Stimulated Raman scattering spectroscopy with quantum-enhanced balanced detection.

Quantum-enhanced stimulated Raman scattering (QE-SRS) is a promising technique for highly sensitive molecular vibrational imaging and spectroscopy surpassing the shot noise limit. However, the previous demonstrations of QE-SRS utilized rather weak optical power which hinders from competing with the sensitivity of state-of-the-art SRS microscopy and spectroscopy using relatively high-power optical pulses. Here, we demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, which works even when using high-power optical pulses. We used 4-ps pulses to generate pulsed squeezed vacuum at a wavelength of 844 nm with a squeezing level of -3.28 ± 0.12 dB generated from a periodically-poled stoichiometric LiTaO3 waveguide. The squeezed vacuum was introduced to an SRS spectrometer employing a high-speed spectral scanner to acquire QE-SRS spectrum in the wavenumber range of 2000-2280 cm-1 within 50 ms. Using SRS pump pulses with an average power of 11.3 mW, we successfully obtained QE-SRS spectrum whose SNR was better than classical SRS with balanced-detection by 2.27 dB.

Phase locking of squeezed vacuum generated by a single-pass optical parametric amplifier.

In high-precision optical measurements, squeezed vacuum states are a promising resource for reducing the shot noise. To utilize a squeezed vacuum, it is important to lock the phase of the local oscillator (LO) to the squeezed light. The coherent control sideband (CCSB) scheme has been established for the precise phase locking, while the previous CCSB scheme was designed for the squeezed vacuum generated with an optical parametric oscillator (OPO). Thus the previous CCSB scheme is not applicable to squeezing by a single-pass optical parametric amplifier (OPA), which is attractive for generating broadband squeezed vacuum states. In this study, we propose a variant of CCSB scheme, which is applicable to the squeezing by single-pass OPA. In this scheme, we inject pump light and frequency-shifted signal light into an OPA crystal in the same way as the previous CCSB scheme. The parametric process in the OPA crystal generates a squeezed vacuum, amplifies the signal light, generates an idler light, and causes the pump depletion reflecting the interference of the amplified signal light and the idler light. Through the lock-in detection of the pump depletion, we can phase-lock the injected signal light to the pump light. Then, after the heterodyne detection of the signal and the idler light, we get the error signal of LO and realize the precise phase locking of LO to the squeezed quadrature. We show the feasibility of the proposed scheme by deriving the signal-to-noise ratio (SNR) of the modulated pump signal. We experimentally demonstrate the proposed scheme on pulsed squeezing by a single-pass OPA.

Development of an interference-filter-type external-cavity diode laser for resonance ionization spectroscopy of strontium.

A frequency tunable external-cavity diode laser (ECDL) using a narrow bandwidth (∼0.3 nm) interference filter has been developed for resonance ionization spectroscopy of strontium (Sr) with high isotopic selectivity. Improved wavelength and single mode stabilities of this interference-filter-type ECDL (IF-ECDL) over a commonly used (also home-made) Littrow-type ECDL were theoretically expected and experimentally confirmed by both a wavelength meter and a home-made Fabry-Perot interferometer. The measured spectral profile of the dominant isotope 88Sr using our IF-ECDL in the 689.4 nm intercombination transition shows that the Lorentzian component (∼1.3 MHz) of the spectrum width is consistent with the obtained fringe width of the interferometer. High 90Sr isotopic selectivity of ∼104 with respect to 88Sr is expected in this transition, which indicates that even if the manufacturing accuracy is not comparable to commercial Littrow-type ECDLs, our compact IF-ECDL having sufficient wavelength stability is a promising laser source for background-free analysis of radioactive 90Sr in marine samples.