Study design

Cesium Rydberg atoms convert the difficult-to-detect THz fields into visible PL spectra. The basic principle is that the electric dipole transitions between the adjacent Rydberg states of cesium atoms lie within the THz frequency range. The spontaneous emission process of two adjacent Rydberg states has different decay pathways owing to their transition probabilities and selection rules. Therefore, the PL spectra of the two neighboring Rydberg states are crucial for detecting THz fields. In our demonstration, a rectangular vapor cell serving as a warm cesium atom Rydberg detector is shown in Fig. 1a. Three laser beams, shaped by plano-convex and plano-concave cylindrical lenses, define an interaction region filled with cesium atoms via thermal atomic motion inside the vapor cell. In the region where the THz field and laser beams coexist, the atoms are excited to the final Rydberg state by the THz field and then fluoresce as they decay in the visible PL signal.

Fig. 1Full-screen View

(Color online) (a) Experiment layout. The THz beam (0.548 THz) is perpendicular to the laser beam passing through the cesium vapor cell. (b) Cesium atomic energy levels and laser excitation scheme. The PL_WithoutTHz and PL_WithTHz signals of cesium atoms decay from the 14P_3/2 state and 13D_5/2 state

Figure 1 a shows schematic diagrams of a 5-level energy ladder of cesium atoms and lasers, where three laser beams excite cesium atoms to a Rydberg state, and the THz field further excites the cesium atoms to the final Rydberg state. The probe laser (5 mW at 852 nm) is tuned between the ground 6P_1/2 state and 6P_3/2 state, while the coupling laser (20 mW at 1470 nm) and Rydberg laser (200 mW at 843 nm) are coupled to the 6P_3/2 →7S_1/2 and 7S_1/2 → 14P_3/2 transitions, respectively. The final Rydberg state 14P_3/2 → 13D_5/2 transition is in the THz field (548.613 GHz at 1.58 mW). In contrast to previously presented THz detection schemes, this scheme is used to sense the THz field through the PL signal.

In the absence of the THz field, the populated 14P_3/2 state emits spontaneous decay containing visible fluorescence, which is referred to as the “PL_WithoutTHz” signal. In contrast, when the THz field is present, the population is transferred to the 13D_5/2 state by the THz field, and the PL signal emitted from the decay of the 13D_5/2 state is designated as the “PL_WithTHz” signal (Fig. 1b).

Experimental set up

To strictly control the quality and availability of the dataset, a detailed introduction to the system parameters is required. The filling vapor cell pressure employed in the cesium atomic vapor was maintained at 10^-4 Pa [25]. The dimensions of the cesium atomic vapor cell were 1 cm ×1 cm × 2 cm, with a laser path length of 1 cm and a quartz wall thickness of 1 mm. The sealed cesium atomic vapor is heated to approximately 62.5 ℃ to increase the vapor pressure and optimize the PL signal [19].

The strategy to excite cesium atoms to Rydberg states involves a three-step excitation process, after which THz further excites these atoms to the final Rydberg state. The probe laser was generated using an external cavity semiconductor laser (ECL801). This laser was operated at a wavelength of 852 nm with a power output of 5 mW. The coupling laser used was a Distributed Bragg Reflector semiconductor laser with a wavelength of 1470 nm and a power of 20 mW. The Rydberg laser used was a Tapered Amplifier laser (TAL801 and ECL801) with a wavelength of 843 nm and a power of 200 mW. The laser beam was shaped using a plano-concave cylindrical lens and a plano-convex cylindrical lens, creating a 1 cm × 1 cm two-dimensional light sheet in atomic vapor. All lasers used in the experiment were purchased from Uniquanta Co., Ltd. The frequency range of the THz source is between 535 GHz and 570 GHz. It is equipped with a WR-1.5 DH rectangular standard waveguide as the output port, which provides a power of 1.58 mW.

The PL spectra were measured using a HORIBA iHR320 spectrometer and a charge-coupled device (CCD, SYNCER-1024 × 256). The spectrometer grating has a groove density of 1800 g/mm, achieving a resolution of ∼0.05 nm. The spectrometer and CCD system were calibrated using a mercury lamp as a standard light source. The testing range for the cesium atomic PL spectra was 450–730 nm.

Quality control and existing use of data

An alkaline Rydberg calculator (ARC) was used to calculate the laser and corresponding energy levels of the excited cesium atoms, ensuring the rationality of the experiment. To ensure data quality, the equipment underwent standard spectral calibration, and multiple measurements were conducted on the obtained data to verify consistency. The evaluation of the test results involved the use of an ARC calculator for the rationality analysis of the PL spectral data, ensuring that the data were valid. The relevant data has been published in the Science Data Bank, and a complete overview of the dataset, including the “title”, “description”, “data”, and “information”.

Photoluminescence spectra

To verify the performance of cesium atom THz detectors and understand the detection mechanism of cesium atoms in the THz field, we conducted experiments on the PL spectra of the adjacent Rydberg states. The PL spectra have also been shown to affect the vapor cell temperature, THz frequency, and THz field intensity. In the differential PL spectra, a positive signal signifies an increase in PL spectra intensity caused by the transfer to the final Rydberg state and an increase in transition probability, whereas negative signals indicate a decrease in PL signal due to the population transfer to the final Rydberg states.

Figure 2a illustrates the excitation of cesium atoms’ PL spectra by the three lasers in the absence of a THz field, covering the range of 450–730 nm. The PL_WithoutTHz signal of the excited cesium atom has peaks at 534.9 nm and 632.2 nm. This result indicates that the population transfers from $11D_{3/2}$ to 6P_1/2 and from 14P_3/2 to 5D_3/2. The PL_WithTHz spectra of the 0.548 THz induced cesium atoms are shown in Fig. 2b, with the amplification of the peak features at 534.9 nm and a typical peak at 632.2 nm. The PL signal peaks at 534.9 nm and 632.2 nm may be attributed to the $13D_{5/2}\to 6P_{3/2}$ and the $14P_{3/2}\to 5D_{3/2}$. The PL_WithTHz signal clearly shows a stronger peak at 534.9 nm, indicating a higher transition efficiency from 13D_5/2 to 6P_3/2.

Fig. 2Full-screen View

(a) PL spectra of cesium atoms without THz. (b) PL spectra of cesium atoms with THz

To better understand the correspondence between the spectra and the decay pathways, the transition channels in the PL spectra were calculated using ARC. The peaks of the PL spectra were identified based on the potential decay pathways and grouped according to the different channels (Fig. 3). The black curve in Fig. 3 represents the PL_WithTHz spectra, which cover the PL spectra in the range of 500–730 nm. The vertical lines in different colors correspond to the wavelengths of the calculated transition channels. The first row of vertical lines illustrates the decay pathways from nS_1/2 to 6P_1/2 and nS_1/2 to 6P_3/2 (n=9-15). The second row shows the decay pathways from nP_1/2 to 5D_3/2 and nP_3/2 to 5D_5/2 (n=10-15). The third row corresponds to the decay pathways from nD_3/2 to 6P_1/2 and from nD_5/2 to 6P_3/2 (n=7-15). The fourth row shows the decay pathways from nF_5/2 to 5D_3/2 and nF_7/2 to 5D_5/2 (n=7-15). Notably, most peaks in the measured spectra were accounted for in the calculations.

Fig. 3Full-screen View

(Color online) Identifying PL lines by spectral series. The vertical lines from top to bottom represent the decay pathways from nS_1/2 to 6P_1/2 and from nS_1/2 to 6P_3/2 (n=9-15), the decay pathways from nP_1/2 to 5D_3/2 and from nP_3/2 to 5D_5/2 (n=10-15), the decay pathways from nD_3/2 to 6P_1/2 and from nD_5/2 to 6P_3/2 (n=7-15), and the decay pathways from nF_5/2 to 5D_3/2 and from nF_7/2 to 5D_5/2 (n=7-15)

It is noteworthy that, at 534.9 nm of cesium atoms, the peaks of PL_WithTHz and PL_WithoutTHz signals display a significant difference in signal intensity, suggesting a diverse transition efficiency for the PL spectra of 14P and 13D states. The THz-induced PL characteristic spectra of cesium atoms can be expressed as the PL_WithTHz spectra subtracted from the PL_WithoutTHz spectra (Differential PL spectra, Fig. 4). The results of representative processed PL spectra are provided at several different intensities in Fig. 4, where the enhanced PL signals are at 519.6 nm and 534.9 nm, while the other PL peaks are all attenuated signals. For the THz field-induced case, the differential PL spectra of the 14P_3/2 and 13D_5/2 states arise from the variations in the transition probabilities and electric dipole moments, resulting in differing efficiencies of the decay channels. Thus, these differential PL spectra can serve as a fingerprint spectrum for a 0.548 THz field.

Fig. 4Full-screen View

Differential PL spectra of cesium atoms with PL_WithTHz spectra subtracted from the PL_WithoutTHz spectra. Differential signal can reveal the PL spectral changes induced by the THz field

Figure 5a shows the PL spectra of cesium atoms without the THz field, spanning the range of 20–62.5 ℃. The PL signal peak at 534.9 nm and 632.2 nm rises with increasing temperature. It is worth noting that the PL signal intensity at 632.2 nm (20–45 ℃) is stronger in the lower temperature range. As the temperature increases, the PL signal at 534.9 nm gradually exceeds the 632.2 nm signal peak at the same temperature (>45 ℃). However, in the case of the THz field, both the 534.9 nm signal and the 632.2 nm signal show a phenomenon where the PL signal becomes stronger as the temperature increases, and the 534.9 nm signal peak is greater than the 632.2 nm signal peak at any temperature in Fig. 5b. Therefore, these results suggest that higher temperatures facilitate the distinction of the 534.9 nm signal from other signals.

Fig. 5Full-screen View

(Color online) The PL signal peak at 534.9 and 632.2 nm of cesium atoms (a) without the THz filed and (b) with the THz field under cesium atomic vapor cell temperatures ranging from 20–62.5 ℃

Evolution of PL with THz Detuning and Detector Sensitivity

To investigate the influence of varying THz detuning on the PL spectra, the differential PL spectra evolution under THz detuning was characterized, as shown in Fig. 6a. The red-shaded area represents signals in which the differential PL signal is positive when the THz field is tuned. The blue-shaded area corresponds to the signal where PL spectra are suppressed and the differential PL signal is negative. When the THz field is present, the maximum enhanced peaks of the PL signal are 534.9 nm and 519.3 nm, corresponding to the strongest emission lines (decay from the 13D state) at a center frequency of 548.613 GHz. The suppressed PL spectra correspond to the emission lines present in the PL spectra without the THz field (decay from the 14P state). To determine the THz-responsive linewidth, differential PL spectra at THz frequency detuning were recorded, extracting the intensity of the 534.9 nm and 632.2 nm signals as a function of the THz frequency, as shown in Fig. 6b. Based on this result, the PL spectra with THz frequency detuning displayed a full width at half maximum (FWHM) value of approximately 14 MHz.

Fig. 6Full-screen View

(Color online) (a) Evolution of differential PL with THz detuning. (b) The dependence of the differential PL signal on THz detuning. The red and blue dashed lines are guide lines

The PL spectra of cesium atomic Rydberg states are sensitive to the THz field, and the PL spectral intensity was measured at different THz intensities, as shown in Fig. 7. The signal intensity of the cesium Rydberg atom at 534.9 nm increases with an increase in terahertz intensity, whereas the 632.2 nm signal diminishes as THz intensity increases. This observation indicates a robust dependence of the decay channels on THz intensity. Furthermore, at a THz intensity of ∼10^-6 mW, the 534.9 nm signal continues to exhibit a distinct difference from the PL_WithoutTHz base, while the 632.2 nm signal demonstrates a significantly lower intensity than the PL_WithoutTHz base, thereby achieving commendable resolution capability with respect to

Fig. 7Full-screen View

The PL_WithTHz signal about 534.9 nm and 632.2 nm as a function of THz power. The red and blue line was PL_WithoutTHz signal about 534.9 nm and 632.2 nm. The red and blue solid lines represent the signal intensities of the peaks at 534.9 nm and 632.2 nm in the PL spectra without the THz field, serving as the baseline

Basic Information of the Dataset

Rydberg atoms have been extensively studied in areas such as THz imaging. PL spectroscopy was combined with the imaging and testing of the PL spectra while simultaneously recording the THz spectroscopic detection. Therefore, the impact of the atomic vapor cell temperature, THz intensity, and THz frequency detuning on the PL spectra is important. The supporting data for a clear presentation of the dataset and experimental operations were recorded, as shown in the Specifications Table 1.

Dataset location

The datasets contained both raw and analyzed data. All datasets for this study have been uploaded to the “Science Data Bank” as a separate .zip file. (https://doi.org/10.57760/sciencedb.16313https://doi.org/10.57760/sciencedb.16313)

Dataset name and format

The data contained a file of two classes: (1) the data, including both raw data and analyzed data (file format: .dat). (2) This file contains all the original files for the drawings and is in. ai format. All files are provided in English. After obtaining written permission from the Science Data Bank, the data were used publicly for academic research and teaching purposes.

Data Content

The “data” index has five files, containing the raw data and analyzed data with corresponding file names of “raw” and “analyzed”. Figure 2 shows the raw and analyzed atomic PL spectra with and without the THz field. Figure 3 includes the raw and analyzed data of the differential PL spectra. The file in Fig. 4 contains the raw data and original PL spectra data at temperatures ranging from 20 to 62.5 ℃, both with and without THz field conditions. Figure 5 shows the analysis data of cesium atomic PL spectra with and without THz detuning. Figure 6 shows the raw cesium atomic PL spectra and the analysis data at different THz field intensities.

Technical Validation

The Rydberg state of cesium atoms was achieved by laser frequency locking using 852-, 1470-, and 843 nm lasers to match the excitation frequency with the cesium atom resonance transition frequency. The 852 nm probe laser was scanned using a piezoelectric ceramic module to adjust the laser output frequency to match the cesium atom resonance transition frequency. For the 1470 nm coupling laser, the EIT locking method was used for frequency locking. A modulation signal was added to the 843 nm laser, and the error signal caused by the laser was extracted by a phase-locked amplifier and fed back into the laser for frequency locking. The PL spectra were collected using a plano-convex lens in free space and entered the spectrometer slit. The spectrometer was set with a 20 μm slit width in Figs. 1, 2, 3 and 4, with an exposure time of 1 s. The slit width in Fig. 5 is 200 μm, with an exposure time of 0.5 s. The THz field was delivered to a vapor cell in free space.

Usage Notes

THz radiation has broad application prospects in imaging and spectroscopy. The use of the PL spectral differences of Rydberg atoms to achieve THz frequency resolution is a new and validated feasible method. By utilizing the differential PL spectroscopy of Rydberg atoms, this method can be used for THz imaging and THz spectroscopy. The dataset includes PL spectra for identifying a 0.548 THz field, differential PL spectra, PL spectra under THz detuning, and provides differential PL spectra at different frequencies, offering reusable data for researchers. The data in this dataset is in .dat format, which can be opened and directly used by the vast majority of data-processing software. The dataset was uploaded to the Science Data Bank, and both raw and processed data in the database are readily usable without the need for further software processing. In the database, the x axis of the raw data represents the PL spectrum wavelength, whereas the y axis represents the PL intensity.