RELATED APPLICATIONS

This application claims priority to GB Application No. 2001865.1 filed on Feb. 11, 2020, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to circular dichroism (CD) spectroscopy in the infrared (IR). CD spectroscopy in the IR region is referred to as vibrational CD (VCD). VCD is a widely used standard method for determination of absolute stereochemistry. VCD is also used for biomolecule characterization and for following dynamic processes. Initial VCD instruments were constructed in the same manner as conventional ultraviolet-to-visible (UV-vis) circular dichroism (CD) spectrometers, but with various components altered to provide functionality in the infrared (IR). Typical CD instruments have bright broadband sources and dispersive optics that utilized a sequentially scanned prism or grating to select the wavelength bandpass that passed through a slit. To operate in the IR and maximize signal-to-noise ratio (S/N), many alterations to conventional CD designs were needed. Most VCD spectrometers use grid polarizers, holographically formed on IR transmitting, isotropic substrates. The monochromator output is passed through a wire-grid polarizer and a photo-elastic modulator (PEM) to induce a fast polarization modulation, where fast means in the tens of kHz range. Semiconductor detectors are used to provide the sensitivity needed to detect very low amplitude modulations, and to give the speed of response necessitated by the high modulation frequencies. The detector signal is then demodulated at the PEM frequency to retrieve the VCD signal [1, 2].

The pioneering work of Nafie and co-workers in the development of Fourier transform (FT) VCD instrumentation shifted the methodology, and such instruments now dominate the VCD field. They commonly utilize a commercial FTIR to which polarization optics as well as processing electronics and software are added for purposes of VCD measurement. The interferometer used is typically of rapid-scan design, incorporating a KBr beam splitter coated for mid-IR use and a glow bar or other heated ceramic source. Its output beam is directed through a polarizer (wire grid) and PEM (ZnSe) and onto a sample, after which it is focused onto a Mercury Cadmium Telluride (MCT) detector, typically with a ZnSe lens [1, 3].

More recently, Lüdeke and co-workers have developed a quantum cascade laser (QCL) VCD system which replaces the broadband IR source, e.g. glow bar, and monochromator of previous systems with a tunable laser source in the form of a QCL. Their results demonstrated that a QCL source can be used to measure VCD and replicate FT-VCD results [1, 4].

Notwithstanding these significant advances in VCD, the main issue preventing the more widespread use of VCD is the long measurement time required for obtaining good signal-to-noise ratio (SNR) VCD data. Typically, VCD signals of fundamental transitions are ten thousand to one hundred thousand times lower than normal absorption signals. Since, for avoidance of non-linearity issues, the normal absorption must be kept below one optical density (OD), the signals to be detected are generally on the order of 10-100 micro optical density (μOD). To obtain a signal to noise ratio of at least SNR=10, measurement times of several hours are common.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of acquiring a circular dichroism spectrum measuring differential absorption of left and right circularly polarized light by a sample over a wavelength range of interest, the method comprising:arranging a sample in a sampling beam path;arranging a polarization modulator in the sampling beam path before the sample, the polarization modulator being switchable between two states to output left and right circularly polarized light respectively;combining first and second light signals containing respective first and second frequency combs into the sampling beam path, the first and second frequency combs consisting of respective finite series of discrete frequencies separated by first and second spacings defined as first and second free spectral ranges respectively;arranging a sample detector operable to output a measurement signal in the sampling beam path after the sample, the sample detector having a detection bandwidth that covers a frequency range corresponding to beat frequencies of a third frequency comb that is detected by the sample detector as the measurement signal as a result of heterodyne mixing of the first and second light signals;periodically switching the polarization modulator between its two states at a polarization modulation frequency to output left and right circularly polarized light respectively; andsetting the frequencies of the first and second light signals such that the beat frequencies of the third frequency comb are higher than the polarization modulation frequency so that for each of the left and right circularly polarized light the sample detector's measurement signal measures transmission through the sample across the wavelength range of interest during each period of the polarization modulation.

Embodiments of the disclosure can thus provide a VCD spectroscopy method and corresponding spectrometer that significantly reduce the measurement time needed to acquire a differential absorption spectrum with good signal-to-noise ratio compared to known VCD methods and apparatus. In our approach, we acquire a full vibrational spectrum within each polarization modulation, instead of only acquiring data for a single wavelength during each modulation of the polarization (as in dispersive or tunable laser VCD) or a single Fourier component of the spectrum (as in FT VCD). Our experimental results with a pair of complimentary test samples show that a VCD spectrum with good signal-to-noise ratio can be acquired in as little as two seconds of measurement time.

In some embodiments, the third frequency comb's beat frequencies, i.e. beat notes, are synchronized with the periodic switching of the polarization modulator, e.g. using a suitable controller, such that each of the beat frequencies is an integer multiple of the polarization modulation frequency, e.g. 2, 3, 4, 5, etc., or 52, 53, 54, 55 etc. or any higher harmonics, e.g. 25, 28, 31, 34, etc. (i.e. increments of more than one).

In other embodiments, the third frequency comb's beat frequencies, i.e. beat notes, are between 1-2 and 4-5 orders of magnitude higher than the polarization modulation frequency. While in most implementations, there will be an orders of magnitude difference in the respective frequencies, the ratio of frequencies could be as low as two, i.e. the beat frequencies of the third frequency comb should be at least twice the polarization modulation frequency; assuming that all beat frequencies are retained in the determination of the differential absorption spectrum. If the lowest beat frequencies were discarded from the collected signal data, then the ratio could be less than two.

According to a second aspect of the invention there is provided a QCL dual comb circular dichroism spectrometer operable to measure a sample's differential absorption of left and right circularly polarized light over a wavelength range of interest. The spectrometer comprises:a polarization modulator that is periodically switchable between two states at a polarization modulation frequency to output left and right circularly polarized light respectively;a sample holder configured to receive a sample to be measured in a sample space so that a sample held in the sample holder receives light output from the polarization modulator;a first semiconductor laser source, serving as a local oscillator, operable to output a first light signal containing a first frequency comb consisting of a finite series of discrete frequencies separated by a first spacing defined as a first free spectral range;a second semiconductor laser source, serving as an interrogator, operable to output a second light signal containing a second frequency comb consisting of a finite series of discrete frequencies separated by a second spacing defined as a second free spectral range;a beam combiner configured to receive and superimpose the first and second light signals to form a combined light signal and to direct the combined light signal to be input into the polarization modulator;a sample detector operable to output a measurement signal and arranged to receive the combined light signal after it has traversed the sample space, the sample detector having a detection bandwidth that covers a frequency range corresponding to beat frequencies of a third frequency comb that is detected by the sample detector as the measurement signal as a result of heterodyne mixing of the first and second light signals;a controller configured to output first and second control signals to the first and second semiconductor laser sources to set the frequencies of the first and second light signals such that the resultant beat frequencies of the third frequency comb are higher than the polarization modulation frequency so that for each of the left and right circularly polarized light the sample detector's measurement signal measures transmission through the sample across the wavelength range of interest during each period of the polarization modulation; anda data acquisition module arranged to collect data from the measurement signal and configured to determine a differential absorption spectrum for a sample over the wavelength range of interest from the collected data.

In some embodiments, the spectrometer further comprises a frequency demodulator arranged to extract an error signal from the measurement signal, the error signal being fed back to the controller, the controller being configured to determine a correction signal from the error signal in a control loop to at least one of the first and second semiconductor laser sources derived from the error signal so as to lock the first laser source to the second semiconductor laser source in a master-slave configuration.

In other embodiments, the spectrometer further comprises a first RF generator operable to generate a first reference signal at a first RF frequency that is matched to the first free spectral range, the first RF generator being connected to inject the first reference signal into the cavity of the first semiconductor laser source to lock the first free spectral range to the first RF frequency and optionally also a second RF generator operable to generate a second reference signal at a second RF frequency that is matched to the second free spectral range, the second RF generator being connected to inject the second reference signal into the cavity of the second semiconductor laser source to lock the second free spectral range to the second RF frequency.

The semiconductor laser sources may be quantum cascade lasers (QCLs) based on transitions between subbands, or interband cascade lasers (ICLs) based on transitions between bands.

In our implementation, the polarization modulator comprises a linear polarizer arranged in series in the sampling beam path with a photo-elastic modulator (PEM). The photo-elastic modulator having a resonance frequency that defines the polarization modulation frequency. A PEM is made of a section of a material that is optically transparent over its operating wavelength range and which exhibits strain-induced birefringence when resonated at its resonance frequency. Suitable materials for infrared operating wavelengths suitable for VCD are calcium fluoride, silicon and zinc selenide, for example. The bar is dimensioned to provide a resonance frequency of a desired magnitude. The PEM also includes a transducer, typically a piezoelectric transducer, which is mechanically connected to the bar of birefringent material and can be driven at the bar's resonance frequency in order to resonate the bar. A PEM is thus only operable at its resonance frequency. Commercially available PEMs that are suitable for use in embodiments of the disclosure typically have resonance frequencies between 20 kHz to 100 kHz, such as the 37 kHz ZnSe PEM we have used in our experiments that are discussed further below.

In practical implementations some kind of data acquisition module will be used to collect data from the measurement signal, wherein the data contained in the measurement signal in each period of the polarization modulation is sufficient to determine a differential absorption spectrum over the wavelength range of interest. The data acquisition module may additionally include a processor operable to determine the differential absorption spectrum from the collected data. The data acquisition module may serve to aggregate the differential absorption spectra that are obtained from the measurement signal in each polarization modulation period. As the data accumulates, the SNR will improve.

In addition to a sample detector, it is useful to also have a normalizing detector. The normalizing detector, like the sample detector, has a detection bandwidth covering the third frequency comb. The normalizing detector is arranged to receive a combined light signal from superimposing the first and second light signals, neither of which has traversed the sample. The signal from the normalizing detector can be used to normalize the signal from the sample detector, e.g. by dividing the amplitudes of the beat notes measured in the sample detector signal by the amplitudes of the beat notes measured in the normalizing detector signal. The data acquisition module is arranged also to collect the signal from the normalizing detector, which we refer to as a normalizing signal. The normalizing signal can then be compared with the simultaneously obtained measurement signal.

In some embodiments, the first and second laser sources are free running. By free running we mean that the lasers are allowed to lase optically independently of each other, rather than being locked together, e.g. by phase locking. In other embodiments, the first and second laser sources may be locked, e.g. in a master-slave relationship.

In some embodiments, the first and second control signals are voltage signals that are input to first and second current drivers, which translate the first and second control signals into a desired setting of the first and second drive currents applied to the first and second laser sources, thereby to fix the first and second frequency combs at desired positions in frequency space. This will likely be the most common implementation, but there are other ways to transfer the control signals onto the laser sources. Instead or as well as the above, the first and second control signals, or first and second components thereof, may be applied to adjust the temperature of the first and second laser sources via first and second heaters arranged in thermal communication with the first and second laser sources. Other possibilities may also be available based on any parameter that can be varied so as to set refractive index or temperature of the sources, e.g. broadband illumination of a part of the source.

