional-integral-derivative (PID) operation. Besides allowing for sinusoidal modulation (up to 50 MHz), triangular ramp scanning, in-phase and quadrature demodulation (1-5 f), infinite impulse response, and PID filtering, Linien features two different algorithms for automatic lock point selection; one of them performs time-critical tasks completely on field-programmable gate arrays. Linien is capable of autonomously optimizing spectroscopy parameters by means of machine learning and can measure the error signal's power spectral density. The software is built in a modular way, providing both a graphical user interface as well as a Python scripting interface. It is based on the RedPitaya STEMLab platform but may be ported to different systems.

High-power, frequency-quadrupled UV laser source resonant with the 1S0-3P1 narrow intercombination transition of cadmium at 326.2 nm

We present a novel high-power, frequency-stabilized UV laser source at 326.2 nm, resonant with the Cd ¹S₀-³P₁ narrow intercombination transition. We achieve a maximum produced power of 1 W at 326.2 nm by two successive frequency doubling stages of a narrow-linewidth (<1 kHz) seed laser at 1304.8 nm. Approximately 3.4 W of optical power at 652.4 nm is produced by a visible Raman fiber amplifier (VRFA) that amplifies and generates the second harmonic of the infrared radiation. The visible light is subsequently frequency-doubled down to 326.2 nm in a nonlinear bow-tie cavity using a Brewster-cut beta-barium-borate (BBO) crystal, with a maximum conversion efficiency of approximately 40% for 2.5 W of coupled red power. Full characterization of the laser source, together with spectroscopy signals of all Cd isotopes, spanning more than 4 GHz in the UV, are shown.

Stable 2 W continuous-wave 261.5 nm laser for cooling and trapping aluminum monochloride

We present a high-power tunable deep-ultraviolet (DUV) laser that uses two consecutive cavity enhanced doubling stages with LBO and CLBO crystals to produce the fourth harmonic of an amplified homebuilt external cavity diode laser. The system generates up to 2.75 W of 261.5 nm laser light with a ∼2 W stable steady-state output power and performs second harmonic generation in a largely unexplored high intensity regime in CLBO for continuous wave DUV light. We use this laser to perform fluorescence spectroscopy on the A¹Π ← X¹Σ⁺ transition in a cold, slow beam of AlCl molecules and probe the A¹Π|v' = 0, J' = 1〉 state hyperfine structure for future laser cooling and trapping experiments. This work demonstrates that the production of tunable, watt-level DUV lasers is becoming routine for a variety of wavelength-specific applications in atomic, molecular and optical physics.

A flexible system-on-a-chip control hardware for atomic, molecular, and optical physics experiments

We have implemented a control system core for experiments in atomic, molecular, and optical physics based on a commercial low-cost board, featuring a field-programmable gate array as part of a system-on-a-chip on which a Linux operating system is running. The board features Gigabit Ethernet, allowing for fast data transmission and operation of remote experimental systems. A single board can control a set of devices generating digital, analog, and radio frequency signals with precise timing given either by an external or internal clock. Contiguous output and input sampling rates of up to 40 MHz are achievable. Several boards can run synchronously with a timing error approaching 1 ns. For this purpose, a novel auto-synchronization scheme is demonstrated, with possible application in complex distributed experimental setups with demanding timing requests.

Watt-level blue light for precision spectroscopy, laser cooling and trapping of strontium and cadmium atoms

High-power and narrow-linewidth laser light is a vital tool for atomic physics, being used for example in laser cooling and trapping and precision spectroscopy. Here we produce Watt-level laser radiation at 457.75 nm and 460.86 nm of respective relevance for the cooling transitions of cadmium and strontium atoms. This is achieved via the frequency doubling of a kHz-linewidth vertical-external-cavity surface-emitting laser (VECSEL), which is based on a novel gain chip design enabling lasing at > 2 W in the 915-928 nm region. Following an additional doubling stage, spectroscopy of the ¹S₀ → ¹P₁ cadmium transition at 228.87 nm is performed on an atomic beam, with all the transitions from all eight natural isotopes observed in a single continuous sweep of more than 4 GHz in the deep ultraviolet. The absolute value of the transition frequency of ¹¹⁴Cd and the isotope shifts relative to this transition are determined, with values for some of these shifts provided for the first time.

XUV frequency comb production with an astigmatism-compensated enhancement cavity

We have developed an extreme ultraviolet (XUV) frequency comb for performing ultra-high precision spectroscopy on the many XUV transitions found in highly charged ions (HCI). Femtosecond pulses from a 100 MHz phase-stabilized near-infrared frequency comb are amplified and then fed into a femtosecond enhancement cavity (fsEC) inside an ultra-high vacuum chamber. The low-dispersion fsEC coherently superposes several hundred incident pulses and, with a single cylindrical optical element, fully compensates astigmatism at the w₀ = 15 µm waist cavity focus. With a gas jet installed there, intensities reaching ∼ 10¹⁴ W/cm² generate coherent high harmonics with a comb spectrum at 100 MHz rate. We couple out of the fsEC harmonics from the 7th up to the 35th (42 eV; 30 nm) to be used in upcoming experiments on HCI frequency metrology.

Frequency ratio of an 115In+ ion clock and a 87Sr optical lattice clock

We report on the first, to the best of our knowledge, frequency ratio measurement of an ¹¹⁵In⁺ singleion clock and a ⁸⁷Sr optical lattice clock. A hydrogen maser serves as a flywheel oscillator to measure the ratio by independent optical combs. From 89,000 s of measurement time, the frequency ratio f_In/f_Sr is determined to be 2.952 748 749 874 863 3(23) with 7.7×10^-16 relative uncertainty. The measurement creates a new connection in the network of frequency ratios of optical clocks.

Analytical approximation of the second-harmonic conversion efficiency

The second-harmonic generation process of a focused laser beam inside a nonlinear crystal is described by the Boyd-Kleinman theory. Calculating the actual conversion efficiency and upconverted power requires the solution to a double integral that is analytically intractable. We provide an expression that predicts the exact gain coefficient within an error margin of less than 2% over several orders of magnitude of the confocal parameter and as a function of the walk-off parameter. Our result allows for readily tuning the beam parameters to optimize the performance of the upconversion process and improve optical system designs.

Digital laser frequency and intensity stabilization based on the STEMlab platform (originally Red Pitaya)

We report on the development, implementation, and characterization of digital controllers for laser frequency stabilization as well as intensity stabilization and control. Our design is based on the STEMlab (originally Red Pitaya) platform. The presented analog hardware interfaces provide all necessary functionalities for the designated applications and can be integrated in standard 19-in. rack mount units. Printed circuit board layouts are made available as an open-source project (T. Preuschoff et al., https://github.com/TU-Darmstadt-APQ/RedPitaya-Lockbox, 2020 and T. Preuschoff et al., https://github.com/TU-Darmstadt-APQ/RedPitaya-IntStab, 2020). A detailed characterization shows that the bandwidth (1.25 MHz) and the noise performance of the controllers are limited by the STEMlab system and not affected by the supplementary hardware. Frequency stabilization of a diode laser system resulting in a linewidth of 52(1) kHz (FWHM) is demonstrated. Intensity control to the 1 × 10^-3 level with sub-microsecond rise and fall times based on an acousto-optic modulator as actuator is achieved.

Robust and Resource-Efficient Microwave Near-Field Entangling ^{9}Be^{+} Gate

Microwave trapped-ion quantum logic gates avoid spontaneous emission as a fundamental source of decoherence. However, microwave two-qubit gates are still slower than laser-induced gates and hence more sensitive to fluctuations and noise of the motional mode frequency. We propose and implement amplitude-shaped gate drives to obtain resilience to such frequency changes without increasing the pulse energy per gate operation. We demonstrate the resilience by noise injection during a two-qubit entangling gate with ^{9}Be^{+} ion qubits. In the absence of injected noise, amplitude modulation gives an operation infidelity in the 10^{-3} range.